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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Magmatism
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.
Articles tagged with the key word magmatism tend to deal with the behavior of magmas within the earth's crust either as mountain building batholiths or volcanic eruptions, which distinguishes them from articles tagged as melting which deal with magma formation. None of these predominantly scientific articles have anything to do with diamonds unless they deal with diamond bearing kimberlitic or lamproitic magmas.
An Experimental and Theoretical Analysis of Partial Melting in the System Kalsio4 Cao Mgo Sio2 Cos and Applications to The Genesis of Potassic Magmas, Carbonatites and Kimberlites.
Proceedings of Third International Kimberlite Conference, Vol. 1, PP. 359-369.
Cawthorn, R.G., Maske, S., de Wit, M., Groves, D.I., Cassidy, K.
Mineralogical geochemical indicators of the formation conditions of apatite bearing carbonatites of the Arbarastakh Massif,Southern Yakutia (USSR).(Russian)
Alkaline and carbonatitic magmatism of the earth and related ore deposits.International Geological Correlation Programme (IGCP)Proposal Project 314. 1991-1995
Graphite melt equilibration temperatures during mantle melting: constraints on CO2 in Mid Ocean Ridge Basalt (Mid Ocean Ridge Basalt (MORB))magmas and carbon content..
Chemical Geology, Vol. 147, No. 1-2, May 15, pp. 89-98.
The megacryst bearng Barium, Strontium, and light rare earth element (LREE) rich lamprophyres from northwest Ladoga, Baltic Shield: melt evolution and nature..
Mineralogical Magazine, Goldschmidt abstract, Vol. 62A, p. 1139-40.
Magma compositions and genesis of the rocks of the Mushugai Khuduk carbonatite bearing alkalic complex ( southern Mongolia): evidence from melt inclusions.
Periodico di Mineralogia, (in english), Vol. LXX11, 1. April, pp. 95-105.
Arzamastsev, A.A., Bea, F., Arzamastseva, L.V., Montero, P.
Devonian plume magmatism in the NE Baltic Shield: rare earth elements in rocks and minerals of ultrabasic alkaline series as indicators of magma evolution.
Deep Seated Magmatism, magmatism sources and the problem of plumes., pp. 42-68.
Geochemistry and mineralogy of HFSE in intracratonic carbonatites: implications for their economic potential (on the example of Kola alkaline province).
GAC Annual Meeting Halifax May 15-19, Abstract 1p.
Crystallization of Cr poor and Cr rich megacrysts suites from the host kimberlite magma: implications for mantle structure and generation of kimberlite magmas.
Contributions to Mineralogy and Petrology, On line
Shang, C.K., Satir, M., Siebel, W., Nsifa, E.N., Taubald, H., Ligeois, J.P., Tchoua, F.M.
TTG Magmatism in the Congo Craton: a view from major and trace element geochemistry, Rb Sr Sm Nd systematics: case of the Sangmelima region, Ntem Complex
Journal of African Earth Sciences, Vol. 39, 3-5, pp. 61-79.
Evidence for the resemblance of the subcontinental lithospheric mantle in the areas of kimberlite lamproite magmatism: constraints on the evolution of the Siberian Craton.
Deep Seated Magmatism, magmatism sources and the problem of plumes., pp. 132-147.
The farthest record of the Central Atlantic Magmatic Province into West Africa craton: precise 40 Ar 39 Ar dating and geochemistry of Taoudenni basin intrusives
Earth and Planetary Science Letters, Vol. 235, 1-2, pp. 391-407.
Magmatic evolution of the differentiated ultramafic, alkaline and carbonatite intrusion of Vuoriyarvi ( Kola Peninsula) Russia, A LA-ICP-MS study of apatite.
Post collisional, potassic and ultrapotassic magmatism of the northern Tibetan Plateau: constraints on characteristics of the mantle source, geodynamic uplift
Geochemistry and petrogenesis of neoArchean high Mg low Ti mafic igneous rocks in and intracratonic setting, Central India craton: evidence for boninite magmatism.
Proterozoic evolution of the western margin of the Wyoming Craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains.
Canadian Journal of Earth Sciences, Vol. 43, 10, pp. 1601-1619,
Trumbull, R.B., Reid, D.L., De Beer, C., Van Acken, D., Romer, R.L.
Magmatism and continental breakup at the west margin of southern Africa: a geochemical comparison of dolerite dikes from northwestern Namibia and the Western Cape.
South African Journal of Geology, Vol. 110, 2-3, Sept. pp. 477-502.
Kaeser, B., Olker, B., Kait, A., Altherr, R., Pettke, T.
Pyroxenite xenoliths from Marsabit ( northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite
Contributions to Mineralogy and Petrology, Vol. 157, 4, pp. 453-472.
Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil: subducted protoliths, carbonated melts and primary kimberlite ..
Contributions to Mineralogy and Petrology, Vol. 160, 4, pp. 489-50.
Interrelated structural chemical model to predict and calculate viscosity of magmatic melts and water diffusion in a wide range of compositions and T-P parameters
Russian Geology and Geophysics, Vol. 50, 12, pp. 1079-1090.
Li, L-M., Sun, M., Wang, Y., Xing, G., Zhao, G., Cai, K., Zhang, Y.
Geochronological and geochemical study of Paleproterozoic gneissic granites and clinopyroxenite xenolths from NW Fujian: implications for crustal evol.
Journal of Asian Earth Sciences, Vol. 41, 2, pp. 204-212.
Azzone, R.G., Enrich, G.E.R., De Barros Gomes, C., Ruberti, E.
Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: the Juquia mafic-ultramafic alkaline-carbonatite massif, SE Brazil.
Journal of South American Earth Sciences, in press available, 17p.
Eocene potassic magmatism of the Milk River area southern Alberta NTS 72E and Sweet Grass Hills, northern Montana: overview and new dat a on mineralogy, geochemistry, petrology and economic potential.
Alberta Geological Survey, Open file report 2012-01, 96p. Free pdf
Late Ediacaran ( 605-580 Ma) post-collisional alkaline magmatism in the Arabian-Nubian shield: a case study of Serbal ring-shaped intrusion, southern Sinai, Egypt.
Journal of Asian Earth Sciences, Vol. 77, pp. 203-223.
Ernst, R.E., Pereira, E., Hamilton, M.A., Pisarevsky, S.A., Rodriques, J., Tasinari, C.C.G., Teixeira, W., Van-Dunem, V.
Mesoproterozoic intraplate magmatic 'barcode' record of the Angola portion of the Congo craton: newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna ( Columbia) supercontinent reconstructions.
Chrome spinel hosted melt inclusions in Paleoproterozoic primitive volcanic rocks, northern Finland: evidence for coexistence and mixing of komatiitic and picritic magmas.
Neoproterozoic alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton: petrography, geochemistry, and geochronology.
Russian Geology and Geophysics, Vol. 53, 11, pp. 1176-1196.
Linking together the dissolution and reaction features of kimberlite hosted diamond and Fe-Ti oxides with magmatic fluid and its role in kimberlite emplacement.
Persikov, E., Bukhtiyarov, P., Skol, A., Palyanov, Y.
Viscosity of kimberlite and basaltic magmas to 10 Gpa and 2000K.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Rooney, T.O., Nelson, W.R., Dosso, L., Furman, T., Hanan, B.
The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates, East African Rift zone.
Abstract: The asthenosphere—derived from the Greek asthen?s, meaning weak—is the uppermost part of Earth's mantle, right below the tectonic plates that make up the solid lithosphere. First proposed by Barrell 100 years ago (1), the asthenosphere has traditionally been viewed as a passive region that decouples the moving tectonic plates from the mantle and provides magmas to the global spreading ridge system. Recent studies suggest that the asthenosphere may play a more active role as the source of the heat and magma responsible for intraplate volcanoes. Furthermore, it may have a major impact on plate tectonics and the pattern of mantle flow.
Progress in Earth and Planetary Science, Vol. 2, 22p.
Mantle
Magmatism
Abstract: Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H 2 O mol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si 4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from 29 Si single-pulse magic-angle spinning nuclear magnetic resonance ( 29 Si SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions.
Abstract: The architecture of subsurface magma plumbing systems influences a variety of igneous processes, including the physiochemical evolution of magma and extrusion sites. Seismic reflection data provides a unique opportunity to image and analyze these subvolcanic systems in three dimensions and has arguably revolutionized our understanding of magma emplacement. In particular, the observation of (1) interconnected sills, (2) transgressive sill limbs, and (3) magma flow indicators in seismic data suggest that sill complexes can facilitate significant lateral (tens to hundreds of kilometers) and vertical (<5 km) magma transport. However, it is often difficult to determine the validity of seismic interpretations of igneous features because they are rarely drilled, and our ability to compare seismically imaged features to potential field analogues is hampered by the limited resolution of seismic data. Here we use field observations to constrain a series of novel seismic forward models that examine how different sill morphologies may be expressed in seismic data. By varying the geologic architecture (e.g., host-rock lithology and intrusion thickness) and seismic properties (e.g., frequency), the models demonstrate that seismic amplitude variations and reflection configurations can be used to constrain intrusion geometry. However, our results also highlight that stratigraphic reflections can interfere with reflections generated at the intrusive contacts, and may thus produce seismic artifacts that could be misinterpreted as real features. This study emphasizes the value of seismic data to understanding magmatic systems and demonstrates the role that synthetic seismic forward modeling can play in bridging the gap between seismic data and field observations.
Journal of Volcanology and Geothermal Research, Vol 304, pp. 1-10.
Mantle
Magmatism
Abstract: For a typical poroelastic shallow crustal magma chamber, about 0.1% of the mafic magma and about 4% of the felsic magma are erupted and/or injected during magma-chamber rupture. Magma chambers with volumes of the order of several tens to several hundred cubic kilometres, as are common, are thus sufficiently large to supply magma to small or moderate eruptions. For large eruptions, however, a much higher percentage of the magma volume must be squeezed out of the chamber. For an ordinary poroelastic chamber, the excess pressure in the chamber falls exponentially during the eruption. For a large eruption to be possible, however, the excess pressure must be essentially maintained until the very end of the eruption. Here I show that caldera collapse can maintain the excess pressure through forced magma-chamber volume reduction, in which case a resulting large eruption would be the consequence (not the cause) of the collapse. I also show that ring-fault dip partly controls the size and intensity (volumetric flow or effusion rate) of the eruption. If the ring-fault dips inward (a normal fault), the displacement is ‘stable’, the volumetric flow rate (intensity) remains essentially constant during the collapse, and the magma chamber remains active following the collapse. By contrast, if the ring-fault dips outward (a reverse fault), the displacement is ‘unstable’, the volumetric flow rate normally increases dramatically during the collapse, and the magma chamber may be entirely destroyed during the collapse.
Abstract: Magma dominantly exists in a slowly cooling crystal-rich or mushy state1, 2, 3. Yet, observations of complexly zoned crystals4, some formed in just one to ten years5, 6, 7, 8, 9, as well as time-transgressive crystal fabrics10 imply that magmas mix and transition rapidly from a locked crystal mush to a mobile and eruptable fluid5, 6. Here we use a discrete-element numerical model that resolves crystal-scale granular interactions and fluid flow, to simulate the open-system dynamics of a magma mush. We find that when new magma is injected into a reservoir from below, the existing magma responds as a viscoplastic material: fault-like surfaces form around the edges of the new injection creating a central mixing bowl of magma that can be unlocked and become fluidized, allowing for complex mixing. We identify three distinct dynamic regimes that depend on the rate of magma injection. If the magma injection rate is slow, the intruded magma penetrates and spreads by porous media flow through the crystal mush. With increasing velocity, the intruded magma creates a stable cavity of fluidized magma that is isolated from the rest of the reservoir. At higher velocities still, the entire mixing bowl becomes fluidized. Circulation within the mixing bowl entrains crystals from the walls, bringing together crystals from different parts of the reservoir that may have experienced different physiochemical environments and leaving little melt unmixed. We conclude that both granular and fluid dynamics, when considered simultaneously, can explain observations of complex crystal fabrics and zoning observed in many magmatic systems.
Geological Society of America Special Paper, No. 514, pp. SPE514-19.
