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SDLRC - Scientific Articles all years by Author - D-Dd
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Abstract: Among mineral inclusions in diamond, sulphides are the most abundant. Also, they are the keel tool for dating diamond formation given their high concentration of highlysiderophile elements. However, the mineralogical nature of these inclusions is not well understood, mainly due to the exsolution of the original, high temperature monosulphide solid solution (Mss) to Fe-, Ni- and Cu-rich endmembers during cooling, obscuring the original composition. This complex exsolution observed in sulphide inclusions in diamonds can also cause problems with Re-Os age determinations if the whole inclusion is not extracted. To overcome this issue, recently, sulphide inclusions have been homogenized at high temperature and controlled oxygen fugacity [1]. However, X-ray diffraction or Raman spectroscopy analyses, required to accurately identify the inclusion phases, and define their degree of crystallographic plus compositional homogeneity, have not been reported. Here we combine for the first time a thorough nondestructive multi-technique characterization of sulphide inclusions in diamonds from the Victor Mine (Canada) with homogenization experiments and isotopic analyses. In particular, we report X-ray diffraction data of the sulphides before and after homogenization, confirming a change from a polycrystalline assemblage of pyrrothite, pentlandite and chalcopyrite to single-crystal Mss. The data are used to reconstruct the Mss’ original bulk composition, define the true bulk isotopic ratios and document any difference in Re- Os isotope systematics.
Abstract: The permafrost zone is expected to be a substantial carbon source to the atmosphere, yet large-scale models currently only simulate gradual changes in seasonally thawed soil. Abrupt thaw will probably occur in <20% of the permafrost zone but could affect half of permafrost carbon through collapsing ground, rapid erosion and landslides. Here, we synthesize the best available information and develop inventory models to simulate abrupt thaw impacts on permafrost carbon balance. Emissions across 2.5?million?km2 of abrupt thaw could provide a similar climate feedback as gradual thaw emissions from the entire 18?million?km2 permafrost region under the warming projection of Representative Concentration Pathway 8.5. While models forecast that gradual thaw may lead to net ecosystem carbon uptake under projections of Representative Concentration Pathway 4.5, abrupt thaw emissions are likely to offset this potential carbon sink. Active hillslope erosional features will occupy 3% of abrupt thaw terrain by 2300 but emit one-third of abrupt thaw carbon losses. Thaw lakes and wetlands are methane hot spots but their carbon release is partially offset by slowly regrowing vegetation. After considering abrupt thaw stabilization, lake drainage and soil carbon uptake by vegetation regrowth, we conclude that models considering only gradual permafrost thaw are substantially underestimating carbon emissions from thawing permafrost.
Diamond and Related Materials, in press available 33p.
Global
DiaMap
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Ventura Santos, R., Souza de Alvarenga, C.J., Babinski, M., Ramos, M.L.S., Cukrov, N., Fonsec, M.A., Da Norbrega
Carbon isotopes of Mesoproterozoic Neoproterozoic sequences from southern Sao Francisco craton and Aracuai Belt, Brazil: paleogeorgraphic implications.
Journal of South American Earth Sciences, Vol. 18, 1, Dec. 30, pp. 27-39.
Journal of South American Earth Sciences, Vol. 94, 102208 13p. Pdf
South America, Brazil
carbonatite
Abstract: Carbonatites are peculiar igneous rocks, consisting mainly of greater than 50% carbonate minerals, which arouse an economic interest due to the potentiality of high phosphate content and Light Rare Earth Elements (LREE) associated with their occurrence. The Passo Feio Carbonatite (PFC) is located 17?km Southwest of Caçapava do Sul city and constitutes NW dipping body, which is interposed with Passo Feio Formation metamorphic rocks. The PFC varies texturally from massive to foliated, being mainly composed of calcites and dolomites and on a smaller scale by apatites, phlogopites and tremolites. The opaque minerals correspond to hematites, magnetites, pyrites and barites, while the accessory minerals are represented by zircons, monazites- (Ce) and aeschynites- (Ce). Probably those REE mineral phases correspond to a hydrothermal stage, with the REE remobilization from apatites into those latter REE-rich mineral phases - this hypothesis is corroborated by geochemistry, mineral chemistry and microtextures found. Considering the results of mineral chemistry and taking into account the textural criteria, it was possible to classify carbonatite as an alvikite, with geochemical patterns that do not indicate economic potential for REE. However, soil geochemistry showed an important enrichment in REE, reflecting a probable concentration of monazites- (Ce) and aeschynites- (Ce), and because of this, it was possible to establish a zone in which the Passo Feio Carbonatite would probably be extended. In the stable isotope analyzes, the ?13C values varied between ?4.14 and ?3.89‰ while those of ?18O between 10.01 and 11.32‰ which can be attributed to the cooling of the magma itself, without suggesting metamorphic processes or subsequent changes. The deformation found in this carbonatite was probably developed in late-magmatic conditions, guided by tectonics associated with horizontal movements in shear zones. Thus, this work suggests that this carbonatite was the product of the reactivation of mantle sources, within a post-collision magmatic context of the Sul-Riograndense Shield.
Brazil Journal of Geology ( www.scielo.br) ENG, 17p. Pdf
South America, Brazil
deposit - Tres Estradas
Abstract: Carbonatites were recently discovered in Southern Brazil, which increased the interest to evaluate the economic potential of these uncommon rocks, especially the Três Estradas Carbonatite. Carbonates are the dominant minerals of fresh rock followed by apatite, but the weathering process makes apatite abundant. We focused on apatite from the carbonatite using conventional petrography and electronic microscopy associated with microprobe, micro-Raman and Fourier-transform infrared spectroscopy. Results demonstrate the existence of four types. The primary type is associated with the rock crystallization and the subsequent three others are associated with weathering processes. The alteration mechanism was favorable for initial carbonate leaching and subsequent increase of phosphate with late precipitation of three new apatite generations. The deduced model involves long exposure during polycyclic climate changes, intercalating periods of warm dry with humid climate. The apatite types differ chemically and morphologically and have distinctive characteristics that are suitable to be used to differentiate them. These properties should be considered in future planes of industrial processes to transform apatite into single superphosphate, a basic input for fertilizer production.
Leal, R.E., Lafon, J.M., da Ros Costa, L.T., Dantas, E.L.
Orosirian magmatic episodes in the erepercuru-trombetas domain ( southeastern Guyana shield: implications for the crustal evolution of the Amazonian craton.
South American Earth Sciences, Vol. 85, pp. 278-297.
Journal of South American Earth Sciences, Vol. 94, 102208 13p. Pdf
South America, Brazil
carbonatite
Abstract: Carbonatites are peculiar igneous rocks, consisting mainly of greater than 50% carbonate minerals, which arouse an economic interest due to the potentiality of high phosphate content and Light Rare Earth Elements (LREE) associated with their occurrence. The Passo Feio Carbonatite (PFC) is located 17?km Southwest of Caçapava do Sul city and constitutes NW dipping body, which is interposed with Passo Feio Formation metamorphic rocks. The PFC varies texturally from massive to foliated, being mainly composed of calcites and dolomites and on a smaller scale by apatites, phlogopites and tremolites. The opaque minerals correspond to hematites, magnetites, pyrites and barites, while the accessory minerals are represented by zircons, monazites- (Ce) and aeschynites- (Ce). Probably those REE mineral phases correspond to a hydrothermal stage, with the REE remobilization from apatites into those latter REE-rich mineral phases - this hypothesis is corroborated by geochemistry, mineral chemistry and microtextures found. Considering the results of mineral chemistry and taking into account the textural criteria, it was possible to classify carbonatite as an alvikite, with geochemical patterns that do not indicate economic potential for REE. However, soil geochemistry showed an important enrichment in REE, reflecting a probable concentration of monazites- (Ce) and aeschynites- (Ce), and because of this, it was possible to establish a zone in which the Passo Feio Carbonatite would probably be extended. In the stable isotope analyzes, the ?13C values varied between ?4.14 and ?3.89‰ while those of ?18O between 10.01 and 11.32‰ which can be attributed to the cooling of the magma itself, without suggesting metamorphic processes or subsequent changes. The deformation found in this carbonatite was probably developed in late-magmatic conditions, guided by tectonics associated with horizontal movements in shear zones. Thus, this work suggests that this carbonatite was the product of the reactivation of mantle sources, within a post-collision magmatic context of the Sul-Riograndense Shield.
Zircon geochronology and Sm Nd isotopic study: further constraints for the Archean and Paleoproterozoic geodynamical evolution of southe eastern Guiana Shield.
International Journal of Earth Sciences, Vol. 108, pp. 89-113.
Africa, Angola
thermochronology
Abstract: There are two main points of view regarding how continental margins evolve. The first one argues that the present-day margins have been developed by long-term denudation since a major exhumation episode, probably driven by rifting or another relevant tectonic event. The second one argues that continental margins underwent alternating burial and exhumation episodes related to crustal tectonic and surface uplift and subsidence. To demonstrate that the proximal domain of the southwestern Angolan margin has evolved in a polycyclic pattern, we present a review of geological and thermochronological information and integrate it with new combined apatite fission-track and (U-Th)/He data from Early Cretaceous volcanic and Precambrian basement samples. We also provide hypotheses on the possible mechanisms able to support the vertical crustal movements of this margin segment, which are also discussed based on some modern rifting models proposed for Central South Atlantic. The central apatite fission-track ages range from 120.6?±?8.9 to 272.9?±?21.6 Ma, with the mean track lengths of approximately 12 µm. The single-grain apatite (U-Th)/He ages vary between 52.2?±?1 and 177.2?±?2.6 Ma. The integration of the thermochronological data set with published geological constraints supports the following time-temperature evolution: (1) heating since the Carboniferous-Permian, (2) cooling onset in the Early Jurassic, (3) heating onset in the Early Cretaceous, (4) cooling onset in the Mid- to Late Cretaceous, (5) heating onset in the Late Cretaceous, and (6) cooling onset in the Oligocene-Miocene. The thermochronological data and the geological constraints, support that the proximal domain of the southwestern Angolan margin was covered in the past by pre-, syn-, and post-rift sediments, which were eroded during succeeding exhumation events. For this margin segment, we show that a development based on long-term denudation is less realistic than one based on burial and exhumation episodes during the last 130 Myr.
Abstract: Mineral chemistry analysis is a valuable tool in several phases of mineralogy and mineral prospecting studies. This type of analysis can point out relevant information, such as concentration of the chemical element of interest in the analyzed phase and, thus, the predisposition of an area for a given commodity. Due to this, considerable amount of data has been generated, especially with the use of electron probe micro-analyzers (EPMA), either in research for academic purposes or in a typical prospecting campaign in the mineral industry. We have identified an efficiency gap when manually processing and analyzing mineral chemistry data, and thus, we envisage this research niche could benefit from the versatility brought by machine learning algorithms. In this paper, we present Qmin, an application that assists in increasing the efficiency of mineral chemistry data processing and analysis stages through automated routines. Our code benefits from a hierarchical structure of classifiers and regressors trained by a Random Forest algorithm developed on a filtered training database extracted from the GEOROC (Geochemistry of Rocks of the Oceans and Continents) repository, maintained by the Max Planck Institute for Chemistry. To test the robustness of our application, we applied a blind test with more than 11,000 mineral chemistry analyses compiled for diamond prospecting within the scope of the Diamante Brasil Project of the Geological Survey of Brazil. The blind test yielded a balanced classifier accuracy of ca. 99% for the minerals known by Qmin. Therefore, we highlight the potential of machine learning techniques in assisting the processing and analysis of mineral chemistry data.
Abstract: The Late Neoproterozoic assembly of western Gondwana played an important role in the subduction of oceanic and continental lithospheres. Such event was also a source of arc magmatism, reworking of cratonic margins and development of ultra-high pressure (UHP) suture zones. In the Borborema province, NE Brazil, we have described for the first time UHP rocks enclosed within gneiss migmatite and calc-silicate rocks. They bear coesite included in atoll-type garnet from metamafic rocks, identified by petrographic study and Raman microspectroscopy analysis. U-Pb zircon dating of the leucosome of the migmatites and the calc-silicate rock displays, concordant ages of 639 ± 10 Ma and 649.7 ± 5 Ma, respectively, here interpreted as the minimum age of the eclogitization event in the region. U-Pb zircon dating of the coesite-bearing rock defined a concordia age of 614. 9 ± 7.9 Ma that comprised the retrograde eclogitic conditions to amphibolite facies. The UHP rocks, mostly retrograded to garnet amphibolites, occur enclosed in the Paleoproterozoic continental block composed of calc-silicate rocks, migmatized sillimanite gneiss, mylonitic augen gneiss and granitic and tonalitic gneiss along a narrow N-S oriented belt between the Santa Quitéria magmatic arc and the Transbrasiliano lineament. This block was involved in the subduction to UHP eclogite depths, and was retrogressed to amphibolite during its exhumation and thrusting. Our data indicate an important Neoproterozoic transcontinental suture zone connecting the Pharusian belt with Borborema Province, and probably with the Brasília belt in central Brazil.
Da Silva Filho, A.F., Guimaraes, I.P., Thompson, R.N.
Shoshonitic and ultrapotassic Proterozoic intrusive suites in the Cachoeirinha-Saigueiro belt, northeast Brasil: a transition collisional to post-collisional magmatism
Precambrian Research, Vol. 62, No. 3, June pp. 323-342
Abstract: T-shaped orogenic triple junctions between mobile belts usually form in two unrelated stages by subsequent and oblique continental collisions separated by a significant time span. Besides these "oblique triple junctions", another type, named "transverse triple junctions", may exist. Such junctions are created by a more complex mechanism of partly contemporaneous convergence of three cratons in a restricted time frame, involving strike slip. The Neoproterozoic-Cambrian Kaoko-Damara junction between the Rio de la Plata, Congo, and Kalahari cratons in Namibia is an example of such a transverse orogenic triple junction, formed by at least four subsequent but partly related deformation events. Initial north-south convergence between the Congo and Kalahari cratons was followed by east-west collision of the Rio de la Plata and Congo cratons. Subsequently, the Kalahari and Congo cratons collided, contemporaneous with sinistral strike-slip motion between the Congo and Rio de la Plata cratons and with the intrusion of large granite-syenite plutons, probably associated with slab detachment aided by the strike-slip movement. Other examples of transverse triple junctions may exist in Gondwana. Transcurrent shear zones, some possibly nucleated on transform faults from the pre-collision setting, are essential for the formation of transverse triple junctions.
Brazil Journal of Geology ( www.scielo.br) ENG, 14p. Pdf
South America, Brazil
deposit - Fazenda Bonfim
Abstract: The Fazenda Bonfim emerald deposit lies within the Seridó Belt. It is a classic example of deposit formed through metasomatic interactions between Be-rich granite intrusions and Cr(± V)-rich mafic-ultramafic rocks. The setting of the emerald mineralization was built under strong strike-slip dynamics, which produced serpentinization and talcification of mafic-ultramafic host-rocks, and was followed by syn-kinematic emplacement of Be-rich albite granite, favoring hydrothermal/metasomatic processes. The structural control and lithological-contrast were fundamental to the fluid flow and the best ore-shoot geometry, developed in the S-foliation intra-plane at the contact zone (phlogopite hornfels) between mafic-ultramafic rocks and the albite granite. Subsequently, an albitization process, linked to the final-stage of magmatic crystallization, led to an overall mineralogical and chemical change of the albite granite. 207U-235Pb data revealed inheritance ages from Archean to Neoproterozoic and a crystallization age of 561 ± 4 Ma for albite granite. However, 40Ar/39Ar data revealed plateau age of 553 ± 4 Ma for phlogopite hornfels, interpreted as the closure time for the metasomatic event responsible for the nucleation and growth of emerald crystals. The short interval of time between U-Pb and Ar-Ar data indicates an intense, but not protracted, metasomatic history, probably due to low volume of intrusive magma.
Contributions to Mineralogy and Petrology, Vol. 176, 10, 28p. Pdf
Canada, Quebec
deposit - Renard
Abstract: This study aims to constrain the nature of kimberlite-xenolith reactions and the fluid origin for Kimberley-type pyroclastic kimberlite (KPK). KPKs are characterized by an abundance of basement xenoliths (15-90%) and display distinct pipe morphology, textures, and mineralogy. To explain the KPK mineralogy deviating from the mineralogy of crystallized kimberlite melt, we study reactions between hypabyssal kimberlite transitional to KPK and felsic xenoliths. Here, we characterize the pectolite-diopside-phlogopite-serpentine-olivine common zonal patterns using petrography, bulk composition, thermodynamic modelling, and conserved element ratio analysis. To replicate the observed mineral assemblages, we extended the thermodynamic database to include pectolite, using calculated density functional theory methods. Our modelling reproduces the formation of the observed distinct mineralogy in reacted granitoid and gneiss. The assimilation of xenoliths is a process that starts from high temperatures (1200-600 °C) with the formation of clinopyroxene and wollastonite, continues at 600-200 °C with the growth of clinopyroxene, garnet, and phlogopite finishing at temperatures?
Abstract: The rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high-resolution seismic reflection data paired with data from industry-drilled wells and proposes a structural model of the Limpopo transform margin in a magma-rich context. Results indicate that the Limpopo transform margin is characterized by an ocean-continent transition that links the Beira-High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano-sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo-Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady-state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE-ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma-rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn-transfer and -transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large-scale differential subsidence along the transform margin.
Watremez, L., Leroy, S., d'Acremont, E., Roche, V., Evain, M., Lepretre, A., Verrier, F., Aslanian, D., Dias, N., Afilhado, A., Schnurle, P., Castilla, R., Despinois, F., Moulin, M.
Abstract: A variety of structures results from the interplay of evolving far-field forces, plate kinematics, and magmatic activity during continental break-up. The east Limpopo transform margin, offshore northern Mozambique, formed as Africa and Antarctica separated during the mid-Jurassic period break-up of the Gondwana supercontinent. The nature of the crust onshore has been discussed for decades in an effort to resolve issues with plate kinematic models. Two seismic refraction profiles with coincident multichannel seismic reflection profiles allow us to interpret the seismic velocity structures across the margin, both onshore and offshore. These seismic profiles allow us to (a) delineate the major regional crustal domains; (b) identify widespread indications of magmatic activity; and (c) map crustal structure and geometry of this magma-rich transform margin. Careful examination of the profiles allows us to make the following observations and interpretations: (a) on land, continental crust is overlain by a >10-km thick volcano-sedimentary wedge related to an early rifting stage, (b) offshore, thick oceanic crust formed due to intense magmatic activity, and between the two (c) a 50-60-km wide transform zone where the crustal structures are affected by intense magmatic activity and faulting. The prominent presence of intrusive and extrusive igneous units may be attributed to the combination of a deep-seated melting anomaly and a trans-tensional fault zone running through thinned lithosphere that allowed melt to reach the surface. A comparison of the crustal thinning along other transform margins shows a probable dependence with the thermal and/or tectonic history of the lithosphere.
Abstract: Subduction of oceanic crust buries an average thickness of 300-500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700-900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2-6 GPa/750-1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1-5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.
Geological Society of London Special Publication Supercontinent Cycles through Earth History., Vol. 424, pp. 167-190.
South America, Brazil
Supercontinents
Abstract: Moderate to high palaeolatitudes recorded in mafic dykes, exposed along the coast of Bahia, Brazil, are partly responsible for some interpretations that the São Francisco/Congo craton was separate from the low-latitude Rodinia supercontinent at about 1050 Ma. We report new palaeomagnetic data that replicate the previous results. However, we obtain substantially younger U-Pb baddeleyite ages from five dykes previously thought to be 1.02- 1.01 Ga according to the 40 Ar/ 39 Ar method. Specifically, the so-called 'A-normal' remanence direction from Salva-dor is dated at 924.2 + 3.8 Ma, within error of the age for the 'C' remanence direction at 921.5 + 4.3 Ma. An 'A-normal' dyke at Ilhéus is dated at 926.1 + 4.6 Ma, and two 'A-normal' dykes at Olivença have indistinguishable ages with best estimate of emplacement at 918.2 + 6.7 Ma. We attribute the palaeomagnetic variance of the 'A-normal' and 'C' directions to lack of averaging of geomagnetic palaeosecular variation in some regions. Our results render previous 40 Ar/ 39 Ar ages from the dykes suspect, leaving late Mesoproterozoic palaeolatitudes of the São Francisco/Congo craton unconstrained. The combined 'A-normal' palaeomagnetic pole from coastal Bahia places the São Francisco/Congo craton in moderate to high palaeolatitudes at c. 920 Ma, allowing various possible positions of that block within Rodinia. Despite more than two decades of intense global research, the configuration of Neoproterozoic supercontinent Rodinia remains enigmatic. Following the first global synthesis by Hoffman (1991), most models include a central location for Laurentia, flanked by 'East' Gondwana-Land cra-tons along its proto-Cordilleran margin and 'West'
40 Ar-39 Ar dating of 1.0-1.1 Ga magnetizations from the Sao Francisco and Kalahari cratons: tectonic implicationsPan-African and Brasiliano mobilebelts
Earth and Planetary Science Letters, Vol. 101, No. 2/4, December pp. 349-367
Heilbron et al. eds. Sao Francisco Craton, eastern Brazil, Chapter 16, 17p. Researchgate
South America, Brazil
geochronology
Abstract: This chapter, based on paleomagnetic and geologic-geochronological evidence, discusses the position of the São Francisco craton and other South American and African cratonic blocks within paleo-continents, since the formation of Columbia supercontinent in the Paleoproterozoic up to the fragmentation of Pangea in the Mesozoic. In Paleoproterozoic times, between ca. 2.0 and 1.8 Ga, two large independent landmasses were formed. The first one involved several cratonic blocks that were leading to the formation of Laurentia. Later, Laurentia, proto-Amazonia, West Africa and Baltica amalgamated to form the nucleus of the supercontinent Columbia at about 1.78 Ga. The second landmass encompassed the São Francisco-Congo, Kalahari, Rio de la Plata and Borborema-Trans-Sahara, forming the Central African block. For the São Francisco-Congo and Kalahari cratons, two robust Paleoproterozoic poles are available. One is from the Jequié charnockites of Bahia (São Francisco Craton), and the other from the Limpopo high-grade metamorphics in South Africa (Kalahari Craton). They support the possible link between these two cratonic blocks at ca. 2.0 Ga. Columbia may have remained united until 1.25 Ga, when Baltica and Amazonia/West Africa broke apart. Their paleomagnetic record seems to indicate that both executed clockwise rotations, until they collided with Laurentia along the Grenville belt at ca. 1.0 Ga., culminating with the formation of Rodinia. For the Central African block, however, there are no reliable paleomagnetic poles available between 1.78 and 1.27 MA. Nevertheless, during this time interval, the geological-geochronological evidence indicates that no continental collisional episodes affected the São Francisco-Congo craton, where important intra-plate tectonic episodes occurred. Most probably, this large continental block drifted alone since the end of the Paleoproterozoic and did not take part of Columbia or Rodinia. At the end of the Mesoproterozoic, ca. 1100 MA, the robust Umkondo pole of the Kalahari craton, as part of the Central African block, and the equally robust Keweenawan pole of Laurentia at the center of Rodinia, indicated that these landmasses were very far apart. At that time a large oceanic realm, the Goiás-Pharusian Ocean, was indeed separating Amazonia-West Africa from the Central African block. This ocean closed by a continued subduction process that started at ca. 900 MA and ended in a collisional belt with Himalayan-type mountains at ca. 615 MA, as part of the few continental collisions which formed Gondwana. However, the age of the final convergence is still a matter of debate, because paleomagnetic measurements for the Araras Group, which occurs within the Paraguay belt at the eastern margin of the Amazonian craton, would indicate that a large ocean was still in existence between it and São Francisco craton close to the Ediacaran/Cambrian boundary. Consensus about this matter awaits for further paleomagnetic data. Gondwana collided with Laurasia during the late Paleozoic, at about 300 Ma, originating Pangea, which not much later started splitting apart, near the Permian/Triassic boundary. As part of this present-time plate tectonic regime, the São Francisco Craton (in South America) started separation from the Congo craton (in Africa) in Jurassic times, giving rise of the present-day oceanic lithosphere of the Atlantic Ocean.
Chapter 6,, Personen et al. ed. Precambrian Supercontinents, 99p. Pdf
South America
craton
Abstract: Here, we reassess the paleomagnetic database for Amazonia and its geodynamic implications for supercontinents. According to paleomagnetic and geological data, Amazonia and West Africa joined at ca. 2.00 Ga defining a single long-lived block. This landmass eventually formed a part of the Columbia supercontinent together with Baltica and Laurentia between 1.78 and 1.42 Ga. For the formation of Rodinia paleomagnetic and geological data permit three different models: an oblique collision at ca. 1.2 Ga, a clockwise rotation of Amazonia/West Africa and Baltica from Columbia to Rodinia joining Laurentia at ca. 1.0 Ga, or a scenario where Amazonia/West Africa was a wandering block that did not take part in Columbia and collided with Laurentia to form Rodinia at ca. 1.0-0.95 Ga. The time Amazonia/West Africa joined Gondwana is also debatable, with contrasting geochronological and geological evidence supporting an early collision at 0.65-0.60 Ga or a late collision at 0.53-0.52 Ga.
Abstract: Here, we reassess the paleomagnetic database for Amazonia and its geodynamic implications for supercontinents. According to paleomagnetic and geological data Amazonia and West Africa joined at c.2.00 Ga defining a single long-lived block. This landmass eventually formed a part of the Columbia supercontinent together with Baltica and Laurentia between 1.78 and 1.42 Ga. For the formation of Rodinia paleomagnetic and geological data permit three different models: an oblique collision at c.1.2 Ga, a clockwise rotation of Amazonia/West Africa and Baltica from Columbia to Rodinia joining Laurentia at c.1.0 Ga, or a scenario where Amazonia/West Africa were a wandering block that did not take part in Columbia and collided with Laurentia to form Rodinia at c.1.0-0.95 Ga. The time Amazonia/West Africa joined Gondwana is also debatable, with contrasting geochronological and geological evidence supporting an early collision at 0.65-0.60 Ga or a late collision at 0.53-0.52 Ga.
Abstract: For approximately the first 2?billion years of the Earth’s history, atmospheric oxygen levels were extremely low. It was not until at least half a billion years after the evolution of oxygenic photosynthesis, perhaps as early as 3?billion years ago, that oxygen rose to appreciable levels during the Great Oxidation Event. Shortly after, marine carbonates underwent a large positive spike in carbon isotope ratios known as the Lomagundi event. The mechanisms responsible for the Great Oxidation and Lomagundi events remain debated. Using a carbon-oxygen box model that tracks the Earth’s surface and interior carbon fluxes and reservoirs, while also tracking carbon isotopes and atmospheric oxygen levels, we demonstrate that about 2.5?billion years ago a tectonic transition that resulted in increased volcanic CO2 emissions could have led to increased deposition of both carbonates and organic carbon (organic?C)?via enhanced weathering and nutrient delivery to oceans. Increased burial of carbonates and organic?C would have allowed the accumulation of atmospheric oxygen while also increasing the delivery of carbon to subduction zones. Coupled with preferential release of carbonates at arc volcanoes and deep recycling of organic?C to ocean island volcanoes, we find that such a tectonic transition can simultaneously explain the Great Oxidation and Lomagundi events without any change in the fraction of carbon buried as organic?C relative to carbonate, which is often invoked to explain carbon isotope excursions.
Abstract: Paleoproterozoic suture zones mark the formation of supercontinent Nuna and provide a record of North America's assembly. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of Laurentia, long after peak metamorphism in the Trans-Hudson orogen. Using paleomagnetic data from the newly dated 1899 ± 5 Ma Sourdough mafic dike swarm (Wyoming craton), we compare the relative positions of Wyoming, Superior, and Slave cratons before, during, and after peak metamorphism in the Trans-Hudson orogen. With these constraints, we refine a collisional model for Laurentia that incorporates Wyoming craton after Superior and Slave cratons united, redefining the Paleoproterozoic sutures that bind southern Laurentia.
Abstract: The chemical composition of Earth's mantle can tell us how our planet formed and how subsequent mantle dynamics have since homogenized the mantle through convective processes. Most terrestrial rocks have a similar tungsten (W) isotope composition (1), but some rocks that have been dated at 2.8 Ga (billion years old) (2), 3.8 Ga (3), and 3.96 Ga (4) have elevated 182W/184W ratios. This is reported as µ182W, in parts per million (ppm) deviation from the bulk silicate Earth. Until now, the outliers have included only these ancient rock samples with a small µ182W excess (?15 ppm) that can be attributed to the final ?0.5% of Earth's mass that accreted late in its accretion history. On page 809 of this issue, Rizo et al. (5) report W isotope data from young mantle-derived rocks with µ182W excesses of 10 to 48 ppm. This result is spectacular because the range of µ182W values in mantle-derived rocks is larger than can be accommodated by late accretion; the implication is that remnants of Earth's earliest mantle have been preserved over the entirety of Earth's history.
New 207 Pb 206 Pb and 40 Ar 39 Ar ages from SW Montana: constraints on the Proterozoic and Archean tectonic and depositional history of the Wyoming Province.
Abstract: Nitrogen availability is a pivotal control on terrestrial carbon sequestration and global climate change. Historical and contemporary views assume that nitrogen enters Earth’s land-surface ecosystems from the atmosphere. Here we demonstrate that bedrock is a nitrogen source that rivals atmospheric nitrogen inputs across major sectors of the global terrestrial environment. Evidence drawn from the planet’s nitrogen balance, geochemical proxies, and our spatial weathering model reveal that ~19 to 31 teragrams of nitrogen are mobilized from near-surface rocks annually. About 11 to 18 teragrams of this nitrogen are chemically weathered in situ, thereby increasing the unmanaged (preindustrial) terrestrial nitrogen balance from 8 to 26%. These findings provide a global perspective to reconcile Earth’s nitrogen budget, with implications for nutrient-driven controls over the terrestrial carbon sink.
Abstract: Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Abstract: The Fen Complex is a 2 km-wide subcircular intrusion composed mainly of sovite, Fe-dolomite carbonatite, damtjernite (lamprophyre) and minor alkaline rocks such as nepheline syenite and ijolite, emplaced at 580 Ma through Mesoproterozoic orthogneisses forming the Fennoscandian Shield. Previous bulk-rock isotopic study indicates that the carbonatite magma originated in the upper mantle [(87Sr/86Sr)i = 0.7029] and underwent contamination during its ascent throughout the crust. This study focuses on two deep cores (1000 m and 700 m), drilled to assess the distribution of REE mineralizations in the Fe-dolomite carbonatite. Hyperspectral data, allowing investigators to log cores objectively and quantify lithologies, were acquired using a SisuRock Gen 2 system composed of three cameras gathering data in the following wavelengths: RGB, Near-Visible Short-Wave Infrared (VN-SWIR) and Long-Wave Infrared (LWIR). In addition, every meter of the first core and 500 m of the second one were analyzed for bulk-rock geochemistry to characterize the distribution of elements. In this study, we compare the results obtained by the imaging technique with the bulk-rock data and present preliminary results of the textural variations observed in rare-earth mineralizations. Preliminary results indicate that neither of the deep bore holes reached the fenitized host-rock and that the Fe-dolomite carbonatite continues at depth. In both cores, the dominant carbonate is Fe-rich dolomite, although calcite and Fe-Mg carbonate have been observed locally. REE-minerals, composed mainly of bastnäsite, parisite/synchisite and monazite, display variable textural relationships and often occur together in clusters associated with barite and minor Fe-oxides, sulfides (pyrite ± sphalerite) and locally thorite.
Mineralogy and Petrology, doi.org/10.1007/ s00710-021-00736-0 11p. Pdf
Europe, Norway
carbonatite
Abstract: Carbonatites in the Fen intrusive complex (southern Norway) contain abundant burbankite (confirmed by Raman microspectroscopy) as inclusions in calcite, dolomite and, less commonly, fluorapatite and pyrochlore. Typically the inclusions occur in the core of calcite or dolomite grains relatively unaffected by subsolidus processes, and are associated with Fe-poor dolomite or Sr-rich calcite, respectively. Burbankite does not exceed 30?×?50 ?m in size and is characteristically absent from the peripheral areas of carbonate grains affected by recrystallization or interaction with fluids. Compositionally, the mineral falls within the following range: (Na1.51-2.16Ca0.58-1.21)(Sr1.50-2.42Ca0.28-0.57LREE0.05-0.64Ba0.06-0.41)(CO3)5 and contains low Th, but no detectable Mg, Fe or F (LREE?=?light rare-earth elements: Ce?>?La?>?Nd?>?Pr?>?Sm). Burbankite inclusions at Fen are interpreted as primary and indicative of Na enrichment in their parental carbonatitic magma. Dissociation of burbankite during subsolidus re-equilibration of its host phases with fluids undoubtedly served as one of the sources of LREE for the development of late-stage mineralization in the Fen complex.
Abstract: From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ?4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.
Lithospheric and asthenospheric sources of lamprophyres in the Jiadong Peninsula: a consequence of rapid lithospheric thinning beneath the North Chin a craton?
Geochimica et Cosmochimica Acta, Vol. 124, pp. 250-271.
Abstract: The North China Craton is a classic case for the destruction of an ancient craton, in that it records the loss of more than 100 km of ancient refractory lithospheric mantle during the late Mesozoic and early Cenozoic. However, the mechanisms for this lithospheric thinning remain controversial in large part due to the lack of any systematic investigations of the Mesozoic asthenospheric mantle via its derived mafic rocks, which are key to understand the thinning processes. In this paper, we present detailed zircon U-Pb geochronology, elemental geochemistry, and Sr-Nd-Hf isotopic data for lamprophyres and diabase-porphyries of the Jiaodong Peninsula, in the eastern North China Craton in order to place constraints on models for lithospheric thinning. Our results show that the lamprophyres and diabase-porphyries are derived from the convective asthenospheric mantle via different degrees of partial melting, and that this mantle source was previously modified by carbonatitic liquids. Zircon LA-ICP-MS U-Pb dating suggests an emplacement age for these rocks of 123-121 Ma, the earliest evidence for asthenospherically-derived melts in the Jiaodong Peninsula so far. This emplacement age indicates that the thickness of the lithosphere in the Jiaodong Peninsula was relatively thin at that time. Co-occurrence of the asthenospheric and lithospheric mantle-derived mafic rocks as well as high-Mg adakites record a rapid transition from lithospheric to asthenospheric mantle sources, indicating that the lithosphere beneath the Jiaodong Peninsula was rapidly detached just prior to ca. 120 Ma. Lithospheric thinning of the North China Craton may have been initiated from the Jiaodong Peninsula and Bohai Sea and then propagated towards the interior of the craton.
