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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Xenoliths
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 announcements called 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 Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
A xenolith is literally a "foreign" fragment of rock enclosed within an igneous rock. A xenolith is a foreign collection of crystals, compared to a "xenocryst" which is an individual diamond. A diamond is thus never a xenolith, but it is a xenocryst within a kimberlitic magma. A piece of harzburgite or eclogite found within a kimberlite is a xenolith, though, ironically, a diamond found inside such a xenolith is not a xenocryst because its crystal formed in conjunction with the less illustrious crystals of elements other than carbon. The operative concept is that of an already formed rock being physically introduced into a rock that is still at the formative stage, which is a magma that has not yet chilled into a igneous rock. Every magmatic intrusion will engulf already crystallized rocks (or cemented ones in the case of sedimentary rocks) as it works its way to the surface of the earth, but what makes the xenoliths found within a rapidly emplaced intrusion such as a kimberlite or lamproite so much more interesting is that they are like little suitcases of original rock plucked from the mantle as deep as 200 km. From an economic perspective mantle xenoliths are relevant to diamonds because they are the source rocks within which diamond crystals formed, but, because of the rapid ascent of kimberlitic magmas which can prevent these "suitcases" from being re-melted or otherwise transformed, xenoliths found within kimberlites are windows into the mantle.
Mineralogie des Megacristaux des Xenolithes Eclogitiques Etgranulitiques et des Inclusions Cristallines dans Les Diamants Provenant de la Kimberlite du Kasai
Ph.d. Thesis, University Cath. Louvain, Louvain-la-neuve, Belgiqu,
Cr- Rich Spinel and Garnet in Two Peridotite Xenoliths From the Frank Smith Mine South Africa: Significance of Al and Chromium Distribution between Spinel and Garnet.
Speculations concerning the importance of metasomatic melt migration In the formation of pyroxenite sheets in garnet peridotite xenoliths from MatsokuLes
Proceedings of the Fourth International Kimberlite Conference, Held Perth, Australia, No. 16, pp. 184-186
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
Diffusion gradients in an eclogite xenolith from theRoberts Victorkimberlite pipe: 1. Mechanism- Evolution of garnet exsolution in Al2O3 richclinopyroxene
Journal of Petrology, Vol. 29, No. 6, December pp. 1325-1352
Samarium-neodymium (Sm-Nd) systematics in eclogite and garnet peridotite nodules fromkimberlites: implications for the early differentiation of the earth #2
Geological Society of Australia Inc. Blackwell Scientific Publishing, No. 14, Vol. 2, pp. 864-876
Osmimum, Strontium, neodymium and lead isotope systematics of Southern african peridotite xenoliths: implications for the chemical evolution of subcontinental mantle
Geochimica et Cosmochimica Acta, Vol. 53, pp. 1583-1595
Comment on lower crustal evolution under central Arizona: Strontium, neodymium, and lead isotopic and geochemical evidence from the mafic xenoliths of Camp Creek
Earth and Planetary Science Letters, Vol. 99, pp. 400-409
Studies on the defect structures and metasomatism of olivine and pryoxenein lherzolite xenoliths from basalts in Fujian and Zhejiang Provinces, southeastern China
International Mineralogical Association Meeting Held June, 1990 Beijing China, Vol. 1, extended abstract p. 363-364
Osmium, Strontium, neodymium and lead isotopic studies of Montana mantle xenoliths: long term preservation of basalt depleted, LIL element-enriched lithosphere
Geological Society of America Annual Meeting Abstract Volume, Vol. 23, No. 5, San Diego, p. A 211
Early to mid-Tertiary inverted metamorphic gradient under the ColoradoPlateau: evidence from eclogite xenoliths in ultramafic microbreccias, Navajo volcanic field
Journal of Geophysical Research, Vol. Paper # 91JB00284
Low -Ca garnet harzburgite xenoliths from southern Africa: abundance, composition, and bearing on the structure and evolution of the subcratoniclithosphere
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 350-352
Strontium, neodymium, lead isotopes in ultramafic xenoliths of Cenozoic volcanic rocks of eastern China: implications for EMI and EMII domains in subcontinental lithosphere
Proceedings of the 29th International Geological Congress. Held Japan, Vol. 2, abstract p. 545
Kimberlite xenocrysts from the Porcupine Dome diatremes east-centralMontana: evidence for multiple sources of garnet and evaluation of diamondpotential
Geological Society of America Annual Abstract Volume, Vol. 25, No. 6, p. A98 abstract only
Ionov, D.A., Ashchepkov, I.V., Stosch, H.G., et al.