Russia, Siberia
Magmatism
Abstract: The Siberian craton was affected by flood basalt volcanism at least twice during the Devonian (Yakutsk-Vilyui province) and Permian-Triassic (Siberian province) periods. In both cases volcanism appeared as brief pulses of flood basalt eruptions, followed by kimberlitic (and lamproitic) emplacement. Pressure estimations for the kimberlite-entrained mantle xenoliths reflect that the lithosphere was 190-230 km thick at the time of the Devonian flood basalt volcanism. Differently from Devonian kimberlites, the majority of Triassic kimberlites are diamond free, but at least one Triassic kimberlite pipe and some lamproites are diamondiferous, suggesting that the Siberian lithosphere remained thick during the Permian-Triassic flood basalt volcanic activity. If both the lithosphere and the asthenosphere were volatile poor, thick cratonic lithosphere prevented melting even at an elevated geotherm. During the Paleozoic, Siberia was surrounded by subduction systems. The water deep cycle in association with fast subduction and slab stagnation in the mantle transition zone is proposed to cause fluxing of the asthenosphere by water plus other fluids via wet diapir formation in the mantle transition zone. Such diapirs started to melt in the asthenosphere beneath thick cratonic lithosphere, producing voluminous melts. Mafic melts probably accumulated beneath cratonic lithosphere and rapidly erupted on the surface in response to stress-induced drainage events, as assumed for some other cratonic flood basalts.
Abstract: Baddeleyite is a frequently found accessory mineral in silica undersaturated lavas. Because it is typically enriched in uranium, while having low initial lead, baddeleyite has long been a prime target for U-Pb geochronology of mafic rocks. The difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence, however, have limited baddeleyite chronology largely to coarse-grained mafic intrusive rocks. Here, the development of U-Th in situ baddeleyite analysis using secondary ionization mass spectrometry (SIMS) is presented together with an assessment of baddeleyite occurrence in Quaternary silica-undersaturated lavas from Campi Flegrei (Naples, Italy). Samples studied comprise the pre- and post Campanian Ignimbrite (ca. 40 ka) lava domes of Cuma and Punta Marmolite, and Astroni and Accademia, respectively. The in situ sample preparation required initial identification of baddeleyite crystals from sawed and polished rock billets using scanning electron microscope (SEM) backscatter imaging and energy-dispersive X-ray analysis. U-Th baddeleyite isochron ages for intra-caldera Astroni and Accademia lava domes are 5.01+2.61?2.55 ka (MSWD = 2.0; n = 17) and 4.36+1.13?1.12 ka (MSWD = 2.9; n = 24), respectively. The ages for Punta Marmolite (62.4+3.9?3.8 ka; MSWD = 1.2; n = 11) and Cuma (45.9+3.6?3.5 ka; MSWD = 2.2; n = 11) predate the Campanian Ignimbrite. Rapid baddeleyite crystallization at the time of emplacement is supported by petrologic observations that >50% of the baddeleyite crystals documented in this study occur either in vesicles or in vesicle-rich regions of the host lavas whose textures developed over timescales of a few years to a few decades based on microlite crystal size distribution (CSD) analysis. Radiometric U-Th baddeleyite ages are mostly in agreement with previously determined K-Ar eruption ages, except for the Punta Marmolite lava dome whose U-Th baddeleyite age predates the K-Ar age by ca. 15 ka. Baddeleyite thus records eruptive emplacement with little evidence for significant pre-eruptive crystal residence, and has potential as an eruption chronometer for Quaternary silica-undersaturated volcanic rocks.
Abstract: The number of studies investigating the vesiculation of natural samples and their implications to volcanic degassing and eruption mechanisms has been growing rapidly within the last decades. To interpret the natural rock textures, the geoscience community has produced a range of experimental and theoretical data sets on bubble nucleation, growth, and coalescence in magmatic systems. A robust experimental database is required to calibrate (theoretical and empirical) modeling approaches, which allow the calculation of magma ascent rates from volcanic ejecta mainly by the determination of the bubble number density (BND). Although, the available data set is still limited, it already shows that variations in melt (and volatile/fluid) composition can have a significant effect. In this manuscript we (re-)evaluate the existing experimental data set, while focusing mainly on the review and discussion of continuous decompression experiments. One aim of this review article is to encourage scientists to fill the gaps in the existing experimental data sets and help to acknowledge, use, and further develop the most promising experimental techniques. Therefore, we highlight different methods and discuss their advantages and possible limitations. We also discuss possible ways of how to better account for the influence of melt composition in models, which link BND to decompression rate.
Abstract: Concentrations of H2O, F, Cl, C, P, and S have been measured by secondary ion mass spectrometry (SIMS) in experimentally produced peridotite phases (including clinopyroxene, orthopyroxene, olivine, garnet, amphibole, and mica) and coexisting basanitic glasses. Because only two experiments produced glasses on quenching (with the melt phase in others reverting to felt-like crystallite masses) H2O concentrations in melts were also separately determined from mass-balance relationships and by assuming constant H2O/La in melts and starting materials. The resulting values were used to calculate mineral/melt partition coefficients (D values) for H2O [where DH2Ocrystal/melt = (mass fraction of H2O in crystal)/(mass fraction of H2O in melt)] for conditions of 1025-1190 °C and 1.0-3.5 GPa. These gave 0.0064-0.0164 for clinopyroxene, 0.0046-0.0142 for orthopyroxene, 0.0015-0.0016 for olivine, and 0.0016-0.0022 for garnet. Although less information was obtained for the other volatiles, F was found to be significantly more compatible than H2O during peridotite melting, whereas Cl is significantly less compatible. S also has small but appreciable solubilities in amphiboles and micas, but not in pyroxenes or olivine. The solubility of C in silicate minerals appears to be negligible, although C was present in coexisting melts (~0.5 wt% as CO2) and as residual graphite during experiments. The D values for H2O in clinopyroxene and orthopyroxene are positively correlated with ivAl but negatively correlated with the H2O concentrations of melts (when considered as wt%). These relationships are consistent with the broad trends of previously published partitioning data. Although some of the concentration dependence can be related to cross-correlation between ivAl in pyroxenes and H2O concentrations in melts (via the latter’s control of liquidus temperatures) this relationship is too inconsistent to be a complete explanation. A concentration dependence for DH2Omineral/melt can also be independently predicted from speciation models for H2O in silicate melts. Thus it is likely that DH2Opyx/melt is influenced by both ivAl and the absolute concentration of H2O in melts. DH2O/DCe for clinopyroxene is inversely correlated with M2 site radii. Because the latter decrease with increasing pressure and temperature, relatively hot and/or deeply derived melts should be enriched in Ce relative to H2O when compared to melts from cooler and shallower mantle sources. Conversely, melts from H2O-rich settings (e.g., subduction zones) should have higher H2O/Ce than their source rocks. When combined with previously obtained partitioning data for non-volatile elements (from the same experiments), our data are consistent with the enrichment of intraplate basalt sources in both volatile and non-volatile incompatible elements by small-degree melts derived from local mid-ocean ridge basalt sources. In this way, volatiles can be seen to play an active role (via their promotion of partial-melting and metasomatic processes) in the auto-regulation of incompatible element concentrations in the depleted upper mantle.
Abstract: Volcanic eruptions often occur simultaneously1, 2, 3, 4 or tap multiple magma reservoirs5, 6. Such lateral interactions between magmatic systems are attributed to stress changes7, 8 or hydraulic connections4, 8, 9, 10 but the precise conditions under which coupled eruptions occur have yet to be quantified. Here we use interferometric synthetic aperture radar satellite data to analyse the surface deformation generated by volcanic unrest in the Kenyan Rift. We identify several magma sources located at depths of 2-5?km; importantly, sources that are spaced less than about 10?km apart interact, whereas those spaced more than about 25?km apart do not. However, volcanoes up to 25?km apart have interacted in the geologic past1, 11. Thus, volcanic coupling is not simply controlled by the distance between the magma reservoirs. We then consider different tectonic settings globally, including intraplate volcanoes such as Hawaii and Yellowstone, arc volcanism in Alaska and Chile, and other rift settings, such as New Zealand, Iceland and Afar. We find that the most closely spaced magmatic interactions are controlled by the extent of a shallow crystal mush layer, stress changes can couple large eruptions over distances of about 20-40?km, and only large dyke intrusions or subduction earthquakes could generate coupled eruptions over distances of about 50-100?km.
Abstract: Knowing the density of silicate liquids at high pressure is essential to answer questions relevant to the presence of magmas at depth, whether that be in the present Earth or in its earliest times, during differentiation of the planet. Melts have unique physical and chemical properties, which vary as a function pressure, and chemical composition. The focus here will be on in situ measurements of the density of magmas, with a presentation of the available methods and of the main results obtained so far, including why some magmas may be trapped at depth. Understanding the macroscopical physical properties of magmas requires an accurate microscopic structural description. Structural descriptions of compressed magmas are becoming more widely available, from experiments and from theoretical calculations. These structural inputs are used to understand the compression mechanisms at stake in the densification of magmas, e.g. the collapse of voids, coordination increase for the major cations, and bond compressibility. These densification processes profoundly affect not only the physical properties of the melt, but also its chemical properties, i.e. the way element partition between the magma and a metallic melt or between the magma and crystals.
Contributions to Mineralogy and Petrology, Vol. 171, 45p.
Mantle
Magmatism
Abstract: The origin and source rocks of alkali-rich and SiO2-undersatured magmas in the Earth’s upper mantle are still under debate. The garnet signature in rare earth element patterns of such magmas suggests a garnet-bearing source rock, which could be garnet lherzolite or garnet pyroxenite. Partial melting experiments were performed at 2.8 GPa and 1345-1445 °C in a piston-cylinder using mixtures of natural lherzolite with either 0.4 wt% H2O and 0.4 wt% CO2 or 0.7 wt% H2O and 0.7 wt% CO2. Different designs of AuPd capsules were used for melt extraction. The most successful design included a pentagonally shaped disc placed in the top part of the capsule for sufficient melt extraction. The degrees of partial melting range from 0.2 to 0.04 and decrease with decreasing temperature and volatile content. All samples contain olivine and orthopyroxene. The amounts of garnet and clinopyroxene decrease with increasing degree of partial melting until both minerals disappear from the residue. Depending on the capsule design, the melts quenched to a mixture of quench crystals and residual glass or to glass, allowing measurement of the volatile concentrations by Raman spectroscopy. The compositions of the partial melts range from basalts through picrobasalts to foidites. Compared to literature data for melting of dry lherzolites, the presence of H2O and CO2 reduces the SiO2 concentration and increases the MgO concentration of partial melts, but it has no observable effect on the enrichment of Na2O in the partial melts. The partial melts have compositions similar to natural melilitites from intraplate settings, which shows that SiO2-undersaturated intraplate magmas can be generated by melting of garnet lherzolite in the Earth’s upper mantle in the presence of H2O and CO2.
Geophysical Research Letters, Vol. 43, 8, pp. 3717-3726.
Europe, Greenland
Magmatism
Abstract: Understanding the evolution of extinct ocean basins through time and space demands the integration of surface kinematics and mantle dynamics. We explore the existence, origin, and implications of a proposed oceanic slab burial ground under Greenland through a comparison of seismic tomography, slab sinking rates, regional plate reconstructions, and satellite-derived gravity gradients. Our preferred interpretation stipulates that anomalous, fast seismic velocities at 1000-1600?km depth imaged in independent global tomographic models, coupled with gravity gradient perturbations, represent paleo-Arctic oceanic slabs that subducted in the Mesozoic. We suggest a novel connection between slab-related arc mantle and geochemical signatures in some of the tholeiitic and mildly alkaline magmas of the Cretaceous High Arctic Large Igneous Province in the Sverdrup Basin. However, continental crustal contributions are noted in these evolved basaltic rocks. The integration of independent, yet complementary, data sets provides insight into present-day mantle structure, magmatic events, and relict oceans.
Annual Review of Earth and Planetary Sciences, Vol. 43, pp. 299-331.
Mantle
Magmatism
Abstract: Continental magmatic arcs form above subduction zones where the upper plate is continental lithosphere and/or accreted transitional lithosphere. The best-studied examples are found along the western margin of the Americas. They are Earth's largest sites of intermediate magmatism. They are long lived (tens to hundreds of millions of years) and spatially complex; their location migrates laterally due to a host of tectonic causes. Episodes of crustal and lithospheric thickening alternating with periods of root foundering produce cyclic vertical changes in arcs. The average plutonic and volcanic rocks in these arcs straddle the compositional boundary between an andesite and a dacite, very similar to that of continental crust; about half of that comes from newly added mafic material from the mantle. Arc products of the upper crust differentiated from deep crustal (>40 km) residual materials, which are unstable in the lithosphere. Continental arcs evolve into stable continental masses over time; trace elemental budgets, however, present challenges to the concept that Phanerozoic arcs are the main factories of continental crust.
Annual Review of Earth and Planetary Sciences, Vol. 43, pp. 431-458.