Abstract: Gemmological and spectroscopic characteristics are reported for two type IIa blue CVD synthetic diamonds from Huzhou SinoC Semiconductor Science and Technology Co. Ltd, China. These are the first relatively large (1.76 and 2.63 ct) blue CVD synthetics examined in NGTC’s laboratories, and their colour was slightly brighter than other blue synthetic diamonds that we have encountered. In the DiamondView, they fluoresced blue (with purple-red in one sample), which is unusual for CVD synthetics. The mid- and near-IR absorption spectra of one sample showed no hydrogen-related features, while the other synthetic diamond showed a weak absorption at 6853 cm?1attributed to hydrogen. The spectra of both samples had a very weak line at 1332 cm?1 due to isolated nitrogen and a distinct band at 9282 cm-1 related to radiation. A very strong GR1 absorption feature was detected by UV-Vis-NIR spectroscopy. Photoluminescence spectra obtained at liquid-nitrogen temperature recorded emissions related to radiation (mainly in the 480-510 nm region), N-V and [Si-V]- centres, and several unassigned weak emissions. This combination of optical centres strongly suggests that these samples underwent post-growth treatment to improve their transparency before they were irradiated to produce blue colouration.
Journal of Petrology, 10.1093/petrology/egaa079 110p. Pdf
China
xenoliths
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ? 90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880 ? 970 oC), and low-Mg# (70 ? 80) plagioclase-bearing websterites with low equilibration temperatures (550 ? 835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd?=?+11.41, 87Sr/86Sr = ?0.7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0.703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14.20 ? -16.74, 87Sr/86Sr?=?0.7070 ? 0.7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr?=?0.706-0.711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian ocean and the Late Mesozoic lithospheric extension of eastern Asia.
Abstract: Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ?90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880-970 oC), and low-Mg# (70-80) plagioclase-bearing websterites with low equilibration temperatures (550-835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (?Nd = +11•41, 87Sr/86Sr = ?0•7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ?0•703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (?Nd = -14•20- -16•74, 87Sr/86Sr = 0•7070-0•7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0•706-0•711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian Ocean and the Late Mesozoic lithospheric extension of eastern Asia.
Abstract: The dynamics of formation and exhumation of high-pressure (HP) and ultra-high pressure (UHP) metamorphic orogens in double subduction-collision zones remain enigmatic. Here we employ two-dimensional thermo-mechanical numerical models to gain insights on the exhumation of HP-UHP metamorphic rocks, as well as their deformation during the collision of a micro-continent with pro- and retro-continental margins along two subduction zones. A three-stage collisional process with different convergence velocities is tested. In the initial collisional stage, a fold-and-thrust belt and locally rootless superimposed folds are developed in the micro-continent and subduction channel, respectively. In the second (exhumation) stage of HP-UHP rocks, a faster convergence model results in upwelling of the asthenosphere, which further leads to a detachment between the crust and lithospheric mantle of the micro-continent. A slower convergence model results in rapid exhumation of HP-UHP rocks along the north subduction channel and a typical piggy-back thrusting structure in the micro-continent. A non-convergence model produces a slab tear-off, leading to the rebound of residual lithosphere of the micro-continent. In the third and final stage, a series of back and ramp thrusts are formed in the micro-continent with the pro-continent re-subducted. Based on an analogy of our numerical results with the Western Dabie Orogen (WDO), we suggest that: (1) slab tear-off results in a rebound of residual lithosphere, which controls the two-stage syn-collisional exhumation process of HP-UHP rocks in the WDO; and (2) in contrast to the single subduction-collision system, the exhumation range of the partially molten rocks with lower viscosity and density is restricted to a specific region of the micro-continent by the Mianlue and Shangdan subduction zones, which generated the complex deformation features in the WDO.
Chen, Y.X., Zheng, Y-F., Chen, R-X., Zhang, S-B., Li, Q., Dai, M., Chen, L.
Metamorphic growth and recrystallization of zircons in extremely 18 O depleted rocks during eclogite facies metamorphism: evidence from U-Pb ages, trace elements and O-Hf isotopes.
Geochimica et Cosmochimica Acta, Vol. 75, 17, pp. 4877-4898.
Abstract: Most carbonatite-related REE (rare earth element) deposits record two stages of REE enrichment: magmatic and magmatic-hydrothermal. It is generally accepted that the first stage of enrichment, which occurs in magmas associated with carbonatite-syenite complexes, is a prerequisite to the formation of REE deposits. The magmatic-hydrothermal process is also important, as demonstrated by the fact that many fertile carbonatite-syenite complexes do not produce REE deposits. The Miaoya carbonatite-syenite complex is prospective for REE and is ideal for studies of the formation of REE deposits. The Miaoya REE prospect lies in the western member of the Wudan Terrane of the Qinling Belt, China, and is hosted by a carbonatite-syenite complex that was intruded along a fault zone between schist of the lower Silurian Meiziya Group and adjacent Proterozoic quartzite. Mineralization at the Miaoya REE prospect includes carbonatite-, syenite-, and mixed-type, all low grade (about 1%). Results of X-ray diffraction (XRD) and electron probe micro-analyzer (EMPA) analyses reveal that modes of REE minerals are low in all samples (<5%), which is consistent with the fact that less monazite, bastnäsite and other REE minerals have been found in the Miaoya REE prospect. REE mineralization is less likely to occur as an overprint on gangue minerals. Results of Photon Laser Ablation Inductively-Coupled-Plasma Mass-Spectrometer (LA-ICP-MS) analyses reveal that apatite and calcite in carbonatite have the highest REE concentrations which are responsible for the relatively high concentration in carbonatite rather than because of the presence of REE minerals. The consistence of Sr-Nd isotopes ratios between altered host rocks and fresh hosted rocks suggested REE mineralization originates directly from the unmineralized carbonatite-syenite complex rather than other host rocks. Carbon and oxygen isotope ratios of hydrothermal calcite are consistent with low-temperature alteration subsequent to ore. Trace element ratios for the Miaoya carbonatite-syenite complex lie in the barren carbonatite field (REEs vs. CaO/MgO, FeO/MgO, Ba and Sr/Ba) compared with those of other giant or large carbonatite-syenite complex related REE deposits, just below the boundary between fields for fertile and barren carbonatites. This suggests that the carbonatite-syenite complex at the Miaoya prospect did not have the potential to produce large or giant REE deposits. The low REE of the Miaoya prospect compared with other carbonatite-syenite hosted deposits may reflect: 1) as supported by petrography, minimal tectonic deformation in the area resulting in 2) restricted cycling of hydrothermal solutions that led to 3) minimal fluid scavenging from REE-rich apatite and calcite for local REE re-deposition and concentration.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3311-3327.
Africa, South Africa
geophysics
Abstract: We conduct a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion from both ambient noise and earthquakes using data from 79 seismic stations in southern Africa, which is home to some of the world's oldest cratons and orogenic belts. The area has experienced two of the largest igneous activities in the world (the Okavango dyke swarm and Bushveld mafic intrusion) and thus is an ideal locale for investigating continental formation and evolution. The resulting 3?D shear wave velocities for the depth range of 0-100 km and crustal thickness measurements show a clear spatial correspondence with known geological features observed on the surface. Higher than normal mantle velocities found beneath the southern part of the Kaapvaal craton are consistent with the basalt removal model for the formation of cratonic lithosphere. In contrast, the Bushveld complex situated within the northern part of the craton is characterized by a thicker crust and higher crustal Vp/Vs but lower mantle velocities, which are indicative of crustal underplating of mafic materials and lithospheric refertilization by the world's largest layered mafic igneous intrusion. The thickened crust and relatively low elevation observed in the Limpopo belt, which is a late Archean collisional zone between the Kaapvaal and Zimbabwe cratons, can be explained by eclogitization of the basaltic lower crust. The study also finds evidence for the presence of a stalled segment of oceanic lithosphere beneath the southern margin of the Proterozoic Namaqua?Natal mobile belt.
Geostandards and Geoanalytical Research, in press available, 16p.
Australia
deposit - Mud Tank
Abstract: Zircon megacrysts from the Mud Tank carbonatite, Australia, are being used in many laboratories as a reference material for LA?ICP?MS U?Pb dating and trace element measurement, and LA?MC?ICP?MS determination of Hf isotopes. We summarise a database of > 10000 analyses of Mud Tank zircon (MTZ), collected from 2000 to 2018 during its use as a secondary reference material for simultaneous U?Pb and trace element analysis, and for Hf?isotope analysis. Trace element mass fractions are highest in dark red?brown stones and lowest in colourless and gem?quality ones. Individual unzoned grains can be chemically homogeneous, while significant variations in trace element mass fraction are associated with oscillatory zoning. Chondrite?normalised trace element patterns are essentially parallel over large mass fraction ranges. A Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) is taken as the age of crystallisation. Some grains show lower concordant to mildly discordant ages, probably reflecting minor Pb loss associated with cooling and the Alice Springs Orogeny (450-300 Ma). Our weighted mean 176Hf/177Hf is 0.282523 ± 10 (2s, n = 9350); the uncertainties on this ratio reflect some heterogeneity, mainly between grains. A few analyses suggest that colourless grains have generally lower 176Hf/177Hf. MTZ is a useful secondary reference material for U?Pb and Hf?isotope analysis, but individual grains need to be carefully selected using CL imaging and tested for homogeneity, and ideally should be standardised by solution analysis.
Abstract: New mineralogical and isotope-geochemical data for zircon megacrysts (n = 48) from alluvium of Kholomolokh Creek (a tributary of the Ebelakh River) are reported. Using the geochemical classification schemes, the presence of zircons of kimberlitic and carbonatitic genesis was shown. The U-Pb dating of zircons revealed two major age populations: the Triassic (258-221 Ma, n = 18) and Jurassic (192-154 Ma, n = 30). Weighted mean 206Pb/238U ages allowed us to distinguish the following age stages: 155 ± 3, 161 ± 2, 177 ± 1.5, 183 ± 1.5, 190 ± 2, 233 ± 2.5, and 252 ± 4 Ma. It is suggested that the Ebelyakh diamonds could have been transported from the mantle depths by kimberlite, as well as by other related rocks, such as carbonatite, lamprophyre, lamproite, olivine melilitite, etc. Diamonds from the Ebelyakh placers most likely have polygenic native sources and may be associated with polychronous and multistage Middle Paleozoic and Mesozoic kimberlite and alkaline-ultrabasic magmatism in the eastern slope of the Anabar Shield (the Ebelyakh, Mayat, and Billyakh river basins).
Abstract: The breakup of continents and their subsequent drifting plays a crucial role in the Earth's periodic plate aggregation and dispersal cycles. While continental aggregation is considered the result of oceanic closure during subduction, what drives sustained divergence in the following stages remains poorly understood. In this study, thermo-mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents. We quantify the drag exerted by subduction-induced mantle flow along the basal surface of continental plates, comparing models of lithospheric slab stagnation above the upper-lower mantle boundary with those where slabs penetrate into the lower mantle. When subduction is upper-mantle confined, divergent basal tractions localise at distances comparable to the effective upper mantle thickness (~ 500 km), causing the opening of a marginal basin. Instead, subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench (~ 3000 km). Sub-continental tractions are higher and more sustained over longer time periods in this case, and progressively increase as the slab sinks deeper. Although relatively low, basal-shear stresses when integrated over large plates, generate tension forces that may exceed the strength of the continental lithosphere, eventually leading to breakup and opening of a distal basin. The models illustrate the emergence of a similar mechanism, which results in the formation of back-arc basins above upper-mantle confined subduction, and scales to much larger distances for deeper subduction. Examples include the Atlantic Ocean formation and drifting of the South and North American plates during the Mesozoic-Cenozoic Farallon plate subduction.
Abstract: The breakup of continents and their subsequent drifting plays a crucial role in the Earth's periodic plate aggregation and dispersal cycles. While continental aggregation is considered the result of oceanic closure during subduction, what drives sustained divergence in the following stages remains poorly understood. In this study, thermo-mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents. We quantify the drag exerted by subduction-induced mantle flow along the basal surface of continental plates, comparing models of lithospheric slab stagnation above the upper-lower mantle boundary with those where slabs penetrate into the lower mantle. When subduction is upper-mantle confined, divergent basal tractions localise at distances comparable to the effective upper mantle thickness (~ 500 km), causing the opening of a marginal basin. Instead, subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench (~ 3000 km). Sub-continental tractions are higher and more sustained over longer time periods in this case, and progressively increase as the slab sinks deeper. Although relatively low, basal-shear stresses when integrated over large plates, generate tension forces that may exceed the strength of the continental lithosphere, eventually leading to breakup and opening of a distal basin. The models illustrate the emergence of a similar mechanism, which results in the formation of back-arc basins above upper-mantle confined subduction, and scales to much larger distances for deeper subduction. Examples include the Atlantic Ocean formation and drifting of the South and North American plates during the Mesozoic-Cenozoic Farallon plate subduction.
Abstract: Thermo-mechanical thinning of the lithosphere by mantle plumes is essential for intra-plate volcanism, the initiation of rifting, the evolution of Earth’s lower continental crust and the genesis of metals, diamonds and hydrocarbons. To develop a new understanding of how a mantle plume thins the overlying lithosphere beneath moving plates, we use 2-D and 3-D numerical models based on a finite-element discretization on anisotropic adaptive meshes. Our models include Earth-like material properties for the upper mantle (e.g. temperature and viscosity contrasts, non-Newtonian rheology) discretised at a local mesh resolution that has previously been considered intractable. In our simulations, a plume is injected at the base of the model (670 km depth) with a prescribed mass flux that is consistent with surface observations of topographic swells: from 0.5 (e.g. Louisville, Bermuda, Darfur) to 7 Mg/s (Hawaii). We undertake a systematic numerical study, across a wide parameter space, to investigate the effect of plume buoyancy flux, plate velocity, rheology law and Rayleigh number on processes leading to a reduction of the depth of the Lithosphere Asthenosphere boundary (LAB), such as small-scale convection (SSC) (‘dripping’), or delamination of the lower lithosphere.
Osmium isotopes in Baffin Island and West Greenland picrites: implications for the 187 Os and 188 Os composition of the convection mantle and nature 3He/4he
Earth and Planetary Interiors, Vol. 278, 3-4, pp. 267-277.
Wittig, N., Webb, M., Pearson, D.G., Dale, C.W., Ottley, C.J., Hutchison, M., Jensen, S.M., Luget, A.
Formation of the North Atlantic craton: timing and mechanisms constrained from Re-Os isotope and PGE dat a of peridotite xenoliths from S.W. Greenland.
Wittig, N., Webb, M., Pearson, D.G., Dale, C.W., Ottley, C.J., Hutchison, M., Jensen, S.M., Luget, A.
Formation of the North Atlantic craton: timing and mechanisms constrained from Re-Os isotope and PGE dat a of peridotite xenoliths from S.W. Greenland.
Methods and Applications in Flouresence, Vol. 8, 1, 01404 htpp:dx.doi.org/10.1088/2050-6120/ab4eac
Global
luminescence
Abstract: We report a multidimensional luminescence microscope providing hyperspectral imaging and time-resolved (luminescence lifetime) imaging for the study of luminescent diamond defects. The instrument includes crossed-polariser white light transmission microscopy to reveal any birefringence that would indicate strain in the diamond lattice. We demonstrate the application of this new instrument to detect defects in natural and synthetic diamonds including N3, nitrogen and silicon vacancies. Hyperspectral imaging provides contrast that is not apparent in conventional intensity images and the luminescence lifetime provides further contrast.
Abstract: The Cretaceous Juína Kimberlite Province (JKP, 95-92 Ma) is located in the southwest of the Amazonian Craton, northwest of Mato Grosso, Brazil. Here we present new geochemical and isotopic data of garnet (n=187) and zircon (n=25) megacrysts collected from the KS2 kimberlite. The magmatic zircon megacrysts have U-Pb ages of 92.1 ± 0.7 Ma. The chondrite-normalized rare earth element (REE) patterns (LREE
Abstract: Super Deep Diamonds (SDD) are known to form at depths between ~300 and ~1000 km in the Earth’s mantle [1]. These diamonds as well as their minerals, melts and fluid inclusions are rare natural materials from deep Earth. The aim of this study is to indentify and characterize mineral inclusions in diamonds from Juína, Mato Grosso, Brazil, and hence classify them as SDD (or not). Twelve diamonds from four different mining sites of Juína were selected according to their inclusions using an Estereo Microscope. The main diamond features were based on crystallographic faces, shape, degrees of resportion, crystal state and intergrowing [2]. Diamond samples are transparent, colorless and present octahedro, octahedro-tetrahexahedral and tetrahexahedral habits. Some diamonds show trigons with positive and negative relief, and hexagons with negative relief. Four diamonds are heavily resorbed and were classified as "unknowing habits", as their shapes are distorced and fragmented. Moreover, three samples show abrasion on the vertices of the quartenary axes, and the others have distinct degrees of resorption. Some crystals present intergorwth, such as contact twins (macle) in {111} or aggregates. All diamonds have mineral inclusions of different colors. Most inclusions are black and could be carbon spots, oxides or even silicates, such as olivine. Other inclusions are yellow to red, which might indicate garnet. In addition, blue inclusions were observed, and could be sulphides. The next steps consists of Fourier Transform Infrared (FTIR) to determine diamond nitrogen impurities, and Micro-Raman spectroscopy and X-Ray Diffraction analyses using Synchrotron radiation to determine in situ the chemical composition of mineral inclusions.
Ferrando, S., Frzzotti, M.L., Dallai, L., Compagnoni, R.
Multiphase solid inclusions in UHP rocks ( Su-Lu, China): remnants of supercritical silicate rich aqueous fluids released during continental subduction.
Chemical Geology, Vol. 223, 1-3, Nov. 22, pp. 68-81.
Abstract: Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr-Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1-2 GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions. The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.
Abstract: The continental margin of the Guiana Shield formed at the intersection of the Central and Equatorial Atlantic Oceans that developed one after the other and, in doing so, achieved the break-up of the Gondwana supercontinent. To form these Ocean, the continent crust was stretched and broke but the way this thinning is actually varying along the margin and the causes are not known so we used offshore industrial data to map it. This allows us showing that the width of the continental margin depends primarily on the direction along which the crust was thinned such that the continental margin width is much wider (200-300 km) in domains where this direction is perpendicular to the margin than in domain where it is oblique (<100 km). This also allow us showing that the continental margin resulting from the opening of the Central Atlantic Ocean is systematically wider than the one resulting from the opening of the Equatorial Atlantic. Additionally, our observations suggest that Central Atlantic Ocean opened under warmer conditions than the Equatorial Atlantic. Finally, the area at the intersection of the Central and Equatorial Atlantic Oceans, individualized a promontory of continental crust that formed the present-day Demerara Plateau.
Abstract: U-Pb age data from southwest Madagascar provide a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580 Ma and 520 Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630-600 Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or which represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580-520 Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and to reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anosyen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.
Abstract: Southern and western Madagascar is comprised of five tectonic provinces that, from northeast to southwest, are defined by the: (i) Ikalamavony, (ii) Anosyen, (iii) Androyen, (iv) Graphite and (v) Vohibory Domains. The Ikalamavony, Graphite and Vohibory Domains all have intermediate and felsic igneous protoliths of tonalite-trondhjemite-granodiorite-granite composition, with positive ?Nd, and low Sr and Pb isotopic ratios. All three domains are interpreted to be the products of intra-oceanic island arc magmatism. The protoliths of the Ikalamavony and Graphite Domains formed repectively between c. 1080-980?Ma and 1000-920?Ma, whereas those of the Vohibory Domain are younger and date to between c. 670-630?Ma. Different post-formation geologic histories tie the Vohibory-Graphite and Ikalamavony Domains to opposite sides of the pre-Gondwana Mozambique Ocean. By contrast, the Androyen and Anosyen Domains record long crustal histories. Intermediate to felsic igneous protoliths in the Androyen Domain are of Palaeoproterozoic age (c. 2200-1800?Ma), of tonalite-trondhjemite-granodiorite-granite composition, and show negative ?Nd, moderate to high 87Sr/86Sr and variable Pb isotopic compositions. The felsic igneous protoliths of the Anosyen Domain are of granitic composition and, when compared to felsic gneisses of the Androyen Domain, show consistently lower Sr/Y and markedly higher Sr and Pb isotope ratios. Like the Vohibory and Graphite Domains, the Androyen Domain can be linked to the western side of the Mozambique Ocean, while the Anosyen Domain shares magmatic and detrital zircon commonalities with the Ikalamavony Domain. It is consequently linked to the opposing eastern side of this ocean. The first common event observed in all domains dates to c. 580-520?Ma and marks the closure of the Mozambique Ocean. The trace of this suture lies along the boundary between the Androyen and Anosyen Domains and is defined by the Beraketa high-strain zone.
Earth and Planetary Science Letters, Vol. 458, 1, pp. 141-151.
Mantle
carbon
Abstract: One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of fO2 from ?IW?0.4 to ?IW?3.5 at 1.2 to 3 GPa, and 1400?°C or 1600?°C, where ?IW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of crystalline Fe and wüstite. N partitioning ( ) depends chiefly on fO2, decreasing from to with decreasing fO2. also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients () show no evidence of a pressure dependence but diminish with temperature. At 1400?°C, partition coefficients increase linearly with decreasing fO2 from to At 1600?°C, however, they increase from ?IW?0.7 to ?IW?2 ( to ) and decrease from ?IW?2 to ?IW?3.3 . Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH4). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (?IW?0.4 to ?2.2), N is more compatible in core-forming metal than in molten silicate ( ), while at more reduced conditions (?IW?2.2 to ?IW?3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated (). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.
Contributions to Mineralogy and Petrology, Vol. 175, 15p. Pdf
Africa, Tanzania
deposit - Oldoinyo Lengai
Abstract: Trace element partitioning between minerals and liquids provides crucial constraints on igneous processes. We quantified trace element concentrations in clinopyroxene (Cpx) phenocrysts and their phonolite melt inclusions from the 2007-08 eruption of Oldoinyo Lengai (Tanzania), and report Cpx-melt partition coefficients (D) and corresponding partitioning equations for rare earth elements (REE) and high field strength elements (HFSE) in alkaline magmas. Heavy REE (HREE: Er, Tm, Yb, Lu) are enriched relative to middle REE in alkaline Cpx and display a specific partitioning behavior that is characteristic of alkaline systems. HFSE (Ti, Zr, Hf) and HREE have similar D values (DHf?=?0.25; DLu?=?0.4) that are significantly higher than MREE (DSm?=?0.06). High DHREE/DMREE are strongly correlated with the high values of DZr and DHf relative to the low DMREE values. In this study, REE partitioning between phonolite melt and Cpx is not consistent with standard models assuming incorporation of all REE in the Cpx M2 site, but rather highlights HREE substitution in both the M1 and M2 sites. Here we highlight the preferential incorporation of HREE in the VI-coordinated M1 site, whereas light REE and MREE remain mostly distributed in the VIII-coordinated M2 site. REE partitioning is strongly dependent on Cpx chemistry: the ideal ionic radius and HREE incorporation in the M1 site increase with increasing Fe3+ content and decrease with increasing Mg2+ and AlVI content. In our study, we focus on alkaline evolved magmas, and update existing models to obtain adequate DHREE for alkaline evolved melts. We provide equations to quantify REE and HFSE partitioning, and HREE enrichment in Cpx that are based on Cpx major element composition and temperature. We propose a new model based on the lattice strain approach that predicts HREE partitioning between Cpx and alkaline magmas. The knowledge of the melt composition or of the trace element contents is not required to obtain DREE from the new model. An improved parameterization of HFSE partitioning between Cpx and phonolite and trachy-phonolite melts is also provided herein. We discuss the potential implications of the new data on our understanding of REE deposits that are commonly associated with igneous alkaline complexes.
Abstract: The first comprehensive and authoritative history of the Koh-i Noor, arguably the most celebrated and mythologised jewel in the world. On 29 March 1849, the ten-year-old Maharajah of the Punjab was ushered into the magnificent Mirrored Hall at the centre of the great Fort in Lahore. There, in a public ceremony, the frightened but dignified child handed over to the British East India Company in a formal Act of Submission to Queen Victoria not only swathes of the richest land in India, but also arguably the single most valuable object in the subcontinent: the celebrated Koh-i Noor diamond.
Geochemistry, Geophysics, Geosystems: G3, Vol. 16, 11, Nov. pp. 3982-4005.
Mantle
Geophysics - seismic
Abstract: Seismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low-velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat flow data and mantle-xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one-dimensional shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1-D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity-depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low-velocity zone in the upper mantle and that temperature structures that are consistent with constraints from mantle xenoliths yield phase-velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km.
Geophysical Research Letters, Vol. 44, 20, pp. 10,189-10,197.
Mantle
craton
Abstract: Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of Sp waves to constrain the lithosphere-asthenosphere velocity gradient at long-lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2-3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere-asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere-asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
Physics of the Earth and Planetary Interiors, Vol. 309, 106600, 20p. Pdf
Mantle
melting
Abstract: Over the last decade, seismological studies have shed new light on the properties of the mantle lithosphere and their physical and chemical origins. This paper synthesizes recent work to draw comparisons between oceanic and continental lithosphere, with a particular focus on isotropic velocity structure and its implications for mantle temperature and partial melt. In the oceans, many observations of scattered and reflected body waves indicate velocity contrasts whose depths follow an age-dependent trend. New modeling of fundamental mode Rayleigh waves from the Pacific ocean indicates that cooling plate models with asymptotic plate thicknesses of 85-95 km provide the best overall fits to phase velocities at periods of 25 s to 250 s. These thermal models are broadly consistent with the depths of scattered and reflected body wave observations, and with oceanic heat flow data. However, the lithosphere-asthenosphere velocity gradients for 85-95 km asymptotic plate thicknesses are too gradual to generate observable Sp phases, both at ages less than 30 Ma and at ages of 80 Ma or more. To jointly explain Rayleigh wave, scattered and reflected body waves and heat flow data, we propose that oceanic lithosphere can be characterized as a thermal boundary layer with an asymptotic thickness of 85-95 km, but that this layer contains other features, such as zones of partial melt from hydrated or carbonated asthenosphere, that enhance the lithosphere-asthenosphere velocity gradient. Beneath young continental lithosphere, surface wave constraints on lithospheric thickness are also compatible with the depths of lithosphere-asthenosphere velocity gradients implied by converted and scattered body waves. However, typical steady-state conductive models consistent with continental heat flow produce thermal and velocity gradients that are too gradual in depth to produce observed converted and scattered body waves. Unless lithospheric isotherms are concentrated in depth by mantle upwelling or convective removal, the presence of an additional factor, such as partial melt at the base of the thermal lithosphere, is needed to sharpen lithosphere-asthenosphere velocity gradients in many young continental regions. Beneath cratons, numerous body wave conversions and reflections are observed within the thick mantle lithosphere, but the velocity layering they imply appears to be laterally discontinuous. The nature of cratonic lithosphere-asthenosphere velocity gradients remains uncertain, with some studies indicating gradual transitions that are consistent with steady-state thermal models, and other studies inferring more vertically localized velocity gradients.
Physics of the Earth and Planetary Interiors, Vol. 309, di.org/10.1016/ jpepi.2020.106600 20p. Pdf
Mantle
geophysics - seismics
Abstract: Over the last decade, seismological studies have shed new light on the properties of the mantle lithosphere and their physical and chemical origins. This paper synthesizes recent work to draw comparisons between oceanic and continental lithosphere, with a particular focus on isotropic velocity structure and its implications for mantle temperature and partial melt. In the oceans, many observations of scattered and reflected body waves indicate velocity contrasts whose depths follow an age-dependent trend. New modeling of fundamental mode Rayleigh waves from the Pacific ocean indicates that cooling plate models with asymptotic plate thicknesses of 85-95 km provide the best overall fits to phase velocities at periods of 25 s to 250 s. These thermal models are broadly consistent with the depths of scattered and reflected body wave observations, and with oceanic heat flow data. However, the lithosphere-asthenosphere velocity gradients for 85-95 km asymptotic plate thicknesses are too gradual to generate observable Sp phases, both at ages less than 30 Ma and at ages of 80 Ma or more. To jointly explain Rayleigh wave, scattered and reflected body waves and heat flow data, we propose that oceanic lithosphere can be characterized as a thermal boundary layer with an asymptotic thickness of 85-95 km, but that this layer contains other features, such as zones of partial melt from hydrated or carbonated asthenosphere, that enhance the lithosphere-asthenosphere velocity gradient. Beneath young continental lithosphere, surface wave constraints on lithospheric thickness are also compatible with the depths of lithosphere-asthenosphere velocity gradients implied by converted and scattered body waves. However, typical steady-state conductive models consistent with continental heat flow produce thermal and velocity gradients that are too gradual in depth to produce observed converted and scattered body waves. Unless lithospheric isotherms are concentrated in depth by mantle upwelling or convective removal, the presence of an additional factor, such as partial melt at the base of the thermal lithosphere, is needed to sharpen lithosphere-asthenosphere velocity gradients in many young continental regions. Beneath cratons, numerous body wave conversions and reflections are observed within the thick mantle lithosphere, but the velocity layering they imply appears to be laterally discontinuous. The nature of cratonic lithosphere-asthenosphere velocity gradients remains uncertain, with some studies indicating gradual transitions that are consistent with steady-state thermal models, and other studies inferring more vertically localized velocity gradients.
Abstract: Kimberlites are silica-poor, volatile-rich (CO2 ± H2O), volcanic rocks that are often described as ‘hybrid’, because their parental magmas include abundant xenocrystic (crustand mantle-derived) components. Unravelling the influence of mantle assimilation on kimberlite melt compositions represents an outstanding question of kimberlite petrology. To address this issue, we have carried out a comprehensive geochemical and petrographic investigation of nine kimberlites from the Kaavi-Kuopio field in Finland, that were emplaced on the southern margin of the Karelian Craton in the Neoproterozoic (~550-600 Ma). Olivine is the dominant mineral phase in kimberlites (~50 vol.%) with cores mainly derived from the disaggregation of mantle peridotite. In contrast, olivine rims crystallise directly from the kimberlitic melt and their Mg# (Mg/(Mg+Fe)) typically show remarkable homogeneity within and between kimberlites of a single cluster and field (e.g., Lac de Gras). The Kaavi-Kuopio kimberlites appear to represent a unique case where there is a (statistically) significant difference between the average Mg# of olivine rims in different pipes (89.9 ± 0.2 to 88.5 ± 0.3). Importantly, the Mg# of olivine rims exhibit a strong correlation with the Mg# of olivine cores. Furthermore, the compositions of olivine cores (and rims) exhibit a strong correlation with those of spinel (e.g., Mg#, TiO2 contents). These geochemical variations correlate with the modal mineralogy of the kimberlites: for example, higher abundances of monticellite and lower abundances of ilmenite are associated with higher Mg# olivine. The robust relationship between entrained and assimilated lithospheric mantle material (i.e. olivine cores) and magmatic components (i.e. olivine rims, spinel, and other groundmass minerals) suggests that assimilation of lithospheric mantle has impacted the compositions of kimberlitic melts to a greater extent than previously recognised. These new data also suggest significant variations in the composition of the mantle lithosphere beneath the Kaavi-Kuopio kimberlites, which are spaced less than 10 km apart.
Abstract: Kimberlites are complex, ‘hybrid’ igneous rocks because their parental magmas entrain abundant crust- and mantle-derived components that can be readily assimilated during ascent to surface. Recent studies of olivine zonation patterns have shown compositional relationships between xenocrystic cores and magmatic rims, suggesting that kimberlite melt compositions might be controlled by assimilation of mantle material during emplacement. However, the nature and extent to which this process, as well as assimilation of crustal material, influences melt compositions within single kimberlite fields remains unclear. To address this issue, we have conducted a comprehensive geochemical and petrographic investigation of kimberlites from eight pipes in the Kaavi-Kuopio field in Finland, which were emplaced on the southern margin of the Karelian craton during the Neoproterozoic (~550-600 Ma). While magmatic olivine rims are usually homogeneous in composition within and between kimberlites of a single cluster and field (e.g., Lac de Gras), the Kaavi-Kuopio kimberlites appear to represent a unique case where there are statistically significant differences between the average Mg# of olivine rims in different pipes (89.9 ± 0.2 to 88.5 ± 0.3). Importantly, the Mg# of magmatic olivine rims exhibit a strong correlation with the Mg# of their mantle-derived xenocrystic cores. Furthermore, the compositions of olivine cores and rims exhibit a robust relationship with those of magmatic spinel (e.g., Mg#, TiO2 contents). These geochemical variations also align with the mineralogy of the kimberlites: whereby abundances of phlogopite and oxides (e.g., spinel) are negatively correlated with olivine rim Mg#. The robust relationship between entrained and assimilated lithospheric mantle material (i.e. olivine cores) and magmatic components (i.e. olivine rims, spinel, and groundmass mineral abundance), combined with numerical modelling suggests that up to 10 wt% assimilation of lithospheric mantle material has modified the compositions of the Kaavi-Kuopio kimberlites. These new data are also consistent with significant variations in the lithospheric mantle composition of the Karelian craton beneath the closely spaced (<10 km) kimberlites. Finally, in addition to mantle assimilation, formation of Si-Fe-rich mica in some of the examined kimberlites might be linked to late-stage increases in oxygen fugacity potentially enhanced by crustal contamination. This study shows for the first time that variable assimilation of mantle and crustal material can generate significant variations in kimberlites derived from seemingly similar sources.
Journal of Petrology, in press available, 79p. Pdf
Europe, Finland
deposit - Kuusamo
Abstract: Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types; yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies (Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi) distributed along a 60?km NE-SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g., Mg-rich ilmenite, Al-Ba-rich, Ti-Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0.8 ± 0.1; Th/Nb = 0.07 ± 0.01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggest derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba-Al-rich cores) to those more typical of orangeites/lamproites (increasing Si-Fe, decreasing Al-Ti-Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe-Ti-rich) and spinel (high Fe2+/Fe2++Mg) compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb-Sr dating of phlogopite in Dike 15 yields an age of 1178.8 ± 4.1?Ma (2?), which is considerably older than the ?750?Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira-Kuhmo-Kostomuksha olivine lamproites emplaced ?100?km to the southeast. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is likely the result of exploitation of similar trans-lithospheric corridors.