Garnet peridotite xenoliths from the Vitim volcanic field, Baikal region:the nature of the garnet-spinel peridotite transition zone in the continentalmantle.
Journal of Petrology, Vol. 34, No. 6, pp. 1141-1175.
O xenolito de Granada lherzolito de Tres Ranchos 4: uma rocha matriz dodiamante na provincia magmatica Cretacea do Alto Paranaiba, Goias. (in Portugese).
Brasiliao Geologi do Diamante UFMT., Esp. 2/93, Cuabe, pp. 3-16.
light rare earth element (LREE) distribution in accessory minerals from southwest Ugand an xenoliths and their kamafugite hosts: an electron microprobe study
Rare earth Minerals: chemistry, origin and ore deposits, International Geological Correlation Programme (IGCP) Project, pp. 73-75. abstract
The nature of Archean and Proterozoic lithospheric mantle and lower crust in West Greenland illustrated by the geochemistry and petrography of xenoliths from kimberl
Ph.D. Thesis University of Copenhangen, Denmark, 289p.
Mineral chemistry of spinel peridotite xenoliths from Baengnyeong Island, South Korea, and its applications for the paleogeotherm of the uppermost mantle.
Comtrasting Group I and Group II eclogite xenolith petrogenesis: petrological, trace element and isotopic evidence from eclogite, garnet websterite and akremite
Journal of Petrology, Vol. 46, 10, Oct. pp. 2059-2090.
Processes and mechanism of Mesozoic lithospheric thinning in eastern North Chin a Craton: evidence from Mesozoic igneous rocks and deep seated xenoliths.
Earth Science Frontiers, Vol. 11, 4, pp. 309-318. Ingenta 1045384777
Os Hf Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: effects of pervasive metasomatism in shallow refractor
Earth and Planetary Science Letters, Vol. 241, 1-2, pp. 47-64.
Hydrogen loss from olivines in mantle xenoliths from Simcoe (USA) and Mexico: mafic alkalic magma ascent rates and water budget of sub-continental lithosphere
Earth and Planetary Science Letters, Vol. 242, 3-4, pp. 302-319.
Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula ( Russia): evidence from spinel lherzolite and wehrlite xenoliths.
Mechanism and timing of lithospheric modification and replacement beneath the eastern North Chin a Craton: peridotitic xenoliths from the 100 Ma Fuxin basalts
Geochimica et Cosmochimica Acta, In press, available
Mechanism and timing of lithospheric modification and replacement beneath the eastern North Chin a Craton: peridotitic xenoliths from the 100 Ma Fuxin basalts...
Geochimica et Cosmochimica Acta, Vol. 71, 21, pp. 5303-5225.
Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths ( Tok Cenozoic volcanic field, SE Siberia).
Earth and Planetary Science Letters, Vol. 272, 1-2, pp. 65-77.
Harvey, J., Yoshikawa, M., Hammond, S.J., Burton, K.W.
Deciphering the trace element characteristics in Kilbourne Hole peridotite xenoliths: melt-rock interaction and metasomatism beneath the Rio Grande rift, SW USA.