Mantle
Magmatism
Abstract: Magma fragmentation is the breakup of a continuous volume of molten rock into discrete pieces, called pyroclasts. Because magma contains bubbles of compressible magmatic volatiles, decompression of low-viscosity magma leads to rapid expansion. The magma is torn into fragments, as it is stretched into hydrodynamically unstable sheets and filaments. If the magma is highly viscous, resistance to bubble growth will instead lead to excess gas pressure and the magma will deform viscoelastically by fracturing like a glassy solid, resulting in the formation of a violently expanding gas-pyroclast mixture. In either case, fragmentation represents the conversion of potential energy into the surface energy of the newly created fragments and the kinetic energy of the expanding gas-pyroclast mixture. If magma comes into contact with external water, the conversion of thermal energy will vaporize water and quench magma at the melt-water interface, thus creating dynamic stresses that cause fragmentation and the release of kinetic energy. Lastly, shear deformation of highly viscous magma may cause brittle fractures and release seismic energy.
Annual Review of Earth and Planetary Sciences, Vol. 43, pp. 167-205.
Mantle
Magmatism
Abstract: The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10 -20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO2. Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO2 contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust.
Annual Review of Earth and Planetary Sciences, Vol. 43, pp. 363-404.
Mantle
Magmatism
Abstract: We review data and recent research on arc composition, focusing on the relatively complete arc crustal sections in the Jurassic Talkeetna arc (south central Alaska) and the Cretaceous Kohistan arc (northwest Pakistan), together with seismic data on the lower crust and uppermost mantle. Whereas primitive arc lavas are dominantly basaltic, the Kohistan crust is clearly andesitic and the Talkeetna crust could be andesitic. The andesitic compositions of the two arc sections are within the range of estimates for the major element composition of continental crust. Calculated seismic sections for Kohistan and Talkeetna provide a close match for the thicker parts of the active Izu arc, suggesting that it, too, could have an andesitic bulk composition. Because andesitic crust is buoyant with respect to the underlying mantle, much of this material represents a net addition to continental crust. Production of bulk crust from a parental melt in equilibrium with mantle olivine or pyroxene requires processing of igneous crust, probably via density instabilities. Delamination of dense cumulates from the base of arc crust, foundering into less dense, underlying mantle peridotite, is likely, as supported by geochemical evidence from Talkeetna and Kohistan. Relamination of buoyant, subducting material—during sediment subduction, subduction erosion, arcarc collision, and continental collision—is also likely.
Geophysical Research Letters, Vol. 43, 8, pp. 3717-3726.
Europe, Greenland
Magmatism
Abstract: Understanding the evolution of extinct ocean basins through time and space demands the integration of surface kinematics and mantle dynamics. We explore the existence, origin, and implications of a proposed oceanic slab burial ground under Greenland through a comparison of seismic tomography, slab sinking rates, regional plate reconstructions, and satellite-derived gravity gradients. Our preferred interpretation stipulates that anomalous, fast seismic velocities at 1000 -1600?km depth imaged in independent global tomographic models, coupled with gravity gradient perturbations, represent paleo-Arctic oceanic slabs that subducted in the Mesozoic. We suggest a novel connection between slab-related arc mantle and geochemical signatures in some of the tholeiitic and mildly alkaline magmas of the Cretaceous High Arctic Large Igneous Province in the Sverdrup Basin. However, continental crustal contributions are noted in these evolved basaltic rocks. The integration of independent, yet complementary, data sets provides insight into present-day mantle structure, magmatic events, and relict oceans.
Omarini, R.H., Gasparon, M., De Min, A.M., Comin-Chiaramonti, P.
An overview of the Mesozoic-Cenozoic magmatism and tectonics of Eastern Paraguay and central Andes ( western Gondwana): implications for the composition of mantle sources.
Journal of South American Earth Sciences, In press available, 19p.
Geochimica et Cosmochimica Acta, Vol. 196, pp. 36-57.
Africa, Namibia, Angola
Paran-Etendeka, Karoo
Abstract: We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Parana ´-Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely re?ect those of the host whole-rocks, except in a small subset a?ected by hydrothermal alteration. The Parana ´-Etendeka picrites and ferropicrites are petrogenet- ically related to the more evolved and voluminous ?ood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation. The incompatible trace element homogeneity of olivine-hosted melt inclusions in Parana ´-Etendeka and Karoo primitive magmatic rocks has also been identi?ed in other CFB provinces and contrasts with ?ndings from studies of basalts from mid- ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Gala ´pagos, Samoa), may re?ect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Parana ´-Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.
Journal of South American Earth Sciences, Vol. 72, pp. 302-314.
South America, Paraguay
Magmatism
Abstract: The amalgamation of the Western Gondwana (including the Greater Gondwana supercraton) occurred at 600 Ma during the Brazilian - Pan African orogeny. A plate junction related to this event is marked by the Transbrazilian lineament which separates the South American continent into two sectors: the Eastern Paraguay-Brazilian and Central Andean domains. An overview of the geodynamic data from these two sectors indicates that the two domains were subjected to distinct evolutions from the Proterozoic to the present. The Andean domain is characterized by long-lived subduction processes linked to the convergence and consequent collision of microplates since the Middle Proterozoic (western Amazonian Craton) with a peak at about 600-580 Ma. The Paraguay-Brazilian domain remained relatively stable but was affected by extension episodes that reactivated ancient (Early and Middle Proterozoic) suture zones. These different geodynamic evolutions seem to reflect broadly distinct mantle compositions. In the subduction zones of the Andean domain the mantle was deeply modified by metasomatic processes following the subduction of oceanic plates. Consequently, the Andean type magma sources show a clear HIMU imprint inherited from the MORB, whereas the Paraguay-Brazilian sector shows a prevalent EMI and subordinate EMII character. The petrological data mainly from Mesozoic and Cenozoic magmatic events in the two sectors are reviewed to investigate the current mantle plume and mantle dome models for the uprising of the asthenospheric (or sub-lithospheric) material.
Abstract: Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150-250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1-6.5 GPa and T = 1200-1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1-6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100-250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.
Abstract: We estimate density and P-wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P-wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P-wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250???300 km. These values correlate with high-velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean Săo Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high-density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re-Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P-wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere. The high-density PMP lithosphere is presently gravitationally unstable and prone to delamination.
Abstract: Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian-Ludicovian LIP in the Fennoscandian Shield, the Permian-Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ?20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.
Geophysical Research Letters, Vol. 7, 10.1002/ 2016GL068424
Europe, Greenland
Magmatism
Abstract: Understanding the evolution of extinct ocean basins through time and space demands the integration of surface kinematics and mantle dynamics. We explore the existence, origin, and implications of a proposed oceanic slab burial ground under Greenland through a comparison of seismic tomography, slab sinking rates, regional plate reconstructions, and satellite-derived gravity gradients. Our preferred interpretation stipulates that anomalous, fast seismic velocities at 1000-1600?km depth imaged in independent global tomographic models, coupled with gravity gradient perturbations, represent paleo-Arctic oceanic slabs that subducted in the Mesozoic. We suggest a novel connection between slab-related arc mantle and geochemical signatures in some of the tholeiitic and mildly alkaline magmas of the Cretaceous High Arctic Large Igneous Province in the Sverdrup Basin. However, continental crustal contributions are noted in these evolved basaltic rocks. The integration of independent, yet complementary, data sets provides insight into present-day mantle structure, magmatic events, and relict oceans.
Abstract: The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (?11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ~40 and ~2,600?Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ~2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between ?8.6‰ and +5.5‰, with all of the young (<300?Ma) carbonatites characterized by more positive ?11B values (>?4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the ?11B value for asthenospheric mantle of ?7 ± 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.
Abstract: This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380-390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume-lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.
Abstract: Crystals within volcanic rocks contain records of the changing chemical and thermal conditions within the magma reservoirs in which they resided before eruption. Observations from these crystal records place fundamental constraints on the processes operating within the reservoirs. Data from volcanic crystals are in accord with recent conceptual models of magma reservoirs being composed dominantly of crystal mushes, with small volumes and/or small fractions of melt present. The implication is that magma reservoirs have differing modes of behavior: magmas are stored over the long term in largely crystalline, quiescent, conditions, punctuated by brief episodes of intense activity during the decades to centuries immediately prior to an eruption.
Abstract: The role of volatiles in magma dynamics and eruption style is fundamental. Magmatic volatiles partition between melt, crystal, and vapor phases and, in so doing, change magma properties. This has consequences for magma buoyancy and phase equilibria. An exsolved vapor phase, which may be distributed unevenly through reservoirs, contains sulfur and metals that are either transported into the atmosphere or into ore deposits. This article reviews the controls on volatile solubility and the methods to reconstruct the volatile budget of magmas, focusing particularly on the exsolved vapor phase to explore the role of volatiles on magma dynamics and on eruption style.
Abstract: Magmas are erupted from a wide range of depths. Olivine compositions, for example, indicate magma storage in the lower crust and upper mantle, while clinopyroxene and amphibole record middle to upper crust storage. Pre-eruptive magmas also often cool by 100-300?°C, frequently at middle-upper crust depths, indicating clogged, ephemeral volcanic pathways. These coolings imply that mafic recharge is not a sufficient cause for eruption and that crystallization-induced vapor saturation is a more proximal eruption trigger. But an improved understanding of eruption mechanisms require precise identifications of what are herein termed "ultimate", "proximal," and "immediate" causes of eruption.
Abstract: Magma systems that supply volcanoes can extend throughout the crust and consist of mush (melt within a crystalline framework) together with ephemeral magma accumulations. Within a crystal-rich mush, slow processes of melt segregation and heat loss alternate with fast processes of destablisation and magma transport. Magma chambers form by two mechanisms: incremental magma intrusion into sub-solidus rocks or the segregation and rapid merging of melt-rich layers within mush regions. Three volcanic states reflect alternations of slow and fast processes: dormancy, unrest and eruption. Monitoring needs to detect processes of melt and fluid movements in the lower and middle crust during destabilisation to improve forecasting.
Abstract: The Hadean-Archaean transition is poorly known because of the dearth of Hadean rocks. A new conceptual model is presented based on variations in mantle potential temperature (Tp) with time. The critical issue is the depth of melting with respect to a negatively buoyant magma sink between 410 and 330 km (14-11 GPa). Hadean plume magmatism begins below the magma sink, leading to generation of a refractory upper mantle reservoir and the minor production of boninite-like magmas near the surface. With cooling, the onset of melting migrates above the magma sink, a situation likely occurring since 3.9 Ga and corresponding to Tps of ~1870°C or less. Therefore, a burst of mafic to ultramafic volcanism was produced at 3.9-3.8 Ga. This extensive volcanism may have triggered gravitational instabilities and favoured the recycling of the Hadean crust into the mantle. Results of this model are discussed in the light of existing isotopic data.
Contributions to Mineralogy and Petrology, in press available 22p.
Mantle
Metasomatism, magmatism, carbonatite
Abstract: Hydrous K-rich kimberlite-like systems are studied experimentally at 5.5-7.5 GPa and 1200-1450 °C in terms of phase relations and conditions for formation and stability of phlogopite. The starting samples are phlogopite-carbonatite-phlogopite sandwiches and harzburgite-carbonatite mixtures consisting of Ol + Grt + Cpx + L (±Opx), according to the previous experimental results obtained at the same P-T parameters but in water-free systems. Carbonatite is represented by a K- and Ca-rich composition that may form at the top of a slab. In the presence of carbonatitic melt, phlogopite can partly melt in a peritectic reaction at 5.5 GPa and 1200-1350 °C, as well as at 6.3-7.0 GPa and 1200 °C: 2Phl + CaCO3 (L)?Cpx + Ol + Grt + K2CO3 (L) + 2H2O (L). Synthesis of phlogopite at 5.5 GPa and 1200-1350 °C, with an initial mixture of H2O-bearing harzburgite and carbonatite, demonstrates experimentally that equilibrium in this reaction can be shifted from right to left. Therefore, phlogopite can equilibrate with ultrapotassic carbonate-silicate melts in a ? 150 °C region between 1200 and 1350 °C at 5.5 GPa. On the other hand, it can exist but cannot nucleate spontaneously and crystallize in the presence of such melts in quite a large pressure range in experiments at 6.3-7.0 GPa and 1200 °C. Thus, phlogopite can result from metasomatism of peridotite at the base of continental lithospheric mantle (CLM) by ultrapotassic carbonatite agents at depths shallower than 180-195 km, which creates a mechanism of water retaining in CLM. Kimberlite formation can begin at 5.5 GPa and 1350 °C in a phlogopite-bearing peridotite source generating a hydrous carbonate-silicate melt with 10-15 wt% SiO2, Ca# from 45 to 60, and high K enrichment. Upon further heating to 1450 °C due to the effect of a mantle plume at the CLM base, phlogopite disappears and a kimberlite-like melt forms with SiO2 to 20 wt% and Ca# = 35-40.
Earth and Planetary science Letters, Vol. 466, pp. 152-167.