Abstract: Quantifying the compositional evolution of mantle-derived melts from source to surface is fundamental for constraining the nature of primary melts and deep Earth composition. Despite abundant evidence for interaction between carbonate-rich melts, including diamondiferous kimberlites, and mantle wall rocks en route to surface, the effects of this interaction on melt compositions are poorly constrained. Here, we demonstrate a robust linear correlation between the Mg/Si ratios of kimberlites and their entrained mantle components and between Mg/Fe ratios of mantle-derived olivine cores and magmatic olivine rims in kimberlites worldwide. Combined with numerical modeling, these findings indicate that kimberlite melts with highly variable composition were broadly similar before lithosphere assimilation. This implies that kimberlites worldwide originated by partial melting of compositionally similar convective mantle sources under comparable physical conditions. We conclude that mantle assimilation markedly alters the major element composition of carbonate-rich melts and is a major process in the evolution of mantle-derived magmas.
Abstract: The Karelian Craton in Finland is host to (at least) two distinct pulses of kimberlite magmatism. Twenty kimberlite occurrences have so far been discovered on the southwest margin of the craton at Kaavi-Kuopio and seven kimberlites are located in the Kuusamo area within the core of the craton. Comprehensive radiometric age determinations (U-Pb, Ar- Ar and Rb-Sr) reveal that all kimberlite activity was restricted to the Proterozoic. The Kaavi-Kuopio field was emplaced over a protracted period from ~610 to 550 Ma and is predated by the Kuusamo cluster that represents a relatively short pulse of magmatism at ~750 to 730 Ma. The emplacement of kimberlites globally has recently been linked to supercontinent reorganisation and we propose a similar scenario for these Finnish occurrences which, at the time of kimberlite emplacement, were situated on the Baltica paleo-continent. This land mass was contiguous with Laurentia in the Proterozoic and together formed part of Rodinia. The breakup of Rodinia is considered to have commenced at ~750 Ma and initiation of the opening of the Iapetus ocean at ~615 Ma. Contemporaneous with Kaavi-Kuopio magmatism, this latter period of Neoproterozoic crustal extension also includes the emplacement of kimberlites and related rocks in areas that were linked with Baltica as part of Rodinia - West Greenland and eastern North America. Both the initial and final periods of Rodinia’s breakup have been linked to mantle upwellings from the core-mantle boundary. We suggest that kimberlite magmatism in Finland was promoted by the influx of heat from mantle upwellings and lithospheric extension associated with the demise of Rodinia. Although both magmatic episodes are potentially linked to the breakup of Rodinia, whole-rock and perovskite radiogenic isotope compositions for the Kuusamo kimberlites (?Nd(i) +2.6 to +3.3, ?Hf(i) +3.1 to +5.6) are distinct from the Kaavi-Kuopio kimberlites (?Nd(i) -0.7 to +1.8, ?Hf(i) -6.1 to +5.2). The spread in Hf isotope compositions for the Kaavi-Kuopio magmas may be linked to variable assimilation of diverse mantle lithologies.
Abstract: The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites—volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a 143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic 143Nd/144Nd and 176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core-mantle boundary. PREMA could have been stored in these structures for most of Earth’s history, partially shielded from convective homogenization.
Proceedings of the National Academy of Sciences PNAS, Vol. 118, 1 e201521118 9p. Pdf
Mantle
kimberlite
Abstract: The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites—volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a 143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic 143Nd/144Nd and 176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core-mantle boundary. PREMA could have been stored in these structures for most of Earth’s history, partially shielded from convective homogenization.
Geostandards and Geoanalysis Research, doi.org/10.1111/GGR.12419 34p. Pdf
Australia
geochemistry
Abstract: To promote a more efficient and transparent geochemistry data ecosystem, a consortium of Australian university research laboratories called the AuScope Geochemistry Network (AGN) assembled to build a collaborative platform for the express purpose of preserving, disseminating, and collating geochronology and isotopic data. In partnership with geoscience-data-solutions company Lithodat Pty Ltd, the open, cloud-based AusGeochem platform (https://ausgeochem.auscope.org.au) was developed to simultaneously serve as a geosample registry, a geochemical data repository, and a data analysis tool. Informed by method-specific groups of geochemistry experts and established international data reporting practices, community-agreed database schemas were developed for rock and mineral geosample metadata and secondary ion mass spectrometry U-Pb analysis, with additional models for laser ablation inductively-coupled mass spectrometry U-Pb and Lu-Hf, Ar-Ar, fission-track and (U-Th-Sm)/He under development. Collectively, the AusGeochem platform provides the geochemistry community with a new, dynamic resource to help facilitate FAIR (Findable, Accessible, Interoperable, Reusable) data management, streamline data dissemination and advanced quantitative investigations of Earth system processes. By systematically archiving detailed geochemical (meta-)data in structured schemas, intractably large datasets comprising thousands of analyses produced by numerous laboratories can be readily interrogated in novel and powerful ways. These include rapid derivation of inter-data relationships, facilitating on-the-fly data compilation, analysis, and visualisation.
Science Advances, doi.10.1126/sciadv.abj1325 1p. Pdf
Mantle
subduction
Abstract: Earth’s carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth’s history, but the impact of these changes on the mantle carbon cycle is unclear. Here, we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The 13C/12C of kimberlites before ~250 Ma preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios-a switch coincident with a recognized surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low 13C/12C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth’s surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.
Abstract: There is much discussion of the timing of the onset of plate tectonics, yet there is increasing evidence that magma types similar to those from recent within plate and subduction related settings were generated in different areas at broadly similar times in the early Archaean. It may therefore be helpful to consider when plate tectonics became the dominant mechanism associated with the generation of continental crust, rather than just when it started. To do this we discuss the geochemical and mechanical characteristics of the lithosphere in the search to discern differences between Early Archaean and younger tectonic environments. Seismic tomography allows increasingly detailed mapping of the lithosphere, and it provides some evidence that the degree of anisotropy is different in different Archaean terrains. Structural styles also appear to vary from basin and swell, or vertical tectonics, as in the Australia Pilbara and southern Africa, to those with more strongly developed regional fabrics and greater seismic anisotropy, as in North America. These terrains tend to be characterized by inferred within-plate and subduction-related magmatism respectively, and we consider possible links between the degree of crustal and mantle anisotropy and the nature of the magmatic record. At least in some areas, terrains with stronger regional fabrics may be younger than those in which such fabrics are less well developed. A model is developed for the generation and stabilization of continental lithosphere in the Archaean. It seeks to reconcile evidence for hot shallow melting with melt fractions up to 40% to generate residual peridotites now preserved as mantle xenoliths, and the lower degrees of melting required to generate the mafic sources of TTGs (fractionated Lu/Hf and Sm/Nd, and perhaps not Rb/Sr).
Abstract: Lithospheric thickness of continents, obtained from Rayleigh wave tomography, is used to make maps of the lithospheric thickness of Pangea by reconstructing the continental arrangement in the Permian. This approach assumes that lithosphere moves with the overlying continents, and therefore that the arrangement of both can be obtained using the poles of rotation obtained from magnetic anomalies and fracture zones. The resulting reconstruction shows that a contiguous arc of thick lithosphere underlay most of eastern Pangea. Beneath the western convex side of this arc, there is a wide belt of thinner lithosphere underlying what is believed to have been the active margin of Pangea, here named the Pangeides. On the inner side of this arc is another large area of thin lithosphere beneath the Pan-African belts of North Africa and Arabia. The arc of thick lithosphere is crossed by bands of slightly thinner lithosphere that lie beneath the Pan-African and Brasiliano mobile belts of South America, Africa, India, Madagascar, and Antarctica. This geometry suggests that lithospheric thickness has an important influence on continental deformation and accretion.
Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: new dat a from the Antarctic Peninsula and southernmostAndes
Geological Society of America (GSA) Bulletin, Vol. 104, No. 11, November pp. 1497-1514
IN: Cycle Concepts in Plate Tectonics, editors Wilson and Houseman , Geological Society of London special publication 470, pp. 18-38.
Mantle
plate tectonics
Abstract: In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian-Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.
Dampare, S.B., Asiedu, D.K., Osea, S., Nyarko, B.J.B., Banoeng-Yakubo, B.
Determination of rare earth elements by neutron activation analysis in altered ultramafic rocks from the Akwatia district of Birim Diamondiferous field.
Journal of Radioanalytical and Nuclear Chemistry , Vol. 265, 1, pp. 101-106.
Geochemistry, Geophysics, Geosystems, in press available pdf 20p.
Global
geodynamics
Abstract: The relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume?push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross?lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20-50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80?km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5-50 MPa. In both homogeneous and orogeny?embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1-2 order of magnitude smaller (0-5 MPa). In our two end?member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents.
Abstract: Understanding the dominant force responsible for supercontinent breakup is crucial for establishing Earth's geodynamic evolution that includes supercontinent cycles and plate tectonics. Conventionally, two forces have been considered: the push by mantle plumes from the sub-continental mantle which is called the active force for breakup, and the dragging force from oceanic subduction retreat which is called the passive force for breakup. However, the relative importance of these two forces is unclear. Here we model the supercontinent breakup coupled with global mantle convection in order to address this question. Our global model features a spherical harmonic degree-2 structure, which includes a major subduction girdle and two large upwelling (superplume) systems. Based on this global mantle structure, we examine the distribution of extensional stress applied to the supercontinent by both sub-supercontinent mantle upwellings and subduction retreat at the supercontinent peripheral. Our results show that: (1) at the center half of the supercontinent, plume push stress is ?3 times larger than the stress induced by subduction retreat; (2) an average hot anomaly of no higher than 50 K beneath the supercontinent can produce a push force strong enough to cause the initialization of supercontinent breakup; (3) the extensional stress induced by subduction retreat concentrates on a ?600 km wide zone on the boundary of the supercontinent, but has far less impact to the interior of the supercontinent. We therefore conclude that although circum-supercontinent subduction retreat assists supercontinent breakup, sub-supercontinent mantle upwelling is the essential force.
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Historical Research, Vol. 89, no 243, pp. 136-157.
Africa, Sierra Leone
History
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Journal of African Earth Sciences, Vol. 146, pp. 28-47.
Africa, Cameroon
craton
Abstract: Field, microstructural, and anisotropy of magnetic susceptibility (AMS, magnetic fabrics) studies assessed the Pan-African deformational history and strain geometry at the southern margin of the Central African Fold Belt (CAFB) against the older, cratonic basement of the Congo Shield (CS). Reflected light microscopy and thermomagnetic studies supported the identification of magnetic minerals. Data cover a low angle thrust margin (Mbengis-Sangmelima area) in the east and high angle shear zones cutting the margin (Kribi area) in the west, at the Atlantic coast. In the CS basement units, magnetic anisotropy is generally higher than in the low grade Pan-African units. In the latter, early D1/D2 shortening produced a flat-lying magnetic foliation parallel with the regional trend of the belt, a shallow magnetic lineation, and mostly oblate fabrics. Subsequent D3 deformation is only of local importance in the Mbengis-Sangmelima area. The magnetic lineation shows distinct maxima in NNE-SSW direction, parallel with the low angle tectonic transport direction. In the Kribi area, the NNE-SSW trending Kribi-Campo shear zone (KCSZ) affected both older rocks and Pan-African high grade metapelites of the Yaoundé unit together with their basal thrust. The early planar fabric (S1) was overprinted during D2 folding under relatively high T conditions, and subsequent D3 wrenching. Magnetic fabrics document a progressive change from oblate towards prolate ellipsoids towards the KCSZ. Magnetic foliations with medium to steep dips curve into the N-S to NE-SW orientation of the KCSZ, lineations follow the same trend with shallow to medium plunges. This fabric implies that the KCSZ is a Pan-African strike-slip shear zone with a subordinate component of compression. Strike-slip tectonics in the west (KCSZ) and thrusting in the east imply N-S to NE-SW convergence during Pan-African terrane assembly against the present northern margin of the CS. In addition, the KCSZ may separate the CS from the São Francisco Craton in Brazil and thus be the northern part of a link connecting the CAFB to the West Congo Belt in the south. This putative Pan-African link separated the São Francisco Craton from the Congo Shield prior to Mesozoic Gondwana break-up.
Abstract: The integrated evaluation of soil geochemistry, aerogammaspectrometry (eTh), geological and structural mapping associated with the description of boreholes and outcrops in the Caçapava do Sul region, southernmost Brazil, led to the discovery of two carbonatite bodies. They are located near the eastern and southeastern border of Caçapava do Sul Granite and intrude the Passo Feio Complex. The carbonatite system is composed of early pink-colored alvikite followed by late white beforsite dikes. The carbonatites are tabular bodies concordant with the deformed host rocks. Petrographic and scanning electron microscopy show that the alvikites are dominantly composed of calcite with subordinate apatite, magnetite, ilmenite, biotite, baddeleyite, zircon, rutile, pyrochlore-like and rare earth element minerals. Beforsite is composed of dolomite and has the same minor and accessory minerals as the alvikite. U-Pb zircon geochronology via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed on a beforsite sample, yielding a 603.2 ± 4.5 Ma crystallization age. The carbonatite was emplaced an Ediacaran post-collisional environment with transpressive tectonism and volcanic activity marked by shoshonitic affinity.
Brazil Journal of Geology ( www.scielo.br) ENG, 17p. Pdf
South America, Brazil
deposit - Tres Estradas
Abstract: Carbonatites were recently discovered in Southern Brazil, which increased the interest to evaluate the economic potential of these uncommon rocks, especially the Três Estradas Carbonatite. Carbonates are the dominant minerals of fresh rock followed by apatite, but the weathering process makes apatite abundant. We focused on apatite from the carbonatite using conventional petrography and electronic microscopy associated with microprobe, micro-Raman and Fourier-transform infrared spectroscopy. Results demonstrate the existence of four types. The primary type is associated with the rock crystallization and the subsequent three others are associated with weathering processes. The alteration mechanism was favorable for initial carbonate leaching and subsequent increase of phosphate with late precipitation of three new apatite generations. The deduced model involves long exposure during polycyclic climate changes, intercalating periods of warm dry with humid climate. The apatite types differ chemically and morphologically and have distinctive characteristics that are suitable to be used to differentiate them. These properties should be considered in future planes of industrial processes to transform apatite into single superphosphate, a basic input for fertilizer production.
Restoration of Laramide right lateral strike slip in northern New Mexico by using Proterozoic piercing points: tectonic implications from theProterozoic...
Earth and Planetary Science Letters, Vol. 433, pp. 360-369.
Mantle
Geophysics - seismics
Abstract: In regions of the mantle where multi-phases coexist like at the olivine-wadsleyite-ringwoodite transitions, the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases, a possible source of dissipation. In such a situation, the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave. Here we report synchrotron-based measurements of the kinetics of the olivine to ringwoodite transformation at pressure-temperature conditions of the co-stability loop, for iron-rich olivine compositions. Both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries. Transformation-time data show an increase of reaction rates with temperature and iron content, and have been fitted to a rate equation for interface-controlled transformation: G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)]G=k0?T?exp?[n?XFa]?exp?[?(?Ha+PV?)/RT]×[1?exp?(?Gr/RT)], where XFaXFa is the fayalite fraction, the exponential factor n=9.7n=9.7, View the MathML sourceln?k0=?9.1 ms?1. View the MathML sourceXFa?1 and ?Ha=199 kJ/mol?Ha=199 kJ/mol, assuming V?=0 cm3/molV?=0 cm3/mol. Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict View the MathML sourceQK?1 and View the MathML sourceQ??1 significantly higher than the PREM values for both body waves and normal modes. This attests that the olivine-wadsleyite transition can significantly contribute to the attenuation of the Earth's mantle transition zone.
The 4th Colloquium on Diamonds - source to use held Gabarone March 1-3, 2010, 14p.
Technology
Crushing
Abstract: The diamond industry is no newcomer to High Pressure Grinding Rolls (HPGR) as it has used the technology for more than 20 years, predominantly in secondary crushing and recrushing roles. In fact it could be argued that the diamond industry has led the way for the wider minerals industry to consider its application. In existing conventional secondary cone crushing applications, large valuable diamonds are "won" through meticulously managing the recovery process within defined particle size ranges. COllventional crushers operate with relatively large closed side sehings, hut have the potential to damage diamonds by making direct contact with the sides of the crusher. Cone crushers also result in steep product size distributions that run a high risk of losing many of the smaller, un-liberated but more abundant diamonds from the rock. This current comminution paradigm of particle size reduction management results in recrushing processing applications or plants where HPGRs are often used. HPGRs operate under the seemingly odd condition where the gap between the rolls is largely a function of the roll diameter irrespective of the feed size. This otTern an opportun'ity for the diamond industry to consider using multiple HPGRs or units with high circulating loads to effectively generate a product with a very high proportion of fine material that can be rejected ahead of the beneficiation step. This in effect results in a new paradigm, a single comminution step, where all diamonds of all sizes are liberated and preserved. The circuit product size distribution will consist mainly of barren kimberlitic fines along with some grits, pebbJes, indicator minerals, as well as the prized lUldamaged diamonds. The HPGR product stream needs to be scrubbed, slurried and screened at t mm resulting in a greatly reduced volume of -diamond-rich particles that progress to the dense medium concentration and/or direct x-ray separation steps_ In this application, the HPGR is viewed "outside the box", but within the context of diamond winning processes. Examples of how "HPGR can go all the way" are presented in the paper. Page
Contrasting termite transported indicator mineral concentrations in the Kgalafadi of central district Botswana: Macrotermes micaelseni vs Hodotermes mossambicus.
Vancouver Kimberlite Cluster, March 9, 1p. Abstract
Mineralogy and Petrology, 10.1007/s00710-018-0608-5 8p.
Africa, Botswana
indicator minerals
Abstract: The majority of the diamond mines in Botswana were discovered as a direct consequence of soil sampling for indicator minerals such as garnet and picroilmenite. Over the past 60 years the application of soil sampling for indicator minerals as a primary exploration tool has declined while aeromagnetic surveys have increased in popularity. The rate of kimberlite discovery in Botswana has declined significantly. The obvious magnetic kimberlites have been discovered. The future of new kimberlite discoveries is once again dependent on soil sampling for kimberlite indicator minerals. It is essential to have an in depth understanding of the transport mechanism of kimberlite indicator minerals from the kimberlite to the modern day surface of the Kalahari Formation, which is solely via termite bioturbation. Field observations indicate that the concentration of indicator minerals at surface is directly dependent on the physical characteristics and capabilities as well as behavioural patterns of the particular termite species dominant in the exploration area. The discovery of future diamond mines in Botswana will be closely associated with an in depth understanding of the relationship between size and concentration of kimberlite indicator minerals in surface soils and the seasonal behaviour, depth penetration capabilities, earthmoving efficiencies and mandible size of the dominant termite species within the exploration area. Large areas in Botswana, where kimberlite indicator minerals recovered from soil samples have been described as distal from source or background, will require re-evaluation. Without detailed termite studies the rate of discovery will continue to decline.
Geochimica et Cosmochimica Acta, Vol. 198, pp. 1-16.
Mantle
Core chemistry
Abstract: A large number of siderophile (iron-loving) elements are also volatile, thus offering constraints on the origin of volatile elements in differentiated bodies such as Earth, Moon, Mars and Vesta. Metal-silicate partitioning data for many of these elements is lacking, making their overall mantle concentrations in these bodies difficult to model and origin difficult to distinguish between core formation and volatile depletion. To address this gap in understanding, we have undertaken systematic studies of four volatile siderophile elements - Sb, As, Ge and In - at variable temperature and variable Si content of metal. Several series were carried out at 1 GPa, and between 1500 and 1900 °C, for both C saturated and C-free conditions. The results show that temperature causes a decrease in the metal/silicate partition coefficient for all four elements. In addition, activity coefficients for each element have been determined and show a very strong dependence on Si content of Fe alloy. Si dissolved in metal significantly decreases the metal/silicate partition coefficients, at both 1600 and 1800 °C. The combination of temperature and Si content of the metal causes reduction of the metal-silicate partition coefficient to values that are close to those required for an origin of mantle As, Sb, Ge, and In concentrations by metal-silicate equilibrium processes. Combining these new results with previous studies on As, Sb, Ge, and In, allowed derivation of predictive expressions for metal/silicate partition coefficients for these elements which can then be applied to Earth. The expressions are applied to two scenarios for continuous accretion of Earth; specifically for constant and increasing fO2 during accretion. The results indicate that mantle concentrations of As, Sb, Ge, and In can be explained by metal-silicate equilibrium during an accretion scenario. The modeling is not especially sensitive to either scenario, although all element concentrations are explained better by a model with variable fO2. The specific effect of Si is important and calculations that include only S and C (and no Si) cannot reproduce the mantle As, Sb, Ge, and In concentrations. The final core composition in the variable fO2 model is 10.2% Si, 2% S, and 1.1% C (or XSi = 0.18, XS = 0.03, and XC = 0.04. These results suggest that core formation (involving a Si, S, and C-bearing metallic liquid) and accretion were the most important processes establishing many of Earth’s mantle volatile elements (indigenous), while post-core formation addition or re-equilibration (exogenous) was of secondary or minor importance.
The equations of state of forsterite, wadsleyite, ringwoodite, akimotoite, Mg2SiO4 perovskite and post perovskite and phase diagram for the Mg2SiO4 system at pressures of up to 130 Gpa.
Russian Geology and Geophysics, Vol. 56, 1-2, pp. 172-189.
Abstract: We present Microsoft Excel spreadsheets for calculation of thermodynamic functions and P-V-T properties of MgO, diamond and 9 metals, Al, Cu, Ag, Au, Pt, Nb, Ta, Mo, and W, depending on temperature and volume or temperature and pressure. The spreadsheets include the most common pressure markers used in in situ experiments with diamond anvil cell and multianvil techniques. The calculations are based on the equation of state formalism via the Helmholtz free energy. The program was developed using Visual Basic for Applications in Microsoft Excel and is a time-efficient tool to evaluate volume, pressure and other thermodynamic functions using T-P and T-V data only as input parameters. This application is aimed to solve practical issues of high pressure experiments in geosciences and mineral physics.
Geodynamics & Tectonophysics, Vol. 11, 4, pp. 678-696.
Russia
deposit - Sayan
Abstract: The study of the Bol’shaya Tagna alkaline-carbonatite massif and adjacent areas was focused on the mineral and chemical compositions of minerals, the distribution of petrogenic and trace elements in pyroxene-free alkaline picrites in veins and dikes dated at the late Riphean (circa 645 Ma), and comparison with the Bushkanai kimberlite-picrite dike. Phenocrysts in the pyroxene-free picrites are represented by olivine (replaced with serpentine) and phlogopite; the bulk is formed by serpentine, phlogopite, monticellite, calcite, etc .; xenocrysts of pyrope and chrome diopside are absent. Phlogopite and Cr-spinel from the picrites are chemically similar to these minerals in kimberlites, but the evolution of the spinel compositions corresponds to the titanomagnetite trend; monticellite is depleted in forsterite (Mg2SiO4). The rocks contain strontianite, burbankite, titanium andradite, calcirtite and Mn-ilmenite, which are not typical of kimberlites, but are inherent in carbonate-bearing ultramafic lamprophyres, ayllikites. The pyroxene-free picrites have low contents (wt %) of SiO2 (28.4?33.2), Al2O3 (3.2?5.6), and Na2O (0.01?0.05); relatively high contents of TiO2 (2.0?3.3), and ?2? (0.45?1.33); varying contents of MgO (16.1?24.1), ??? (12.9?22.8), ??2 (1.1?12.2), Ni (260?850 ppm), and Cr (840?2200 ppm); and Mg#=0.73?0.80. The contents of Th, U, Nb, Ta, La, and Ce in the veins are approximately two orders higher than those in the primitive mantle; the spectra of trace elements differ from the spectra of the South African and Yakuian kimberlites. In the pyroxene-free picrites and the rocks of the Bushkanai dike, the Nb/U, Nb/Th, Th/Ce, La/Nb, and Zr/Nb ratios are similar to those in ocean island basalts (OIB) and thus give evidence of the leading contribution of the recycled component into the source melt. In experiments conducted to investigate melting of carbonated garnet lherzolite, the pyroxene-free alkaline picrites melted at 5-6 GPa.
Pure and Applied Geophysics, Vol. 178, 10, pp, 3933-3952.
Russia, Arkangelsk
deposit - Lomonsov
Abstract: Kimberlite pipes are difficult to investigate due to their vertical orientation, conic shape and diverse physical characteristics and petrological compositions, all of which obstruct the use of magnetic methods, reflection and refraction seismic surveys to examine kimberlite pipes. Wherein the emplacement model for kimberlite pipes has important significance in resource geology and in mine design process. As a result, the development of new methods of investigating kimberlite pipes remains necessary. To that end, because the most stable characteristic of kimberlite pipes is their downward-tapering structure, the pipes can be more effectively examined by using methods offering high resolution and new indicators for prospecting. Herein, we present the results of jointly using passive seismic and radiometric methods to study the structure of a kimberlite pipe and its enclosing environment. In particular, we employed a microseismic sounding method, passive seismic interferometry, the H/V method, gamma spectrometry and emanation mapping to model the kimberlite pipe named after M. Lomonosov of the Arkhangelsk diamondiferous province. The combined use of those methods revealed an ore-controlled fault and probably a supply channel (i.e. dyke). The obtained model is correspondent to drill whole data and includes additional information about the structure and elastic properties of the studied pipe. Amongst its principal benefits, the proposed technique affords the possibility of discerning the primary elements of the kimberlite pipes and enclosing environments at depths from 30 m to 2 km, which can significantly increase the effectiveness of investigations into kimberlite pipes.
Journal of Volcanology and Seismology, Vol. 10, 5, pp. 339-346.
Russia, Kola Peninsula, Archangel
Deposit- Lomonsov
Abstract: This paper presents results from a study of the Lomonosov volcanic pipe as derived from anomalies of the microseismic field. Microseismic sounding revealed that this volcanic pipe is a cone-shaped body with a small gradient of microseismic intensity motion (2 to 5 dB). Discontinuities generally show greater contrasts compared with the variations of microseismic motion in the pipe body. Comparison of the results of this microseismic sounding with other geological and geophysical data showed that the intensities of the micro-seismic field along lines that traversed the pipe reflect realistic structures of a kimberlite pipe and the host rocks. The method of microseismic sounding was used to reconstruct the deeper structure of the volcanic pipe and the host rocks down to depths greater than 2 km. We estimated the velocity contrast and the errors involved in the identification of vertical boundaries of the pipe. The volcanic pipe has a shape that is consistent with a nearly vertical source situated at a depth of a few hundred meters. This is hypothesized to be a typical occurrence for other diamond-bearing pipes as well.
Geodynamics & Tectonophysics, Vol. 11, 4, pp. 678-696.
Russia
deposit - Sayan
Abstract: The study of the Bol’shaya Tagna alkaline-carbonatite massif and adjacent areas was focused on the mineral and chemical compositions of minerals, the distribution of petrogenic and trace elements in pyroxene-free alkaline picrites in veins and dikes dated at the late Riphean (circa 645 Ma), and comparison with the Bushkanai kimberlite-picrite dike. Phenocrysts in the pyroxene-free picrites are represented by olivine (replaced with serpentine) and phlogopite; the bulk is formed by serpentine, phlogopite, monticellite, calcite, etc .; xenocrysts of pyrope and chrome diopside are absent. Phlogopite and Cr-spinel from the picrites are chemically similar to these minerals in kimberlites, but the evolution of the spinel compositions corresponds to the titanomagnetite trend; monticellite is depleted in forsterite (Mg2SiO4). The rocks contain strontianite, burbankite, titanium andradite, calcirtite and Mn-ilmenite, which are not typical of kimberlites, but are inherent in carbonate-bearing ultramafic lamprophyres, ayllikites. The pyroxene-free picrites have low contents (wt %) of SiO2 (28.4?33.2), Al2O3 (3.2?5.6), and Na2O (0.01?0.05); relatively high contents of TiO2 (2.0?3.3), and ?2? (0.45?1.33); varying contents of MgO (16.1?24.1), ??? (12.9?22.8), ??2 (1.1?12.2), Ni (260?850 ppm), and Cr (840?2200 ppm); and Mg#=0.73?0.80. The contents of Th, U, Nb, Ta, La, and Ce in the veins are approximately two orders higher than those in the primitive mantle; the spectra of trace elements differ from the spectra of the South African and Yakuian kimberlites. In the pyroxene-free picrites and the rocks of the Bushkanai dike, the Nb/U, Nb/Th, Th/Ce, La/Nb, and Zr/Nb ratios are similar to those in ocean island basalts (OIB) and thus give evidence of the leading contribution of the recycled component into the source melt. In experiments conducted to investigate melting of carbonated garnet lherzolite, the pyroxene-free alkaline picrites melted at 5-6 GPa.
Abstract: A dike -vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite -ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar -39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite -ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5 -11) and carbonatite (4.5 -17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9 -15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.
Doklady Earth Sciences, Vol. 471, 1, pp. 1140-1143.
Russia
Carbonatite
Abstract: Apatite and biotite from dolomite?ankerite and calcite?dolomite carbonatite dikes emplaced into the Paleoproterozoic metamorphic rock complex in the southern part of the Siberian Craton are dated by the U-Pb (LA-ICP-MS) and 40Ar-39Ar methods, respectively. Proceeding from the lower intercept of discordia with concordia, the age of apatite from calcite?dolomite carbonatite is estimated to be 972 ± 21 Ma and that for apatite from dolomite?ankerite carbonatite, as 929 ± 37 Ma. Values derived from their upper intercept have no geological sense. The ages obtained for biotite by the 40Ar-39Ar method are 965 ± 9 and 975 ± 14 Ma. It means that the formation of carbonatites reflects the earliest phases of the Neoproterozoic stage in extension of the continental lithosphere.
Abstract: The use of confocal HR-Raman mapping opens new perspectives in studying melt inclusions. Our major goal is to show advantages of this powerful technique through case studies carried out on alkaline and carbonatite rocks of Kerimasi volcano (East African Rift). Raman spectrometry is one of the few methods that enable qualitative nondestructive analysis of both solid and fluid phases, therefore it is widely used for the identification of minerals and volatiles within melt and fluid inclusions. For better understanding of petrogenetic processes in carbonatite systems it is essential to find all mineral phases in the melt inclusions trapped in intrusive or volcanic rocks. Previous Raman spectroscopic point measurements in melt inclusions revealed the presence of daughter phases (e.g. alkali carbonates, hydrocarbonates) [1] but utilizing Raman mapping on them even provides information on their size, shape and distribution. Raman 3D mapping were applied on unheated multiphase melt inclusions of intrusive and volcanic rocks with high spatial resolution (XY plane < 1 micron) with a depth scan (Z step) as low as 0.5 micron at every XY point, parallel to the surface of the host minerals. Analysis below the surface of the host mineral is especially useful because we can avoid the loss of sensitive (e.g. water soluble) phases and contamination of the melt inclusions, moreover unexposed melt inclusions are suitable for further analytical measurements (e.g. EPMA, microthermometry). By scanning multiple layers 2D or 3D Raman images can be gained, thus we can get an insight into post entrapment crystallization processes that contribute to a more precise description of the evolution of alkaline and carbonatite rocks.
Harvey, S., Read, G., DesGagnes, B., Shimell, M., Danoczi, J., Van Breugel, B., Fourie, L., Stilling, A.
Utilization of olivine macrocryst grain size and abundance dat a as a proxy for diamond size and grade in pyroclastic deposits of the Orion South kimberlite Fort a la Corne, Sasakatchewan, Canada.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
South African Institute of Mining and Metallurgy, Vol. 116, 8, pp. 737-745.
Technology
microdiamond
Abstract: Estimating the size frequency distribution of the macrodiamonds on a new deposit is important for both economic reasons and for the design of the processing plant. Millions of dollars can be lost due to incorrectly sized comminution circuits. This report analyses an alternative methodology for macrodiamond grade estimation using the cumulative results from small parcels of microdiamonds and plotting them on a log-log scale. The method was first evaluated mathematically for diamond populations to assess the confidence for data extrapolation. Macrodiamond size distributions and grades were predicted using microdiamond data from three kimberlites, and the actual macrodiamond grades compared to the the predicted grades. The predicted grades were found to replicate the actual grades closely, showing that a high degree of confidence can be ascribed to the results from this method of analysis. This analysis can be used both for resource estimates and for predicting the diamond size distribution information needed for designing a new operation.
Journal of South American Earth Sciences, Vol. 80, pp. 1-17.
South America, Brazil
carbonatite
Abstract: We present results of U-Pb dating (by MC-ICP-MS) of zircons from samples that cover all of the known lithotypes in the Seis Lagos Carbonatite Complex and associated lateritic mineralization (the Morro dos Seis Lagos Nb deposit). The host rock (gneiss) yielded an age of 1828 ± 09 Ma interpreted as the crystallization time of this unit. The altered feldspar vein in the same gneiss yielded an age of 1839 ± 29 Ma. Carbonatite samples provided 3 groups of ages. The first group comprises inherited zircons with ages compatible with the gneissic host rock: 1819 ± 10 Ma (superior intercept), 1826 ± 5 Ma (concordant age), and 1812 ± 27 Ma (superior intercept), all from the Orosirian. The second and the third group of ages are from the same carbonatite sample: the superior intercept age of 1525 ± 21 Ma (MSWD ¼ 0.77) and the superior intercept age of 1328 ± 58 Ma (MSWD ¼ 1.4). The mineralogical study indicates that the ~1.3 Ga zircons have affinity with carbonatite. It is, however, a tendence rather than a well-defined result. The data allow state that the age of 1328 ± 58 Ma represents the maximum age of the carbonatite. Without the same certainty, we consider that the data suggest that this age may be the carbonatite age, whose emplacement would have been related to the evolution of the K'Mudku belt. The best age obtained in laterite samples (a superior intercept age of 1828 ± 12 Ma) is considered the age of the main source for the inherited zircons related to the gneissic host rock.
De Oliveira Cordeiro, P.F., Brod, J.A., Ventura Santos, R., Dantas, E.L., Gouvieia de Oliveira, C., Soares Rocha, Barbosa, E.
Stable ( C,O) and radiogenic (Sr, Nd) isotopes of carbonates as indicators of magmatic and post magmatic processes of phoscorite series rocks and carbonatites f
Contributions to Mineralogy and Petrology, In press available, 14p.