Economic Geology Research Institute 2015, Vol. 17,, # 1147, 1p. Abstract
China
Peridotite, xenoliths
Abstract: The mantle process during the Early Permian Tarim plume event is revealed by flood basalt and mantle xenoliths. Permian Tarim flood basalts have typical two pulses' eruption. The first pulse of the Tarim flood basalt was erupted at 291-290Ma, characterized by OIB-like Zr/Nb (~5.83), Nb/La and Ce/Pb ratios, and PUM-like initial 187Os/188Os ratios (0.1308-0.1329). They're plotted along a 290±11Ma isochron, implying a pristine "plume mantle" source. The second pulse of the Tarim flood basalt was erupted at 283-281 Ma, with Zr/Nb (~13.6), Nb/La and Ce/Pb ratios similar or close to the lower crust and initial 187Os/188Os ratios (0.1743~19.6740) that deviated from the ~290 Ma isochron line, indicative of significant crustal assimilation. Mantle-derived peridotite and pyroxenite xenoliths hosted in Cenozoic alkali basalts (~20 Ma) are found in the Xikeer, western Tarim Block. Based on their petrographic and geochemical characteristics, peridotite xenoliths can be divided into three groups. Group 1 peridotites, with the presence of the high Mg-number of olivines (91-93) and spinel-pyroxenes clusters, experienced high-degree melt extraction (~17% fractional melting) from garnet- to spinel-stable field. Groups 2 and 3 peridotites, characterized by the clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively, underwent extensive silicate melt metasomatism at low melt/rock ratios (15) and that the host basanite is incapable of being the metasomatic agent. The Re-Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an isochron of 290±11 Ma, virtually identical to the age of Tarim flood basalts. Their PUM-like Os initial ratios and convecting mantle-like É?Nd(t=290 Ma) strongly suggest that the Xikeer mantle xenoliths derive from the plume mantle. We propose that the Xikeer xenolith suite recorded mantle 'auto-refertilization' process, i.e., they may have been initially formed by melt extraction from the convecting mantle and, shortly after, was refertilized by plume melts during the Early Permian.
The effects of melt depletion and metasomatism on highly siderophile and strongly chalcophile elements: S-Se-Te-Re-PGE systematics of peridotite xenoliths from Kilbourne Hole, New Mexico.
Geochimica et Cosmochimica Acta, Vol. 166, pp. 210-233.
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.
Geological Society of London, Chapter 2, pp. 27-48.
Australia, Victoria
xenoliths
Abstract: Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53–13.63 wt%) and crystallized at ~1300–1500 °C and 16–30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950–1100 °C and 12–18 kba
Physics and Chemistry of Minerals, Vol. 45, 3, pp. 237-247.
Technology
xenolths
Abstract: We determined elastic constants of a single-crystal chromian spinel at temperatures from ?15 to 45 °C through the Rectangular Parallelepiped Resonance method. The sample is a natural chromian spinel, which was separated from a mantle xenolith. Elastic constants at an ambient temperature (T = 24.0 °C) are C 11 = 264.8(1.7) GPa, C 12 = 154.5(1.8) GPa and C 44 = 142.6(0.3) GPa. All the elastic constants decrease linearly with increasing temperature. The temperature derivatives are dC 11/dT = ?0.049(2) GPa/°K, dC 12/dT = ?0.019(1) GPa/°K and dC 44/dT = ?0.020(1) GPa/°K. As an implication of the elastic constants, we applied them to the correction of a fluid inclusion geobarometry, which utilizes residual pressure of fluid inclusion as a depth scale. Before entrainment by a magma, the fluid inclusions must have the identical fluid density in constituent minerals of a xenolith. It has been, however, pointed out that fluid density of fluid inclusions significantly varies with host mineral species. The present study elucidates that elastic constants and thermal expansion coefficients cannot explain the difference in fluid density among mineral species. The density difference would reflect the difference in the degree of plastic deformation in the minerals.
Geochimica et Cosmochimica Acta, doi.org/10.1016/j.gca.2018.08.010 78p.
Canada, Nunavut, Parry Peninsula. Central Victoria Island
xenoliths
Abstract: While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ?2?Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ?2?Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.
Abstract: The mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, yet much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions surrounding these cratons. However, it is critically important to establish the nature of the relationship between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula (PP) and Central Victoria Island (CVI), Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Slave craton, where the presence of cratonic mantle has been proposed. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle geotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the PP and CVI terranes extended well into the diamond stability field at the time of kimberlite eruption, consistent with the recovery of diamonds from both kimberlite fields. Bulk Se, Te, and highly siderophile element abundance systematics, plus Re-Os isotope age data suggest that the mantle beneath these parts of Arctic Canada formed at ~2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of evidence for peridotitic diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, the highly depleted mantle compositions beneath both regions developed during mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay- Great Bear-Hottah arc systems. These terranes were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with significant diamond potential.
Abstract: We present a global compilation of major element, as well as Re-Os isotope, data on mantle xenoliths from continental lithosphere to constrain the secular evolution of mantle depletion since the early Archean. Whereas a temporal dichotomy in the degree of mantle depletion has long been recognized in previous regional studies of mantle xenoliths, this global compilation reveals, for the first time, a smooth secular trend in mantle depletion, which is in remarkable agreement with what is expected from the secular cooling of the ambient mantle as inferred from the petrology of non-arc basalts. Depleted mantle now composing continental lithosphere is likely to have been originally formed beneath mid-ocean ridges or similar spreading environments, and a greater degree of depletion in the past can be seen as a corollary of the secular cooling of the mantle.