Mantle
Metasomatism, magma, carbonatite
Abstract: Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ??Hf??Hf coupled with View the MathML source>+5‰?7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (?3.6 to View the MathML source?2.4‰?13C for carbonatites versus ?5.7 to View the MathML source?3.6‰?13C for kimberlites) require open-system fractionation at magmatic temperatures.
Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of ‘pure’ carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly differentiated ‘cratonic carbonatites’ have only little in common with those of metasomatic agents that act on the deeper lithosphere. Consequently, carbonatite trace element systematics should only be used with caution when constraining carbon mobility and metasomatism at mantle depths. Regardless of the exact nature of carbonate-bearing melts within the mantle lithosphere, they play an important role in enrichment processes, thereby decreasing the stability of buoyant cratons and promoting rift initiation - as exemplified by the Mesozoic-Cenozoic breakup of the North Atlantic craton.
Abstract: Volcanic eruptions of basalt are fed by intrusions of magma, called dikes, which advance through Earth’s crust for a few hours or days before reaching the surface. Although many never make it that far, those that do can pose a serious threat to people and infrastructure, so forecasting when and where a dike will erupt is important to assessing volcanic hazards. However, the migration of magma below a volcano is complex, and its simulation is numerically demanding, meaning that efforts to model dike propagation have so far been limited to models that can quantify either a dike’s velocity or its trajectory but not both simultaneously. To overcome this limitation, Pinel et al. have developed a hybrid numerical model that quantifies both by dividing the simulations into two separate steps, one that calculates a two-dimensional trajectory and a second that runs a one-dimensional propagation model along that path. The results indicate that the migration of magma is heavily influenced by surface loading—the addition or removal of weight on Earth’s surface—such as that caused by the construction of a volcano or its partial removal via a massive landslide or caldera eruption. The team confirmed previous research that showed that increasing surface load attracts magma while also reducing its velocity, whereas unloading diverts much of the magma. To test their approach, the team applied their model to a lateral eruption that occurred on Italy’s Mount Etna in July 2001. The eruption was fed by two dikes, including one that in its final stages clearly slowed down and bent toward the west while still 1-2 kilometers below the surface. The results showed that the two-step model was capable of simulating that dike’s velocity and trajectory and thus offers a new means of constraining the local stress field, which partially controls these properties. In the future, report the authors, more complex versions of this model that incorporate information on local topography and magmatic properties could be integrated with real-time geophysical data to improve forecasts of when and where a propagating dike could erupt at the surface.
Abstract: Carbon isotope compositions and the distribution of nitrogen and hydrogen in diamonds from 18 eclogites from Nurbinskaya kimberlites were studied in situ in polished plates. Cathodoluminescence images show that most of the diamonds have complex growth structures with distinctive cores, intermediate and rim zones. In some diamonds the cores display dissolution features, and intermediate growth zones are separated from the cores by narrow rounded oscillatory zones. At least three crystals show interrupted multistage diamond growth; variations in ? ąłC of 2–3‰ occur across the contacts between distinct zones. Generally, ?ąłC within the diamond cores varies only by 1–2‰, in rare cases up to 3.3‰. ?ąłC values are usually lower in the intermediate zones and drop further towards the rims by up to 3‰. High-resolution SIMS profiles show that variations in ?ąłC across the diamond growth zones are sharp with no evidence of diffusive relaxation.
Abstract: Magmas forming large igneous provinces (LIP) on continents are generated by extensive melting in the deep crust and underlying mantle and associated with break-up of ancient supercontinents, followed by formation of a new basaltic crust in the mid-oceanic rifts. A lack of the unifying model in understanding the sources of LIP magmatism is justified by lithological and geochemical complexity of erupted magmas on local (e.g. a cross-section) and regional (a single and different LIP) scales. Moreover, the majority of LIP rocks do not fit general criteria for recognizing primary/primitive melts (i.e. < 8 wt% MgO and absence of high-Fo olivine phenocrysts). This study presents the mineralogical (olivine, Cr-spinel, orthopyroxene), geochemical (trace elements and Sr-Nd-Hf-Pb isotopes) and olivine-hosted melt inclusion compositional characteristics of a single primitive (16 wt% MgO), high-Ti (2.5 wt% TiO2) picrite with high-Mg olivine (up to 91 mol% Fo) from the Letaba Formation in the ~ 180 Ma Karoo LIP (south Africa). The olivine compositions (unusually high ?18O (6.17‰), high NiO (0.36–0.56 wt%) and low MnO and CaO (0.12–0.20 and 0.12–0.22 wt%, respectively)) are used to argue for a non-peridotitic mantle source. This is supported by the enrichment of the rock and melts in most incompatible trace elements and depletion in heavy rare earth elements (e.g. high Gd/Yb) that reflects residual garnet in the source of melting. The radiogenic isotopes resemble those of the model enriched mantle (EM-1) and further argue for a long-term enrichment of the source in incompatible trace elements. The enriched high-Ti compositions, strongly fractionated incompatible trace elements, presence of primitive olivine and high-Cr spinel in the Letaba picrites are closely matched by olivine-phyric rocks from the ~ 260 Ma Emeishan (Yongsheng area, SW China) and ~ 250 Ma Siberian (Maimecha-Kotuy region, N Siberia) LIPs. However, many other compositional parameters (e.g. trace element and ?18O compositions of olivine phenocrysts, Fe2 +/Fe3 + in Cr-spinel, Sr-Nd-Hf isotope ratios) only partially overlap or even diverge. We thus imply that parental melts of enriched picritic rocks with forsteritic olivine from three major continental igneous provinces – Karoo, Emeishan and Siberia cannot be assigned to a common mantle source and similar melting conditions. The Karoo picrites also exhibit some mineralogical and geochemical similarities with rocks and glasses in the south Atlantic Ridge and adjacent fracture zones. The geodynamic reconstructions of the continental plate motions since break-up of the Gondwanaland in the Jurassic support the current position of the source of the Karoo magmatism in the southernmost Atlantic. Co-occurrence of modern and recent anomalous rocks with normal mid-ocean ridge basalts in this region can be related to blocks/rafts of the ancient lithosphere, stranded in the ambient upper mantle and occasionally sampled by rifting-related decompressional melting.
Abstract: The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30–40 million years after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in the protoliths to the melting assemblages. Here, we report on a series of high-pressure experiments at 1–3 GPa and 900 to 975 °C on the melting behaviour of natural phyllite from Serbia, which serves as a proxy for Balkan upper continental crust. Hydrous granitic melts are present in all runs (68 wt% SiO2, ~ 4–5 wt% K2O, Mg# < 54 and ~ 5 wt% H2O). Garnet, quartz/coesite, plagioclase, K-feldspar, biotite/phengite, clinopyroxene and sillimanite/kyanite, and accessory phases including zircon, rutile, ilmenite, apatite and monazite occur in the charges. LA-ICP-MS analyses establish that the melts are extremely enriched in LILE (except for Sr), Th and U, but depleted in Nb and Zr, with LREE higher than HREE. Accessory phases accommodate several trace elements, especially HFSE and REE. Partition coefficients for some trace elements between residue and crustal melts are close to 1, contrasting strongly with melts of peridotite. Our dataset indicates that the direct melting of upper continental crust alone would generate siliceous, high-K magmas with enriched LILE, Th and U, but cannot explain the high Th/La fingerprint of K-rich lavas of the Alpine-Himalayan orogenic belt. We demonstrate that the Alpine-Himalayan orogenic volcanics attribute their unusual trace element geochemistry to the involvement of lawsonite blueschists that are imbricated together with extremely depleted fore-arc peridotites to form new lithosphere in the source region. There is no need or evidence for deep subduction in which a succession of additional reactions would only serve to modify and dilute the high Th/La signature.
Journal of the Geological Society, Vol. 174, pp. 609-626.
South America, Uruguay
magmatism
Abstract: The crystalline basement of Uruguay was assembled during the Brasiliano Orogeny in the Neoproterozoic Era and was later affected by discrete tectonic activity. A new multi-method low-temperature dataset including (U–Th)/He ages from both zircon and apatite, T–t modelling and K–Ar dating of fine sericite fractions and fault gouge reveal a detailed post-orogenic geological history spanning the Phanerozoic Eon. The juxtaposition of the terranes that compose the area was achieved in the Ediacaran Period, and post-collision was marked by intense exhumation, in which the crystalline basement reached near-surface conditions by the early to mid-Palaeozoic. Regional subsidence promoted sedimentation in the Paraná Basin until the Permian, covering and reheating much of the basement that is at present exposed. Afterwards, deposition and volcanism were mostly confined to its current limits. Regional exhumation of the shield during the Permo-Triassic exposed much of the northern portion of the basement, and the south was further affected by the opening of the South Atlantic Ocean during the Mesozoic. Little exhumation affected the Uruguayan Shield during the Cenozoic, as reflected in its modest topography. The reactivation of inherited Neoproterozoic structures influenced the development of Mesozoic basins and the present-day landscape.
Abstract: Magmatic processes on Earth govern the mass, energy and chemical transfer between the mantle, crust and atmosphere. To understand magma storage conditions in the crust that ultimately control volcanic activity and growth of continents, an evaluation of the mass and heat budget of the entire crustal column during magmatic episodes is essential. Here we use a numerical model to constrain the physical conditions under which both lower and upper crustal magma bodies form. We find that over long durations of intrusions (greater than 105 to 106?yr), extensive lower crustal mush zones develop, which modify the thermal budget of the upper crust and reduce the flux of magma required to sustain upper crustal magma reservoirs. Our results reconcile physical models of magma reservoir construction and field-based estimates of intrusion rates in numerous volcanic and plutonic localities. Young igneous provinces (less than a few hundred thousand years old) are unlikely to support large upper crustal reservoirs, whereas longer-lived systems (active for longer than 1 million years) can accumulate magma and build reservoirs capable of producing super-eruptions, even with intrusion rates smaller than 10?3 to 10?2?km3?yr?1. Hence, total duration of magmatism should be combined with the magma intrusion rates to assess the capability of volcanic systems to form the largest explosive eruptions on Earth.
Abstract: Located at the northwestern border of the Bohemian Massif in the eastern part of the European Variscides, the Erzgebirge-Krušné hory is one of the most important metallogenic provinces in Europe with a 800-year history of mining. The following rare metal resources are associated with late-Variscan (315 - 280 Ma), postmagmatic mineralization pulses in the Erzgebirge-Krušné hory and surrounded areas: 900 kt Sn, 230 kt W, 10 kt Mo, 1 kt Ta, 300 kt Li, 200 kt Rb, 2 kt Cs, 1.5 kt In, 230 t Ge, 320 t Sc, 14 kt Sb, 10 kt Bi, and 3 kt Ag. At the end of the Variscan Orogeny the regional tectonic regime in Central Europe changed, indicating the beginning of the break-up of the supercontinent. The Late Carboniferous-Early Permian in Europe was a period of widespread basin formation that was associated in many areas with mantle-derived magmatic activity. 300 Ma-old dike swarms in NE England and the Scottish Midland Valley, the Oslo Graben and Scania, radiate from a triple junction in the northernmost part of Jutland. This triple junction marked the axis of a deep-mantle plume centered in this area. In this context it is important to note that the mantle plume center is surrounded by significant lamprophyre intrusions which show in some districts spatial-time relationships to Sn-W-polymetallic, Ag-base metal, and U mineralization. During the Late Carboniferous and Early Permian an extensive magmatic province developed within the present northern and central Europe, intimately with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into Northern Germany. Peak magmatic activity was concentrated in a narrow time-span from 300 to 280 Ma. Simultaneously in Stephanian-Early Permian an intensive bimodal magmatism associated with intra-continental extensional setting occurs in the European Variscides. Permo-Carboniferous volcanism in the Spanish Central System, Iberian Ranges, Cantabrian Chain, Pyrenees and the French Massif Central includes a range of mafic calc-alkaline and shoshonitic rock types, and lamprophyres (spessartites and camptonites) with age data between 300-270 Ma. The Mid-European Variscides show a large number of Permo-Carboniferous magmatic complexes with similar ages (Halle Volcanic Complex, Saar-Nahe Basin, Thuringian Forest, Harz Mts., Northwest-Saxonian Volcanic Complexes, bimodal volcanic rocks of the Sub-Erzgebirge basin and the Rhyolite Complex of Tharandt as well as Li-F-Sn small intrusion granites and lamprophyric intrusions in the Erzgebirge. It is important to note that the late-Variscan W-Mo, Sn-W-Mo, Ag-bearing Sn-In-base metal, Ag-Sb-base metal, and U mineralizations in the Erzgebirge-Krušné hory are spatially and temporal associated with intrusion centers of Permo-Carboniferous post-collisional mafic and rhyolitic (sub)volcanic bimodal magmatism (315-290 Ma) along deep-rooted NW-SE fault zones, especially at the intersections with NE-SW, E-W, and N-S major regional structural zones. The bimodal lamprophyre-rhyolite assemblage in the Erzgebirge / Sub-Erzgebirge basin area was formed during intracontinental rifting in a 'Fast Extension' setting by melting of a metasomatic enriched mantle source. The emplacement of fluid-enriched lamprophyres and F-rich rhyolitic intrusions at the same time is probably associated with decompression melting of updoming asthenosphere which is possibly associated with the above mentioned mantle plume.