De Oliveire Cordeiro, P.F., Brod, J.A., Ventura Santos, R., Dantas, E.L., Gouveia de Oliveira, C., Soares Rochas Barbosa, E.
Stable (C,O) and radiogenic (Sr,Nd) isotopes of carbonates as indicators of magmatic and post-magmatic processes of phoscorite series rocks and carbonatites from Catalao 1, central Brazil.
Contributions to Mineralogy and Petrology, Vol. 161, 3, pp. 451-464.
Soares Rocha Barbosa, E., Brod, J.A., Junqueira-Brod, T.C., Dantas, E.L., De Oliveira Cordeiro, P.F., Siqueira Gomide, C.
Bebdourite from its type area Sailtre 1 complex: a key petrogenetic series in the Late-Cretaceous Alto Paranaiba kamafugite carbonatite phoscorite association, central Brazil.
Abstract: The Late Neoproterozoic assembly of western Gondwana played an important role in the subduction of oceanic and continental lithospheres. Such event was also a source of arc magmatism, reworking of cratonic margins and development of ultra-high pressure (UHP) suture zones. In the Borborema province, NE Brazil, we have described for the first time UHP rocks enclosed within gneiss migmatite and calc-silicate rocks. They bear coesite included in atoll-type garnet from metamafic rocks, identified by petrographic study and Raman microspectroscopy analysis. U-Pb zircon dating of the leucosome of the migmatites and the calc-silicate rock displays, concordant ages of 639 ± 10 Ma and 649.7 ± 5 Ma, respectively, here interpreted as the minimum age of the eclogitization event in the region. U-Pb zircon dating of the coesite-bearing rock defined a concordia age of 614. 9 ± 7.9 Ma that comprised the retrograde eclogitic conditions to amphibolite facies. The UHP rocks, mostly retrograded to garnet amphibolites, occur enclosed in the Paleoproterozoic continental block composed of calc-silicate rocks, migmatized sillimanite gneiss, mylonitic augen gneiss and granitic and tonalitic gneiss along a narrow N-S oriented belt between the Santa Quitéria magmatic arc and the Transbrasiliano lineament. This block was involved in the subduction to UHP eclogite depths, and was retrogressed to amphibolite during its exhumation and thrusting. Our data indicate an important Neoproterozoic transcontinental suture zone connecting the Pharusian belt with Borborema Province, and probably with the Brasília belt in central Brazil.
Abstract: The Late Cretaceous Catalão I carbonatite complex consists of ultramafic silicate rocks, phoscorites, nelsonites and carbonatites. The latest stages of the evolution of the complex are characterized by several nelsonite (magnetite-apatite rock) and carbonatite dykes, plugs and veins crosscutting earlier alkaline rocks. The interaction between the latter and late-stage carbonatites and/or carbo-hydrothermal fluids, converted the original dunites and bebedourites to metasomatic phlogopitites. Late-stage nelsonites (N1), pseudonelsonites (N2) and various types of dolomite carbonatites (DC) including norsethite-, magnesite- and/or monazite-bearing varieties show significant whole-rock Nd and Sr isotopic variations. To elucidate whether magmatic or metasomatic processes, or both, were responsible for these isotope variations we characterized the Nd and Sr isotope compositions of major mineral phases (i.e. apatite, dolomite, norsethite, pyrochlore and tetraferriphlogopite) in these late-stage rocks. Mineral isotope data recorded the same differences observed between N1 and N2 whole-rocks with N2 minerals showing more enriched isotopic signatures than minerals from N1. Sr isotopic disequilibrium among minerals from N2 pseudonelsonites and spatially related dolomite carbonatite pockets implies formation from batches of carbonate melts with distinct isotopic compositions. A detailed investigation of Nd and Sr isotopes from whole-rocks and minerals suggests that the most evolved rocks of the Catalão I complex probably derive from two different evolution paths. We propose that an earlier magmatic trend (path A) could be explained by several batches of immiscible and/or residual melts derived from carbonated-silicate parental magma (e.g. phlogopite picrite) contaminated with continental crust to a variable extent, in an AFCLI-like process. A second trend (path B) comprises highly variable 143Nd/144Ndi at nearly constant 87Sr/86Sri coupled with high ?18O in carbonates. This is interpreted here as the result of the interaction of previously-formed dolomite carbonatites with carbo-hydrothermal fluids.
Leal, R.E., Lafon, J.M., da Ros Costa, L.T., Dantas, E.L.
Orosirian magmatic episodes in the erepercuru-trombetas domain ( southeastern Guyana shield: implications for the crustal evolution of the Amazonian craton.
South American Earth Sciences, Vol. 85, pp. 278-297.
Brazil Journal of Geology ( www.scielo.br) ENG, 14p. Pdf
South America, Brazil
deposit - Fazenda Bonfim
Abstract: The Fazenda Bonfim emerald deposit lies within the Seridó Belt. It is a classic example of deposit formed through metasomatic interactions between Be-rich granite intrusions and Cr(± V)-rich mafic-ultramafic rocks. The setting of the emerald mineralization was built under strong strike-slip dynamics, which produced serpentinization and talcification of mafic-ultramafic host-rocks, and was followed by syn-kinematic emplacement of Be-rich albite granite, favoring hydrothermal/metasomatic processes. The structural control and lithological-contrast were fundamental to the fluid flow and the best ore-shoot geometry, developed in the S-foliation intra-plane at the contact zone (phlogopite hornfels) between mafic-ultramafic rocks and the albite granite. Subsequently, an albitization process, linked to the final-stage of magmatic crystallization, led to an overall mineralogical and chemical change of the albite granite. 207U-235Pb data revealed inheritance ages from Archean to Neoproterozoic and a crystallization age of 561 ± 4 Ma for albite granite. However, 40Ar/39Ar data revealed plateau age of 553 ± 4 Ma for phlogopite hornfels, interpreted as the closure time for the metasomatic event responsible for the nucleation and growth of emerald crystals. The short interval of time between U-Pb and Ar-Ar data indicates an intense, but not protracted, metasomatic history, probably due to low volume of intrusive magma.
Journal of South American Earth Sciences, Vol. 100, 10.1016/j.jsames.2020.102583 7p. Pdf
South America, Brazil, Rondonia
deposit - Carolina
Abstract: In this study, we characterize pectolite that occurs in a Carolina kimberlitic intrusion from the Pimenta Bueno Kimberlite Field (PBKF). The PBKF is the only kimberlite field of Permo-Carboniferous age in Brazil and is found on the southern Amazonian Craton. Pectolite, an Na-Ca-silicate usually identified in alkaline rocks as a primary mineral, is not common in the mineral paragenesis of kimberlites and is described here for the first time in Brazil. The genesis of pectolite in kimberlite has been well-studied and can be interpreted as a primary or secondary mineral resulting from the infiltration of an Na-rich fluid into metasomatic reactions. In the rocks from the PBKF, pectolite mainly occurs as fibrous and radial aggregates enriched in K2O that grow between olivine partially altered to serpentine and phlogopite. The results of field and petrographic observations suggest that the PBKF pectolite is of secondary origin, having formed during the hydrothermal alteration of the Carolina kimberlitic intrusion.
Abstract: The São Francisco Craton, in Brazil, together with adjacent orogenic systems formed during Gondwana assemblage, are well-suited for the study of crustal growth processes. The region's geological history is marked by a series of complete tectono-metamorphic cycles, from the Archean to late Neoproterozoic, comprising arc-related magmatism followed by continental collisions and ultimately post-tectonic igneous events and rifting. In this contribution, a comprehensive isotopic database was compiled from the literature, composed mainly of high-quality U-Pb magmatic and metamorphic ages (ca. 1000), together with Lu-Hf (ca. 1300) and Sm-Nd (ca. 300) data. Using this database, combined with a tectonic/geochemical synthesized review of the region, it is possible to test which of the available contending models can better explain the apparent periodicity in the formation of the continental crustal. Some interpreted the peaks and troughs in the crustal age record as periods of increased magmatic production, controlled by periodic mantellic events. Another hypothesis is that subduction-related rocks are shielded from tectonic erosion after continental amalgamation, the peaks thus reflecting enhanced preservation potential. The latter hypothesis is favored, as the variability regarding the timing of arc-related peak magmatic production (U-Pb age peaks) from different tectonic provinces around the globe and in the considered regions, coupled to the fact that peak arc-production is always closely followed in time by major continental amalgamations (supercontinent formation), precludes a unified global causation effect, such as mantellic overturns or slab avalanches, and supports the preservation bias hypothesis. Furthermore, the worldwide (including the São Francisco Craton) occurrence of plume-related magmatism is concentrated during the periods of supercontinent break-up (i.e. after major collisions), which better relates to a top-down control on mantle convection and opposes most of the models that advocate for the primary periodicity of magmatic production, which predict enhanced plume activity slightly prior or concomitant to supercontinent formation events.
Abstract: 2.9 Ga is an uncommon magmatic age in Archean evolution worldwide, especially in West Gondwana. We identified so far unknown 2.97-2.92 Ga high?K calc?alkaline magmatism in the Borborema Province, northeast Brazil. It appears to indicate that the transition to high?K magmas occurred before c. 2.7 Ga in Earth's history. The 2.9 Ga protoliths were reworked and progressively changed composition to 2.65 Ga and 2.25 Ga higher?K granites in early magmatic arcs. Therefore, despite several reworking events from the Archean to Proterozoic times, these rare relicts of K?rich magmatism indicate that reworking of felsic components was significant for the growth and differentiation of continental crust from c. 2.9 Ga onwards in West Gondwana.
Journal of South American Earth Sciences, Vol. 111, 19p. Pdf
South America, Brazil
geophysics - seismics
Abstract: The Orocaima SLIP consists of an association of acid-intermediate volcanic-plutonic rocks. The volcanic rocks were generated in explosive eruptions through low eruptive columns, probably associated with fissural volcanism in the north of the Amazonian Craton, Brazil, between 2.0 and 1.98 Ga. It generated ignimbrites, whose facies (volcanic breccia rich in lithic, lapilli-tuff and lithic lapilli-tuff) show the proximity of the source. The extensive area of ca. 200.000 km2 of ignimbrite, rhyolite and dacite deposits, as well as the age range (2.0-1.98 Ga) and geochemical signatures suggest that the Orocaima volcano-plutonism may correspond to one of the oldest silicic LIPs in the world. The silicic volcanism is essentially subaerial and characterized by high-grade ignimbrites (densely welded) and subordinate lava, the ages of which indicate the longevity of the volcanic event in the Orosirian. They are included in the Surumu Group and comprise rocks with high-K calc-alkaline affinities and were emplaced in a subduction-related setting, similar to the rocks that extend through Venezuela, Guyana and Suriname (Cuchivero-Surumu-Iwokrama-Dalbana metavolcanic belt - CSID). The occurrence of mafic fragments disseminated in volcanic and granitic rocks in the north of Roraima, Brazil and in other segments of the CSID belt suggests the coexistence of acid and basic magmas. Except for one sample (?Nd(t) = -2.3), the Nd isotopic data of analyzed Surumu Group volcanic rocks yielded positive ?Nd(t) values (0.5-4.48; TDM = 2.0-2.47 Ga), suggesting generation from magmas derived from the mantle or from the melting of new juvenile crust. The Orocaima volcanism bears no evidence of involvement of Archean sources in the generation of the rocks. Thus, the Orocaima volcano-plutonism may represent one of the most significant ignimbrite eruption events during the Palaeoproterozoic in the world.-
Journal of South American Earth Sciences, Vol. 114, 103712, 20p. Pdf
South America, Brazil, Mato Grosso
deposit - Itiquira, alluvials
Abstract: The Itiquira River, Mato Grosso state (western Brazil), hosts several diamond placer deposits, mined intermittently over the last century. It runs over volcanic and sedimentary rocks of the Paraná Basin until it discharges in the northern Pantanal Basin. The bedrocks were deposited in marine, continental desertic, alluvial deltaic, and fluvial environments. The meanders of the Itiquira River are controlled by NE-SW, ENE-WSW, NNW-SSE, N-S, and NW-SE fractures and normal faults, developed in response to the evolution of the Paraná Basin and by neotectonics, linked with the development of the Pantanal Basin since the Paleogene. The Itiquira River middle valley, in which the diamondiferous placers are found, is controlled by NE-SW structures inherited from the Neoproterozoic Transbrasiliano Lineament. The landscape comprises dissected plateaus and structure-controlled valleys formed by Cretaceous, Paleogene, and Neogene planation processes. The diamonds occur in the muddy-sandy matrix of the current stream bed and older terraces gravels. In the Itiquira River, the diamond deposits are related to the following traps: point bars, cut-and-fill channels, pockets, and potholes. Sapphire, garnet (including kimberlitic), ilmenite, zircon, rutile, gold, and iron oxides are documented as heavy minerals in the gravels. Detrital zircon dating of grains extracted from the Itiquira River diamond deposits resulted in the ages of 2057, 1184, 873, 645-508, 307-207, and 144-142 Ma. The potential zircon sources are the Goiás Magmatic Arc granitoids, Paraguay Belt metavolcanics and granites, and Serra Geral Formation volcanics. The ages between 307 and 207 Ma are likely to be from an unknown (possibly kimberlitic) source. The Itiquira River tectonic, geomorphological, and sedimentological evolutions suggest potential sources for the diamond placers and paleoplacers.
Journal of South American Earth Sciences, Vol. 94, 102203 14p. Pdf
South America, Brazil
geophysics - magnetics
Abstract: Accretionary orogens are considered as the result of the major crust production process, and terranes are accreted material representing distinctive assemblages regarding age and evolution. Scientific advances in the last years show that the southernmost São Francisco Craton includes Archean, Paleoproterozoic and Neoproterozoic areas. We used aerogeophysical data and field geology to understand the extension of individual pieces of this tectonic puzzle. We described five magnetometric lineaments. A-, B- and C-lineaments are related to dyke swarms of different ages. D- and E-lineaments represent regional-scale tectonic structures. In a tectonic perspective, we have identified the following terranes: (i) the Archean São Tiago crust (2.67?Ga), which is part of the São Francisco proto-craton; two Siderian juvenile arcs, represented by the (ii) Cassiterita (2.47?Ga) and the (iii) Resende Costa/Lagoa Dourada (2.36-2.35?Ga) orthogneisses; and two Rhyacian arcs, the (iv) juvenile Serrinha (2.22-2.20?Ga) and the continental Ritápolis (2.19-2.10?Ga) arcs. Tectonic terranes and five magnetic subdomains were identified on the basis of (i) association of shear zones/faults with quartz veins in the field; (ii) low- and high-intensity magnetic anomalies; (iii) sharp contrast in Euler solution intensities; and (iv) high-contrast in radioelement contents in the gammaspectrometric maps. Processing of aerogeophysical data permitted us to propose a new scenario on the evolution of the southern São Francisco Craton, and in particular of the Mineiro belt. The integration between aerogeophysical, new and compiled geologic information, provides a robust model for the understanding of individual tectonic pieces of the studied area.
Journal of African Earth Sciences, Vol. 174, 104059, 17p. Pdf
Africa, Madagascar
melilitites
Abstract: The olivine melilitites from the southern part of the 6.8 Ma-old Takarindiona volcanic field (Eastern Madagascar) are olivine ± chromite -phyric lavas, with zoned titanaugite, perovskite, melilite, nepheline, monticellite, Ba-Ti-mica and Fe-Ti oxides as microphenocrysts and groundmass phases. The rocks are very primitive, rich in incompatible trace elements (e.g., Ba = 1049 ± 153 ppm, Sr = 1050 ± 167 ppm, Nb = 98 ± 13 ppm; La/Ybn = 41 ± 5; La/Nb = 0.88 ± 0.05), and have restricted ranges of initial 87Sr/86Sr (0.70391-0.70410) and 143Nd/144Nd (0.51272-0.51282). The rocks follow a differentiation trend controlled by ab. 20% removal/addition of phenocryst olivine ± chromite. The olivine melilititic magmas are the product of small degrees of partial melting (1-3%) of a peridotitic source, enriched in highly incompatible trace elements by CO2-, F-, and H2O-rich melts, located within the garnet stability field (3-3.5 GPa and ~100 km depth) of sub-continental lithospheric mantle, where carbonates (dolomite) and possibly phlogopite were stable phases. Mantle xenoliths within the volcanics are mostly spinel harzburgites having mineral modes and chemical compositions suggesting variable degrees of "basalt" melt extraction. Based on textural and chemical evidence, and quantitative thermobarometric estimates, the xenoliths were incorporated at a pressure of ~1.1 GPa (~35-40 km depth), far shallower than the source of the melilititic magmas, and along a predictably cool geotherm beneath Archean continental lithosphere. Highly resorbed orthopyroxene xenocrysts mantled by augite indicate that the melilitites may have also entrained lower crustal materials or underplated subalkaline rocks. The mantle sources of the lavas and mantle xenoliths of the Takarindiona district indicate stratification of the lithospheric mantle, and help constraining the lithospheric features and the magmatic history of the Eastern Madagascar craton.
Abstract: The extent to which water and halogens in Earth’s mantle have primordial origins, or are dominated by seawater-derived components introduced by subduction is debated. About 90% of non-radiogenic xenon in the Earth’s mantle has a subducted atmospheric origin, but the degree to which atmospheric gases and other seawater components are coupled during subduction is unclear. Here we present the concentrations of water and halogens in samples of magmatic glasses collected from mid-ocean ridges and ocean islands globally. We show that water and halogen enrichment is unexpectedly associated with trace element signatures characteristic of dehydrated oceanic crust, and that the most incompatible halogens have relatively uniform abundance ratios that are different from primitive mantle values. Taken together, these results imply that Earth’s mantle is highly processed and that most of its water and halogens were introduced by the subduction of serpentinized lithospheric mantle associated with dehydrated oceanic crust.
Abstract: Abundant multiphase solid inclusions (MSI) were found in garnet in an ultrahigh-pressure (UHP) paragneiss from the Kokchetav complex, Kazakhstan. The MSI are composed of mineral associations that include rock-forming and accessory minerals, which crystallized during exhumation. We present experimental and analytical protocols for how such inclusions can be homogenized to glass and analysed for major and trace elements. After homogenization we identified two types of glass. One type is present in garnet porphyroblasts in the melanocratic part of the sample and represents a high-pressure melt formed close to peak conditions of >45 kbar, 1000°C. These inclusions are characterized by high concentrations of light rare earth elements (LREE), Th and U. Extraction of these melts resulted in a pronounced depletion of the Kokchetav gneisses in those elements. Measured partition coefficients of large ion lithophile elements (LILE) between phengite inclusions and melt inclusions are DRb?=?1•9-2•5, DBa?=?1•1-6•9 and DCs?=?0•6-0•8, resulting in limited depletion of these elements during partial melting in the presence of phengite. The Nb concentration in melts (27?ppm) is about double that in the restite (15?ppm), indicating slightly incompatible behaviour during UHP anatexis, despite the presence of residual accessory rutile and phengite. A second type of inclusion occurs in garnet from the leucocratic part of the rock and represents a late-stage melt formed during exhumation at 650-750°C and crustal pressures. These inclusions are characterized by low LREE and Nb and high U. Zircon domains formed during high-temperature melting are characterized by high Ti content (100-300?ppm) and unfractionated Th/U (0•4-0•8), whereas the low-temperature domains display low Ti (10?ppm) and Th/U (0•08). The composition of UHP melts with moderate enrichment in LILE, no depletion in Nb and extreme enrichment in LREE and Th is remarkably different from the trace element signature of arc basalts, arguing against involvement of this type of melting in the generation of arc crust. The composition of the UHP melt inclusions is similar to that of melt inclusions from HP crustal xenoliths from Pamir and also to some shoshonites from Tibet. UHP anatexis, as observed in the Kokchetav massif, might be related to the formation of shoshonitic alkaline igneous rocks, which are common in collisional settings.
Geostandards and Geoanalytical Research, in press available, 21p.
Asia, Mongolia
olivine
Abstract: A new olivine reference material - MongOL Sh11?2 - for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn?Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5-2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA?ICP?MS, SIMS and bulk analytical methods (ID?ICP?MS for Mg and Fe, XRF, ICP?MS) for major, minor and trace elements at six institutions world?wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1-2). The presence of some mineral and fluid?melt micro?inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty?seven major, minor and trace elements.
Lithos, doi.org/10.1016/ jlithos.2020.105880, 26p. Pdf
Africa, South Africa
deposit - Bultfontein
Abstract: The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/? PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80-120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Experimental investigation and peridotite oxybarometers: implications for spinel thermodynamic models and Fe3+ compatibility during generation of upper mantle melts.
Geological Society of America, SPE 526 pp. 107-132.
United States
craton
Abstract: The North American continent consists of a set of Archean cratons, Proterozoic orogenic belts, and a sequence of Phanerozoic accreted terranes. We present an ~1250-km-long seismological profile that crosses the Superior craton, Grenville Province, and Appalachian domains, with the goal of documenting the thickness, internal properties, and the nature of the lower boundary of the North American crust using uniform procedures for data selection, preparation, and analysis to ensure compatibility of the constraints we derive. Crustal properties show systematic differences between the three major tectonic domains. The Archean Superior Province is characterized by thin crust, sharp Moho, and low values of Vp/Vs ratio. The Proterozoic Grenville Province has some crustal thickness variation, near-uniform values of Vp/Vs, and consistently small values of Moho thickness. Of the three tectonic domains in the region, the Grenville Province has the thickest crust. Vp/Vs ratios are systematically higher than in the Superior Province. Within the Paleozoic Appalachian orogen, all parameters (crustal thickness, Moho thickness, Vp/Vs ratio) vary broadly over distances of 100 km or less, both across the strike and along it. Internal tectonic boundaries of the Appalachians do not appear to have clear signatures in crustal properties. Of the three major tectonic boundaries crossed by our transect, two have clear manifestations in the crustal structure. The Grenville front is associated with a change in crustal thickness and crustal composition (as reflected in Vp/Vs ratios). The Norumbega fault zone is at the apex of the regional thinning of the Appalachian crust. The Appalachian front is not associated with a major change in crustal properties; rather, it coincides with a zone of complex structure resulting from prior tectonic episodes, and thus presents a clear example of tectonic inheritance over successive Wilson cycles.
Geological Society of London, Chapter 6, pp. 107-132.
United States, Canada
tectonics
Abstract: The North American continent consists of a set of Archean cratons, Proterozoic orogenic belts, and a Sequence of Phanerozoic accreted terranes. We present an ~1250-km-long seismological profile that crosses the Superior craton, Grenville Province, and Appalachian domains, with the goal of documenting the thickness, internal properties, and the nature of the lower boundary of the North American crust using uniform procedures for data selection, preparation, and analysis to ensure compatibility of the constraints we derive. Crustal properties show systematic differences between the three major tectonic domains. The Archean Superior Province is characterized by thin crust, sharp Moho, and low values of Vp/Vs ratio. The Proterozoic Grenville Province has some crustal thickness variation, near-uniform values of Vp/Vs, and consistently small values of Moho thickness. Of the three tectonic domains in the region, the Grenville Province has the thickest crust. Vp/Vs ratios are systematically higher than in the Superior Province. Within the Paleozoic Appalachian orogen, all parameters (crustal thickness, Moho thickness, Vp/Vs ratio) vary broadly over distances of 100 km or less, both across the strike and along it. Internal tectonic boundaries of the Appalachians do not appear to have clear signatures in crustal properties. Of the three major tectonic boundaries crossed by our transect, two have clear manifestations in the crustal structure. The Grenville front is associated with a change in crustal thickness and crustal composition (as reflected in Vp/Vs ratios). The Norumbega fault zone is at the apex of the regional thinning of the Appalachian crust. The Appalachian front is not associated with a major change in crustal properties; rather, it coincides with a zone of complex structure resulting from prior tectonic episodes, and thus presents a clear example of tectonic inheritance over successive Wilson cycles.
Earth and Planetary Science Letters, Vol. 479, pp. 170-178.
Canada, Nunavut, Baffin Island
geophysics - seismics
Abstract: The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian-Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan-Karakoram-Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.
Journal of Geophysical Research, Vol. 123, 7, pp. 5690-5709.
Canada, Nunavut
Geophysics - seismic
Abstract: The geology of northern Hudson Bay, Canada, documents more than 2 billion years of history including the assembly of Precambrian and Archean terranes during several Paleoproterozoic orogenies, culminating in the Trans?Hudson Orogen (THO) ?1.8 Ga. The THO has been hypothesized to be similar in scale and nature to the ongoing Himalaya?Karakoram?Tibetan orogen, but the nature of lithospheric terrane boundaries, including potential plate?scale underthrusting, is poorly understood. To address this problem, we present new P and S wave tomographic models of the mantle seismic structure using data from recent seismograph networks stretching from northern Ontario to Nunavut (60-100?W and 50-80?N). The large size of our network requires careful mitigation of the influence of source side structure that contaminates our relative arrival time residuals. Our tomographic models reveal a complicated internal structure in the Archean Churchill plate. However, no seismic wave speed distinction is observed across the Snowbird Tectonic Zone, which bisects the Churchill. The mantle lithosphere in the central region of Hudson Bay is distinct from the THO, indicating potential boundaries of microcontinents and lithospheric blocks between the principal colliders. Slow wave speeds underlie southern Baffin Island, the leading edge of the generally high wave speed Churchill plate. This is interpreted to be Paleoproterozoic material underthrust beneath Baffin Island in a modern?style subduction zone setting.
Abstract: Two-station surface-wave analysis was used to measure Rayleigh-wave phase velocities between 105 station pairs in western Canada, straddling the boundary between the tectonically active Cordillera and the adjacent stable craton. Major variations in phase velocity are seen across the boundary at periods from 15 to 200 s, periods primarily sensitive to upper mantle structure. Tomographic inversion of these phase velocities was used to generate phase velocity maps at these periods, indicating a sharp contrast between low-velocity Cordilleran upper mantle and high-velocity cratonic lithosphere. Depth inversion along selected transects indicates that the Cordillera-craton upper mantle contact varies in dip along the deformation front, with cratonic lithosphere of the Taltson province overthrusting Cordilleran asthenosphere in the northern Cordillera, and Cordilleran asthenosphere overthrusting Wopmay lithosphere further south. Localized high-velocity features at sub-lithospheric depths beneath the Cordillera are interpreted as Farallon slab fragments, with the gap between these features indicating a slab window. A high-velocity feature in the lower lithosphere of the Slave province may be related to Proterozic or Archean subduction.
The lithospheric root beneath Hudson Bay, Canada from Rayleigh wave dispersion: no clear seismological distinction between Archean and Proterozoic mantle.
P wave tomography of eastern North America: evidence for mantle evolution from Archean to Phanerozoic, and modification during subsequent hotspot tectonism.
Journal of Geophysical Research, Vol. 117, B12302, 15p.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 41-67.
Canada, Ontario, Quebec
Geotectonics
Abstract: Hudson Bay Lithospheric Experiment (HuBLE) was designed to understand the processes that formed Laurentia and the Hudson Bay basin within it. Receiver function analysis shows that Archaean terranes display structurally simple, uniform thickness, felsic crust. Beneath the Palaeoproterozoic Trans-Hudson Orogen (THO), thicker, more complex crust is interpreted as evidence for a secular evolution in crustal formation from non-plate-tectonic in the Palaeoarchaean to fully developed plate tectonics by the Palaeoproterozoic. Corroborating this hypothesis, anisotropy studies reveal 1.8 Ga plate-scale THO-age fabrics. Seismic tomography shows that the Proterozoic mantle has lower wavespeeds than surrounding Archaean blocks; the Laurentian keel thus formed partly in post-Archaean times. A mantle transition zone study indicates ‘normal’ temperatures beneath the Laurentian keel, so any cold mantle down-welling associated with the regional free-air gravity anomaly is probably confined to the upper mantle. Focal mechanisms from earthquakes indicate that present-day crustal stresses are influenced by glacial rebound and pre-existing faults. Ambient-noise tomography reveals a low-velocity anomaly, coincident with a previously inferred zone of crustal stretching, eliminating eclogitization of lower crustal rocks as a basin formation mechanism. Hudson Bay is an ephemeral feature, caused principally by incomplete glacial rebound. Plate stretching is the primary mechanism responsible for the formation of the basin itself.
Journal of Geophysical Research,, Vol. 121, 7, pp. 5013-5030.
Canada
Subduction
Abstract: The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (?250 km), cold, chemically depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P wave and S wave relative arrival-time tomography to constrain upper mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW-SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian age rocks up to ?300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wave speed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less depleted Proterozoic composition at formation.
Journal of Geophysical Research: Solid Earth, in press available, 24p. Pdf
Canada
geophysics - seismics
Abstract: The geological record of SE Canada spans more than 2.5Ga, making it a natural laboratory for the study of crustal formation and evolution over time. We estimate the crustal thickness, Poisson's ratio, a proxy for bulk crustal composition, and shear velocity (Vs) structure from receiver functions at a network of seismograph stations recently deployed across the Archean Superior craton, the Proterozoic Grenville and the Phanerozoic Appalachian provinces. The bulk seismic crustal properties and shear velocity structure reveal a correlation with tectonic provinces of different ages: the post-Archean crust becomes thicker, faster, more heterogenous and more compositionally evolved. This secular variation pattern is consistent with a growing consensus that crustal growth efficiency increased at the end of the Archean. A lack of correlation among elevation, Moho topography, and gravity anomalies within the Proterozoic belt is better explained by buoyant mantle support rather than by compositional variations driven by lower crustal metamorphic reactions. A ubiquitous ?20km thick high-Vs lower-crustal layer is imaged beneath the Proterozoic belt. The strong discontinuity at 20km may represent the signature of extensional collapse of an orogenic plateau, accommodated by lateral crustal flow. Wide anorthosite massifs inferred to fractionate from a mafic mantle source are abundant in Proterozoic geology and are underlain by high Vs lower crust and a gradational Moho. Mafic underplating may have provided a source for these intrusions and could have been an important post-Archean process stimulating mafic crustal growth in a vertical sense.
Abstract: The Canadian High Arctic preserves a long and complex tectonic history, including craton formation, multiple periods of orogenesis, extension and basin formation, and the development of a passive continental margin. We investigate the possible preservation of deformational structures throughout the High Arctic subcontinental lithosphere using measurements of seismic anisotropy from shear wave splitting at 11 seismograph stations across the region, including a N-S transect along Ellesmere Island. The majority of measurements indicate a fast-polarisation orientation that parallels tectonic trends and boundaries, suggesting that lithospheric deformation is the dominant source of seismic anisotropy in the High Arctic; however, a sub-lithospheric contribution cannot be ruled out. Beneath Resolute in the central Canadian Arctic, distinct back-azimuthal variations in splitting parameters can be explained by two anisotropic layers. The upper layer is oriented E-W and correlates with tectonic trends and the inferred lithospheric deformation history of the region. The lower layer has a ?NNE-SSW orientation and may arise from present-day convective mantle flow beneath locally-thinned continental lithosphere. In addition to inferences of anisotropic structure beneath the Canadian High Arctic, measurements from the far north of our study region suggest the presence of an anisotropic zone in the lowermost mantle beneath northwest Alaska.
Minerals MDPI, Vol. 10, 267 doi: 10.23390/min10030267 14p. Pdf
Mantle
Melililite, carbon
Abstract: Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure-temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate-silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa•s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T-T and T-O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km•yr?1 in the present-day or the Archaean mantle, respectively.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 201-222.
Tuffisitic kimberlite from Eastern Dharwar craton, Undraldoddi area, Raichur District, Karnataka, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 109-128.
Bose, S., Dunkley, D.J., Dasgupta, S., Das, K., Arima, M.
India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: evidence from new zircon U Pb and monzazite chemical age data
Geological Society of America Bulletin, Vol. 123, 9/10 pp. 2031-2049.
Geophysical Journal International, Vol. 203, pp. 910-926.
India
Geophysics - seismics
Abstract: We present a comprehensive study of thickness and composition of the crust; and the nature of crust-mantle boundary beneath Southern India using P-wave receiver function from 119 seismic stations. Data from distributed network of seismograph location encompass geological domains like mid to late Archean Dharwar craton, Archean and Proterozoic metamorphic terrains, Proterozoic basin, rifted margins and escarpments, and Deccan volcanics. Except for the mid to lower crust exhumed Archean terrains (of West Dharwar and Southern Granulite) all other geological domains have crustal thickness in the range 33-40 km. In the western Dharwar, crustal thickness increases from ?40 km in the north to over 50 km in the south. The Archean domain of granulite terrain is thicker (40-45 km) and more mafic compared to its counterpart in south deformed at 550 Ma. Most of the crustal blocks have low to moderate Vp/Vs (1.72-1.76) representing a felsic to intermediate composition. Exception to the above include Archean granulite terrain with high Vp/Vs (1.76–1.81) suggestive of more mafic crust beneath them. When accounted for the paleo burial depth of 15-25 km, the study suggests a possible Himalaya-Tibet like scenario beneath the mid-late Archean in southwestern Dharwar and north granulite terrain whose deeper crust has progressively densified. This led to a gradational crust-mantle transition that is otherwise sharp elsewhere. The study suggests a more homogenized and felsic nature of the Precambrian crust beneath the terrains formed after 2.6 Ga, possibly due to delamination of the mafic lower crust. Our study does not suggest any distinction between late Archean and Proterozoic crust. The Deccan volcanism at 65 Ma does not appear to have altered the crustal character beneath it and is similar to the adjoining late Archean east Dharwar craton. The western Ghat escarpment and the coastal plain formed due to separation of India from Madagascar are underlain by mafic lower crust.
Das, S., Nasipuri, P., Bhattachaya, A., Swaminathan, S.
The thrust contact between the Eastern Ghats belt and the adjoining Bastar craton, Eastern India: evidence from mafic granulites and tectonic implications.
Immiscible transition from carbonate rich to silicate rich melts in the 3 GPa melting interval of eclogite + CO2 and genesis of silica undersaturated Oceanic lavas.
Journal of Petrology, Vol. 47, 4, April pp. 647-671.
Abstract: In ore processing, electric-pulse disaggregation (EPD) is used for the liberation of mineral crystals from host rocks. Since 2019, EPD technology has been used exclusively to recover emeralds produced from the Kagem mine in Zambia. This article compares the differences in the recovery of emeralds from micaceous schist host rock at the Kagem mine by EPD technology versus the conventional hand-cobbing method. The amount of emeralds obtained using both methods was similar, but EPD had numerous advantages in terms of liberation speed, ease of performing the process and the characteristics of the liberated emeralds.
Geochemistry, Geophysics, Geosystems, Vol. 19, 3, pp. 595-620.