Abstract: Small amounts of H2O, on the order of tens to hundreds of parts per million, can significantly influence the physical properties of mantle rocks. Determining the H2O contents of nominally anhydrous minerals (NAMs) is one relatively common technique that has been applied to estimate mantle H2O contents. However, for many mantle NAMs, the relation between H2O activity and H2O content is not well known. Furthermore, certain mantle minerals may be prone to H2O loss during emplacement on Earth’s surface. The goal of this study is to apply mineral equilibria to estimate values of aH2O in rocks that originated below the Moho. The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel ± garnet were used to estimate values of temperature (T), pressure (P), aH2O, hydrogen fugacity (fH2), and oxygen fugacity (fO2) in 11 amphibole-bearing mantle xenoliths from the southwestern U.S.A. Application of amphibole dehydration equilibria yields values of aH2O ranging from 0.05 to 0.26 for these 11 samples and the compositions of coexisting spinel + olivine + orthopyroxene yield ?logfO2 (FMQ) of -1 to +0.6. For nine of the samples, values of fH2 were estimated using amphibole dehydrogenation equilibria, and these values of fH2 ranged from 6 to 91 bars. Values of fH2 and fO2 were combined, using the relation 2H2O = 2H2 + O2, to estimate a second value of aH2O that ranged from 0.01 to 0.57 for these nine samples. Values of aH2O, estimated using these two methods on the same sample, generally agree to within 0.05. This agreement indicates that the amphibole in these samples has experienced little or no retrograde H-loss and that amphibole equilibria yields robust estimates of aH2O that, in these xenoliths, are generally <0.3, and are often 0.1 or less.
Geochemistry, Geophysics, Geosystems, Vol. 20, 2, pp. 952-973.
United States, New Mexico
xenoliths
Abstract: Elemental and isotopic compositions of volatile species such as halogens, noble gases, hydrogen, and carbon can be used to trace the evolution of these species in the Earth. Halogens are important tracers of subduction recycling of surface volatiles into the mantle: however, there is only limited understanding of halogens in the mantle. Here we provide new halogen data of mantle xenoliths from intraplate settings. The mantle xenoliths show a wide range of halogen elemental ratios, which are expected to be related to later processes after the xenoliths formed. A similar primary halogen component is present in the xenoliths sampled from different localities. This suggests that the mantle has the uniform halogen composition over a wide scale. The halogen composition in the convecting mantle is expected to have remained constant over more than 2 billion years, despite subduction of iodine?rich halogens. We used mass balance calculations to gain understanding into evolution rate of I/Cl ratio in the mantle. Calculations suggest that, in order to maintain the I/Cl ratio of the mantle over 2 Gyr, the I/Cl ratio of the subducted halogens must be no more than several times higher than the present?day mantle value.
Geochemical Perspective Letters, Vol. 10, pp. 43-48. doi:10.7185/ geochemlet.1904
Mantle
xenoliths
Abstract: Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.
Contributions to Mineralogy and Petrology, Vol. 174, 5, 27p.