Abstract: Direct constraints on the ascent, storage and eruption of mantle melts come primarily from exhumed, long-frozen intrusions. These structures, relics of a dynamic magma transport network, encode how Earth’s crust grows and differentiates over time. Furthermore, they connect mantle melting to an evolving distribution of surface volcanism. Disentangling magma transport processes from the plutonic record is consequently a seminal but unsolved problem. Here we use field data analyses, scaling theory and numerical simulations to show that the size distribution of intrusions preserved as plutonic complexes in the North American Cordillera suggests a transition in the mechanical response of crustal rocks to protracted episodes of magmatism. Intrusion sizes larger than about 100?m follow a power-law scaling expected if energy delivered from the mantle to open very thin dykes and sills is transferred to intrusions of increasing size. Merging, assimilation and mixing of small intrusions into larger ones occurs until irreversible deformation and solidification dissipate available energy. Mantle magma supply over tens to hundreds of thousands of years will trigger this regime, a type of reverse energy cascade, depending on the influx rate and efficiency of crustal heating by intrusions. Identifying regimes of magma transport provides a framework for inferring subsurface magmatic processes from surface patterns of volcanism, information preservation in the plutonic record, and related effects including climate.
Geochemistry, Geophysics, Geosystems: G3, Vol. 18, 7, pp. 2727-2747.
Canada, Somerset Island, Saskatchewan, United States, Kansas
magmatism, convection, diamond genesis
Abstract: Thirty new high-precision U-Pb perovskite and zircon ages from kimberlites in central North America delineate a corridor of mid-Cretaceous (115–92 Ma) magmatism that extends ?4000 km from Somerset Island in Arctic Canada through central Saskatchewan to Kansas, USA. The least contaminated whole rock Sr, Nd, and Hf isotopic data, coupled with Sr isotopic data from groundmass perovskite indicates an exceptionally limited range in Sr-Nd-Hf isotopic compositions, clustering at the low ?Nd end of the OIB array. These isotopic compositions are distinct from other studied North American kimberlites and point to a sublithospheric source region. This mid-Cretaceous kimberlite magmatism cannot be related to mantle plumes associated with the African or Pacific large low-shear wave velocity province (LLSVP). All three kimberlite fields are adjacent to strongly attenuated lithosphere at the edge of the North American craton. This facilitated edge-driven convection, a top-down driven processes that caused decompression melting of the transition zone or overlying asthenosphere. The inversion of ringwoodite and/or wadsleyite and release of H2O, with subsequent metasomatism and synchronous wet partial melting generates a hot CO2 and H2O-rich protokimberlite melt. Emplacement in the crust is controlled by local lithospheric factors; all three kimberlite fields have mid-Cretaceous age, reactivated major deep-seated structures that facilitated kimberlite melt transit through the lithosphere.
Earth and Planetary Science Letters, Vol. 484, pp. 1-14.
Mantle
kimberlite, origin, magmatism
Abstract: Kimberlite magmatism has occurred in cratonic regions on every continent. The global age distribution suggests that this form of mantle melting has been more prominent after 1.2 Ga, and notably between 250-50 Ma, than during early Earth history before 2 Ga (i.e., the Paleoproterozoic and Archean). Although preservation bias has been discussed as a possible reason for the skewed kimberlite age distribution, new treatment of an updated global database suggests that the apparent secular evolution of kimberlite and related CO2-rich ultramafic magmatism is genuine and probably coupled to lowering temperatures of Earth's upper mantle through time. Incipient melting near the CO2- and H2O-bearing peridotite solidus at >200 km depth (1100-1400?°C) is the petrologically most feasible process that can produce high-MgO carbonated silicate melts with enriched trace element concentrations akin to kimberlites. These conditions occur within the convecting asthenospheric mantle directly beneath thick continental lithosphere. In this transient upper mantle source region, variable CHO volatile mixtures control melting of peridotite in the absence of heat anomalies so that low-degree carbonated silicate melts may be permanently present at ambient mantle temperatures below 1400?°C. However, extraction of low-volume melts to Earth's surface requires tectonic triggers. Abrupt changes in the speed and direction of plate motions, such as typified by the dynamics of supercontinent cycles, can be effective in the creation of lithospheric pathways aiding kimberlite magma ascent. Provided that CO2- and H2O-fluxed deep cratonic keels, which formed parts of larger drifting tectonic plates, existed by 3 Ga or even before, kimberlite volcanism could have been frequent during the Archean. However, we argue that frequent kimberlite magmatism had to await establishment of an incipient melting regime beneath the maturing continents, which only became significant after secular mantle cooling to below 1400?°C during post-Archean times, probably sometime shortly after 2 Ga. At around this time kimberlites replace komatiites as the hallmark mantle-derived magmatic feature of continental shields worldwide. The remarkable Mesozoic-Cenozoic ‘kimberlite bloom’ between 250-50 Ma may represent the ideal circumstance under which the relatively cool and volatile-fluxed cratonic roots of the Pangea supercontinent underwent significant tectonic disturbance. This created more than 60% of world's known kimberlites in a combination of redox- and decompression-related low-degree partial melting. Less than 2% of world's known kimberlites formed after 50 Ma, and the tectonic settings of rare ‘young’ kimberlites from eastern Africa and western North America demonstrate that far-field stresses on cratonic lithosphere enforced by either continental rifting or cold subduction play a crucial role in enabling kimberlite magma transfer to Earth's surface.
University of Helsinki, Scientific Report, Vol. 8, 1, DOI:10.1038/s41598-018-23661-3
Africa, Europe, Finland
magmatism
Abstract: In the Karoo large igneous province, the geochemical assessment of mantle source variability and structure is hampered by probable crustal contamination overprinting of compositionally diverse flood basalts. Mantle source characteristics have been defined only for exceptional, primitive rock types. Here I use a compiled dataset for over 800 samples to demonstrate that the abundance of Nb relative to Zr, Ti, and Y provides a useful geochemical tracer of mantle sources for variably contaminated rock types of the Karoo province. Variations in the relative abundance of Nb reveal emplacement of distinctive, Nb-undepleted and Nb-depleted magmas in the North Karoo and South Karoo sub-provinces, respectively, and clarify correlation between flood basalts and previously proposed mantle source components. Judging from plate tectonic reconstructions and the compositions of plausible mantle source components, the geochemical bilateral asymmetry in Karoo may reflect tapping of contrasting plume and upper mantle reservoirs in the two sub-provinces.
Journal of the Indian Institute of Science, 15p. available in press
India
magmatism
Abstract: We report rare occurrence of lamprophyre xenoliths within a host lamprophyre from the Dongargaon area, Deccan Large Igneous Province, NW India. The lamprophyre xenoliths are distinct in texture (grain size) as well in mineralogy from those of their host rock. The clinopyroxene (diopside) in the xenoliths is depleted in Ca and Mg but substantially enriched in Fe compared to those in the host lamprophyre. Mica in the xenoliths is a phlogopite whereas that present in the host rock is compositionally a biotite; spinels in the host lamprophyre are relatively enriched in TiO2. As the host lamprophyre dyke has been dated to be of Eocene (ca. 55 Ma) age, the entrained lamprophyre xenoliths are inferred to represent an earlier pulse of lamprophyre emplacement. The recurrent lamprophyre emplacement in this domain is consistent with the recently brought out polychronous nature of Late Cretaceous alkaline magmatism at the Mundwara and Sarnu Dandali complexes in the NW India and is related to the extensional events linked with the reactivation of the Narmada rift zone.
Geophysical Research , Vol. 20, EGU2018-13349 1p. Abstract
Iceland
magmatism
Abstract: Porous rocks represent the products of all explosive volcanic eruptions. As magma ascends to the Earth’s surface, bubbles form as a consequence of the evolving saturation state of volatiles dissolved in the melt. The presence of pores (either filled with pressurized volatiles or not) strongly controls the rheological behaviour of magma and thus influences all volcanic processes (pre- syn- and post-eruptive). Nevertheless, the effects of porosity on the rheology of magma are not well characterised, and a general parameterization is not available yet. Here we present a new set of experiments designed to investigate the rheology of porous melts at high temperature (750-800
C), low strain rates (10^6-10^7 s^-1) and variable porosity. Experiments were performed at 1 atm using a Setaram Setsys vertical dilatometer. The starting materials are 5 x 5 mm cores of natural rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland (vesicle and crystal-free) initially containing 0.11(4) wt% dissolved H2O. The experimental procedure is composed by two steps: 1) synthesis of bubble-bearing materials by heating and expansion due to foaming; 2) deformation of the foamed samples. During the first step, the obsidian cores are heated above the glass transition temperature to 900- 1050
C and held for set amounts of time (2-24 h); the volume of the foamed samples increases because H2O vapour-filled bubbles nucleate and expand. The change in volume (measured by He-pycnometry) is linked to the change in porosity (10-50 vol%). For the second step, two different experimental strategies are employed, hereafter “single-stage” and “doublestage” measurements. Single-stage measurements involve deformation of the samples directly after foaming (without quenching). The sample is cooled down from the foaming T to different target T (750-800
C), a constant load (150 g) is applied by silica or alumina probes to the core, and the cores deform isothermally for 5-20 hours. Conversely, double-stage measurements involve deformation of previously synthesised and quenched pore-bearing cores. In this case the sample is heated up to the target T and deformed under an applied load for similar amount of time (5-20 hours). In both cases, the variation in length (displacement) and volume (porosity) is continuously recorded and used to calculate the viscosity of the foamed cores using Gent’s equations. Preliminary results suggest for single-stage measurements a lower effect of bubbles on the bulk viscosity, compared to double-stage measurements. We suggest that the different behaviour may be related to the different microstructure of the experimental materials. For single-stage measurements, closed and H2O vapour-filled bubbles contribute to the observed higher viscosity, whereas in double-stage measurements, possible gas leaking and melt micro-cracking during quenching are able to weaken the porous material and markedly lower suspension viscosity.
Abstract: The two most important magmatic differentiation series on Earth are the Fe-enriching tholeiitic series, which dominates the oceanic crust and island arcs, and the Fe-depleting calc-alkaline series, which dominates the continental crust and continental arcs. It is well known that calc-alkaline magmas are more oxidized when they erupt and are preferentially found in regions of thick crust, but why these quantities should be related remains unexplained. We use the redox-sensitive behavior of europium (Eu) in deep-seated, plagioclase-free arc cumulates to directly constrain the redox evolution of arc magmas at depth. Primitive arc cumulates have negative Eu anomalies, which, in the absence of plagioclase, can only be explained by Eu being partly reduced. We show that primitive arc magmas begin with low oxygen fugacities, similar to that of mid-ocean ridge basalts, but increase in oxygen fugacity by over two orders of magnitude during magmatic differentiation. This intracrustal oxidation is attended by Fe depletion coupled with fractionation of Fe-rich garnet. We conclude that garnet fractionation, owing to its preference for ferrous over ferric iron, results in simultaneous oxidation and Fe depletion of the magma. Favored at high pressure and water content, garnet fractionation explains the correlation between crustal thickness, oxygen fugacity, and the calc-alkaline character of arc magmas.
Abstract: Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry, and electromagnetic data can identify contemporary melt zones, magma reservoirs, and, or, crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs), and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.
Abstract: The beginning of plate tectonics on Earth remains the subject of fundamental debate. Also, future evolution of plate tectonics has not yet been addressed adequately in the literature. Here I develop models to extrapolate secular trends of plate tectonics closely associated with mantle potential temperature; intensity of magmatic activities to estimate the lifetime of plate tectonics. First a fractal model is utilized to analyze global igneous and detrital zircon U-Pb age datasets to characterize nonlinear intensity and periodicity of peaks in magmatic activities associated with deep-seated avalanche-type events (slab break off, lithospheric root detachments and mantle plumes). The results show descending trends of both nonlinear intensity and duration of peaks in magmatism observed from 3?Ga to the present day, suggesting a general trend of mantle cooling. This relation when extrapolated yields 1.45?Gyr of time for when nonlinear intensity of major magmatic activity would vanish. Further it is demonstrated by an independent polynomial model fitted to the relation between mantle potential temperature and future age of the earth that this result conforms to the time estimated for when mantle potential temperature is reduced to mantle solidus temperature.
Journal of Structual Geology, Vol. 11, pp. 329-335.