Mantle
peridotites
Abstract: We present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab?derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950-1175°C and 2-4 GPa using a piston?cylinder and a multi?anvil apparatus, to test the hypothesis that volatile?bearing mineral?phases produced during craton formation can cause reduction in aggregate shear?wave velocities (VS) at mid?lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3-7.6 wt.%; Bulk 2: 2.6-5 wt.%) at 2-4 GPa, and amphibole (Bulk 1: 3-9 wt.%; Bulk 2: 2-6 wt.%) at 2-3 GPa (?1050°C) are also present. Magnesite (Bulk 1: ?1 wt.% and Bulk 2: ?0.6 wt.%) is present at 2-4 GPa (<1000°C at 3 and?100 km depth.
Journal of Earth Science System, doi:10.1007/s12040- 019-1228-0
India
geochemistry
Abstract: The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE-SSW to NE-SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5-25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of ~2.75-2.8 Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India.
Dasgupta, R., Hirschmann, M.M., McDonough, W.F., Spiegelman, M., Withers, A.C.
Trace element partitioning between garnet lherzolite and carbonatite at 6.6 and 8.6 GPa with application to the geochemistry of the mantle and mantle derived melts
Chemical Geology, Vol. 262, 1-2, May 15, pp. 57-77.
Fe Ni Cu C S phase relations at high pressures and temperatures - the role of sulfur in carbon storage and diamond stability at mid to deep upper mantle.
Earth and Planetary Science Letters, Vol. 412, pp. 132-142.
Abstract: The abundances of volatile elements in the Earth’s mantle have been attributed to the delivery of volatile-rich material after the main phase of accretion1, 2, 3. However, no known meteorites could deliver the volatile elements, such as carbon, nitrogen, hydrogen and sulfur, at the relative abundances observed for the silicate Earth4. Alternatively, Earth could have acquired its volatile inventory during accretion and differentiation, but the fate of volatile elements during core formation is known only for a limited set of conditions4, 5, 6, 7, 8. Here we present constraints from laboratory experiments on the partitioning of carbon and sulfur between metallic cores and silicate mantles under conditions relevant for rocky planetary bodies. We find that carbon remains more siderophile than sulfur over a range of oxygen fugacities; however, our experiments suggest that in reduced or sulfur-rich bodies, carbon is expelled from the segregating core. Combined with previous constraints9, we propose that the ratio of carbon to sulfur in the silicate Earth could have been established by differentiation of a planetary embryo that was then accreted to the proto-Earth. We suggest that the accretion of a Mercury-like (reduced) or a sulfur-rich (oxidized) differentiated body—in which carbon has been preferentially partitioned into the mantle—may explain the Earth’s carbon and sulfur budgets.
Earth and Planetary Science Letters, Vol. 459, pp. 183-195.
Mantle
Peridotite
Abstract: Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380?°C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420?°C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO?FeO? and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ?10-15%?10-15% melting with bulk S of 100-150 ppm in the mantle when TPTP is as low as 1300?°C. We also show that although variation of View the MathML sourceDCuperidotite/melt and initial Cu in the mantle can all affect the Cu concentration of primitive MORBs, 100-200 ppm S in the MORB source mantle can satisfy both S and Cu geochemistry of partial melts parental to ocean floor basalts.
Abstract: The net flux of carbon between the Earth’s interior and exterior, which is critical for redox evolution and planetary habitability, relies heavily on the extent of carbon subduction. While the fate of carbonates during subduction has been studied, little is known about how organic carbon is transferred from the Earth’s surface to the interior, although organic carbon sequestration is related to sources of oxygen in the surface environment. Here we use high pressure–temperature experiments to determine the capacity of rhyolitic melts to carry carbon under graphite-saturated conditions in a subducting slab, and thus to constrain the subduction efficiency of organic carbon, the remnants of life, through time. We use our experimental data and a thermodynamic model of CO2 dissolution in slab melts to quantify organic carbon mobility as a function of slab parameters. We show that the subduction of graphitized organic carbon, and the graphite and diamond formed by reduction of carbonates with depth, remained efficient even in ancient, hotter subduction zones where oxidized carbon subduction probably remained limited. We suggest that immobilization of organic carbon in subduction zones and deep sequestration in the mantle facilitated the rise (~103–5 fold) and maintenance of atmospheric oxygen since the Palaeoproterozoic and is causally linked to the Great Oxidation Event. Our modelling shows that episodic recycling of organic carbon before the Great Oxidation Event may also explain occasional whiffs of atmospheric oxygen observed in the Archaean.
Geochimica et Cosmochimica Acta, Vol. 238, pp. 477-495.
Mantle
carbon
Abstract: Constraining carbon (C) fractionation between silicate magma ocean (MO) and core-forming alloy liquid during early differentiation is essential to understand the origin and early distribution of C between reservoirs such as the crust-atmosphere, mantle, and core of Earth and other terrestrial planets. Yet experimental data at high pressure (P)-temperature (T) on the effect of other light elements such as sulfur (S) in alloy liquid on alloy-silicate partitioning of C and C solubility in Fe-alloy compositions relevant for core formation is lacking. Here we have performed multi-anvil experiments at 6-13?GPa and 1800-2000?°C to examine the effects of S and Ni on the solubility limit of C in Fe-rich alloy liquid as well as partitioning behavior of C between alloy liquid and silicate melt (). The results show that C solubility in the alloy liquid as well as decreases with increasing in S content in the alloy liquid. Empirical regression on C solubility in alloy liquid using our new experimental data and previous experiments demonstrates that C solubility significantly increases with increasing temperature, whereas unlike in S-poor or S-free alloy compositions, there is no discernible effect of Ni on C solubility in S-rich alloy liquid. Our modelling results confirm previous findings that in order to satisfy the C budget of BSE, the bulk Earth C undergoing alloy-silicate fractionation needs to be as high as those of CI-type carbonaceous chondrite, i.e., not leaving any room for volatility-induced loss of carbon during accretion. For Mars, on the other hand, an average single-stage core formation at relatively oxidized conditions (1.0 log unit below IW buffer) with 10-16?wt% S in the core could yield a Martian mantle with a C budget similar to that of Earth’s BSE for a bulk C content of ?0.25-0.9?wt%. For the scenario where C was delivered to the proto-Earth by a S-rich differentiated impactor at a later stage, our model calculations predict that bulk C content in the impactor can be as low as ?0.5?wt% for an impactor mass that lies between 9 and 20% of present day Earth’s mass. This value is much higher than 0.05-0.1?wt% bulk C in the impactor predicted by Li et al. (Li Y., Dasgupta R., Tsuno K., Monteleone B., and Shimizu N. (2016) Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary embryos. Nat. Geosci.9, 781-785) because C-solubility limit of 0.3?wt% in a S-rich alloy predicted by their models is significantly lower than the experimentally derived C-solubility of ?1.6?wt% for the relevant S-content in the core of the impactor.
American Journal of Science, Vol. 318, 1, pp. 141-165.
Mantle
melting
Abstract: Besides depth and temperature, CO2 and H2O, are the two most important variables in stabilizing partial melts in the Earth's mantle. However, despite decades of experimental studies on the roles of these two volatile species in affecting mantle melting, ambiguity remains in terms of the stability, composition, and proportion of volatile-bearing partial melts at depths. Furthermore, the difference in the influence of H2O versus CO2 in production of mantle melts is often inadequately discussed. Here I first discuss how as a function of depth and concentration of volatiles, the peridotite + H2O versus peridotite + CO2 near-solidus melting conditions differ - discussing specifically the concepts of saturation of volatile-bearing phases and how the mode of storage of ‘water’ and carbon affects the near solidus melting relations. This analysis shows that for the Earth's mantle beneath oceans and continents, deep, volatile-induced melting is influenced mostly by carbon, with water-bearing carbonated silicate melt being the key agent. A quantitative framework that uses the existing experimental data, allows calculation of the loci, extent of melting, and major element compositions of volatile-bearing partial melts beneath oceans and continents. How the domains of volatile-bearing melt stability are affected when possible oxygen fugacity variation at depths in the mantle is taken into account is also discussed. I show that trace amount hydrous carbonated silicate melt is likely stabilized at two or more distinct depths in the continental lithospheric mantle, at depths ranges similar to where mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have been estimated from seismology. Whereas beneath oceans, hydrous carbonated silicate melt likely remain continuously stable from the base of the thermal boundary layer to at least 200 km or deeper depending on the prevailing oxygen fugacity at depths. Hotter mantles, such as those beneath oceans, prevent sampling strongly silica-undersaturated, carbonated melts such as kimberlites as shallower basaltic melt generation dominates. Thick thermal boundary layers, such as those in cratonic regions, on the other hand allow production of kimberlitic to carbonatitic melt only. Therefore, the increasing frequency of occurrence of kimberlites starting at the Proterozoic may be causally linked to cooling and growth of sub-continental mantles through time.
Abstract: A model based on a thermodynamic framework for CO2 concentrations and speciation in natural silicate melts at graphite/diamond-saturated to fluid-saturated conditions is presented. The model is simultaneously calibrated with graphite-saturated and fluid-saturated conditions allowing for consistent model predictions across the CCO buffer. The model was calibrated using water-poor (?1?wt% H2O) silicate melts from graphite- to CO2-fluid-saturation over a range of pressure (P?=?0.05-3?GPa), temperature (T?=?950-1600?°C), composition (foidite-rhyolite; NBO?=?0.02-0.92; wt% SiO2?~?39-77, TiO2?~?0.1-5.8, Al2O3?~?7.5-18, FeO?~?0.2-24 MgO?~?0.1-24, CaO?~?0.3-14, Na2O~1-5, K2O?~?0-6), and fO2 (~QFM +1.5 to ~QFM ?6). The model can predict CO2 concentrations for a wide range of silicate melt compositions from ultramafic to rhyolitic compositions, i.e., melts that dissolve carbon only as carbonate anions CO32- and those that dissolve carbon both as CO32- and as molecular CO2mol as a function of pressure, temperature, and oxygen fugacity. The model also does a reasonable job in capturing CO2 solubility in hydrous silicate melts with ?2-3?wt% H2O. New CO2 solubility experiments at pressures >3?GPa suggest that the newly developed CO2 solubility model can be satisfactorily extrapolated to ~4-5?GPa. Above 5?GPa the model poorly reproduces experimental data, likely owing to structural change in silicate melt at pressures above 5?GPa. An Excel spreadsheet and a Matlab function are provided as online supplementary materials for implementing the new CO2 solubility model presented here.
Earth and Planetary Science Letters, Vol. 506, pp. 38-52.
Mantle
kimberlite genesis
Abstract: Low-degree partial melts from deeply subducted, carbonated ocean crust are carbonatite liquids with ?35-47 wt% CO2. Their reactions with the overlying mantle regulate the slab-mantle interaction and carbon transport in the deep upper mantle but have not been investigated systematically. Here we present new multi-anvil experiments and parameterized phase relation models to constrain the fate of slab-derived carbonatite melts in the upper mantle. The experiments were conducted at 7 GPa/1400 °C and 10 GPa/1450 °C, and used starting compositions mimicking the ambient mantle infiltrated by variable carbonatite fluxes (0-45 wt%) from the slab surface. Kimberlitic melts (CO2 = 14-32 wt%, SiO2 = 15-33 wt%, and MgO = 20-29 wt%) were produced from experiments with 5.8-25.6 wt% carbonatite influxes. Experimental phase relations demonstrate a reactive melting process in which the carbonatite influx increases in proportion by dissolution of olivine, orthopyroxene, garnet and precipitation of clinopyroxene. This manifests a feasible mechanism for slab-derived carbonatite melts to efficiently transport in the ambient mantle through high-porosity channels. The melt and mineral fractions from this study and previous phase equilibria experiments in peridotite + O systems were empirically parameterized as functions of temperature (900-2000 °C), pressure (3-20 GPa), and bulk compositions (e.g., CO2 = 0.9-17.1 wt% and Na2O + K2O = 0.27-2.51 wt%). Applications of the phase relation models to prescribed melting processes indicate that reactive melting of a carbonatite-fluxed mantle source could produce kimberlitic melts with diverse residual lithologies under various melting conditions. However, reactive melting at the slab-mantle interface can only commence when the slab-released carbonatite melt conquers the carbonation freezing front, i.e., the peridotite solidi suppressed by infiltration of CO2-rich melts in an open system. Depending on temperatures and local influxes, reactive melting and carbonation/redox freezing can occur simultaneously above the slab-mantle interface, yielding heterogeneous lithologies and redox conditions as well as various time-scales of carbon transport in Earth's mantle.
Abstract: Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.
Geochimica et Cosmochimica Acta, Vol. 251. pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Geochimica et Cosmochimica Acta, Vol. 251, pp. 87-115.
Mantle
nitrogen
Abstract: Nitrogen, the most dominant constituent of Earth’s atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth’s major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is not well understood. Here we present equilibrium partitioning data of N between alloy and silicate melt () from 67 new high pressure (P?=?1-6?GPa)-temperature (T?=?1500-2200?°C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO2????IW ?4.2 to ?0.8), mafic to ultramafic silicate melt compositions (NBO/T?=?0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1?wt.% and 0-3.1?wt.%, respectively). Under relatively oxidizing conditions (??IW ?2.2 to ?0.8) nitrogen acts as a siderophile element ( between 1.1 and 52), where decreases with decrease in fO2 and increase in T, and increases with increase in P and NBO/T. Under these conditions remains largely unaffected between S-free conditions and up to ?17?wt.% S content in the alloy melt, and then drops off at >?20?wt.% S content in the alloy melt. Under increasingly reduced conditions (
Geochimica et Cosmochimica Acta, in press, 61p. Pdf
Mantle
petrology
Abstract: Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep global sulfur cycle. Given that sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur concentration at sulfide saturation (SCSS) of mantle-derived magmas. CO2?±?H2O-induced melting beneath oceanic and continental mantle produces incipient CO2-rich melts. Although, SCSS of silicate melts of a variety of compositions is extensively studied, the SCSS of carbonatitic and carbonated silicate melts have not received much attention. Here we present experiments in graphite capsules at pressures (P) of 2.5-6.0 GPa and temperatures (T) of 1350-1650?°C investigating the SCSS of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe?±?Ni-sulfide melt blobs?+?carbonated melt matrix?±?ol?±?cpx?±?opx?±?gt, with melt composition on a CO2-free basis varying from 7 to 40 wt.% SiO2, 0.5 to 7 wt.% Al2O3, and 9 to 17 wt.% FeO* (total FeO). SCSS measured using EPMA increases with SiO2 and T but is not affected by P; the effect of composition being more pronounced than P-T. The composition of sulfide melt phase also affects SCSS. With increasing Ni in the molten sulfide phase, the SCSS changes from 2000 to 4000?ppm (Ni-free) to is 800-3000?ppm (33 wt.% Ni). Comparison of our measured SCSS with the existing SCSS models for nominally CO2-free silicate melts and with one study for carbonated melts show that these parameterizations fail to capture the sulfide saturation values in CO2-rich melts from our study. Using our new SCSS data and previous SCSS data for melt compositions that span the range from carbonatite to basalts via carbonated silicate melts, we develop a new empirical SCSS parameterization. Unlike a previous model, which suggested SCSS of carbonated melt is only affected by melt FeO* (other than P-T) and did not constrain how SCSS evolves from low-silica carbonatitic melt to low-CO2 basaltic melt, our new parameterization captured complex effects of many melt compositional parameters, including silica on SCSS. Using our new SCSS model, we constrained the efficiency of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated melts beneath ridges are expected to mobilize 5-15% of the initial sulfur before nominally-volatile-free peridotite melting begins. In continental mantle, deep kimberlitic melt can act as an agent to mildly enrich the shallow mantle in sulfide as it evolves to a carbonatitic melt upon reactive cooling. Application of our data to subduction zones suggests that low degree carbonatitic melt is not an efficient agent to extract residual sulfide from the subducting oceanic crust.
Journal of Geophysical Research: Solid Earth, Vol. 124, 10, pp. 10089-10106.
Mantle
peridotite
Abstract: We present phase?equilibria experiments of a K?bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2?H2O fluid (0.5 wt.% CO2 and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile?bearing partial melts versus volatile?bearing mineral phases in a depleted peridotite system. Experiments were performed at 850-1150 °C and 2-4 GPa using a piston?cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900-1000 °C at 2 GPa, 850-950 °C at 3 GPa, and 950-1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900-1000 °C at 2 GPa, 950-1000 °C at 3 GPa, and 1000-1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2?H2O content introduced via a silicic melt show that introduction of CO2?H2O as fluid lowers the temperature of phlogopite breakdown by 150-200 °C at 2-4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile?bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile?bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.
Earth and Planetary Letters, Vol.. 553, 116602, 12p. Pdf
Mantle
cratons
Abstract: A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ?2-6% reductions in seismic shear wave velocity (VS) at depths of 60 km to ?150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite + H2O, peridotite + CO2, and peridotite + H2O + CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2 content in SCLM, ??10 vol.% pargasitic amphibole, ??2.1 vol.% phlogopite and ??0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate VS for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate VS at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in VS. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.
Earth and Planetary Letters, Vol. 550, 116549, 13p.
Global, United States, Wyoming, Canada, Northwest Territories, Europe, Baltic, India
geothermometry
Abstract: Cratonic lithosphere is believed to have been chemically buoyant and mechanically resistant to destruction over billions of years. Yet the absence of cratonic roots at some Archean terrains casts doubt on the craton stability and longevity on a global scale. As unique mantle-derived melts at ancient continents, silica-poor, kimberlitic melts are ideal tools to constrain the temporal variation of lithosphere thickness and the processes affecting the lithosphere root. However, no reliable thermobarometer exists to date for strongly silica-undersaturated, mantle-derived melts. Here we develop a new thermobarometer for silica-poor, CO2-rich melts using high-temperature, high-pressure experimental data. Our barometer is calibrated based on a new observation of pressure-dependent variation of Al2O3 in partial melts saturated with garnet and olivine, while our thermometer is calibrated based on the well-known olivine-melt Mg-exchange. For applications to natural magmas, we also establish a correction scheme to estimate their primary melt compositions. Applying this liquid-based thermobarometer to the estimated primary melt compositions for a global kimberlite dataset, we show that the equilibration depths between primary kimberlite melts and mantle peridotites indicate a decrease of up to ?150 km in cratonic lithosphere thickness globally during the past ?2 Gyr. Together with the temporal coupling between global kimberlite frequency and cold subduction flux since ?2 Gyr ago, our results imply a causal link between lithosphere thinning and supply of CO2-rich melts enhanced by deep subduction of carbonated oceanic crusts. While hibernating at the lithosphere root, these melts chemically metasomatize and rheologically weaken the rigid lithosphere and consequently facilitate destruction through convective removal in the ambient mantle or thermo-magmatic erosion during mantle plume activities.
Bose, S., Dunkley, D.J., Dasgupta, S., Das, K., Arima, M.
India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: evidence from new zircon U Pb and monzazite chemical age data
Geological Society of America Bulletin, Vol. 123, 9/10 pp. 2031-2049.
Geological Society of America Annual Meeting, Vol. 50, 4, 1p. Abstract
United States, Kansas
kimberlite
Abstract: The Critical Zone is the realm where rocks meet life. This study examines the physicochemical interactions that occur when interbedded limestone-shale systems and kimberlitc eruptive materials weather to form soils. Fast weathering with extensive soil loss has been a major environmental concern in the Flint Hills. Knowledge of soil formation processes, rates of formation and loss and understanding how these processes differ in different systems are critical for managing soil as a resource. The kimberlites of Riley County, KS, are CO2-rich igneous rocks that are high in Mg and Fe; they are compositionally distinct from the Paleozoic limestones and shales found throughout the rest of the region. Bulk composition and mineralogy of the soils overlying these different bedrock types have been analyzed using X-Ray Fluorescence (XRF), X-Ray diffraction of <2mm soil fraction and <2µm soil clay fraction, bulk elemental extraction, and particle size analyses. Results show that the kimberlitic soils have higher concentrations of Fe, Mg, Ca, K and some trace elements (e.g. Ti, Ni, Cu). The weathering products differ mineralogically as well, e.g. lizardite is abundant in kimberlitic soils and absent from the limestone terrane. As a result, kimberlite-sourced soils have significantly different physical properties than the thin limestone-sourced soils surrounding them. Particle size analysis shows that the limestone-shale soils have a higher proportion of silt-sized particles whereas the kimberlitic soils have more clay (10.55% vs. 8.06%) and significantly more sand (36.12% vs. 14.83%). Mineralogy was determined for all <2mm fractions and for some <2µm soil clay fraction to understand the association and mobility of these major and trace elements in the respective soils. Interestingly some of the similarities between the kimberlite and limestone-shale soils suggests that loess/wind-blown sediment is making a significant contribution to the soil profile. Kimberlite-sourced and limestone-shale-sourced soils produce different weathering products and could potentially have agricultural significance in terms of ionic and nutrient mobility.
Recherches Experimentales sur le Role Possible des Gaz a Hautes Temperatures Doues de Tres Fortes Pressions et Animes D'un Mouvement Fort Rapide dans Divers Phenomenes Geologiques.
Geological Society FRANCE (PARIS) Bulletin., Vol. 19, PP. 313-354.
Geochemical Perspectives Letters, Vol. 10, pp. 51-55.
Mantle
redox
Abstract: Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth’s redox evolution. The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated. In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases in?uences the carbon isotopic compositions of super-deep diamonds. Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe3C under pressure and temperature conditions relevant to super-deep diamond formation. Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe3C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3. Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compo- sitions observed in some super-deep diamonds. Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
Abstract: Diamonds are key messenger from the deep Earth because someare sourced from the longest isolated and deepest accessible regions of the Earth’s mantle. They are prime recorders of the carbon isotopic compositionof the Earth. The C isotope composition (d13C) of natural diamonds showsa widevariationfrom -41‰ to +3‰ with the primary mode at -5 ± 3‰ [1]. In comparison, the d13C values of chondrites and other planetary bodies range between -26‰ and -15‰ [2]. It is possible that some of the low d13C values were inherited from the Earth’s building blocks,but this is unlikely to be the sole explanation for all low d13C values that can reach as low as -41‰. Organic matter at the Earth’s surface that has low d13C values[3] has been regarded as a possible origin for low d13C values. However, organic carbon is usually accompanied by carbonate with higher d13C values (~0 ‰),and it is not clear why this d13C value does not appear frequently in diamonds. Low d13C diamonds were also formed by deposition from C-O-H fluids,but the equilibrium fractionationinvolved between diamonds and fluids issmall at mantle temperatures [1] and the low d13C values of diamonds can only be achieved after extensive Rayleigh distillation. One unique feature of the Earth isactive plate tectonics driven by mantle convection. Relatively oxidized iron and carbon species at the surface, such as carbonate, Fe2+-and Fe3+-bearing silicatesand oxides, are transported to the deep mantle by subducted slabs and strongly involved inthe redox reactions that generatediamonds [4]. The extent to which the isotopic compositionof C duringdiamond formation recordsredox processes that shaped the Earth is still controversial. Here we report onvibration properties of C andFe at high pressure in carbonates, diamond and Fe3C,based on nuclear resonant inelastic X-ray scattering measurements and density functional theory calculationsand further calculate equilibrium C isotope fractionations among these C-bearing species. Our results demonstrate that redox reactions in subducted slabs could generate eclogitic diamonds with d13C values as low as -41‰ if C in diamonds was sourced from the oxidation of a Fe-C liquid. The large C isotopic fractionation and potentially fast separation between diamonds and a Fe-C melt could enable diamond formation as high as 2%with d13C lower than -40‰.
Abstract: We reconstruct the history of topographic growth in southern Africa on both sides of the Orange River valley from an integrated analysis of erosion surfaces, crustal deformation and climate change. First, we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process, i.e. chemical weathering or mechanical erosion. Among the various land units observed we define a new class of landform: the pedivalley, which corresponds to a wide valley with a flat erosional floor. In the Orange River valley, we mapped three low-relief erosion surfaces, each bevelling a variety of lithologies. The oldest and most elevated is (1) a stripped etchplain evolving laterally into (2) a stepped pediplain bearing residual inselbergs; (3) a younger pediplain later formed in response to a more recent event of crustal deformation. These are all Cenozoic landforms: the etchplain is associated with a late Palaeocene to middle Eocene weathering event, and the two pediplains are older than the middle Miocene alluvial terraces of the Orange River. Landscape evolution was first driven by slow uplift (10 m/Ma), followed by a second interval of uplift involving a cumulative magnitude of at least 200 m. This event shaped the transition between the two pediplains and modified the drainage pattern. A final phase of uplift (magnitude: 60 m) occurred after the Middle Miocene and drove the incision of the lower terraces of the Orange River. Climate exerted a major control over the denudation process, and involved very humid conditions responsible for lateritic weathering, followed by more arid conditions, which promoted the formation of pedivalleys. Collectively, these produce pediplains.
Abstract: There are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively. Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa. (i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift. (ii) Central Africa is an extensive etchplain-type weathering surface - called the African Surface - from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift. (iii) The present-day topography of Central Africa is younger than 40 -30 Ma and records very long wavelength deformations (1000 -2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15 -12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10 -3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20 -16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10 -3 Ma.
Abstract: Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790-1165?°C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction - promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing - resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.
International Geology Review, Vol. 57, 11-12, pp. 1341-1348.
Mantle
Supercontinents
Abstract: There is a correlation of global large igneous province (LIP) events with zircon age peaks at 2700, 2500, 2100, 1900, 1750, 1100, and 600 and also probably at 3450, 3000, 2000, and 300?Ma. Power spectral analyses of LIP event distributions suggest important periodicities at 250, 150, 100, 50, and 25?million years with weaker periodicities at 70-80, 45, and 18-20?Ma. The 25?million year periodicity is important only in the last 300?million years. Some LIP events are associated with granite-forming (zircon-producing) events and others are not, and LIP events at 1900 and 600?Ma correlate with peaks in craton collision frequency. LIP age peaks are associated with supercontinent rifting or breakup, but not dispersal, at 2450-2400, 2200, 1380, 1280, 800-750, and ?200?Ma, and with supercontinent assembly at 1750 and 600?Ma. LIP peaks at 2700 and 2500?Ma and the valley between these peaks span the time of Neoarchaean supercraton assemblies. These observations are consistent with plume generation in the deep mantle operating independently of the supercontinent cycle and being controlled by lower-mantle and core-mantle boundary thermochemical dynamics. Two processes whereby plumes can impact continental assembly and breakup are (1) plumes may rise beneath supercontinents and initiate supercontinent breakup, and (2) plume ascent may increase the frequency of craton collisions and the rate of crustal growth by accelerating subduction.
Abstract: Zircon age peaks are commonly interpreted either as crustal production peaks or as selective preservation peaks of subduction-produced crust selectively preserved during continent-continent collision. We contribute to this ongoing debate, using the Nd isotopic compositions of felsic igneous rocks and their distribution during the accretionary and collisional phases of orogens. The proportion of juvenile input into the continental crust is estimated with a mixing model using arc-like mantle and reworked continental crust end members. Orogen length and duration proxies for juvenile crustal volume show that the amount of juvenile crust produced and preserved at zircon age peaks during the accretionary phase of orogens is ?3 times that preserved during the collisional phase of orogens. The fact that most juvenile crust is both produced and preserved during the accretionary phase of orogens does not require craton collisions for its preservation, thus favoring the interpretation of zircon age peaks as crustal production peaks. Most juvenile continental crust older than 600 Ma is produced and preserved before final supercontinent assembly and does not require supercontinent assembly for its preservation. Episodic destabilization of a compositionally heterogeneous layer at the base of the mantle may produce mantle plume events leading to enhanced subduction and crustal production. Our Nd isotope model for cumulative continental growth based on juvenile crust proxies for the past 2.5 b.y. suggests a step-like growth curve with rapid growth in accretionary orogens at the times of zircon age peaks.
Abstract: Before 2.7?Ga, 14 igneous and detrital zircon age peaks and 9 large igneous province (LIP) age peaks are robust and statistically significant. Correlation analysis indicates a synchronous association among these peaks and power spectral analysis shows 91, 114-127 and 182-Myr cycles. These age cycles may be related to mantle plume or mantle overturn events, and to the time it takes to reach threshold temperature gradients for thermo-chemical destabilization in the lowermost mantle. Most zircon age peaks are transferred into younger detrital sediments, which does not favor an origin of the peaks by selective erosion. Correlation of eight pre-2.7-Ga LIP age peaks with zircon age peaks is consistent with a genetic relationship between mantle melting events and felsic crustal production and supports an interpretation of pre-2.7-Ga age peaks as growth rather than preservation peaks produced during craton collisions. Also consistent with the growth peak interpretation is the apparent absence of collisional orogens older than 2.7?Ga. An increasing number of geographic age peak sites from 4 to 2.8?Ga suggests production and survival of only small volumes of continental crust during this time and supports an episodic model for continental crustal growth.
Abstract: Based on SEMUCB?WM1 tomographic model, validated by other recent models, and fluid mechanics constraints, we show that the large low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa do not extend as compact, uniform structures very high above the core?mantle boundary. In contrast, they contain a number of well?separated, low?velocity conduits that extend vertically throughout most of the lower mantle. The conceptual model of compact piles, continuously covering the areal extent of the LLSVPs, is therefore not correct. Instead, each LLSVP is composed of a bundle of thermochemical upwellings probably enriched in denser than average material. It is only when the tomographic model is filtered to long wavelengths that the two bundles of plumes appear as uniform provinces. Furthermore, the overall shape of the LLSVPs is probably controlled by the distribution of subducted slabs, and due to their thermochemical nature, the position of both LLSVPs and individual upwelling dynamics should be time dependent. There is also evidence for smaller plumes originating near the CMB in the faster than average regions of the voting map of Lekic et al. (2012, https://doi.org/10.1016/j.epsl.2012.09.014) as well as other, barely resolved, weaker plumes within the LLSVPs. These finer?scale features are starting to be resolved tomographically owing to improvements in full waveform modeling of body waves, including diffracted S waves (Sdiff) and waves multiply reflected on the core?mantle boundary (ScS) and their codas.
Lithos, doi.org/10.1016/ jlithos.2020.105880, 26p. Pdf
Africa, South Africa
deposit - Bultfontein
Abstract: The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage. The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/? PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80-120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Abstract: Cratons are old and strong continental cores where the lithosphere is thick and remains largely undeformed for 2-3 b.y. Unlike typical cratons, the Wyoming craton underwent pervasive deformation ca. 80-55 Ma during the Laramide orogeny in the west-central United States, and has been subsequently encroached upon by the Yellowstone hotspot since 2.0 Ma. However, the mechanism for the deformation and the craton-hotspot interaction are not well understood. We present here a three-dimensional shear wave velocity model beneath the Wyoming craton constrained from Rayleigh wave data, which reveal new details about the cratonic lithosphere. The average lithosphere thickness beneath the craton is ?150 km, significantly thinner than a normal cratonic root (>200 km). Continuous low velocities are observed beneath the Yellowstone hotspot and the Cheyenne belt. A low-velocity column is also present in the central-eastern craton at depths of 115-250 km. These low velocities can be explained by hot temperature and partial melting, implying mantle upwelling. A high-velocity anomaly with a dripping shape in central Wyoming extends to 200-250 km depth, indicating mantle downwelling and lithosphere erosion. Our model provides the first seismic evidence for complex small-scale mantle convection beneath the Wyoming craton. The convection probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere.
Abstract: Diamonds represent one of the few witnesses of our planet interior. They are mainly formed in the first 200 km of the lithospheric mantle, and, more rarely from the transition zone to 700 km deep. Diamonds contain a lot of information about global evolution, however their mode of formation remains poorly understood. Recent studies in high-pressure mineralogy suggest that diamonds precipitate from oxidized metasomatic fluids. The study of inclusions trapped in diamonds may provide precise information on composition, pressure, temperature and redox conditions. The aim of this study is to use the inclusions trapped in diamond as probes of the deep cycling of volatiles (C, H, halogens). Therefore, we investigate inclusions in diamonds with a systematic study of diamonds from collections. We selected 73 diamonds from three museums: National Museum of Natural History, School of Mines and Sorbonne University. The selected diamonds are studied with the help of a large range of in situ methods: RAMAN and FTIR spectrometry and X-Ray Diffraction. These analyses allow us to identify the nature of the different inclusions without damaging the gems. First results indicate silicate minerals inclusions as pyrope garnet, olivine and enstatite pyroxene. This assemblage is typical of peridotitic-type diamonds in the lithosphere.
Abstract: Rifting, breakup, and subsequent collision related to the ca. 1.92-1.79?Ga Svecofennian orogeny fragmented and deformed the western margin of the Archean Karelia-Kola craton into four crustal blocks: Pudasjärvi, Iisalmi, Kuhmo, and Taivalkoski. Detailed quantification of Svecofennian deformation is limited due to poorly exposed basement geology and an as yet incomplete dyke swarm record. New U-Pb ID-TIMS geochronological results on baddeleyite and zircon are presented for three key mafic dykes from the Pudasjärvi block, namely the Uolevinlehto, Myllykangas, and Sipojuntti dykes. The age of the 325°-trending Uolevinlehto dyke is estimated at ca. 2400?±?12?Ma from discordant multigrain baddeleyite fractions, showing it to be younger than ca. 2450?Ma dykes across Karelia. The 350°-trending Myllykangas dyke has a minimum age of 2135.2?+?3.6/?3.7?Ma based on chemically abraded zircon. Results from single baddeleyite grains provide a precise upper intercept age of 2128.9?±?1.2?Ma for the 320°-trending Sipojuntti dyke. Our new U-Pb ages are integrated with those from the literature to define six major dyke swarms in the Pudasjärvi block: the WNW-trending ca. 2.45?Ga Pääjärvi, NW-trending ca. 2.40?Ga Uolevinlehto, NW-trending ca. 2.13-2.10?Ga Tohmajärvi, WNW-trending ca. 2.07?Ga Palomaa, NNW-trending ca. 1.98?Ga Paukkajanvaara and undated"East-West" dykes. Trends of contemporaneous dyke swarms in the Taivalkoski and Kuhmo blocks, however, are systematically offset by 35°. With subvertical dips, offset dyke swarms record 35° clockwise vertical-axis rotation of the Pudasjärvi block relative to the interior of Karelia, consistent with dextral transpression during the Svecofennian orogeny. Structural restoration of the Pudasjärvi blocks improves the constraints on regional dyke swarm patterns, and these are used to revise the position of the Karelia-Kola craton within the context of the paleogeographic reconstruction of supercraton Superia.