Africa, Ethiopia
xenoliths
Abstract: The behavior of sub-continental lithospheric mantle (SCLM) in extensional settings, up to successful rifting, plays an important role in geodynamics and in the global carbon cycle, yet the underlying processes and rates of lithosphere destruction remain poorly constrained. We determined platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) abundances and Re-Os-isotope systematics for well-characterized mantle xenoliths hosted in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia to provide new insights on the nature and timing of processes leading to the formation and transformation of the off-cratonic lithospheric mantle beneath the East Africa rift system (EARS). The whole-rock PGE concentrations are highly variable, with total PGE abundances ranging from 6.6 to 12.6 ppb for Gundeweyn, 11.5 to 23.3 ppb for Dillo, and 9.9 to 19.4 ppb for Megado mantle xenoliths. The 187Os/188Os ratios of the whole-rock mantle xenoliths vary from 0.1180 to 0.1287 for Gundeweyn, 0.1238 to 0.1410 for Dillo and 0.1165 to 0.1277 for Megado, compared to 0.130 for the Afar plume and???0.14 for the Kenya plume, with Re depletion ages up to 1.45 Ga for Gundeweyn, 0.64 Ga for Dillo, and 1.65 Ga for Megado mantle xenoliths. The regional differences between refertilizing agents recorded in mantle xenoliths from the plateau area and the rift systems reflect distinct tectonomagmatic settings: (1) low PGE abundances, with some retention of low 187Os/188Os in Gundeweyn peridotites, are ascribed to scavenging by early small-volume oxidizing melts, generated in the convecting mantle ahead of the arrival of the Afar plume. (2) Percolation of late-stage silicate/basaltic melts, associated with the arrival of hot mantle plume and lithosphere thinning in the rift setting, locally led to refertilization and sulfide precipitation and partial replenishment of the PGE (Dillo), with convecting mantle-like 187Os/188Os. Local enclaves of older, cryptically metasomatised mantle with unradiogenic Os (Megado) attest to the heterogeneous nature of melt-peridotite interaction at this stage (pervasive vs. focused melt flow). Highly depleted abundances of the compatible PGE are characteristic of SCLM affected by incipient rifting and percolation of oxidizing melts, here associated with the Afar and Kenya plume beneath the East Africa rift, and may be precursors to advanced degrees of lithosphere destruction/transformation.
Geochemistry, Geophysics, Geosystems, Vol. 20, 9, pp. 4426-4456.
Russia
xenoliths
Abstract: This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500-700 °C. These fluids are derived from seawater that was transported to 30? to 50?km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka?Pacific Plate boundary that led to an increase in temperature and the formation of small quantities of melt that crystallized to a distinctive group of secondary minerals that are present in the samples and that postdate (overprint) the initial effects of fluid addition. The final step in the evolution of the samples was infiltration by an Fe? and Mg?rich magma that crystallized principally amphibole?group minerals.
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.
Geochimica et Cosmochimica Acta, Vol. 286, pp. 29-83. pdf
Canada, Northwest Territories
xenoliths
Abstract: Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (?145?km), oxidized ultra-depleted layer; the deeper (?145-180?km), reduced less depleted layer; and an ultra-deep (?180?km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30-145, 110-225, 105-285, 2-105?ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138?ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
Abstract: The accretionary mobile belts surrounding ancient cratonic cores are an important facet of the growth and preservation of continental landmasses. Peridotites from Nuominhe in the Xing'an Mongolia Orogenic Belt (XMOB) provide an additional opportunity to examine the age, structure and evolution of mantle lithosphere separating two of the largest existing ancient continental nuclei: the North China Craton and the Siberian Craton. This suite of mantle rocks comprises fertile to refractory garnet- and spinel-facies harzburgites and lherzolites. Their lithophile element systematics show that the peridotites were metasomatized to variable extent by silicate?carbonate melts. Despite this, the highly siderophile element and Os isotope systematics appear to have been largely undisturbed. The Nuominhe peridotites have Re-depletion Os model ages (TRD) that range from 0.5 Ga to 2.4 Ga, with three peaks/major ranges at ~2.0-2.4 Ga, ~1.4-1.5 Ga and ~ 0.8 Ga, of which the latter two are closely similar to those data from other XMOB localities reported in the literature. The only section of the mantle that appears to have ages which correlate with crust formation is the suite with Neoproterozoic (~0.8 Ga) depletion ages, while the older mantle domains document older episodes of mantle depletion. Given the lack of correlation between equilibrium temperatures and bulk composition or TRD ages, the Nuominhe peridotites were inter-mixed in the mantle column, most likely as a result of incorporation of recycled older continental mantle fragments into juvenile Neoproterozoic mantle during the orogenic processes responsible for new lithosphere formation. Geothermobarometry of the Nuominhe peridotites indicates a conductive geotherm of ~60 mWm?2 and therefore a lithosphere thickness of ~125 km, which is thicker than most Phanerozoic continental terranes, and even thicker than Proterozoic regions that comprise the larger cratonic unit of the Siberian craton. This thick Proterozoic lithosphere sandwiched between the converging North China and Siberian cratons was evidently partly constructed from recycled refractory continental mantle fragments, perhaps extant in the convecting mantle, or in-part derived from the surrounding cratons, leading to a composite nature of the mantle in this re-healed continental suture. Re-accretion of recycled refractory old continental mantle fragments plays a significant role in affecting mantle composition and controlling the thickness of circum-cratonic landmasses between cratonic blocks.