Mantle
magmatism
Abstract: The Earth's continental crust constitutes a major interface between the inner and outer envelops of the planet, controlling the differentiation of magmas produced in the mantle and their transfer to the surface. This close link facilitates the use of different chemical proxies to qualitatively unravel the crustal thickness related to fossil magmatic systems based on the message carried by magmas. This paper aims to bridge different results of statistical petrology, recently obtained at different scales of observation, in a global geodynamic model. Statistical analyses applied to a large multidimensional database of magmatic rocks show that crustal thickness could actually exert a first-order control on the composition of magmas, which become more calc-alkaline and comparatively less tholeiitic with increasing crustal thickness. Using this correlation, we document the progressive build-up of a thick (>40?km) Jurassic to Cretaceous accretionary belt along the Circum-Pacific Orogenic Belts (CPOB) that bounded the Panthalassa Ocean. The destruction of this thick belt started at ca. 125 Ma and was initially recorded by the thinnest magmatic systems hosting amphibole-bearing magma. Thinning of the CPOB became widespread in the northern regions of western America and in the western Pacific after ca. 75 Ma, possibly in response to oceanic plate segmentation, which triggered slab rollback and overriding plate extension. This chemical evolution is superimposed on a more global evolution of magma controlled by the temperature of the mantle that has gradually decreased since 200 Ma. Although the relative contribution of crust vs mantle cooling in the chemical signature of magmatic rocks should be further explored in the future, our results offer a new global perspective of the magmatic history of Pangea, the last supercontinent.
Abstract: This book sheds valuable new light on the genetic mineralogy of lower-mantle diamonds and syngenetic minerals. It presents groundbreaking experimental results revealing the melting relations of ultrabasic and basic associations and a physicochemical peritectic mechanism of their evolution. The experimental investigations included here reveal the key multicomponent, multiphase oxide-silicate-carbonate-carbon parental media for lower-mantle diamonds and syngenetic minerals. Consequently, readers will find extensive information on the diamond-parental oxide-silicate-carbonate-carbon melts-solutions that supplement the general features of lower-mantle diamond genesis and the most efficient ultrabasic-basic evolution. The experimental results on physicochemical aspects, combined with analytical mineralogy data, make it possible to create a generalized composition diagram of the diamond-parental melts-solutions, there by completing the mantle-carbonatite concept for the genesis of lower-mantle diamonds and syngenetic minerals. This book addresses the needs of all researchers studying the Earth’s deepest structure, super-deep mineral formation including diamonds, and magmatic evolution.
Contributions to Mineralogy and Petrology, Vol. 173, 22p. Doi.org/10.1007/s00410-018-1512-z
Africa, Tanzania
magmatism
Abstract: Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine?+?clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.
Abstract: The formation, storage and chemical differentiation of magma in the Earth’s crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high-melt-fraction ‘magma chamber’ paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low-melt-fraction "mush reservoirs". A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1,10. However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts. Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, caused by buoyant melt percolating upwards through, and reacting with, the crystals. Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface. These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data. Changes in local bulk composition caused by reactive melt flow, rather than large increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool- or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid- to lower-crustal reservoirs. Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface. We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallization in magma chambers.
Abstract: We present an extensive study of rehomogenized olivine?hosted melt inclusions, olivine phenocrysts, and chromian spinel inclusions to explore the link between geodynamic conditions and the origin and composition of Pliocene-Quaternary intraplate magmatism in Anatolia at Kula, Ceyhan?Osmaniye, and Karacada?. Exceptional compositional variability of these products reveals early and incomplete mixing of distinct parental melts in each volcanic center, reflecting asthenospheric and lithospheric mantle sources. The studied primitive magmas consist of (1) two variably enriched ocean island basalt (OIB)?type melts in Kula; (2) both OIB?type and plume mid?ocean ridge basalt (P?MORB)?like melts beneath Toprakkale and Üçtepeler (Ceyhan?Osmaniye); and (3) two variably enriched OIB?type melts beneath Karacada?. Estimated conditions of primary melt generation are 23-9 kbar, 75-30 km, and 1415-1215 °C for Kula; 28-19 kbar, 90-65 km, and 1430-1350 °C for Toprakkale; 23-18 kbar, 75-60 km, and 1400-1355 °C for Üçtepeler; and 35-27 kbar, 115-90 km, and 1530-1455 °C for Karacada?, the deepest levels of which correspond to the depth of the lithosphere?asthenosphere boundary in all regions. Although magma ascent was likely facilitated by local deformation structures, recent Anatolian intraplate magmatism seems to be triggered by large?scale mantle flow that also affects the wider Arabian and North African regions. We infer that these volcanics form part of a much wider Arabian?North African intraplate volcanic province, which was able to invade the Anatolian upper plate through slab gaps.
Earth and Planetary Science Letters, Vol. 510, pp. 161-172.
Mantle
Magmatism
Abstract: Tabular intrusions are common features in the Earth's brittle crust. They exhibit a broad variety of shapes, ranging from thin sheet intrusions (sills, saucer-shaped sills, cone sheets), to more massive intrusions (domed and punched laccoliths, stocks). Such a diversity of intrusion shapes reflects different emplacement mechanisms caused by contrasting host rock and magma rheologies. Most current models of tabular intrusion emplacement assume that the host rock behaves purely elastically, whereas numerous observations show that shear failure plays a major role. In this study, we investigate the effects of the host rock's Coulomb properties on magma emplacement by integrating (1) laboratory models using dry Coulomb granular model hosts of variable strength (cohesion) and (2) limit analysis numerical models. Our results show that both sheet and massive tabular intrusions initiate as a sill, which triggers shear failure of its overburden along an inclined shear damage zone at a critical sill radius, which depends on the emplacement depth and the overburden's cohesion. Two scenarios are then possible: (1) if the cohesion of the overburden is significant, opening of a planar fracture along the precursory weakened shear damage zones to accommodate magma flow, leads to the formation of inclined sheets, or (2) if the cohesion of the overburden is negligible, the sill inflates and lifts up the overburden, which is dissected by several faults that control the growth of a massive intrusion. Finally, we derive a theoretical scaling that predicts the thickness-to-radius aspect ratios of the laboratory sheet intrusions. This theoretical prediction shows how sheet intrusion morphologies are controlled by a mechanical equilibrium between the flowing viscous magma and Coulomb shear failure of the overburden. Our study suggests that the emplacement of sheet and massive tabular intrusions are parts of the same mechanical regime, in which the Coulomb behavior of the Earth's brittle crust plays an essential role.
Geochimica et Cosmochimica Acta, dor:10.1016/j.gca.2019.02.021
Mantle
magmatism
Abstract: Sector-zoned clinopyroxene is common in igneous rocks, but has been overlooked in the study of magmatic processes. Whilst concentric zoning is commonly used as a record of physicochemical changes in the melt feeding crystal growth, clinopyroxene is also highly sensitive to crystallisation kinetics. In sector-zoned crystals, the fidelity of compositional changes as recorders of magma history is dubious and the interplay between thermodynamic and kinetic controls remains poorly understood. Here we combine electron probe and laser ablation micro-chemical maps of titanaugite crystals from Mt. Etna (Sicily, Italy) to explore the origin of sector zoning at the major and trace element levels, and its implications for the interpretation of magmatic histories. Elemental maps afford the possibility to revisit sector zoning from a spatially controlled perspective. The most striking observation is a clear decoupling of elements into sectors vs. concentric zones within single crystals. Most notably, Al-Ti enrichments and Si-Mg depletions in the prism sectors {1?0?0}, {1?1?0} and {0?1?0} relative to the hourglass (or basal) sectors {?1?1?1} correlate with enrichments in rare earth elements and highly charged high field strength elements due to cation exchanges driven by kinetic effects. In contrast, transition metals (Cr, Ni, Sc) show little partitioning into sectors and strong enrichments in concentric zones following resorbed surfaces, interpreted as evidence of mafic recharge and magma mixing. Our results document that kinetic partitioning has minor effects on the compositional variations of cations with low charge relative to the ideal charge/radius of the structural site they occupy in the clinopyroxene lattice. We suggest that this may be due to a lower efficiency in charge balance mechanisms compared to highly charged cations. It follows that compatible metals such as Cr can be considered trustworthy recorders of mafic intrusions and eruption triggers even in sector-zoned crystals. We also observe that in alkaline systems where clinopyroxene crystallisation takes place at near-equilibrium conditions, sector zoning should have little effect on Na-Ca partitioning and in turn, on the application of experimentally calibrated thermobarometers. Our data show that whilst non-sector-zoned crystals form under relatively stagnant conditions, sector zoning develops in response to low degrees of undercooling, such as during slow magma ascent. Thus, we propose that the chemistry of sector-zoned crystals can provide information on magma history, eruption triggers, and possibly ascent rates.
Abstract: Many populated areas in the world (e.g., Flagstaff, AZ; Auckland, NZ; Mexico City, MEX) lie within active monogenetic volcanic fields that typically contain small volcanic cones and explosive maar craters formed over the course of a single eruptive cycle. Although much work has focused on the eruptive behaviour of monogenetic volcanoes, little geological information exists about their subsurface development and how the movement of magma through Earth’s shallow crust modulates the location and style of hazardous volcanic eruptions. Determination of the dynamics of magma intrusion and the transition from a coherent magma's ascent to its explosive fragmentation is crucial to our understanding of the controls on explosive versus effusive eruptive behaviour, thus to better evaluation of risks in a certain area. This study aims to determine the processes and relative timing of activity that took place below the ground surface of the deeply-eroded but well-preserved Jagged Rocks Complex, a cluster of monogenetic volcanoes within the Miocene Hopi Buttes Volcanic Field in northeastern Arizona, by combining detailed structural mapping, volcanological observation, paleomagnetic and geochemical analysis. The Jagged Rocks Complex, exposed at ~ 350 m below the pre-eruptive surface, comprises a well-preserved intrusive network, including dikes, sills and inclined sheets, associated with different type of fragmental bodies including buds, pyroclastic massifs and a diatreme, that represent different extents of shallow-depth fragmentation. These exposures at the Jagged Rocks Complex provide an excellent natural laboratory for examining the subsurface record of volcano initiation, and for constraining interpretations of processes controlling upward migration of magma from intrusion to eruption. This multidisciplinary approach allows an investigation at different levels from the source region to the surface, and aims to shed the light on the processes that regulate eruptions not only within monogenetic volcanic fields but also within small basaltic volcanoes in general.
Geochemical Perspective Letters, Vol. 10, pp. 8-13. doi:10.7185/ geochemlet.1907
Canada, Northwest Territories
magmatism
Abstract: Ancient rock samples are limited, hindering the investigation of the processes operative on the Earth early in its history. Here we present a detailed study of well-exposed crustal remnants in the central Slave craton that formed over a 1 billion year magmatic history. The tonalitic-granodioritic gneisses analysed here are broadly comparable to common suites of rocks found in Archean cratons globally. Zircon Hf isotope data allow us to identify a major change in the way continental crust was formed in this area, with a shift to distinctly positive ?Hf starting at ~3.55 Ga. The crust production processes and spatial distribution of isotopic compositions imply variable interaction with older crust, similar to the relationships seen in modern tectonic settings; specifically, long-lived plate margins. A majority of the Slave craton might have been formed by a similar mechanism.
Geochemistry, Geophysics, Geosystems, Vol. 20, 4, pp. 1836-1853.
Mantle
magmatism
Abstract: During the final stage of continental rifting, stretching localizes in the future distal domain where lithospheric necking occurs resulting in continental breakup. In magma?poor margins, the lithospheric necking is accompanied by crustal hyperextension, serpentinization, and exhumation of mantle lithosphere in the continent?ocean transition domain. In magma?rich margins, the necking is accomplished by the emplacement of large amounts of volcanics in the continental?ocean transition, in the form of seaward dipping wedges of flood basalts (seaward dipping reflections). This study examines the factors controlling the final crustal architecture observed in rifted margins and the magmatic budget during continental breakup, using observations from the Labrador Sea. The latter shows magma?rich breakup with seaward dipping reflections documented in the north and magma?poor breakup with a wide domain of exhumed serpentinized mantle recorded in the south. The pre?rift strength of the lithosphere, defined by the inherited thermal structure, composition, and thickness of the lithospheric layers, controls the structural evolution during rifting. While variations in the magmatic budget associated with breakup are controlled primarily by the interaction between the pre?rift inheritance, the timing and the degree of mantle melting, in relation to lithospheric thinning and mantle hydration.
Abstract: Magmatic pulses in intraplate sedimentary Basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe. Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.
Journal of African Earth Sciences, Vol. 158, available 14p. pdf
Africa, South Africa
magmatism
Abstract: Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof. The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180?Ma), mainly sill complexes to later (182-174?Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.