Contribution to Mineralogy and Petrology, Vol. 176, 99 16p. Pdf
Mantle
olivine
Abstract: We report experimental measurements of volume and grain boundary diffusion of 26Mg in Mg2SiO4 bi-crystals at asthenosphere temperatures as a ground reference for olivine. By analysis of literature and combination with previous data, we provide Arrhenius laws D = D0 exp(- E/RT) at ambient pressure for volume diffusion of Mg in Mg2SiO4 in the intrinsic regime along the three crystallographic axes as well as grain boundary diffusion.
Abstract: Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth’s upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content1,2,3. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves2,3. Structurally bound water (hydroxyl) is believed to affect seismic properties2,3 but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth’s interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere-asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.
The origin and evolution of the lithospheric mantle beneath the Makondi fold belt in Botswana: an extensive geochemical study of peridotite xenoliths from the Lethlakane diamond mine.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
Abstract: Carbonatites (sensu stricto) are igneous rocks typically associated with continental rifts, being emplaced at relatively shallow crustal levels or as extrusive rocks. Some carbonatites are, however, related to subduction and lithospheric collision zones, but so far no carbonatite has been reported from ultrahigh-pressure (UHP) metamorphic terranes. In this study, we present detailed petrological and geochemical data on carbonatites from the Tromsø Nappe—a UHP metamorphic terrane in the Scandinavian Caledonides. Massive to weakly foliated silicate-rich carbonate rocks, comprising the high-P mineral assemblage of Mg-Fe-calcite?±?Fe-dolomite?+?garnet?+?omphacitic clinopyroxene?+?phlogopite?+?apatite?+?rutile?+?ilmenite, are inferred to be carbonatites. They show apparent intrusive relationships to eclogite, garnet pyroxenite, garnet-mica gneiss, foliated calc-silicate marble and massive marble. Large grains of omphacitic pyroxene and megacrysts (up to 5?cm across) of Cr-diopside in the carbonatite contain rods of phlogopite oriented parallel to the c-axis, the density of rods being highest in the central part of the megacrysts. Garnet contains numerous inclusions of all the other phases of the carbonatite, and, in places, composite polyphase inclusions. Zircon, monazite and allanite are common accessory phases. Locally, veins of silicate-poor carbonatite (up to 10?cm across) occur. Extensive fenitization by K-rich fluids, with enrichment in phlogopite along contacts between carbonatite and silicate country rocks, is common. Primitive mantle-normalized incompatible element patterns for the carbonatite document a strong enrichment of light rare earth elements, Ba and Rb, and negative anomalies in Th, Nb, Ta, Zr and Hf. The carbon and oxygen isotope compositions of the carbonatite are distinctly different from those of the spatially associated calc-silicate marble, but also from mantle-derived carbonatites elsewhere. Neodymium and Sr isotope data coupled with the trace element distribution indicate a similarity of the Tromsø carbonatite to orogenic (off-craton) carbonatites rather than to anorogenic (on-craton) ones. U-Pb dating of relatively U-rich prismatic, oscillatory-zoned zircon gives an age of 454•5?±?1•1?Ma. We suggest that the primary carbonatite magma resulted from partial melting of a carbonated eclogite at UHP, in a deeply subducted continental slab.
De Wit, M., Bhebhe, Z., Davidson, J., Haggerty, S.E., Hundt, P., Jacob, J., Lynn, M., Marshall, T.R., Skinner, C., Smithson, K., Stiefenhofer, J., Robert, M., Revitt, A., Spaggiari, R., Ward, J.
Abstract: From the discovery of diamonds in South Africa in 1866 until the end of 2013, Africa is estimated to have produced almost 3.2 Bct out of a total global production of 5.03 Bct, or 63.6% of all diamonds that have ever been mined. In 2013 African countries ranked 2nd (Botswana), 3rd (DRC), 6th (Zimbabwe), 7th (Angola), 8th (South Africa), and 9th (Namibia), in terms of carat production and 1st (Botswana), 4th (Namibia), 5th (Angola), 6th (South Africa), 7th (Zimbabwe), and 9th (DRC), in terms of value of the diamonds produced. In 2013 Africa produced 70.6 Mct out of a global total of 130.5 Mct or 54.1%, which was valued at US$ 8.7 billion representing 61.5% of the global value of US$ 14.1 billion.
Abstract: The mantle beneath the Cullinan kimberlite (formerly known as "Premier") is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion - the 2056?Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40?mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450-1550?°C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327?°C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ? 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2?Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300-650?km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2?Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.
Nestola, F., Korolev, N., Kopylova, M., Rotiroti, N., Pearson, D.G., Pamato, M.G., Alvaro, M., Peruzzo, L., Gurney, J.J., Moore, A.E., Davidson, J.
CaSiO3 perovskite in diamond indicates the recycling of oceanic crust into the lower mantle.
Nature, Vol. 555, March 8, pp. 237-241.
Mantle
deposit - Cullinan
Abstract: Laboratory experiments and seismology data have created a clear theoretical picture of the most abundant minerals that comprise the deeper parts of the Earth’s mantle. Discoveries of some of these minerals in ‘super-deep’ diamonds—formed between two hundred and about one thousand kilometres into the lower mantle—have confirmed part of this picture1,2,3,4,5. A notable exception is the high-pressure perovskite-structured polymorph of calcium silicate (CaSiO3). This mineral—expected to be the fourth most abundant in the Earth—has not previously been found in nature. Being the dominant host for calcium and, owing to its accommodating crystal structure, the major sink for heat-producing elements (potassium, uranium and thorium) in the transition zone and lower mantle, it is critical to establish its presence. Here we report the discovery of the perovskite-structured polymorph of CaSiO3 in a diamond from South African Cullinan kimberlite. The mineral is intergrown with about six per cent calcium titanate (CaTiO3). The titanium-rich composition of this inclusion indicates a bulk composition consistent with derivation from basaltic oceanic crust subducted to pressures equivalent to those present at the depths of the uppermost lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond, together with the pristine high-pressure CaSiO3 structure, provides evidence for the recycling of oceanic crust and surficial carbon to lower-mantle depths.https://www.nature.com/articles/nature25972
Mineralogy and Petrology, doi.org/10.1007/s710-018-0601-z 15p.
Africa, South Africa
deposit - Cullinan
Abstract: We studied a suite of Cullinan diamonds (<0.3 ct) with mineral inclusions, which comprised 266 Type I and 75 blank Type II (<20 ppm N) diamonds, as classified by infrared spectroscopy. More than 90% (n?=?68) of Type II diamonds do not luminesce. In contrast, 51.9% (n?=?177) of Type I diamonds luminesce, with blue colors of different intensity. Carbon isotopic compositions of Type I and II diamonds are similar, with ?13CVPDB ranging from ?2.1 to ?7.7‰for Type I diamonds (n?=?25), and from ?1.3 to ?7.8- for Type II diamonds (n?=?20). The Type II diamonds are sourced from three parageneses, lithospheric lherzolitic (45%), lithospheric eclogitic (33%), and sublithospheric mafic (22%). The lherzolitic suite contains Cr-pyrope, forsterite, enstatite, clinopyroxene and Cr-spinel formed at 1090-1530 °C and P?=?4.6-7.0 GPa. Lithospheric eclogitic diamonds containing garnet, omphacite, kyanite and coesite comprise 33% of Type II diamonds. The sublithospheric mafic paragenesis is mainly represented by Cr-free majorite, various CaSiO3 phases and omphacite equilibrated at 11.6-26 GPa, in the transition zone and the lower mantle. The lherzolitic paragenesis predominates in Type II diamonds, whereas 79% Type I diamonds are sourced from eclogites. The higher incidence of sublithospheric inclusions was found in Type II diamonds, 22% against 6% in Type I diamonds. The similarity of the mineral parageneses and C isotopic compositions in the small Cullinan Type II and Type I diamonds indicate the absence of distinct mantle processes and carbon sources for formation of studied Type II diamonds. The parent rocks and the carbon sources generally vary for Type II diamonds within a kimberlite and between kimberlites.
Botswana Journal of Earth Sciences, Vol. 7, pp. 29-34. pdf
Africa, Botswana
deposit - KX36
Abstract: The KX36 kimberlite pipe is situated in the southeastern part of Central Kalahari Game Reserve (CKGR), Botswana, approximately 60 km from the known Gope and Kikao kimberlite fields (see figure 1).The kimberlite is covered by 75m of Kalahari sand, has a surface area of 3.6 Ha at the base of the sand cover and was discovered by Petra Diamonds Botswana (Pty) Ltd in 2008. Application of modern geophysical techniques (Ultra hi-resolution low level flying Xcalibur magnetics) and improved sampling method led to the discovery of KX36.The kimberlite was emplaced into the Karoo Supergroup, which comprised the older sedimentary rocks (300 - 185 Ma) overlain by the flood basalts (185Ma). The Karoo Supergroup rocks are overlain by approximately 80m of Kalahari Group sediments.
An unmetasomatized source for the Malaitan alnoite:petrogenesis involving zone refining, megacryst fract- ionation, and assimilation of oceaniclithosphere
Geochimica et Cosmochimica Acta, Vol. 53, pp. 1975-1990
New age determinations of central Colorado Plateau laccoliths, Utah:recognizing disturbed K-Ar systematics and re-evaluating tectonomagmaticrelationships.
Geological Society of America Bulletin, Vol. 194, No. 12, December pp. 1547-1560.
Characterising modal metasomatic processes in young continental lithospheric mantle: a microsampling isotopic and trace element study on xenoliths from the Middle Atlas Mountains, Morocco.
Contributions to Mineralogy and Petrology, Vol. 162, 2, pp. 289-302.
Abstract: The emerald mineralization in the Habachtal (Austria) is geologically and tectonically complex, and previous investigators have identified fluid inclusion evidence for a hydrothermal/metamorphic origin for the emeralds. In this paper we report the discovery of emeralds with a distinctly different inclusion population including melt inclusions, which demonstrates that at least some and probably most of the emerald mineralization in the Habachtal occurred from an extremely fluid-rich pegmatite-like aluminosilicate melt under supercritical conditions, at high temperatures and moderate pressures (~700 °C, 5 kbar). This conclusion is based on the presence of very highly-ordered graphite, and extremely water-rich melt inclusions in emerald. The Lorentz distribution of MgCO3 against the water concentration is a very robust proof for the supercritical state. We suggest that the purely metamorphic model, based on the extrapolation of fluid inclusion data to the regional metamorphic conditions (550 °C and 5 kbar) by some previous investigators are inconsistent with our finding of high-temperature indications (well-ordered graphite, high-temperature fluid inclusions and melt inclusions). This apparent conflict suggests a more complex situation and requires a re-investigation of the emerald genesis in the Habachtal deposit.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 143-156.
Alteration of Mn ilmentite in Horton area of Lena West.
2018 Yellowknife Geoscience Forum , p. 102-103. abstract
Canada, Northwest Territories
deposit - Lena West
Abstract: Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information.
Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron?
As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron.
The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information. Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron? As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron. The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused on the cluster of smaller anomalies to the east. “Pseudorutile” is found over the cluster of anomalies that lie within the Horton River drainage and in the area north towards Darnley Bay. It was especially abundant with some unaltered Mn-ilmenite grains in the cuttings of a Packsack drill hole that penetrated a few feet of rusty coloured clay coincident with one of the anomalies. It does not appear to travel far. “Ferropseudobrookite” is found mostly west of the Horton River drainage, about 100 kilometers down-ice in the trains coming off the Seahorse anomaly and the cluster of smaller anomalies respectively. It represents the weathering of Mn-ilmenite in the upper part of anomalies that was carried furthest by glaciation indicating a resistance to mechanical wear. Mn-ilmenite is a useful KIM in areas of tropical weathering and is also an indicator of rare large high value diamonds. The distribution of pseudorutile and ferropseudobrookite suggests that the initial alteration of Mn-ilmenite is to pseudorutile and then ferropseudobrookite. The presence of one or the other is therefore a measure of the distance to the source.
Abstract: The Earth’s magnetic field is powered by energy supplied by the slow cooling and freezing of the liquid iron core. Efforts to determine the thermal and chemical history of the core have been hindered by poor knowledge of the properties of liquid iron alloys at the extreme pressures and temperatures that exist in the core. This obstacle is now being overcome by high-pressure experiments and advanced mineral physics computations. Using these approaches, updated transport properties for FeSiO mixtures have been determined at core conditions, including electrical and thermal conductivities that are higher than previous estimates by a factor of two to three. Models of core evolution with these high conductivities suggest that the core is cooling much faster than previously thought. This implies that the solid inner core formed relatively recently (around half a billion years ago), and that early core temperatures were high enough to cause partial melting of the lowermost mantle. Estimates of core-mantle boundary heat flow suggest that the uppermost core is thermally stratified at the present day.
Abstract: arth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core-mantle boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core-mantle boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with heterogeneous core-mantle boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core-mantle boundary—an apparent thermal stratification despite the average heat flux across the core-mantle boundary being strongly superadiabatic.
Abstract: Hotspots are anomalous regions of volcanism at Earth’s surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian-Emperor chain and the Yellowstone-Snake River Plain province. The majority are believed to form as Earth’s tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth’s deep mantle to its surface1. It has long been recognized that lithospheric thickness limits the rise height of plumes2, 3, 4 and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth’s longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago5, 6, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.
Earth and Planetary Science Letters, Vol. 434, pp. 10-17.
Mantle
Tectonics
Abstract: It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.
Earth and Planetary Science Letters, Vol. 460, pp. 112-122.
Mantle
Archean - Boundary
Abstract: The changes that occur at the boundary between the Archean and Proterozoic eons are arguably the most fundamental to affect the evolution of Earth's continental crust. The principal component of Archean continental crust is Granite-Greenstone Terranes (GGTs), with granites always dominant. The greenstones consist of a lower sequence of submarine komatiites and basalts, which erupted onto a pre-existing Tonalite-Trondhjemite-Granodiorite (TTG) crust. These basaltic rocks pass upwards initially into evolved volcanic rocks, such as andesites and dacites and, subsequently, into reworked felsic pyroclastic material and immature sediments. This transition coincides with widespread emplacement of granitoids, which stabilised (cratonised) the continental crust. Proterozoic supra-crustal rocks, on the other hand, are dominated by extensive flat-lying platform sequences of mature sediments, which were deposited on stable cratonic basements, with basaltic rocks appreciably less abundant. The siliceous TTGs cannot be produced by direct melting of the mantle, with most hypotheses for their origin requiring them to be underlain by a complimentary dense amphibole-garnet-pyroxenite root, which we suggest acted as ballast to the early continents. Ubiquitous continental pillow basalts in Archean lower greenstone sequences require the early continental crust to have been sub-marine, whereas the appearance of abundant clastic sediments, at higher stratigraphic levels, shows that it had emerged above sea level by the time of sedimentation. We hypothesise that the production of komatiites and associated basalts, the rise of the continental crust, widespread melting of the continental crust, the onset of sedimentation and subsequent cratonisation form a continuum that is the direct result of removal of the continent's dense amphibole-garnet-pyroxenite roots, triggered at a regional scale by the arrival of a mantle plume at the base of the lithosphere. Our idealised calculations suggest that the removal of 40 km of the amphibole-garnet-pyroxenite root would have raised the average level of the continental crust by ?3 km. The emergence of the continental crust was an essential precursor to the rise of oxygen, which started some 200 Myr later.
Earth and Planetary Science Letters, Vol. 494, pp. 226-238.
Mantle
perovskite
Abstract: Lower mantle tomography models consistently feature an increase in the ratio of shear-wave velocity () to compressional-wave velocity () variations and a negative correlation between shear-wave and bulk-sound velocity () variations. These seismic characteristics, also observed in the recent SP12RTS model, have been interpreted to be indicative of large-scale chemical variations. Other explanations, such as the lower mantle post-perovskite (pPv) phase, which would not require chemical heterogeneity, have been explored less. Constraining the origin of these seismic features is important, as geodynamic simulations predict a fundamentally different style of mantle convection under both scenarios. Here, we investigate to what extent the presence of pPv explains the observed high ratios and negative - correlation globally. We compare the statistical properties of SP12RTS with the statistics of synthetic tomography models, derived from both thermal and thermochemical models of 3-D global mantle convection. We convert the temperature fields of these models into seismic velocity structures using mineral physics lookup tables with and without pPv. We account for the limited tomographic resolution of SP12RTS using its resolution operator for both and structures. This allows for direct comparisons of the resulting velocity ratios and correlations. Although the tomographic filtering significantly affects the synthetic tomography images, we demonstrate that the effect of pPv remains evident in the ratios and correlations of seismic velocities. We find that lateral variations in the presence of pPv have a dominant influence on the / ratio and - correlation, which are thus unsuitable measures to constrain the presence of large-scale chemical variations in the lowermost mantle. To explain the decrease in the / ratio of SP12RTS close to the CMB, our results favour a pPv-bearing CMB region, which has implications for the stability field of pPv in the Earth's mantle.
Earth and Planetary Science Letters, Vol. 501, pp. 67-77.
Mantle
subduction
Abstract: The mechanisms underpinning the formation of a focused volcanic arc above subduction zones are debated. Suggestions include controls by: (i) where the subducting plate releases water, lowering the solidus in the overlying mantle wedge; (ii) the location where the mantle wedge melts to the highest degree; and (iii) a limit on melt formation and migration imposed by the cool shallow corner of the wedge. Here, we evaluate these three proposed mechanisms using a set of kinematically-driven 2D thermo-mechanical mantle-wedge models in which subduction velocity, slab dip and age, overriding-plate thickness and the depth of decoupling between the two plates are systematically varied. All mechanisms predict, on the basis of model geometry, that the arc-trench distance, D, decreases strongly with increasing dip, consistent with the negative D-dip correlations found in global subduction data. Model trends of sub-arc slab depth, H, with dip are positive if H is wedge-temperature controlled and overriding-plate thickness does not exceed the decoupling depth by more than 50 km, and negative if H is slab-temperature controlled. Observed global H-dip trends are overall positive. With increasing overriding plate thickness, the position of maximum melting shifts to smaller H and D, while the position of the trenchward limit of the melt zone, controlled by the wedge's cold corner, shifts to larger H and D, similar to the trend in the data for oceanic subduction zones. Thus, the limit imposed by the wedge corner on melting and melt migration seems to exert the first-order control on arc position.
Earth and Planetary Science Letters, Vol. 502, pp. 244-252.
Russia, Canada, Ontario, Attawapiskat
kimberlite, core boundary
Abstract: Mantle plumes are hot buoyant upwellings that rise from Earth's core-mantle-boundary to its surface where they can produce large igneous provinces (LIPs) and volcanic tracks, such as the Siberian Traps and the Hawaiian Emperor chain, respectively. We show that flattened mantle plume heads, which can have radii of >1200 km in the uppermost mantle, can heat the overlying lithospheric mantle to temperatures above the diamond stability field. As a consequence, they can destroy diamonds within the roots of Archean cratons, the principal source of diamonds in kimberlites. We quantitatively demonstrate that there is a ‘sour spot’ for this effect that occurs when lithospheric thicknesses are 165-185 km and the plume has a temperature of >150?°C above background mantle. Our model explains why the kimberlites associated with the 370 Ma Yakutsk-Vilyui plume in the Siberian craton are diamondiferous whilst those associated with the younger 250 Ma Siberian Traps plume are barren. We also show that the time required to restore the pre-plume thermal structure of the lithosphere is ca. 75-120 Myr, and that destroyed diamonds may regrow once the plume's thermal effect dissipates. The 1100 Ma Kyle Lake and adjacent 180-150 Ma Attawapiskat kimberlites in the southern Superior craton exemplify this, where the older kimberlites are associated with a narrower diamond window (<30 km) in comparison with the ca. 85 km diamond window of the younger Attawapiskat field.
Abstract: Earth’s surface topography is a direct physical expression of our planet’s dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a substantial proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth’s deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth’s oceanic residual topography power spectrum. This indicates water-loaded power of 0.5?±?0.35?km2 and peak amplitudes of up to ~0.8?±?0.1?km at long wavelengths (~104?km), decreasing by roughly one order of magnitude at shorter wavelengths (~103?km). We show that geodynamical simulations can be reconciled with observational constraints only if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-wavelength) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth’s lithosphere.
van den Heuvel, Q., Matveev, S., Drury, M., Gress, M., Chinn, I., Davies, G.
Genesis of diamond inclusions: an integrated cathodluminescence ( CL) and electron backscatter diffraction (EBSD) study on eclogitic and peridotitic inclusions and their diamond host.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 6564 Abstract
Abstract: Diamond inclusions are potentially fundamental to understanding the formation conditions of diamond and the volatile cycles in the deep mantle. In order to fully understand the implications of the compositional information recorded by inclusions it is vital to know whether the inclusions are proto-, syn-, or epigenetic and the extent to which they have equilibrated with diamond forming fluids. In previous studies, the widespread assumption was made that the majority of diamond inclusions are syngenetic, based upon observation of cubo-octahedral morphology imposed on the inclusions. Recent work has reported the crystallographic relationship between inclusions and the host diamond to be highly complex and the lack of crystallographic relationships between inclusions and diamonds has led some to question the significance of imposed cubo-octahedral morphology. This study presents an integrated EBSD and CL study of 9 diamonds containing 20 pyropes, 2 diopsides, 1 forsterite and 1 rutile from the Jwaneng and Letlhakane kimberlite clusters, Botswana. A new method was developed to analyze the crystallographic orientation of the host diamond and the inclusions with EBSD. Diamonds plates were sequentially polished to expose inclusions at different levels in the diamond. CL imaging at different depths was performed in order to produce a 3D view of diamond growth zones around the inclusions. Standard diamond polishing techniques proved too aggressive for silicate inclusions as they were damaged to such a degree that EBSD measurements on the inclusions were impossible. The inclusions were milled with a Ga+ focused ion beam (FIB) at a 12° angle to clean the surface for EBSD measurements. Of the 24 inclusions, 9 have an imposed cubo-octahedral morphology. Of these inclusions, 6 have faces orientated parallel to diamond growth zones and/or appear to have nucleated on a diamond growth surface, implying syngenesis. In contrast, other diamonds record resorption events such that inclusions now cut diamond growth zones. In most cases, the growth zonation around inclusions is not well defined due to CL haloes but some inclusions clearly disrupt diamond growth. Crystallographic orientations of diamond and the inclusions, determined using EBSD, revealed that each inclusion has a homogeneous orientation and record no compositional zonation. The diamonds also showed no angular deviations despite many having multiple growth and resorption zones; implying epitaxial growth of diamond. Crystallographic alignment between diamond and inclusions was not recorded for the principle planes and limited to 3 possible coincidences on minor planes from the 24 inclusions studied. The CL data show no evidence of syngenesis for these 3 inclusions. Analyses of two diamonds with inclusion clusters in different growth zones, 400 µm apart, revealed the same chemical composition and orientation, potentially implying they originated from an original larger inclusion. Combined EBSD and CL data suggest that there is no direct orientational correlation (epitaxial growth) between silicate inclusions and the host diamond, even when the mineral phases are of the same symmetry group. The presentation will provide a detailed evaluation of the genesis of individual inclusions.
Abstract: Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major-trace element geochemistry and garnet Sr-Nd-Hf isotope data are presented. Equilibration pressures range from 3.8-6.1?GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115-185?km). High olivine Mg# (~93.4, n?=?39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ?90%, orthopyroxene ?10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5-3.2?wt%) and SiO2 (43.1-49.1?wt%), however, indicate heterogeneous re-introduction of garnet (?13%) and orthopyroxene (?50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7-13% and?~?9-30%; n?=?6) are characterised by mildly sinusoidal garnet REE patterns (Tbsingle bondDy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7-3.3?Ga; ?Hfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5-7.5% and?~?25-50%; n?=?19) are characterised by highly sinuous REE patterns (Hosingle bondYb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7-1.6?Ga; ?Hfe: ?16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7?Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lusingle bondHf isochron age of 2702?±?64?Ma (?Hfi?=?+44, n?=?31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nbsingle bondTa anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Srsingle bondNd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217?±?58?Ma, ?Ndi?=?+4; n?=?34), consistent with recent G10 garnet transformation into G9 garnets (Ca?+?Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.
Metasomatized lower crustal and upper mantle xenoliths from North Queensland -chemical and isotopic evidence bearing on the composition and source of fluid phase
Geochimica et Cosmochimica Acta, Vol. 53, No. 3, March pp. 649-660
Oxygen isotope evidence for the origin of pyroxenites in the Beni Bousera peridotite massif, North Morocco: derivation from subducted oceaniclithosphere
Earth and Planetary Science Letters, Vol. 102, No. 3/4, March pp. 289-301
Evidence for diamond grade ultra high pressure metamorphism and fluid interaction in the Svartberget Fe Ti garnet peridotite websterite body, western Gneiss region, Norway.
Mineralogy and Petrology, Vol. 88, 1-2, pp. 381-405.
Vrijmoed, J.C., Van Roermund, H.L.M., Davies, G.R.
Evidence for diamond grade ultra high pressure metamorphism and fluid interaction in the Svartberget Fe Ti garnet peridotite websterite body, Western Gneiss
The origin and evolution of the lithospheric mantle beneath the Makondi fold belt in Botswana: an extensive geochemical study of peridotite xenoliths from the Lethlakane diamond mine.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
Re-Os dating of sulphide inclusions zonally distributed in single Yakutian diamonds: evidence for multiple episodes of Proterozoic formation and protracted timescales of diamond growth.
Geochimica et Cosmochimica Acta, Vol. 120, pp. 363-394.
Earth and Planetary Science Letters, Vol. 463, pp. 178-188.
Africa, Botswana
Deposit - Lethakane
Abstract: Monocrystalline diamonds commonly record complex internal structures reflecting episodic growth linked to changing carbon-bearing fluids in the mantle. Using diamonds to trace the evolution of the deep carbon cycle therefore requires dating of individual diamond growth zones. To this end Rb-Sr and Sm-Nd isotope data are presented from individual eclogitic silicate inclusions from the Orapa and Letlhakane diamond mines, Botswana. ?13C?13C values are reported from the host diamond growth zones. Heterogeneous 87Sr/86Sr ratios (0.7033-0.7097) suggest inclusion formation in multiple and distinct tectono-magmatic environments. Sm-Nd isochron ages were determined based on groups of inclusions with similar trace element chemistry, Sr isotope ratios, and nitrogen aggregation of the host diamond growth zone. Diamond growth events at 0.14±0.090.14±0.09, 0.25±0.040.25±0.04, 1.1±0.091.1±0.09, 1.70±0.341.70±0.34 and 2.33±0.022.33±0.02 Ga can be directly related to regional tectono-magmatic events. Individual diamonds record episodic growth with age differences of up to 2 Ga. Dated diamond zones have variable ?13C?13C values (?5.0 to ?33.6‰ vs PDB) and appear to imply changes in subducted material over time. The studied Botswanan diamonds are interpreted to have formed in different tectono-magmatic environments that involve mixing of carbon from three sources that represent: i) subducted biogenic sediments (lightest ?13C?13C, low 87Sr/86Sr); ii) subducted carbonate-rich sediments (heavy ?13C?13C, high 87Sr/86Sr) and iii) depleted upper mantle (heavy ?13C?13C, low 87Sr/86Sr). We infer that older diamonds from these two localities are more likely to have light ?13C?13C due to greater subduction of biogenic sediments that may be related to hotter and more reduced conditions in the Archaean before the Great Oxidation Event at 2.3 Ga. These findings imply a marked temporal change in the nature of subducted carbon beneath Botswana and warrant further study to establish if this is a global phenomenon.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 5540 Abstract
Africa, Botswana
Deposit - Letlhakane
Abstract: The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (˜ 93.1 Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual eclogitic sulphide inclusions (3.0 to 35.7?g) from Letlhakane diamonds along with their rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content and N-aggregation data from the corresponding growth zones of the host diamonds. For the first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013? amplifiers. The analysed inclusion set currently encompasses pairs of individual sulphides from two diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic clinopyroxene (LK040) inclusion. TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement of crustal carbon (?13C between -19.3 to -22.7 ± 0.2 per mill) during diamond precipitation. Cathodoluminescence imaging of central plates from LK040 and LK113 displays homogenous internal structure with no distinct zonation. The two sulphide inclusions from LK040 define an 'isochron' of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os = 1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os = 0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including these inclusions (or any from other diamonds) on the same isochron and implies an extremely heterogeneous diamond crystallisation environment that incorporated recycled Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1) in comparison to eclogitic xenolith data from Orapa that are depleted relative to chondrite. The Re-Os isochrons determined in this study are within error of previously reported ages from the adjacent (˜40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions. Together with additional new Sm-Nd isochron age determinations from individual silicate inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n = 3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning >2.0 Ga.
Geochimica et Cosmochimica Acta, in press available, 22p.
Africa, South Africa
deposit - Venetia
Abstract: Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35–3.28 Ga. The group I samples derive from ?50 to ?170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on the formation and subsequent evolution of Venetia lithosphere. First, melt depletion and remobilisation of Re and Os must have occurred within error of the 3.28 Ga mean TMA age. Second, the refractory peridotites contain significant Re despite recording >40 % melt extraction. Third, assuming that Si-enrichment and Re-Os mobility in the Venetia lithospheric mantle were linked, this process must have occurred within ?100 Myr of initial melt depletion in order to preserve the isochronous relationship. Based on the regional geological evolution, we propose a rapid recycling model with initial melt depletion at ?3.35 Ga to form a tholeiitic mafic crust that is recycled at ?3.28 Ga, resulting in the intrusion of a TTG suite and Si-enrichment of the lithospheric mantle. The non-zero primary Re contents of the Venetia xenoliths imply that TRD model ages significantly underestimate the true depletion age even for highly depleted peridotites. The overlap of the ?2.6 Ga TRD ages with the time of the Kaapvaal-Limpopo collision is purely fortuitous and has no geological significance. Hence, this study underlines the importance of scrutiny if age information is to be derived from whole rock Re-Os analyses.
Abstract: Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95?Ga) and one Proterozoic (1.15?Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1?Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.
Van der Meer, Q., Klaver, M., Reisberg, L., Riches, A. J.V., Davies, G.R.
Preservation of an Archean whole rock Re-Os isochron for the Venetia lithospheric mantle: evidence for rapid crustal recycling and lithosphere stabilization at 3.3 Ga.
Geochimica et Cosmochimica Acta, Vol. 216, pp. 242-263.
Earth and Planetary Science Letters, Vol. 482, pp. 253-264.
Africa, South Africa
metasomatism
Abstract: Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (?800–900?°C), strongly metasomatised mantle xenoliths from the ?84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ?180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomatic assemblages seemingly generated in a single event may instead result from the infiltration of broadly coeval fluids with variable compositions. This in turn implies that the isotopic variations recorded in mantle rocks may be an inherent feature of metasomatism, and that hot fluids infiltrating a rock do not necessarily cause equilibration at the cm scale, as has been assumed previously. Simple modelling of solid-state diffusion in mantle minerals shows that isotopic disequilibrium may be preserved for up to hundreds of Myr at mantle lithosphere temperatures (?1100–1200?°C), unless subsequently affected by transient heating and/or fluid infiltration events. Radiogenic isotope disequilibrium associated with mantle metasomatism may therefore be a common feature of mantle xenoliths.
Geophysical Research Abstracts www.researchgate.net, Vol. 20, EGU2018-128291p. Abstract
Africa, South Africa
diamond inclusions
Abstract: Changing recycling budgets of surface materials and volatiles by subduction of tectonic plates influence the compositions of Earth’s major reservoirs and affect climate throughout geological time. Fluids play a key role in processes governing subduction recycling, but quantifying the exact fate of volatiles introduced into the mantle at ancient and recent destructive plate boundaries remains difficult. Here, we report on the role of fluids and the fate of volatiles and other elements at two very different tectonic settings: 1) at subduction settings, and 2) within the subcontinental lithospheric mantle (SCLM). We will show how olivine-hosted melt inclusions from subduction zones and mineral inclusions in diamond from the SCLM are used to reveal how changing tectonic settings influence volatile cycles with time. Melt inclusions from the complex Italian post-collisional tectonic setting are used to identify changing subduction recycling through time. The use of CO2 in deeply trapped melt inclusions instead of in lavas or volcanic gases provides a direct estimate of deep recycling, minimizing possible effects of contamination during transfer through the crust. The aim is to distinguish if increased recycling of sediments from the down-going plate at continental subduction settings results in increased deep CO2 recycling or if the increased CO2 flux results from crustal degassing of the overriding plate. Both processes likely affected climate through Earth history but could thus far not be discriminated. The study of mineral inclusions and their host diamonds from the SCLM can link changes in the cycling of carbon-rich fluids and the time and process through which the carbon redistribution took place. We use Sm-Nd isotope techniques to date the mineral inclusions and use the carbon isotope data of the host diamonds to investigate the growth conditions. I will present case-studies of peridotitic and eclogitic diamonds from three mines in Southern Africa.
Abstract: The diamondiferous Jwaneng kimberlite cluster (~240 Ma) is located on the NW rim of the Archaean Kaapvaal Craton in central Botswana. Previous studies report eclogitic diamond formation in the late Archean (2.9 Ga) and in the Middle Proterozoic (1.5 Ga) involving different mantle and sedimentary components [1;2;3]. Here we report newly acquired Sm- Nd ages of individual eclogitic pyrope-almandine and omphacite inclusions along with their major element data and nitrogen data from the diamond hosts to re-examine Jwaneng’s diamond formation ages. The Sm-Nd isotope analyses were performed via TIMS using 1013? resistors [4]. An initial suite of three pyropealmandine and 14 omphacite inclusions yield 143Nd/144Nd from 0.51102±7 to 0.5155±5. 147Sm/144Nd vary from 0.024 to 0.469. Major element data defines two inclusion populations: (1) seven omphacites with high Mg#, high Cr# and one pyropealmandine with low-Ca define an isochron age of 1.93±0.16 Ga with ?Ndi= +3.5; (2) seven omphacites with low Mg#, low Cr# and two pyrope-almandines with low-Ca define an isochron age of 0.82±0.06 Ga with ?Ndi= +3.7. Nitrogen contents of corresponding diamond host growth zones in Group (1) are ? 50 at.ppm whereas Group (2) range between 50 to 700 at.ppm with N-aggregation > 70 %B. Additional data used to define “co-genetic” inclusion suites include Sr-isotopes and trace elements of the inclusions and carbon isotopes of the diamond hosts. Re-Os data of coexisting sulphide inclusions from the same silicate-bearing diamonds further validates the ages and indicates more periods of diamond formation at Jwaneng than previously assumed. The integrated data indicate the possibility of an extensive Paleoproterozoic diamond-forming event in southern Africa.