Abstract: Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle-ductile transition around a magma chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2?±?0.5?kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures ?1.5?kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures >rsim2.5?kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only within a relatively narrow range of pressures around 2?±?0.5?kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.
The dynamics of kimberlite magma intrusions: the role of dykes.
Vancouver Kimberlite Cluster, Oct. 16, 1/4p. Abstract
Global
magmatism
Abstract: Eruptions of kimberlite magma are often characterised by explosive activity, creating a conical-shaped cavity at the surface that tapers down to the underlying volcanic plumbing system. This diatreme is infilled by largely fragmented rock and ash, potentially having sampled up to 200 km of lithospheric material, and with diamondiferous mantle nodules ending up in close proximity to shallow crustal lithics. A kimberlite eruption would inevitably be something to see, and yet (as is the case with all volcanoes) it is ultimately the dynamics of the subsurface dykes and sills that feed the eruption which are responsible for what material reaches the surface. In this seminar I will present some geological observations of fossil kimberlite dykes from South Africa and explore how scaled analogue experiments in the laboratory can be used to reconstruct their dynamics of intrusion.
Journal of Volcanology and Geothermal Research, Vol. 354, pp. 87-101.
Mantle
magmatism
Abstract: Volcanic eruptions are fed by plumbing systems that transport magma from its source to the surface, mostly fed by dykes. Here we present laboratory experiments that model dyke ascent to eruption using a tank filled with a crust analogue (gelatine, which is transparent and elastic) that is injected from below by a magma analogue (dyed water). This novel experimental setup allows, for the first time, the simultaneous measurement of fluid flow, sub-surface and surface deformation during dyke ascent. During injection, a penny-shaped fluid-filled crack is formed, intrudes, and traverses the gelatine slab vertically to then erupt at the surface. Polarised light shows the internal stress evolution as the dyke ascends, and an overhead laser scanner measures the surface elevation change in the lead-up to dyke eruption. Fluorescent passive-tracer particles that are illuminated by a laser sheet are monitored, and the intruding fluid's flow dynamics and gelatine's sub-surface strain evolution is measured using particle image velocimetry and digital image correlation, respectively. We identify 4 previously undescribed stages of dyke ascent. Stage 1, early dyke growth: the initial dyke grows from the source, and two fluid jets circulate as the penny-shaped crack is formed. Stage 2, pseudo-steady dyke growth: characterised by the development of a rapidly uprising, central, single pseudo-steady fluid jet, as the dyke grows equally in length and width, and the fluid down-wells at the dyke margin. Sub-surface host strain is localised at the head region and the tail of the dyke is largely static. Stage 3, pre-eruption unsteady dyke growth: an instability in the fluid flow appears as the central fluid jet meanders, the dyke tip accelerates towards the surface and the tail thins. Surface deformation is only detected in the immediate lead-up to eruption and is characterised by an overall topographic increase, with axis-symmetric topographic highs developed above the dyke tip. Stage 4 is the onset of eruption, when fluid flow is projected outwards and focused towards the erupting fissure as the dyke closes. A simultaneous and abrupt decrease in sub-surface strain occurs as the fluid pressure is released. Our results provide a comprehensive physical framework upon which to interpret evidence of dyke ascent in nature, and suggest dyke ascent models need to be re-evaluated to account for coupled intrusive and extrusive processes and improve the recognition of monitoring signals that lead to volcanic eruptions in nature.
Abstract: The late evolution of kimberlite magmas, i.e., during and following emplacement into the upper crust, remains a controversial aspect of kimberlite petrogenesis. Likewise, it is unclear whether or not there is a link between melt composition and the emplacement mechanism of intrusive kimberlites (i.e., planar dykes/sills vs. irregular intrusions in the root zone of pipes). Resolving these issues is hampered by the absence of comparative studies of late-magmatic kimberlite phases (e.g., apatite, monticellite, mica) in dykes, sills, and root-zone intrusions from the same locality. Here we report petrographic and mineral-chemical results for groundmass phases in samples of dykes, sills, and root zone intrusions from the Kimberley kimberlites (South Africa). Early crystalised phases (e.g., olivine, spinel) in dykes/sills and root-zone intrusions have indistingushable compositions, and hence crystallised from similar primitive melts. Conversely, apatite major element compositions are variable and can discriminate dykes/sills (i.e., low and constant Sr, with increasing Si) from root zone intrusions (high but variable Sr, low and constant Si). The Sr depletion in root zone apatite is interpreted to represent fractional crystallisation of earlier apatite, perovskite, and calcite from a broadly similar parental melt. Silica enrichment of apatite from dykes/sills may be attributed to the coupled incorporation of CO32- and Si into the apatite structure, reflecting higher CO2 contents in the melts that formed dykes/sills. CO2 enrichment in the dykes/sills is consistent with petrographic obervations. Dykes/sills are enriched in carbonates, may contain dolomite, and are depleted in mica and monticellite compared to the groundmass of rootzone kimberlites. This suggests the melts parental to dykes/sills have a higher CO2/H2O ratio compared to those parental to root zone intrusions. These two distinct melt evolution paths cannot be due to crustal contamination before emplacement because the Sr-isotope compositions of latecrystallised carbonates are indistinguishable in dykes/sills and root-zone intrusions. We speculate that CO2 is better retained in dykes/sills due to a higher confining pressure (i.e., lack of breakthrough to the surface).
Dyke swarms of the world: a modern perspective., 10.1007/978-981-13-1666-1_4
South America, Guiana, Brazil
magmatism
Abstract: We review geochronological data including U-Pb baddelyite ages of Proterozoic mafic dyke swarms and sills of the Amazonian Craton, as well as their geochemical character and geological settings, in order to arrive at an integrated tectonic interpretation. The information together with the characteristics of coeval volcanic-plutonic suites indicates a cyclicity of the mafic-felsic activity through time and space. At least four LIP/SLIP events are apparent, and each one appears to accompany the stepwise accretionary crustal growth of Amazonia. The oldest two, the Orocaima (1.98-1.96 Ga) and Uatumă (c. 1.89-1.87 Ga) SLIPs, comprise calc-alkaline I-type and subordinate A-type plutonic and volcanic rocks. Synchronous mafic intraplate activity occurs across the Guiana and Central-Brazil Shields. These two events may be caused by interaction between subduction-related processes and mantle plumes with synchronous lithosphere extension during the two time periods. The Avanavero (1.79 Ga) LIP event mostly consists of mafic dykes and sills which are intrusive into the Roraima platform cover, in the Guiana Shield. They show tholeiitic chemistry and similarities with E-MORB and subcontinental lithospheric mantle-derived basalts, whereas the REE pattern suggests affinity with intraplate settings. The age of the Avanavero rocks is identical to the Crepori Diabase, located ca. 1800 km away to the south (Central-Brazil Shield). The youngest LIP event (1.11 Ga), the Rincón del Tigre-Huanchaca, has the Rio Perdido Suite as a component in the Rio Apa Terrane, which is ca. 300 km away from the Rincón del Tigre Complex, located in the SW portion of the Amazonian Craton. Furthermore, the Central-Brazil and Guiana Shields boasts widespread intraplate mafic activity, highlighted by the Mata-Matá (1.57 Ga), Salto do Céu (1.44 Ga) and Nova Floresta (1.22 Ga) mafic sills and the Cachoeira Seca Troctolite (1.19 Ga). Contemporaneous A-type, rapakivi granites with roughly similar ages also occur elsewhere. These particular episodes are extension specific steps of the Mesoproterozoic Amazonia, and the quite large distribution is consistent with LIP events. In a broader perspective, the intermittent Proterozoic intracratonic activity has a barcode that matches LIP/SLIP events in Columbia and Rodinia.
Yellowknife Forum NWTgeoscience.ca, abstract volume poster p.103.
Mantle
magmatism
Abstract: Kimberlites are the deepest mantle magmas to reach the surface of the Earth and the hosts of the major primary diamond deposits. The enigmatic nature of kimberlites owning to poorly constrained triggers of kimberlite magmatism, melt composition, and crystallization conditions, are mostly limited by unknown content and composition of volatiles. The content and ratio of H2O and CO2 are important parameters of magma emplacement. They are essential for the mantle melting and rapid ascent of kimberlites. Volatiles also greatly affect diamond preservation in kimberlites during the ascent to the surface. Apatite has been used as an indicator of volatiles in different magmatic systems. Partitioning of trace elements between apatite and a melt is also sensitive to the carbonate component in the melt. This makes apatite potentially very useful indicator of evolution of melt composition and fluid during kimberlite ascent and emplacement. However, trace element partition coefficients between apatite and melts have been determined for compositions much more SiO2-rich than kimberlites, whereas coefficients in carbonate melts show large discrepancies. This study presents experimentally determined trace element partition coefficients (D) between apatite and synthetic analogues of kimberlite (SiO2 content ranging from 11 to 23 wt%) and carbonatite melts (SiO2 content <1 wt% and 4.5 wt%). The experiments were conducted in piston-cylinder apparatus at 1150o, 1250o, 1350o, at 1 and 2 GPa, and at oxygen fugacity -3.97, 0.27, 4.83 log units relative to fayalite-magnetite-quartz (FMQ) buffer. We demonstrate the increase of DREE for Rare Earth Elements (REE) with increase in SiO2 of the melt and the effect of apatite composition (natural Durango apatite vs. synthetic apatite) on D and substitution mechanism. We found no effect of temperature, pressure, water content, and oxygen fugacity on D in these low SiO2 melts. This allows us to use trace element content of natural apatite to examine the composition of kimberlite magma. We apply the results to apatite from two kimberlite classes: Class 1 composite kimberlite pipe from Orapa cluster (Botswana) with hypabyssal coherent and volcaniclastic kimberlite facies and Class 3 kimberlite pipes from Ekati Mine (Northwest Territories, Canada) filled with effusive coherent kimberlites in Leslie pipe and both volcaniclastic and coherent kimberlites in Boa pipe, and discuss how changes in SiO2 content of kimberlite melt affect apatite saturation. We further apply our experimentally determined DREE to model the composition of evolved kimberlite melt using published bulk rock and apatite compositions in kimberlites.
Abstract: The paper reports the results of an experimental study of phase relations and distribution of elements in silicate melt-salt melt systems (carbonate, phosphate, fluoride, chloride), silicate melt I - silicate melt II, and fluid-magmatic systems in the presence of alkali metal fluorides. Extraction of a number of ore elements (Y, REE, Sr, Ba, Ti, Nb, Zr, Ta, W, Mo, Pb) by salt components was studied in liquid immiscibility processes within a wide temperature range of 800-1250°? and pressure of 1-5.5 kbar. It is shown that partition coefficients are sufficient for concentration of ore elements in amounts necessary for the genesis of ore deposits. In a fluid-saturated trachyrhyolite melt, the separation into two silicate liquids has been determined. The partition coefficients of a number of elements (Sr, La, Nb, Fe, Cr, Mo, K, Rb, Cs) between phases L1 and L2 have been obtained. The interaction processes of a heterophase fluid in the granite (quartz)-ore mineral-heterophase fluid (Li, Na, K-fluoride) system were studied at 650-850°C and P = 1 kbar. The formation of the phase of a highly alkaline fluid-saturated silicate melt concentrating Ta and Nb is shown as a result of the interaction of the fluid with rock and ore minerals.
Abstract: Late Cretaceous global plate reorganization associated with the inception of counterclockwise rotation of Africa relative to Europe initiated in the Balkan region small-volume magmatism of diverse geochemical signature along the enigmatic Sava-Vardar Zone. We study a Late Cretaceous lamprophyric sill in Ripanj village near Belgrade to constrain this magmatic episode. The lamprophyre is characterized by high contents of Na, P, Fe and Al, and low contents of K, Ca and Mg. Its original nature (Na, K, Ca and Mg) is concealed by intense alteration (albitization of feldspar and partial chloritization of phlogopite) that erased the ultrapotassic affinity of the rocks and resulted in extremely low K/Na ratios. The recalculated chemical composition demonstrates that the rocks are ultrapotassic, with K2O and MgO > 3 wt % and K2O/Na2O > 2, and belong to the durbachite-vaugnerite series, i. e., the plutonic equivalents of minettes and kersantites. Two phlogopite concentrates gave Ar-Ar ages of 86.80 ± 0.5 Ma and 86.90 ± 0.5 Ma. Our combined elemental and Sr-Nd-Pb isotope data (87Sr/86Sr 0.70667-0.70677, 143Nd/144Nd 0.512426-0.512429, 206Pb/204Pb 18.82-19.13, 207Pb/204Pb 15.67-15.68, 208Pb/204Pb 38.92-39.19) for representative lamprophyric samples suggests magma derivation from a light rare earth elements (LREE) and K enriched, metasomatized mantle source. The content of LREE of the rocks is enriched, whereas heavy rare earth elements (HREE) is depleted. Rare earth elements (REE) of the whole rock and REE of diopside all indicate that garnet was present in their source. There are two viable and mutually-excluding geodynamic scenarios for the Late Cretaceous magmatism in the Balkans: (i) If the Sava-Vardar ocean still existed in the Late Cretaceous and was subducted under the European plate with arc volcanism along the Apuseni-Banat-Timok-Panagyurishte-Srednjogorje belt, coeval magmatism in the Sava-Vardar Zone occurred in a fore-arc setting, and may be related to ridge subduction; (ii) If the Mesozoic ocean closed already during the Upper Jurassic or Lower Cretaceous, the Late Cretaceous volcanism within the Sava-Vardar Zone represents intracontinental volcanism associated with transtensional tectonics.