Mineralogy and Petrology, 10.1007/ s710-018-0592 -9, 10p.
Africa, Botswana
deposit - Orapa, Damtshaa, BK09, BK12,AK01
Abstract: Twenty eclogitic diamonds from Orapa Mine (Botswana) with an unusual yellow colour are characterised for their growth structure, N systematics, and C isotope composition, and the major element composition of their silicate inclusions. The diamonds show complex luminescence with green, blue and non-luminescent zones and occasional sector zonation. All parts of the diamonds have low total N concentrations (<50 at.ppm, with one exception of <125 at.ppm) and a limited range in C isotope composition (?5.7 to ?10.6‰). Fourier Transform Infrared spectra show bands at 1334, 1332, 1282, and 1240 cm?1 typical for Ib-IaA diamonds. Relict unaggregated N defects (Nso and Ns+) are present and the preservation is likely caused by the low N concentrations and possible low mantle residence temperatures rather than young diamond formation (inclusion ages of 140, 1096, 1699 Ma; Timmerman et al. Earth Planet Sc Lett 463:178-188, 2017). Garnet and clinopyroxene inclusions extracted from 14 diamonds have an eclogitic composition with relatively low Ca contents and based on all characteristics, these diamonds form a distinct population from Orapa.
Africa, Zimbabwe. Australia, Canada, Northwest Territories, South Africa, Botswana
deposit - Murowa, Argyle, Diavik, Venetia, Orapa
Abstract: FTIR maps of diamond plates, cut through the centre of growth, contain abundant information about changing defect concentrations from core to rim. These data can, in principle, be interpreted in terms of the variation in conditions of diamond growth and the temperatures experienced by the diamond during the period of mantle residence between growth and exhumation. Many diamonds show multiple growth zones that can be observed by cathodoluminescence. Importantly, the combination of nitrogen concentration and nitrogen aggregation measured by FTIR can be used to determine whether the growth zones are of similar or very different ages (Kohn et al., 2016). In this study, we use automated fitting of several thousand individual spectra within each FTIR map to define a model temperature for each pixel using the Python program, QUIDDIT. We then use a two-stage aggregation model to constrain potential temperature-time histories for each diamond. To take full advantage of the temperature history recorded by zoned diamonds, radiometric ages of inclusions are required. If the growth ages of each zone and the date of exhumation are well-known, then a model temperature can be calculated for each zone. The combination of zone-specific ages and improved quality and processing of FTIR spectra is able to provide unique new insights into the thermal history of diamondbearing lithospheric mantle. For the first time we will be able to use the N defects in diamonds to work out whether a particular location in the lithosphere has heated or cooled over long periods of geological time. The implications for the mechanism of formation of lithosphere will be discussed. We will illustrate the approach using examples of zoned diamonds from Murowa (Zimbabwe), Argyle (Australia), Diavik (Canada), Venetia (South Africa) and Orapa (Botswana).
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0634-3 12p.
Africa, Botswana
deposit - Jwaneng
Abstract: To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.
Abstract: The Earth’s mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of mid-ocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle endmember. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here, we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high-spatial-resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection and melt transport in the mantle and the crust.
American Geophysical Union, Fall Meeting. , V23A-05 1p.
Mantle
geophysics
Abstract: Variations in radiogenic isotopes in mid-ocean ridge basalts (MORB) are interpreted to reflect the presence of enriched and depleted mantle components in their source regions and have been used to infer the abundance and time scales of crustal recycling. However, MORB are homogenized via magma mixing prior to eruption and may not capture the full heterogeneity of melts generated in their upper mantle source. Here we show that primitive cumulate minerals, formed by crystallization of mantle melts in the lower crust, retain the signature of the recycled material. We performed high spatial resolution Nd and Sr isotopic analyses on clinopyroxene and plagioclase of gabbroic cumulates from the Atlantis massif, located on a depleted ridge segment on the northern Mid-Atlantic Ridge, and compared these data with whole rock isotopic compositions of diabase and microgabbros collected on the same core, associated basalts flows, and MORB data from the literature. We find that cumulate minerals: (1) are significantly more isotopically heterogeneous than the associated diabase and lavas, exceeding the range of 143Nd/144Nd in MORB by a factor of seven; and (2) contain the full Nd isotopic heterogeneity of all of North Atlantic MORB. Furthermore, we find that isotopic heterogeneity occurs down to the sample scale, with plagioclase and clinopyroxene from individual samples commonly not in isotopic equilibrium. We further demonstrate that the MORB and cumulate mineral data can be reconciled with constant high magnitude, small length scale heterogeneity through the North Atlantic upper mantle, with limited magma mixing in the mantle and extensive mixing in the oceanic crust.The isotopic heterogeneity revealed in the lower oceanic crust provides strong evidence that MORB is not an accurate representation of the heterogeneity of its mantle source. Hence, the true isotopic variation of the upper mantle requires rigorous further examination, and models of convective thinning and stretching and melt migration must be re-evaluated to account for greater local variation.
Nature Communications, doi.org/10.1038/ s41467-019-11072-5 10p. Pdf
Europe, Italy
subduction
Abstract: Recycling of Earth’s crust through subduction and delamination contributes to mantle heterogeneity. Melt inclusions in early crystallised magmatic minerals record greater geochemical variability than host lavas and more fully reflect the heterogeneity of magma sources. To date, use of multiple isotope systems on small (
Abstract: Precise analysis of 20xPb/204Pb ratios is challenging when the amount of Pb is limited by sample volume or elemental concentration. The current precision impedes meaningful analyses of analytes with sub-nanogram Pb contents, such as individual melt inclusions with typical diameters (<100 µm). Decreasing this lower limit whilst maintaining precision and accuracy is crucial for studies aiming to understand the composition and heterogeneity of melt source regions, and the effects of magma transport from the Earth’s interior. The preferred method for precise analysis of sub-nanogram Pb samples combines miniaturised ion-exchange separation, a Pb double spike, and thermal ionisation mass spectrometry (TIMS) with 10^13 ? amplifier technology. This approach allows for interference-free, instrumental mass fractionation-corrected isotope measurements, and therefore provides precision superior to in situ measurements. As a result, reliable analyses can be conducted on samples which contain only a few hundred picograms of Pb. The principal obstacle at the lower limit is the analytical blank, which usually adds a few pg Pb—and thus up to a few percent—to the sample of interest. This contribution may differ for the 207Pb-204Pb-spiked and unspiked runs of one sample, which in turn convolutes the algebraic inversion of the spike. It is therefore imperative to evaluate the magnitude, isotope composition, and homogeneity of the blanks, and constrain how the uncertainty and potential variability within these parameters affect the inversion. Here, we describe the optimised analytical techniques, and discuss the present feasibility and limitations in obtaining precise Pb isotope compositions of rock reference materials and olivine-hosted melt inclusions with sub-nanogram Pb contents. In addition, we discuss the effect of different blank contributions on double-spike analyses using numerical simulations, and evaluate the potential of accurate blank corrections. We find that the optimised technique allows accurate Pb analyses to be conducted on melt inclusions with >200 pg Pb, which will ultimately help to better constrain mantle heterogeneity beneath mid-ocean ridges, oceanic islands, and volcanic arcs.
Geochimica et Cosmochimica Acta, 10.1016/j.gca.2020.10.010 35p. Pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (> 100’s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (< 10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C < -21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.
Geochimica et Cosmochimica Acta, Vol. 293, pp. 1-17. pdf
Africa, Botswana
deposit - Jwaneng
Abstract: Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8?Ga) and one Neoproterozoic (0.85?Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100?s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10?km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (?13C?
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich "fibrous" and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Appendix to previous Lithos article in March 2021, 11p. Pdf
Africa, Botswana
deposit - Damtshaa, Orapa
Abstract: Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics. Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.
Science Advances, doi.10.1126/sciadv.abj1325 1p. Pdf
Mantle
subduction
Abstract: Earth’s carbon cycle is strongly influenced by subduction of sedimentary material into the mantle. The composition of the sedimentary subduction flux has changed considerably over Earth’s history, but the impact of these changes on the mantle carbon cycle is unclear. Here, we show that the carbon isotopes of kimberlite magmas record a fundamental change in their deep-mantle source compositions during the Phanerozoic Eon. The 13C/12C of kimberlites before ~250 Ma preserves typical mantle values, whereas younger kimberlites exhibit lower and more variable ratios-a switch coincident with a recognized surge in kimberlite magmatism. We attribute these changes to increased deep subduction of organic carbon with low 13C/12C following the Cambrian Explosion when organic carbon deposition in marine sediments increased significantly. These observations demonstrate that biogeochemical processes at Earth’s surface have a profound influence on the deep mantle, revealing an integral link between the deep and shallow carbon cycles.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
Abstract: Due to the acute scarcity of very ancient rocks, the composition of Earth’s embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today’s oceanic crust1, 2, 3, 4, or included significant amounts of continental crust5, 6, 7, 8 remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth—a 4,019.6 ± 1.8?Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.
45th. Annual Yellowknife Geoscience Forum, p. 65 abstract
Canada, Northwest Territories
deposit - Giraffe
Abstract: Several Lac de Gras kimberlite pipes host thick accumulations of stratified post-eruptive lacustrine sediment and peat. Given the range of Lac de Gras kimberlite emplacement ages, these fills - though rare - provide a unique sedimentary archive of paleoenvironments during the sustained Early Cenozoic “greenhouse” interval, in a high-latitude region otherwise devoid of Phanerozoic sediment cover. Extensive exploration drilling has provided a valuable window into this unique sedimentary record, which would have otherwise remained covered by Quaternary glacial deposits. Our focus to date has been multidisciplinary study of the Giraffe pipe sediment fill: an ~80 m-thick sequence of post-eruptive lacustrine silt overlain by peat, which paints a remarkable picture of a humid-temperate Middle Eocene forest ecosystem on the Canadian Shield. Post-eruptive chronology is provided by interbedded distal tephra horizons, likely sourced from Alaska, that have been dated by glass fission-track and zircon U-Pb techniques. Paleoclimate proxies derived from pollen, wood cellulose oxygen isotopes, and biomarkers converge on reconstructed mean annual temperatures >17 °C warmer than present, with mean winter temperatures above freezing, and mean annual precipitation ~4x present. Two independent reconstructions of CO2 from well preserved conifer foliage suggest that this warming occurred under relatively modest atmospheric CO2 concentrations of 430-630 ppm. These findings provide direct field-based evidence for dramatic past arctic warming at CO2 concentrations that were well within the range of projections under “business-as-usual” emissions scenarios, underscoring the capacity for exceptional polar amplification of climate change under modest CO2 concentrations once both fast and slow feedbacks processes become expressed. Our studies at Giraffe pipe also highlight the scientific value of archived exploration drill core in the Lac de Gras kimberlite field, particularly with respect to pipes that are unremarkable for the purpose of diamond exploration.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 143-156.
Abstract: Thermo-mechanical thinning of the lithosphere by mantle plumes is essential for intra-plate volcanism, the initiation of rifting, the evolution of Earth’s lower continental crust and the genesis of metals, diamonds and hydrocarbons. To develop a new understanding of how a mantle plume thins the overlying lithosphere beneath moving plates, we use 2-D and 3-D numerical models based on a finite-element discretization on anisotropic adaptive meshes. Our models include Earth-like material properties for the upper mantle (e.g. temperature and viscosity contrasts, non-Newtonian rheology) discretised at a local mesh resolution that has previously been considered intractable. In our simulations, a plume is injected at the base of the model (670 km depth) with a prescribed mass flux that is consistent with surface observations of topographic swells: from 0.5 (e.g. Louisville, Bermuda, Darfur) to 7 Mg/s (Hawaii). We undertake a systematic numerical study, across a wide parameter space, to investigate the effect of plume buoyancy flux, plate velocity, rheology law and Rayleigh number on processes leading to a reduction of the depth of the Lithosphere Asthenosphere boundary (LAB), such as small-scale convection (SSC) (‘dripping’), or delamination of the lower lithosphere.
Alteration of Mn ilmentite in Horton area of Lena West.
2018 Yellowknife Geoscience Forum , p. 102-103. abstract
Canada, Northwest Territories
deposit - Lena West
Abstract: Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information.
Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron?
As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron.
The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information. Mn-ilmenites found as inclusions in diamonds range from 51 wt.% TiO2 (total wt.% 100) to 56 wt.% TiO2 (total wt.% 95). The shortfall in wt.% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron? As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt.% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt.% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron. The Horton area consists of a cluster of magnetic anomalies averaging ~ 200 m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused on the cluster of smaller anomalies to the east. “Pseudorutile” is found over the cluster of anomalies that lie within the Horton River drainage and in the area north towards Darnley Bay. It was especially abundant with some unaltered Mn-ilmenite grains in the cuttings of a Packsack drill hole that penetrated a few feet of rusty coloured clay coincident with one of the anomalies. It does not appear to travel far. “Ferropseudobrookite” is found mostly west of the Horton River drainage, about 100 kilometers down-ice in the trains coming off the Seahorse anomaly and the cluster of smaller anomalies respectively. It represents the weathering of Mn-ilmenite in the upper part of anomalies that was carried furthest by glaciation indicating a resistance to mechanical wear. Mn-ilmenite is a useful KIM in areas of tropical weathering and is also an indicator of rare large high value diamonds. The distribution of pseudorutile and ferropseudobrookite suggests that the initial alteration of Mn-ilmenite is to pseudorutile and then ferropseudobrookite. The presence of one or the other is therefore a measure of the distance to the source.
Geochemistry, Geophysics, Geosystems: G3, Vol. 17, 2, pp. 5036-5055.
Africa, Madagascar
Tectonics
Abstract: Accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models. Here, we provide a new plate tectonic reconstruction of the opening of the Western Somali Basin during the breakup of East and West Gondwana. The model is constrained by a new comprehensive set of spreading lineaments, detected in this heavily sedimented basin using a novel technique based on directional derivatives of free-air gravity anomalies. Vertical gravity gradient and free-air gravity anomaly maps also enable the detection of extinct mid-ocean ridge segments, which can be directly compared to several previous ocean magnetic anomaly interpretations of the Western Somali Basin. The best matching interpretations have basin symmetry around the M0 anomaly; these are then used to temporally constrain our plate tectonic reconstruction. The reconstruction supports a tight fit for Gondwana fragments prior to breakup, and predicts that the continent-ocean transform margin lies along the Rovuma Basin, not along the Davie Fracture Zone (DFZ) as commonly thought. According to our reconstruction, the DFZ represents a major ocean-ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as Madagascar drifted southwards, and offshore Tanzania is an obliquely rifted, rather than transform, margin. New seismic reflection evidence for oceanic crust inboard of the DFZ strongly supports these conclusions. Our results provide important new constraints on the still enigmatic driving mechanism of continental rifting, the nature of the lithosphere in the Western Somali Basin, and its resource potential.
Construction of geological controls for three dimensional orebodymodeling
American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) Preprint, Annual Meeting held Phoenix Arizona Feb. 24-27th. 1992, Preprint No. 92-151, 7p
Hf isotopes in zircon from western Superior province, Canada: implications for Archean crustal development and evolution of the depleted mantle reservoir.
Paleomagnetic geochemical and U Pb geochronological study of Proterozoic dykes in Greenland and Arctic Canada and their role in plate tectonic reconstruction.
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Paleomagnetism and U Pb dating of Proterozoic dykes: a new radiation swarm and an increase in post Archean crustal rotation westwards from the Kapuskasing zone.
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Halls, H.C., Davis, D.W., Stott, G.M., Ernst, R.E., Hamilton, M.A.
The Paleoproterozoic Marathon large igneous province: new evidence for a 2.1 Ga long lived mantle plume event along the southern margin of the N.A. Superior Province
Schulze, D.J., Davis, D.W., Helmstaedt, H., Joy, B.
Timing of the Cenozoic " Great Hydration" event beneath the Colorado Plateau: Th-Pb dating of monazite in Navajo volcanic field metamorphic eclogite xenoliths.
Geology, Vol. 43, pp. 727-730.
United States, Colorado Plateau
Diatremes - Moses Rock, Mule's Ear, Garnet Ridge, Cane Valley, Red Mesa, Buell Park, Green Knobs
The Canadian Mineralogist, Vol. 56, pp. 267-286. pdf
Canada, Quebec
deposit - Certac
Abstract: Xenoliths and xenocrysts of mantle material from kimberlite dikes located underground at the Certac Au mine, Québec, in the eastern Superior Craton, were studied in terms of the major element composition of their constituent minerals. The kimberlite was dated at 1151 ± 46 Ma by the U-Pb perovskite method. This suite thus provides a rare glimpse into the Mesoproterozoic mantle of the Superior Craton. Two parageneses of mantle material unrelated to the kimberlite magmatism occur: (1) an olivine + ilmenite ± magnetite association characterized by relatively Fe-rich olivine (Mg# = 0.68-0.84) and ilmenite enriched in Mg and Cr (4-13 wt.% MgO, Cr2O3 up to 3 wt.%), and (2) spinel peridotite characterized by Mg-rich olivine (Mg# = 0.91-0.94). The Fe-rich association is interpreted as a magmatic cumulate likely unrelated to the kimberlite. No mantle-derived garnet occurs in the xenoliths or as xenocrysts. The presence of Cr-rich spinel (Cr# = 0.84-0.98) in high temperature (860-953 °C) chromite peridotite indicates bulk compositions too depleted in Al for garnet to be stable, although geothermometry suggests they equilibrated at depths corresponding to garnet stability (90-131 km, depending on the geothermal gradient). Alternatively, the presence of phlogopite in two of the three high temperature (i.e., deepest) chromite peridotites suggests the absence of garnet and presence of low-Al chromite may have been caused by metasomatism from a K-rich fluid that replaced garnet with phlogopite + clinopyroxene ± chromite. Less depletion at shallower depths is indicated by a chromite (Cr# = 0.60) dunite that equilibrated at 831 °C and a low temperature (752 °C) Mg-Al-spinel lherzolite.
Journal of Metamorphic Geology, doi:10.1111/jmg.12599
Canada, Quebec
cratons
Abstract: Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans?Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans?Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton-that is, Saglek Block (upper plate)-using a multi?chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu-Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu-Hf), indistinguishable from U-Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu-Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga.
Journal of Metamorphic Geology, Vol. 39, 8, 31p. Pdf
Canada
geochronology
Abstract: Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans-Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans-Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton—that is, Saglek Block (upper plate)—using a multi-chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu-Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu-Hf), indistinguishable from U-Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu-Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga.
Earth and Planetary Science Letters, Vol. 494, pp. 172-189.
Mantle
peridotites
Abstract: The oxygen fugacity (fO2) of the oceanic upper mantle has fundamental implications for the production of magmas and evolution of the Earth's interior and exterior. Mid-ocean ridge basalts and peridotites sample the oceanic upper mantle, and retain a record of oxygen fugacity. While fO2 has been calculated for mid-ocean ridge basalts worldwide (>200 locations), ridge peridotites have been comparatively less well studied (33 samples from 11 locations), and never in the same geographic location as basalts. In order to determine whether peridotites and basalts from mid-ocean ridges record congruent information about the fO2 of the Earth's interior, we analyzed 31 basalts and 41 peridotites from the Oblique Segment of the Southwest Indian Ridge. By measuring basalts and peridotites from the same ridge segment, we can compare samples with maximally similar petrogenetic histories. We project the composition and oxygen fugacity of each lithology back to source conditions, and evaluate the effects of factors such as subsolidus diffusion in peridotites and fractional crystallization in basalts. We find that, on average, basalts and peridotites from the Oblique Segment both reflect a source mantle very near the quartz-fayalite-magnetite (QFM) buffer. However, peridotites record a significantly wider range of values (nearly 3 orders of magnitude in fO2), with a single dredge recording a range in fO2 greater than that previously reported for mid-ocean ridge peridotites worldwide. This suggests that mantle fO2 may be heterogeneous on relatively short length scales, and that this heterogeneity may be obscured within aggregated basalt melts. We further suggest that the global peridotite fO2 dataset may not provide a representative sample of average basalt-source mantle. Our study motivates further investigation of the fO2 recorded by ridge peridotites, as peridotites record information about the fO2 of the Earth's interior that cannot be gleaned from analysis of basalts alone.
Abstract: Peridotites dredged from mid-ocean ridges and glassy mid-ocean ridge basalts (MORB) transmit information about the oxygen fugacity (fO2) of Earth's convecting upper mantle to the surface. Equilibrium assemblages of olivine+orthopyroxene+spinel in abyssal peridotites and Fe3+/?Fe ratios in MORB glasses measured by X-ray absorption near-edge structure (XANES) provide independent estimates of MORB source region fO2, with the former recording fO2 approximately 0.8 log units lower than the latter relative to the quartz-fayalite-magnetite (QFM) buffer. To test cross-compatibility of these oxybarometers and examine the compositional effects of changing fO2 on a peridotite plus melt system over a range of Earth-relevant fO2, we performed a series of experiments at 0.1 MPa and fO2 controlled by CO-CO2 gas mixes between QFM-1.87 and QFM+2.23 in a system containing basaltic andesite melt saturated in olivine, orthopyroxene, and spinel. Oxygen fugacities recorded by each method are in agreement with each other and with the fO2 measured in the furnace. Measurements of fO2 from the two oxybarometers agree to within 1? in all experiments. These results demonstrate that the two methods are directly comparable and differences between fO2 measured in abyssal peridotites and MORB result from geographic sampling bias, petrological processes that change fO2 in these samples after separation of melts and residues, or abyssal peridotites may not be residues of MORB melting. As fO2 increases, spinel Fe3+ concentrations increase only at the expense of Cr from QFM-1.87 to QFM-0.11. Above QFM, Al is also diluted in spinel as the cation proportion of Fe3+ increases. None of the three spinel models tested, MELTS (Ghiorso and Sack 1995), SPINMELT (Ariskin and Nikolaev 1996), and MELT_CHROMITE (Poustovetov and Roeder 2001), describe these compositional effects, and we demonstrate that MELTS predicts residues that are too oxidized by >1 log unit to have equilibrated with the coexisting liquid phase. Spinels generated in this study can be used to improve future thermodynamic models needed to predict compositional changes in spinels caused by partial melting of peridotites in the mantle or by metamorphic reactions as peridotites cool in the lithosphere. In our experimental series, where the ratio of Fe2O3/FeO in the melt varies while other melt compositional parameters remain nearly constant, experimental melt fraction remains constant, and Fe3+ becomes increasingly compatible in spinel as fO2 increases. Instead of promoting melting, increasing the bulk Fe3+/?Fe ratio in peridotite drives reactions analogous to the fayalite-ferrosilite-magnetite reaction. This may partly explain the absence of correlation between Na2O and Fe2O3 in fractionation-corrected MORB.
Lamproite diatreme in the Dubawnt Lake area, Northwest Territories
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 67
Abstract: Earth’s mantle has cooled by 6-11?°C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7?km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15-20?°C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.
Abstract: Earth’s mantle has cooled by 6-11?°C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7?km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15-20?°C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Magnetic inversion of three airborne dat a sets over the Tli Kwi Cho kimberlite complex.
SEG Annual Meeting Denver, pp. 1790-1794 extended abstract
Fournier, D., Heagy, L., Corcoran, N., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Multi-EM systems inversion - towards a common conductivity model for Tli Kwi Cho complex.
SEG Annual Meeting Denver, pp. 1795-1798. Extended abstract
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO- 18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Marchant, M., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Society of Exploration Geophysicists, Interpretation, August T 299, 13p.
Canada, Northwest Territories
deposit - Tli Kwi Cho
Abstract: The Tli Kwi Cho (TKC) kimberlite complex contains two pipes, called DO-27 and DO-18, which were discovered during the Canadian diamond exploration rush in the 1990s. The complex has been used as a testbed for ground and airborne geophysics, and an abundance of data currently exist over the area. We have evaluated the historical and geologic background of the complex, the physical properties of interest for kimberlite exploration, and the geophysical surveys. We have carried out 3D inversion and joint interpretation of the potential field data. The magnetic data indicate high susceptibility at DO-18, and the magnetic inversion maps the horizontal extent of the pipe. DO-27 is more complicated. The northern part is highly magnetic and is contaminated with remanent magnetization; other parts of DO-27 have a low susceptibility. Low densities, obtained from the gravity and gravity gradiometry data, map the horizontal extents of DO-27 and DO-18. We combine the 3D density contrast and susceptibility models into a single geologic model that identifies three distinct kimberlite rock units that agree with drilling data. In further research, our density and magnetic susceptibility models are combined with information from electromagnetic data to provide a multigeophysical interpretation of the TKC kimberlite complex.
Critical evaluation of the status of the areas for future research regarding the wide band GAP semi-conductors diamond, gallium nitride and silicon carbide
Material Sci. Eng. B. Solid State Adv. Technol, Vol. B1, No. 1, Aug. pp. 77-104
Age and evolution of the lower crust beneath the western Churchill Province: U-Pb zircon geochronology of kimberlite hosted granulite xenoliths, Nunavut.
Abstract: Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.
Canadian Journal of Earth Sciences, Vol. 54, pp. 203-232.
Canada, Northwest Territories
kimberlite
Abstract: The Wopmay orogen is a Paleoproterozoic accretionary belt preserved to the west of the Archean Slave craton, northwest Canada. Reworked Archean crystalline basement occurs in the orogen, and new bedrock mapping, U–Pb geochronology, and Sm–Nd isotopic data further substantiate a Slave craton parentage for this basement. Detrital zircon results from unconformably overlying Paleoproterozoic supracrustal rocks also support a Slave craton provenance. Rifting of the Slave margin began at ca. 2.02 Ga with a second rift phase constrained between ca. 1.92 and 1.89 Ga, resulting in thermal weakening of the Archean basement and allowing subsequent penetrative deformation during the Calderian orogeny (ca. 1.88–1.85 Ga). The boundary between the western Slave craton and the reworked Archean basement in the southern Wopmay orogen is interpreted as the rifted cratonic margin, which later acted as a rigid backstop during compressional deformation. Age-isotopic characteristics of plutonic phases track the extent and evolution of these processes that left penetratively deformed Archean basement, Paleoproterozoic cover, and plutons in the west, and “rigid” Archean Slave craton to the east. Diamond-bearing kimberlite occurs across the central and eastern parts of the Slave craton, but kimberlite (diamond bearing or not) has not been documented west of ?114°W. It is proposed that while the crust of the western Slave craton escaped thermal weakening, the mantle did not and was moved out of the diamond stability field. The Paleoproterozoic extension–convergence cycle preserved in the Wopmay orogen provides a reasonable explanation as to why the western Slave craton appears to be diamond sterile.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 253-266.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 425-
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 169-190.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 267-286.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 65-88.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 237-252.
Abstract: Raman spectroscopy has been foreseen as a simple and non-destructive characterization method to determine the boron concentration in heavily boron-doped diamond with metallic conductivity. However, currently available empirical studies are not fully satisfactory for enabling accurate determination of the boron concentration in diamond. Here, we study Raman spectra of epitaxial boron-doped diamond as a function of the boron concentration and the excitation wavelength. The zone center phonon and the phonon density of state maximum (at ca. 1200?cm?1) lines are analyzed using a decoupled double Fano-function. This analysis method accurately describes the observed variation of the asymmetric parameters with atomic boron concentration and the photon excitation energy and enables the determination of the atomic boron concentration from the parameters of the examined Raman lines.
Modification of mineral inclusions in garnet under high pressure conditions: experimental simulation and application to carbonate silicate rocks of Kokchetetav
Russian Geology and Geophysics, Vol. 50, 12, pp. 1153-1168.
Precambrian-Paleozoic geology of Smith Sound, Canada and Greenland: key constraint to paleogeographic reconstructions northern Laurentia and North Atlantic region
Abstract: Compared to all published data from carbonatites and granitoids, the fluorapatite compositions in the Songwe Hill carbonatite, determined by EPMA and LA ICP-MS, have the highest heavy (H)REE concentration of any carbonatite apatite described so far. A combination of this fluorapatite and the REE fluorocarbonates, synchysite-(Ce) and parisite-(Ce), which are the other principal REE bearing minerals at Songwe, gives a REE deposit with a high proportion of Nd and a higher proportion of HREE (Eu-Lu including Y) than most other carbonatites. Since Nd and HREE are currently the most sought REE for commercial applications, the conditions that give rise to this REE profile are particularly important to understand. Multiple apatite crystallisation stages have been differentiated texturally and geochemically at Songwe and fluorapatite is divided into five different types (Ap-0-4). While Ap-0 and Ap-1 are typical of apatite found in fenite and calcite-carbonatite, Ap-2, -3 and -4 are texturally atypical of apatite from carbonatite and are progressively HREE-enriched in later paragenetic stages. Ap-3 and Ap-4 exhibit anhedral, stringer-like textures and their REE distributions display an Y anomaly. These features attest to formation in a hydrothermal environment and fluid inclusion homogenisation temperatures indicate crystallisation occurred between 200-350 °C. Ap-3 crystallisation is succeeded by a light (L)REE mineral assemblage of synchysite-(Ce), strontianite and baryte. Finally, late-stage Ap-4 is associated with minor xenotime-(Y) mineralisation and HREE-enriched fluorite. Fluid inclusions in the fluorite constrain the minimum HREE mineralisation temperature to approximately 160 °C. A model is suggested where sub-solidus, carbonatite-derived, (carbo)-hydrothermal fluids remobilise and fractionate the REE. Chloride or fluoride complexes retain LREE in solution while rapid precipitation of apatite, owing to its low solubility, leads to destabilisation of HREE complexes and substitution into the apatite structure. The LREE are retained in solution, subsequently forming synchysite-(Ce). This model will be applicable to help guide exploration in other carbonatite complexes.
Journal of African Earth Sciences, Vol. 134, pp. 10-23.
Africa, Malawi
carbonatite - Songwe Hill
Abstract: Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.
Abstract: The geochemistry and mineralogy of REE deposits is diverse, from carbonatite-related deposits, alkaline rocks, mineral sands and ion adsorption clays to potential by-products of phosphate and bauxite, and reuse of waste materials. Despite the large number of prospects that have been explored recently, very little additional REE production has started. A major challenge is to design effective, cost-efficient and environmentally-friendly processing and extraction. Processing flow sheets have to be constructed carefully for each deposit. Translating geochemistry and mineralogy studies, including quantitative mineralogy results, into processing characteristics can be illustrated using results from the Songwe Hill carbonatite, Malawi. Combining results with other published data then allows us to make some general conclusions about the common REE ore minerals and their geological environment, including the REE fluorcarbonate series, monazite and xenotime. The use of chemicals for REE extraction is often the largest environmental burden to mitigate. A new issue is that certain REE, such as Ce, are in oversupply, and are not being recovered in some proposed processing flowsheets. It will be important to understand the environmental and commercial implications of this development.
Abstract: Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1-2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g?1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid - country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
Abstract: The Harrat Kishb area of western Saudi Arabia is part of the Cenozoic volcanic fields in the western margin of the Arabian Shield. Numerous fresh ultramafic xenoliths are entrained in the basanite lava of Harrat Kishb, providing an opportunity to study the nature and petrogenetic processes involved in the evolution of the lithospheric mantle beneath the Arabian Shield. Based on the petrological characteristics and mineralogical compositions, the majority of the mantle xenoliths (~ 92%) are peridotites (lherzolites and pyroxene-bearing harzburgites); the remaining xenoliths (~ 8%) are unusual spinel-rich wehrlites containing black Al-spinel micropods. The two types of mantle xenoliths display magmatic protogranular texture. The peridotite xenoliths have high bulk-rock Mg#, high forsterite (Fo90-Fo92) and NiO (0.24-0.46 wt.%) contents of olivine, high clinopyroxene Mg# (0.91-0.93), variable spinel Cr# (0.10-0.49, atomic ratio), and approximately flat chondrite-normalized REE patterns. These features indicate that the peridotite xenoliths represent residues after variable degrees of melt extraction from fertile mantle. The estimated P (9-16 kbar) and T (877-1227 °C) as well as the oxidation state (?logfO2 = ? 3.38 to ? 0.22) under which these peridotite xenoliths originated are consistent with formation conditions similar to most sub-arc abyssal-type peridotites worldwide. The spinel-rich wehrlite xenoliths have an unusual amount (~ 30 vol.%) of Al-spinel as peculiar micropods with very minor Cr2O3 content (< 1 wt.%). Olivines of the spinel-rich wehrlites have low-average Fo (Fo81) and NiO (0.18 wt.%) contents, low-average cpx Mg# (0.79), high average cpx Al2O3 content (8.46 wt.%), and very low-average spinel Cr# (0.01). These features characterize early mantle cumulates from a picritic melt fraction produced by low degrees of partial melting of a garnet-bearing mantle source. The relatively high Na2O and Al2O3 contents of cpx suggest that the spinel-rich wehrlites are formed under high P (11-14 kbar), T (1090-1130 °C), and oxidation state (?logfO2 FMQ = + 0.14 to + 0.37), which occurred slightly below the crust-mantle boundary. The REE patterns of spinel-rich wehrlites are almost similar to those of the associated peridotite xenoliths, which confirm at least a spatial genetic linkage between them. Regarding the formation of Al-spinel micropods in spinel-rich wehrlite cumulates, it is suggested that the melt-rock reaction mechanism is not the only process by which podiform chromitite is formed. Early fractionation of picritic melts produced by partial melting of a mantle source under high P-T conditions could be another mechanism. The cpx composition, not opx, as it was assumed, seems to be the main control of the size and composition of spinel concentrations.
CAT scanning the earth... supercomputers are enabling geophysicists to visualize the earth's churning interior - and gain new insights into how itworks
Metasomatised and veined upper mantle xenoliths from Pello Hill, Tanzania: evidence for anomalously light mantle beneath the Tanzanian sectorof the East African
Mineralogy and Petrology, Vol. 100, No. 4, pp. 510-527
Short lived decay series disequilibration temperatures in the natrocarbonatite lavas of Oldoinyo Lengai, Tanzania: constraints on the timing of magma genesis
Earth and Planetary Science Letters, Vol. 105, pp. 378-396
The 24th September 2007 ash eruption of the carbonatite volcano Oldoinyo Lengai: mineralogy of the ash and implications for formation of a new hybrid magma type.
Mineralogical Magazine, Vol. 71, 5, Oct, pp. 483-492.
Carbonate-silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo-Lengai Volcano, Tanzania.
Contrasting garnet lherzolite xenoliths suites from the Letseng kimberlite pipes: inferences for the northern Lesotho geotherm.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 29-44.
Abstract: This study presents compositional data for a statistically significant number (n=180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene are very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Abstract: This study presents compositional data for a statistically significant number (n = 180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene is very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Abstract: This study presents compositional data for a statistically significant number (n = 180) of heated and quenched (recreated) carbonate melt inclusions trapped in magnetite and clinopyroxene in jacupirangite from Kerimasi volcano (Tanzania). On the basis of homogenization experiments for clinopyroxene-hosted melt inclusions and forsterite-monticellite-calcite phase relations, a range of 1000 to 900 °C is estimated for their crystallization temperatures. Petrographic observations and geochemical data show that during jacupirangite crystallization, a CaO-rich and alkali-"poor" carbonate melt (relative to Oldoinyo Lengai natrocarbonatite) existed and was entrapped in the precipitating magnetite, forming primary melt inclusions, and was also enclosed in previously crystallized clinopyroxene as secondary melt inclusions. The composition of the trapped carbonate melts in magnetite and clinopyroxene is very similar to the parental melt of Kerimasi calciocarbonatite; i.e., enriched in Na2O, K2O, F, Cl and S, but depleted in SiO2 and P2O5 relative to carbonate melts entrapped at an earlier stage and higher temperature (1050-1100 °C) during the formation of Kerimasi afrikandite. Significant compositional variation is shown by the major minerals of Kerimasi plutonic rocks (afrikandite, jacupirangite and calciocarbonatite). Magnetite and clinopyroxene in the jacupirangite are typically transitional in composition between those of afrikandite and calciocarbonatite. These data suggest that the jacupirangite represents an intermediate stage between the formation of afrikandite and calciocarbonatite. Jacupirangite most probably formed when immiscible silicate and carbonate melts separated from the afrikandite body, although the carbonate melt was not separated completely from the silicate melt fraction. In general, during the evolution of the carbonate melt at Kerimasi, concentrations of P2O5 and SiO2 decreased, whereas volatile content (alkalis, S, F, Cl and H2O) increased. Volatiles were incorporated principally in nyerereite, shortite, burbankite, nahcolite and sulfohalite as identified by Raman spectrometry. These extremely unstable minerals cannot be found in the bulk rock, because of alteration by secondary processes. On the basis of these data, an evolutionary model is developed for Kerimasi plutonic rocks.
Abstract: A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe-Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na-Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn-Mg-magnetite, Mn-Fe-sphalerite and Nb-perovskite. Individual Na-Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7-23.6 Na2O; 0.5-2.8 K2O, with minor SO3 (1.1-2.2) and SrO (0.34-1.0). The Na-Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na-Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional variation and contains 2-4 wt.% MnO and 12 wt.% FeO and is Mn-poor relative to monticellite in Oldoinyo Lengai natrocarbonatite. Periclase is Fe-bearing with up to 13 wt.% FeO. Spinels are Cr-free, Mn-poor and belong to the magnetite-magnesioferrite series in contrast to Mn-rich spinels of the magnetite-jacobsite series occurring in Oldoinyo Lengai natrocarbonatite. The matrix in which the ‘corduroy’ clasts and phenocrystal calcite are set consists of closely packed small prisms of calcite lacking melt inclusions, with interstitial fine-grained apatite, baryte, strontianite and minor fluorite. Pore spaces are filled with secondary Mn-Fe hydroxides/oxides, anhydrite and gypsum. The hypothesis that flow-aligned calcite in volcanic calciocarbonatites from Kerimasi, Tinderet, Homa and Catanda is altered nyerereite is discussed and it is considered that these calcite are either primary phases or altered melilite. The nyerereite alteration hypothesis is discussed with respect to the volumetric and compositional aspects of pseudomorphism by dissolution-precipitation replacement mechanisms. This study concludes that none of the volcanic calciocarbonatites containing flow-aligned calcite phenocrysts are altered natrocarbonatite.
Abstract: Abstract A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe-Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na-Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn-Mg-magnetite, Mn-Fe-sphalerite and Nb-perovskite. Individual Na-Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7-23.6 Na2O; 0.5-2.8 K2O, with minor SO3 (1.1-2.2) and SrO (0.34-1.0). The Na-Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na-Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional variation and contains 2-4 wt.% MnO and 12 wt.% FeO and is Mn-poor relative to monticellite in Oldoinyo Lengai natrocarbonatite. Periclase is Fe-bearing with up to 13 wt.% FeO. Spinels are Cr-free, Mn-poor and belong to the magnetite-magnesioferrite series in contrast to Mn-rich spinels of the magnetite-jacobsite series occurring in Oldoinyo Lengai natrocarbonatite. The matrix in which the ‘corduroy’ clasts and phenocrystal calcite are set consists of closely packed small prisms of calcite lacking melt inclusions, with interstitial fine-grained apatite, baryte, strontianite and minor fluorite. Pore spaces are filled with secondary Mn-Fe hydroxides/oxides, anhydrite and gypsum. The hypothesis that flow-aligned calcite in volcanic calciocarbonatites from Kerimasi, Tinderet, Homa and Catanda is altered nyerereite is discussed and it is considered that these calcite are either primary phases or altered melilite. The nyerereite alteration hypothesis is discussed with respect to the volumetric and compositional aspects of pseudomorphism by dissolution-precipitation replacement mechanisms. This study concludes that none of the volcanic calciocarbonatites containing flow-aligned calcite phenocrysts are altered natrocarbonatite.
Timescales and mechanisms of plume-lithosphere interactions: Ar/Ar geochronology and geochemistry of alkaline igneous rocks from the Parana Etendeka igneous
Earth and Planetary Science Letters, Vol. 251, 1-2, Nov. 15, pp. 1-17.
Geochimica et Cosmochimica Acta, Vol. 275, pp. 140-162.
Mantle
craton
Abstract: The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on "off-craton" sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low (<5 ppmw) in all analysed phases and both Cl and B are below detection levels (14 ppmw and 0.03 ppmw, respectively). Unique to our study is the large variation in major- and trace-element concentrations of the clinopyroxenes, which allows us to place quantitative constraints on how volatiles are stored in the mantle. We demonstrate that: (i) F contents of clinopyroxenes closely correlate with Ti and (ii) and is systematic and inversely correlated with temperature. Despite the redistribution of volatiles during sub-solidus re-equilibration, we show that the first order control on the concentration of volatiles in clinopyroxene is the style of metasomatism, i.e. channellised flow versus reactive percolation. The mean bulk volatile contents of peridotites from Pali Aike and the Antarctic Peninsula (H2O?=?89?±?31 ppmw, F?=?16?±?11.2 ppmw and Li?=?2?±?0.7 ppmw) are within the range previously published for continental "off-craton" mantle. The pyroxenites have significantly higher mean bulk concentrations of H2O (260?±?59 ppmw), F (86?±?43 ppmw) and Li (1.0?±?0.35 ppmw). While the greater capacity of mantle pyroxenites to host H2O relative to the associated peridotites has previously been observed in global "off-craton" mantle xenolith suites (e.g. Oahu, Hawaii; eastern China and the Rio Grande Rift, SW USA), here we show for the first time that pyroxenites are also major hosts of F (but not Cl, Li or B). Because of their relatively low solidus temperatures, pyroxenites in "off-craton" settings will be readily re-mobilised during lithospheric extension (and heating). We suggest these pyroxene-rich mantle lithologies may be responsible for the elevated concentrations of H2O and F observed in basalts and volcanic gasses from major continental rift zones and flood basalt provinces, and hence an important consideration in models of global volatile cycles.
Abstract: Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3 He/ 4 He) and concentra-tion data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kim-berlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3 He/ 4 He ratios span a range from 0.1 to 9.8 R A (where R A = air 3 He/ 4 He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3 He/ 4 He = 0.41 ± 0.30 R A (1?); n = 7 excluding 1 outlier), and 2) 'mantle-like' post plume Obnazhennaya samples (mean clinopyroxene 3 He/ 4 He = 4.20 ± 0.90 R A (1?); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3 He/ 4 He than clinopyroxenes (or garnet). Helium con-tents in Udachnaya samples ([He] = 0.13–1.35 ?cm 3 STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 ?cm 3 STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49– 349 ?cm 3 STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3 He/ 4 He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre-and post-plume 3 He/ 4 He ra-tios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).
Geochimica et Cosmochimica Acta, Vol. 200, pp. 232-254.
Mantle
Peridotite
Abstract: Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5 Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history. Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356 ± 21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361 ± 20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292 ± 25), and 186Os/188Os of 0.1198388 ± 29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.
Abstract: The origin of carbonatites—igneous rocks with more than 50% of carbonate minerals—and whether they originate from a primary mantle source or from recycling of surface materials are still debated. Calcium isotopes have the potential to resolve the origin of carbonatites, since marine carbonates are enriched in the lighter isotopes of Ca compared to the mantle. Here, we report the Ca isotopic compositions for 74 carbonatites and associated silicate rocks from continental and oceanic settings, spanning from 3 billion years ago to the present day, together with O and C isotopic ratios for 37 samples. Calcium-, Mg-, and Fe-rich carbonatites have isotopically lighter Ca than mantle-derived rocks such as basalts and fall within the range of isotopically light Ca from ancient marine carbonates. This signature reflects the composition of the source, which is isotopically light and is consistent with recycling of surface carbonate materials into the mantle.
Science Advances, Vol. 6, eaba3269 June 3, 7p. Pdf
Global, Africa, Tanzania
carbonatites
Abstract: The origin of carbonatites-igneous rocks with more than 50% of carbonate minerals-and whether they originate from a primary mantle source or from recycling of surface materials are still debated. Calcium isotopes have the potential to resolve the origin of carbonatites, since marine carbonates are enriched in the lighter isotopes of Ca compared to the mantle. Here, we report the Ca isotopic compositions for 74 carbonatites and associated silicate rocks from continental and oceanic settings, spanning from 3 billion years ago to the present day, together with O and C isotopic ratios for 37 samples. Calcium-, Mg-, and Fe-rich carbonatites have isotopically lighter Ca than mantle-derived rocks such as basalts and fall within the range of isotopically light Ca from ancient marine carbonates. This signature reflects the composition of the source, which is isotopically light and is consistent with recycling of surface carbonate materials into the mantle.
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 69
A review of diamond occurrences and potential in Alberta
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 98-99.
Preliminary release of kimberlite indicator mineral dat a from National geochemical Reconnaissance stream sediment samples in the Jackpine Lake area Buffalo Head Hills
Geological Survey of Canada Open File, 5267, 23p. 1 CD $ 26.00
GAC MAC Meeting Special Session SS11: Cratons, kimberlites and diamonds., abstract 1/4p.
Canada, Northwest Territories
Diamond indicators
Abstract: The Central Mackenzie Valley (CMV) area of the Northwest Territories (NWT) comprises a Phanerozoic sedimentary basin that lies between the western margin of the Slave craton and the Cordillera. Although the region is considerably outside the bounds of the exposed Slave craton, both LITHOPROBE and more recent regional-scale surface wave studies (e.g., Priestley and McKenzie, 2006) indicate the likely presence of lithospheric mantle extending into the diamond stability field. Recent work conducted by Olivut Resources Ltd. led to the discovery of 29 kimberlites in the CMV. However, the indicator mineral chemistry of discovered kimberlites does not appear to be a good match (www.olivut.ca) with those during regional till and stream sediment sampling by the Geologic Survey of Canada (GSC) and Northwest Territories Geologic Survey (NTGS) in August 2003 and July 2005. We present new geochemical data on the regional indicator minerals with the aim of obtaining geotherm and depth of mantle sampling constraints on those indicator minerals discovered to date. A statistical evaluation of the data will compare the similarities to indicator mineral chemistry with parts of the Slave craton to evaluate whether the CMV indicators may ultimately be derived from that region. In total 3600 kimberlite indicator mineral grains were picked from the 0.25-2.0 mm size fractions. Peridotitic garnet grains dominate (46%), followed by magnesium ilmenite (26%), with decreasing individual proportions >15% of chromite, low-chrome diopside, olivine, chrome-diopside and eclogitic garnet. A sub-sample of these grains (3143) were analysed by EPMA. Garnet grains classify (after Grütter et al., 2004) as 1015 (62.1%) G9, 270 (16.5%) G11, 113 (6.9%) G10, 103 (6.3%) G12, 57 (3.5%) G1, 46 (2.8%) G10D, and the remaining 31 (1.9%) as G0, G3, G3D, G4, and G5. A sub-set of garnet grains (~700) were selected for LA-ICP-MS trace element analysis. Of the grains selected 74% G9, 14% G10 (and G10D), and 8% G11, with only 4% G12 and G0 (Grütter et al., 2004). Nickel concentrations from these grains range from 2.6-168.2 ppm, with the majority (>80%) between 20-100 ppm, yielding TNi (Canil, 1999) values ranging from 643-1348°C, with the majority between ~1000-1200°C. Using a central Slave craton geothermal gradient (Hasterok and Chapman, 2011), equilibration pressures for these garnet grains range from 20-80 kbars with the majority between 40-60 kbars (120-185 km). Preliminary analysis has 581 (81%) of the erupted peridotitic mantle garnet grains plotting within the diamond stability field (Kennedy and Kennedy, 1976). Of the 128 clinopyroxene grains analysed, only a few represent garnet peridotite (lherzolite) facies KIM clinopyroxene grains following compositional screening. Thermobarometry of these grains (Nimis and Taylor, 2000), assuming they were all derived from the same lithospheric section, yields P-T arrays identical to the central Slave geotherm that was 220 km thick at the time of eruption. These results are encouraging for diamond exploration. We thank Overburden Drilling Management Ltd. for grain picking and recovery of the small diamond, SGS Lakefield Research for mounting grains, and the GSC for probing of the grains.
National geochemical Reconnaissance: regional stream sediment and water data: Travaillant Lake area.. analytical, mineralogical kimberlite indicator data
Geological Survey of Canada Open File, 4951, 1 CD May 17, $ 9.10
Geochemical, mineralogical and kimberlite indicator mineral electron microprobe dat a from sills, heavy mineral concentrates and waters Buffalo Head Hills.
Geological Survey of Canada Open File, No. 5057, 16p.
iron-copper-rare earth elements (REE) deposits in Middle Proterozoic rocks of the Midcontinent region of the United States..are they Olympic Dam-type deposits?
The Gangue, Geological Association of Canada (GAC)/Mineral Deposits Newsletter, No. 42, April pp. 1-4
Mine Water and the Environment, in press available, 19p.
Canada, Northwest Territories
Deposit - Gahcho Kue
Abstract: Eight water models were used to assess potential aquatic environmental effects of the proposed Gahcho Kué diamond mine on groundwater and surface water flow and quality in the Northwest Territories, Canada. This sequence of models was required to cover different spatial and temporal domains, as well as specific physico-chemical processes that could not be simulated by a single model. Where their domains overlapped, the models were interlinked. Feedback mechanisms amongst models were addressed through iterative simulations of linked models. The models were used to test and refine mitigation plans, and in the development of aquatic component monitoring programs. Key findings generated by each model are presented here as testable hypotheses that can be evaluated after the mine is operational. This paper therefore offers a record of assumptions and predictions that can be used as a basis for post-validation.
Mine Water and the Environment, Vol. 35, pp. 350-368.
Canada, Northwest Territories
Deposit - Gahcho Kue
Abstract: Eight water models were used to assess potential aquatic environmental effects of the proposed Gahcho Kué diamond mine on groundwater and surface water flow and quality in the Northwest Territories, Canada. This sequence of models was required to cover different spatial and temporal domains, as well as specific physico-chemical processes that could not be simulated by a single model. Where their domains overlapped, the models were interlinked. Feedback mechanisms amongst models were addressed through iterative simulations of linked models. The models were used to test and refine mitigation plans, and in the development of aquatic component monitoring programs. Key findings generated by each model are presented here as testable hypotheses that can be evaluated after the mine is operational. This paper therefore offers a record of assumptions and predictions that can be used as a basis for post-validation.
Barry, P.H., de Moor, J.M., Giovannelli, D., Schrenk, M., Hummer, D.R., Lopez, T., Pratt, C.A., Alpizar Segua, Y., Battaglia, A., Beaudry, A., Bini, G., Cascante, M., d'Errico, G., di Carlo, M., Fattorini, D., Fullerton, K., H+Gazel, E., Gonzalez, G., Hal
Abstract: Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.
Abstract: Kimberlites of the Udachnaya-East pipe (Siberia) include a uniquely dry and serpentine-free rock type with anomalously high contents of chlorine (Cl ? 6.1 wt%), alkalies (Na2O + K2O ? 10 wt%) and sulfur (S ? 0.50 wt%), referred to as a “salty” kimberlite. The straightforward interpretation is that the Na-, K-, Cl- and S-rich components originate directly from a carbonate-chloride kimberlitic magma that is anhydrous and alkali-rich. However, because brines and evaporites are present on the Siberian craton, previous studies proposed that the kimberlitic magma was contaminated by the assimilation of salt-rich crustal rocks. To clarify the origin of high Cl, alkalies and S in this unusual kimberlite, here we determine its sulfur speciation and isotopic composition and compare it to that of non-salty kimberlites and kimberlitic breccia from the same pipe, as well as potential contamination sources (hydrothermal sulfides and sulfates, country-rock sediment and brine collected in the area). The average ?34S of sulfides is ? 1.4 ± 2.2‰ in the salty kimberlite, 2.1 ± 2.7‰ in the non-salty kimberlites and 14.2 ± 5.8‰ in the breccia. The average ?34S of sulfates in the salty kimberlites is 11.1 ± 1.8‰ and 27.3 ± 1.6‰ in the breccia. In contrast, the ?34S of potential contaminants range from 20 to 42‰ for hydrothermal sulfides, from 16 to 34‰ for hydrothermal sulfates, 34‰ for a country-rock sediment (Chukuck suite) and the regional brine aquifer. Our isotope analyses show that (1) in the salty kimberlites, neither sulfates nor sulfides can be simply explained by brine infiltration, hydrothermal alteration or the assimilation of known salt-rich country rocks and instead, we propose that they are late magmatic phases; (2) in the non-salty kimberlite and breccia, brine infiltration lead to sulfate reduction and the formation of secondary sulfides – this explains the removal of salts, alkali-carbonates and sulfates, as well as the minor olivine serpentinization; (3) hydrothermal sulfur was added to the kimberlitic breccia, but not to the massive kimberlites. In situ measurements of sulfides confirm this scenario, clearly showing the addition of two sulfide populations in the breccia (pyrite-pyrrhotites with average ?34S of 7.9 ± 3.4‰ and chalcopyrites with average ?34S of 38.0 ± 0.4‰) whereas the salty and non-salty kimberlites preserve a unique population of djerfisherites (Cl- and K-rich sulfides) with ?34S values within the mantle range. This study provides the first direct evidence of alkaline igneous rocks in which magmatic sulfate is more abundant than sulfide. Although sulfates have been rarely reported in mantle materials, sulfate-rich melts may be more common in the mantle than previously thought and could balance the sulfur isotope budget of Earth's mantle.
Bulletin de la Societe Geologique de France *eng, Vol. 188, 1-2, 8p.
Russia, Siberia
deposit - Udachnaya-East
Abstract: The Udachnaya-East pipe in Yakutia in Siberia hosts a unique dry (serpentine-free) body of hypabyssal kimberlite (<0.64wt% H2O), associated with a less dry type of kimberlite and a serpentinized kimberlitic breccia. The dry kimberlite is anomalously rich in salts (Na2O and Cl both up to 6wt%) whereas the slightly less dry and the breccia kimberlite are salt free. Yet the Udachnaya kimberlite is a group-I kimberlite, as is the archetypical kimberlite from Kimberley, South Africa. Samples were studied from the three different types of kimberlite (dry-salty, n=8, non-salty, n=5 and breccia, n=3) regarding their mineralogy, geochemistry, and more specifically their sulfur content. Our results show the salty kimberlite is unprecedentedly rich in sulfur (0.13-0.57wt%) compared to the non-salty kimberlite (0.04-0.12wt%) and the breccia (0.29-0.33wt%). In the salty kimberlite, most of the sulfur is present as sulfates (up to 97% of Stotal) and is disseminated throughout the groundmass in close association with Na-K-bearing carbonates. Sulfates occur within the crystal structure of these Na-K-bearing carbonates as the replacement of (CO3) by (SO3) groups, or as Na- and K-rich sulfates (e.g. aphtitalite, (K,Na)3Na(SO4)2). The associated sulfides are djerfisherite; also Na- and K-rich species. The close association of sulfates and carbonates in these S-rich alkaline rocks suggests that the sulfates crystallized from a mantle-derived magma, a case that has strong implication for the oxygen fugacity of kimberlite magmatism and more generally for the global S budget of the mantle.
Geochimica et Cosmochimica Acta, Vol. 315, pp. 295-316.
Russia, Siberia
deposit - Udachnaya
Abstract: Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks. Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n = 3). We then compared the results with the same geochemical data on host kimberlites (n = 4), sedimentary cover (n = 3) and hydrothermal veins (n = 3). Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and ?34S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with ?34S > 0‰ are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value (?34S of 0‰) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites).
Mineralogy and Petrology, doi.org/10.1007/ s00710-018- 0587-6 16p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Zimmi diamonds (Sierra Leone) have 500 million year mantle residency times whose origin is best explained by rapid tectonic exhumation to shallower depths in the mantle, associated with continental collision but prior to kimberlite eruption. Here we present spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the link between their spectroscopic features and their unusual geological history. Cathodoluminesence (CL) imaging of these diamonds revealed irregular patterns with abundant deformation lamellae, associated with the diamonds' tectonic exhumation. Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/- centres and H3 (NVN0). The brownish-yellow to greenish-yellow colours observed in Zimmi Ib-IaA diamonds result from visible absorption by a combination of isolated substitutional nitrogen ( {N}S^0 ) and deformation-related vacancy clusters. Colour-forming centres and other spectroscopic features can all be attributed to the unique geological history of Zimmi Ib-IaA diamonds and their rapid exhumation after formation.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265.
Global
DiamondView
Abstract: Diamond, known for its splendor in exquisite jewelry, has been synthesized since the 1950s. In the last six decades, the perfection of laboratory-grown single-crystal diamond has vastly improved through the research and development of two main synthesis techniques. One replicates Earth’s natural process, where the diamond is grown in the laboratory under conditions of diamond stability at high temperature and high pressure (HPHT). The other technique relies on the dissociation of methane (or other carbon-containing source gas) and hydrogen and the subsequent deposition of diamond at low pressures from the gaseous phase in a process known as chemical vapor deposition (CVD). In the latter case, diamond is not the stable form of carbon, but the kinetics in the CVD process are such that diamond wins out. Large gem-quality synthetic diamonds are now possible, and a 6 ct CVD (2018) and a 15.32 ct HPHT (2018) have been reported. It is of course possible to differentiate laboratory-grown from natural diamond based on how extended and point defects are incorporated into the crystal. Furthermore, treated diamond can be identified utilizing knowledge of how defects are produced and how they migrate and aggregate in both natural and synthetic diamond samples. Room-temperature confocal photoluminescence microscopy can be used to image the emission of light from defects in diamond with a spatial resolution limited only by the diffraction limit; a lateral spatial resolution approaching 300 nm is routinely achieved (figure 1). It is possible with this tool to identify point defects with concentrations less than 1 part per trillion (1011 cm–3). This talk will outline the experimental setup, how this tool has been used to identify the decoration of dislocations with point defects in CVD lab-grown diamond, and how different mechanisms for defect incorporation operate at growth sector boundaries in HPHT synthetic diamond.
GSA Annual Meeting, Paper 300-12, 1p. Abstract only Booth
Technology
Synthetic diamonds
Abstract: Defect of [Si-V]- is common in CVD synthetic diamonds, and its occurrence was also reported in some rare natural diamonds (Breeding and Wang, 2008). It is an important feature employed for gem diamond identification, and also has great potential for applications in industry. However little is known about how the silicon impurity gets into diamond lattice either in synthetic or natural diamonds. In this study, we discovered the occurrence of [Si-V]- in HPHT synthetic diamonds and the correlation between its precipitation and diamond growth sectors was successfully determined.
Total 20 samples, HPHT grown diamond wafers from NDT (New Diamond Technology) were studied in addition to one type IIb HPHT synthetic diamond submitted to GIA Laboratory for grading. Distributions of defects in these samples were carefully mapped using infrared microscopy at room temperature and an imaging Raman microscope at liquid nitrogen temperature.
Defect of [Si-V]- has doublet emissions at 736.6/736.9 nm (Clark et al., 1995), and can be effectively excited using 633 nm laser. Analyses were conducted at Liquid Nitrogen temperature as the detection of the Si related emissions peak is temperature dependent (Feng and Schwartz 1993). Additionally, the solvent catalysts used in the HPHT methods to grow synthetic diamond either intentionally or unintentionally contain nickel in varying quantities. Nickel impurity creates optical centers which emit a doublet peak at 882.6/884.3nm, and can be easily excited using 780 nm laser.
The [SiV]- is clearly observed in only certain growth sectors of the synthetic crystal and the distribution is not homogeneous. By comparing the two acquired maps one acquired at 633nm excitation showing the [Si-V]- distribution and one acquired with 780nm excitation showing the nickel defect distribution, it was found that the [Si-V]- is confined to the same growth sector as Ni related defect with higher concentrations/intensity at the edges of these sectors. Since it is well known that the Ni defect is confined exclusively to the octahedral growth sectors {111} of diamond (Lawson et al., 1993), this study for the first time confirmed that [Si-V]- is confined to the {111} octahedral growth sectors.
This new discovery leads to discussion as to the incorporation of silicon in diamond and the relationship to other impurities.
Abstract: A freely moving diamond trapped inside another diamond was discovered in Siberia by Alrosa in 2019. The unusual diamond, nicknamed the “Matryoshka” after the traditional Russian nesting dolls, attracted widespread interest in how this feature formed.
Abstract: The Russian company New Diamond Technology is producing colorless and near-colorless HPHT-grown synthetic diamonds for the gem trade. Forty-four faceted samples synthesized using modified cubic presses were analyzed using a combination of spectroscopic and gemological techniques to characterize the quality of the material and determine the means of distinguishing them from natural, treated, and alternative laboratory-grown diamonds. These samples, with weights ranging from 0.20 to 5.11 ct, had color grades from D to K and clarity grades from IF to I2. Importantly, 89% were classified as colorless (D-F), demonstrating that HPHT growth methods can be used to routinely achieve these color grades. Infrared absorption analysis showed that all were either type IIa or weak type IIb, and photoluminescence spectroscopy revealed that they contained Ni-, Si-, or N-related defects. Their fluorescence and phosphorescence behavior was investigated using ultraviolet excitation from a long-wave/short-wave UV lamp, a DiamondView instrument, and a phosphorescence spectrometer. Key features that reveal the samples’ HPHT synthetic origin are described.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 273-4.
Africa, Botswana
deposit - Karowe
Abstract: In November 2015, Lucara Diamond’s operation at the Karowe mine in Botswana gained notoriety due to the extraction of a series of large colorless diamonds, including the 1,109 ct Lesedi La Rona and the 812 ct Constellation. The Lesedi La Rona marks the largest gem diamond recovered since the Cullinan (3,106 ct) in 1905. The Constellation, considered to be the seventh-largest recorded diamond, attained the highest price ever paid for a rough, selling for $63.1 million ($77,649 per carat). Additionally, three other significant colorless diamonds were recovered during the same period, weighing 374, 296, and 183 ct. Due to the similarity in their external characteristics— which include cleavage faces—as well as their extraction locations and dates, it was suspected that these stones might have originated from a larger rough that had broken. Lucara demonstrated that the 374 ct diamond and the Lesedi La Rona fit together, yet a large cleavage plane is still unaccounted for. GIA was able to study several rough and/or faceted pieces of these five diamonds using a range of spectroscopic and imaging techniques to gain insight into the presence and distribution of point defects in these diamonds. Diamonds are commonly classified according to their nitrogen content measured by Fourier-transform infrared (FTIR) spectroscopy: Type I diamonds contain nitrogen in either isolated (Ib) or aggregated (IaAB) forms, while type II diamonds do not contain detectable nitrogen concentrations (IIa) but may contain boron (IIb). Analysis of faceted stones cut from the Lesedi La Rona indicates that the rough is a mixed-type diamond, containing both type IIa and pure type IaB regions. These types of diamonds, though exceedingly unusual, have been observed at GIA and reported by Delaunay and Fritsch (2017). The Constellation and the 374, 296, and 183 ct diamonds were determined to be type IaB, containing 20 ± 4 ppm B-aggregates (N4V), in agreement with the concentration for the type IaB pieces of the Lesedi La Rona. Pure type IaB diamonds such as these are actually quite rare, accounting for only 1.2% of a random suite of 5,060 large (>10 ct) D-to-Z diamonds submitted to GIA, whereas 24.6% were type II. Photoluminescence spectra further confirmed analogous defect content for the five large Karowe diamonds, with emissions from H4 (N4V2 0, 496 nm), H3 (NVN0, 503 nm), 505 nm, NV– (637 nm), and GR1 (V0, 741 nm) defects showing similar relative intensities and peak widths. Even for diamonds of the same type, parallel defect content and characteristics across such a variety of defects is unlikely for unrelated stones. The external morphologies of the diamonds showed primary octahedral, resorbed, and fractured faces, with the Constellation and the 296 ct diamond featuring fractures containing metallic inclusions and secondary iron oxide staining. Deep UV fluorescence (< 230 nm) imaging elucidated the internal growth structures of the samples. For the Constellation and the 374, 296, and 183 ct diamonds, at least two growth zones with differing blue fluorescence intensities were observed within single pieces. Combined with the spectroscopic data, these results provide compelling evidence that the Lesedi La Rona, the Constellation, and the 374, 296, and 183 ct diamonds from Karowe had comparable growth histories and likely originated from the same rough, with a combined weight of at least 2,774 ct.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307.
Africa, Sierra Leone
deposit - zimmi
Abstract: Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/? centers, and H3 (NVN0). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.
Diamonds & Related Materials, In press available, 30p. Pdf
Global
synthetics
Abstract: Type IIb diamonds are those that contain more boron than nitrogen. The presence of this uncompensated boron gives rise to absorption in the infrared part of the electromagnetic spectrum, extending into the visible region and often resulting in blue colouration. Here we report on the expansion of the DiaMap freeware (for the automated spectral deconvolution of Type I [nitrogen containing] diamonds) to work on Type IIb diamonds, returning concentrations from three boron-related absorption bands, and determining which band provides the most reliable value. The program uses the calibration coefficients of Collins (2010), which show good relative agreement between the three bands, but might require some further study to confirm their absolute accuracy to the uncompensated boron concentration. The methodology of DiaMap_IIb is applicable to all Type IIb diamonds, both natural and synthetic. Analysis of high-resolution Fourier-transform infrared (FTIR) maps of two high-pressure high-temperature (HPHT) synthetic diamonds using DiaMap_IIb, confirm the growth sector dependence of the boron incorporation. Partitioning of boron strongly favours the octahedral {111} sectors.
Abstract: [Si-V]- is a well-known defect in diamond. It has a great potential in electronics application and also is a very important feature for gem diamond identification in separating natural from synthetic. It is common in CVD diamonds (Martineau et al., 2004), can be doped into HPHT synthetic diamonds (Sittas, et al., 1996), and also occurs in natural type IIa diamonds (Breeding and Wang, 2008). Recent study of [Si-V]- distribution (emissions at 736.6 and 736.9 nm) in HPHT synthetic diamonds confirmed that it was concentrated in the {111} growth sectors. Identical distribution pattern of Nii+, which has doublet emissions at 882.1 and 883.7 nm, was observed (Johnson et al., 2015), strongly indicating that [Si-V]- and Nii+ have the same behavior during HPHT diamond growth. However, little is known how [Si-V]- is incorporated in natural diamond lattice. In this report, we studied the distribution of [Si-V]- defect in natural diamonds and its correlation with other emissions (defects). Seven natural type IIa gem diamonds were analyzed in this study. A common feature of this group of diamonds is occurrence of euhedral olivine inclusions, which is very rare among natural type IIa diamonds and good evidence that these IIa diamonds were formed in the lithospheric mantle. Occurrence of [Si-V]- in these stones were detected among enormous natural type IIa diamonds routinely analyzed in GIA laboratories. Distribution of [Si-V]-, Nii+ and many other emissions were mapped over the table faces using various laser excitations at liquid nitrogen temperature. It was found that intensities of [Si-V]- emission varied significantly over a small area. The distribution patterns were irregular and changed from stone to stone. Distribution of Nii+ emissions showed irregular patterns, but entirely different from that of [Si-V]-. There is no correlation in spatial distribution of these two defects in natural type IIa diamonds, in drastic contract to that observed in HPHT synthetics. Possible causes of the contrast behaviors of [Si-V]- distributions in natural and HPHT synthetic diamonds, and their implications in diamond formation and gem diamond identification will be discussed.
Contributions to Mineralogy and Petrology, Vol. 175, 12 pdf
Mantle
peridotites
Abstract: Experiments were performed from 950 to 1250 °C and 1.5-2.4 GPa to determine the effect of pressure (P) on the temperature (T)-dependent partitioning of Al between olivine and spinel, using mixtures of natural spinel, olivine, clino- and ortho-pyroxene. When compared to 100 kPa experiments, the results show that there is no discernible effect of pressure on the Al-in-olivine thermometer at PT conditions relevant to the spinel peridotite facies. In our experiments with high-Cr spinel, we see no change in Al in olivine from starting values, likely due to the refractory nature of high-Cr spinel. Phase boundary flourescence prevented accurate quantification of Ca in olivine in the run products by electron microprobe analysis but measurements by laser ablation are consistent with the Köhler and Brey (Geochim Cosmochim Acta 54:2375-2388, 1990) Ca-in-olivine thermobarometer. The combination of Al (for T) and Ca (for P) in olivine thus has great potential for thermobarometry in spinel facies peridotites. As a test we apply this approach to published high precision Ca and Al data for olivine from the Ray Pic spinel peridotite xenoliths from the Massif Central (De Hoog et al. Chem Geol 270:196-215, 2010). Reassuringly, the calculated PT conditions (1.0-1.8 GPa; 900-1080 °C) for all samples lie beneath the Moho, within the spinel peridotite facies and fall along a geophysically constrained geotherm.