Contributions to Mineralogy and Petrology, Vol. 175, 13p. pdf
Mantle
magmatism
Abstract: Magmatic superheating is a condition with relevance to natural systems as well as experimental studies of crystallization kinetics. Magmas on Earth and other planetary bodies may become superheated during adiabatic ascent from the mantle or as a consequence of meteorite impact-generated crustal melting. Experimental studies of igneous processes commonly employ superheating in the homogenization of synthetic starting materials. We performed 1-atmosphere dynamic crystallization experiments to study the effects of superliquidus thermal history on the morphologies and compositions of subsequently grown olivine crystals. An ultramafic volcanic rock with abundant olivine was fused above the experimentally determined liquidus temperature (1395 °C), held for 0, 3, or 12 h, cooled at 25 °C h?1, and quenched from 200 °C below the liquidus, all at constant fO2, corresponding to FMQ-2?±?0.2 log units. An increase in olivine morphologic instability is correlated with superheating magnitude, parameterized as the integrated time the sample is held above the liquidus (“TtL”; °C h). We infer that a delay in nucleation, which intensifies monotonically with increasing TtL, causes crystal growth to be increasingly rapid. This result indicates that the structural relaxation time scale controlling the formation of crystal nuclei is (a) far longer than the time scale associated with viscous flow and (b) exceeds the liquidus dwell times typically imposed in crystallization experiments. The influence of magmatic superheating on crystal morphology is similar in sense and magnitude to that of subliquidus cooling rate and thus, both factors should be considered when interpreting the thermal history of a volcanic rock containing anhedral olivine.
Geology of Ore Deposits, Vol. 61, 7, pp. 581-588. pdf
Russia, Kola Peninsula
magmatism
Abstract: The Neoarchean subalkaline magmatism of the Keivy structure is expressed in the formation of the volcanoplutonic latite-monzonite-granite association (LMGA). The formation of LMGA magmas is assumed to occur due to melting of metasomatically altered mafic rocks during intrusion into the lower crust of basaltic melts initial for rocks of the dike complex and gabbro-labradorite massifs. The alkaline granites associated with LMGA have a close U-Pb age but a later formation time based on the geological data. With respect to LMGA, alkali granites have increased concentrations of SiO2, alkalis (K2O/Na2O = 1.1-1.4), iron (F# = 84-98%), a high agpaitic index (Kagp = 0.86-1.2), and lower quantities of TiO2, MgO, Fetot, and Al2O3, which probably resulted from the higher degree of differentiation of their initial melts compared to LMGA.
Abstract: The Hopi Buttes volcanic field (HBVF) is located on the Colorado Plateau, Northern Arizona. In this Miocene volcanic field, the erosion level increases southward, allowing the study of maar-diatreme volcanoes from top (posteruptive crater infill and ejecta ring) to bottom (lower diatreme). The Twin Peaks volcanic complex consists mostly of two hills (North Peak and South Peak) with thick lavas at their summits and pyroclastic rocks underneath. In the HBVF, such volcanic remnants have received little scientific attention so far, despite their relative abundance. Our field observations allow us to interpret the North and South Peaks as remnants of two maar-diatreme volcanoes which evolved into lava lakes filling the craters. Within the complex, we distinguish four volcanic units (from unit 1 at the bottom to unit 4 at the top). On the basis of the field description of the deposits and the componentry measurements, we suggest that unit 1 is phreatomagmatic, unit 2 is phreato-strombolian (with mixed phreatomagmatic and strombolian characteristics), unit 3a is phreato-hawaiian (with mixed phreatomagmatic and hawaiian characteristics), unit 3b is hawaiian (formed by lava fountains) and unit 4 consists of lava lakes filling the maar craters. There is therefore a progressive evolution from a purely phreatomagmatic eruptive style, which excavated the craters and diatremes and partly filled them, to magmatic explosive to nonexplosive eruptive styles, which filled the maar craters up to the pre-eruptive surface. We discuss traditional criteria used to distinguish phreatomagmatic from magmatic eruptive styles in ultramafic to mafic maar-diatreme volcanoes.
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.
Stratigraphy and Geological Correlation, Vol. 28, 1, pp. 1-34.
Europe, Fennoscandia
magmatism
Abstract: The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak-Pechenga-Imandra-Varzuga belt and dikes and sills of the Murmansk and Kola-Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06-1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra-Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra-Varzuga zones intruded at 2010-1970 and 1970-1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra-Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts.
Abstract: The Singhbhum Craton is built up by successive pulses of discrete granitic magmatism at ~3.52 Ga, ~3.47-3.43 Ga, and ~3.40-3.35 Ga that produced tonalitetrondhjemite-granodiorite (TTG)-type suites and were followed by younger pulses at 3.32-3.35 Ga, and 3.31-3.28 Ga producing voluminous granitic-granodioritic magma. There is enough evidence to indicate that continental crust building activity started in the Hadean time and continued through Eoarchean. But the rocks of this period were fully recycled to generate the Paleoarchean and younger crust. The different pulses of granitic magmatism during the Paleoarchean were interspersed with the formation of supracrustal rocks which are now preserved as supracrustal belts peripheral to the craton or as internal screens within the craton. Halfnium isotopic record suggests that the Hadean and Eoarchean granitoids were sourced in an enriched reservoir, probably some form of early mafic protocrust. From ~3.6-3.5 Ga a shift in the isotopic composition of Hf is noticed, marked by upward excursion of ?Hf(t) plots towards suprachondritic values, signifying that the early mantle reservoir was serially modified by contamination by a juvenile melt derived from a depleted source. This probably signals a change in the geodynamic scenario, major depletion of the mantle and generation of voluminous TTG melts. There are contending hypotheses of plume-driven and subduction-driven mechanisms of continental crust formation. In the Singhbhum Craton during Hadean and Eoarchean times episodic mantle plumes probably operated in a stagnant lid tectonic setting. Repeated plume activities and the formation of oceanic plateaus might have triggered the onset of subduction which at the initial stages might have been of short duration. The transition from plume-driven tectonics to subduction-driven tectonics might have taken place at about 3.5 Ga. The supracrustal belts of the Older Metamorphic Group (OMG) and the Iron Ore Group (IOG) are thought to have formed in supra-subduction settings. Widespread metamorphism and deformation affected the craton during 3.34-3.26 Ga. By 3.1 Ga the Singhbhum Craton had stabilized and emerged as a landmass. Paleosols developed on the surface; rift basins were formed which were receptacles of siliciclastic sediments and mafic volcanics; anorogenic K-feldspar bearing granites were emplaced. Swarms of mafic dykes of Paleo- to Meso-Proterozoic age intruded the craton marking a tensional regime that was probably related to the initial stage of basin formation in the North Singhbhum Mobile Belt.
Abstract: The Săo 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 Săo 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.
Journal of Petrology, Vol. 61, 7, doi.org/10.1093 /petrology/egaa072
Europe
magmatism
Abstract: The orogenic development after the continental collision between Laurussia and Gondwana, led to two contrasting associations of mantle-derived magmatic rocks on the territory of the Bohemian Massif: (i) a 340-310?Ma lamprophyre-lamproite orogenic association; and (ii) a 300-275?Ma lamprophyre association of anorogenic affinity. Major types of potassic mantle-derived magmatic rocks recognized in the orogenic and anorogenic associations include: (i) calc-alkaline to alkaline lamprophyres; (ii) alkaline ‘orthopyroxene minettes’ and geochemically related rocks grouped here under the new term lampyrite; and (iii) peralkaline lamproites. These three types significantly differ with respect to mineral, whole-rock and Sr-Nd-Pb-Li isotope composition and spatial distribution. The calc-alkaline lamprophyres occur throughout the entire Saxo-Thuringian and Moldanubian zones, whereas the different types of malte-derived potassic rocks are spatially restricted to particular zones. Rocks of the Carboniferous lamprophyre-lamproite orogenic association are characterized by variable negative ?Nd(i) and variably radiogenic Sr(i), whereas the rocks of the Permian lamprophyre association of anorogenic affinity are characterized by positive ?Nd(i) and relatively young depleted-mantle Nd-model ages reflecting increasing input from upwelling asthenospheric mantle. The small variation in the Pb isotopic composition of post-collisional potassic mantle-derived magmatic rocks (of both the orogenic and anorogenic series) implies that the Pb budget of the mantle beneath the Bohemian Massif is dominated by the same crust-derived material, which itself may include material derived from several sources. The source rocks of ‘orthopyroxene minettes’ are characterized by isotopically light (‘eclogitic’) Li and strongly radiogenic (crustal) Sr and may have been metasomatized by high-pressure fluids along the edge of a subduction zone. In contrast, the strongly Al2O3 and CaO depleted mantle source of the lamproites is characterized by isotopically heavy Li and high SiO2 and extreme K2O contents. This mantle source may have been metasomatized predominantly by melts. The mantle source of the lamprophyres may have undergone metasomatism by both fluids and melts.
Abstract: This paper presents some data of the Jiaopingdu gabbro and Caiyuanzi granite at the southwestern margin of the Yangtze Block, on the geochemical compositions, zircon LA-ICP-MS U-Pb ages and Hf isotopic data. The Jiaopingdu gabbro gives the age of 1721 ± 5 Ma, the Caiyuanzi granite 1732 ± 6 Ma and 1735 ± 4 Ma, and the Wenjiacun porphyry granite 1713 ± 4 Ma, suggesting nearly contemporaneous formation time of the gabbro and granite. The bimodal feature is demonstrated by the gabbro SiO2 content of 44.64-46.87 wt% and granite 73.81-77.03 wt%. In addition, the granite has high content of SiO2 and Na2O + K2O, low content of Al2O3 and CaO, enriched in REEs (except Eu) and Zr, Nb, Ga and Y, depleted in Sr, implying it belongs to A?type granite geochemistry and origin of within?plate environment. The zircon ?Hf(t) of the granite and gabbro is at the range of 2-6, which is near the 2.0 Ga evolution line of the crust, implying the parent magma of the gabbro being derived from the depleted mantle and a small amount of crustal material, and the parent magma of the granite from partial melting of the juvenile crust and some ancient crustal material at the same time. Compared with 1.8-1.7 Ga magmatism during breakup of other cratons in the world, we can deduce that the Columbia has initially broken since ca. 1.8 Ga, and some continental marginal or intra?continental rifts occurred at ca. 1.73 Ga.
Abstract: The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8?Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U-Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85?Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89-1.85?Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0?Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570?Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate-Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.
Journal of Petrology, Vol. 61, 9, egaa067, 35p. Pdf
Mantle
magmatism
Abstract: Besides standard thermo-mechanical conservation laws, a general description of mantle magmatism requires the simultaneous consideration of phase changes (e.g. from solid to liquid), chemical reactions (i.e. exchange of chemical components) and multiple dynamic phases (e.g. liquid percolating through a deforming matrix). Typically, these processes evolve at different rates, over multiple spatial scales and exhibit complex feedback loops and disequilibrium features. Partially as a result of these complexities, integrated descriptions of the thermal, mechanical and chemical evolution of mantle magmatism have been challenging for numerical models. Here we present a conceptual and numerical model that provides a versatile platform to study the dynamics and nonlinear feedbacks inherent in mantle magmatism and to make quantitative comparisons between petrological and geochemical datasets. Our model is based on the combination of three main modules: (1) a Two-Phase, Multi-Component, Reactive Transport module that describes how liquids and solids evolve in space and time; (2) a melting formalism, called Dynamic Disequilibirum Melting, based on thermodynamic grounds and capable of describing the chemical exchange of major elements between phases in disequilibrium; (3) a grain-scale model for diffusion-controlled trace-element mass transfer. We illustrate some of the benefits of the model by analyzing both major and trace elements during mantle magmatism in a mid-ocean ridge-like context. We systematically explore the effects of mantle potential temperature, upwelling velocity, degree of equilibrium and hetererogeneous sources on the compositional variability of melts and residual peridotites. Our model not only reproduces the main thermo-chemical features of decompression melting but also predicts counter-intuitive differentiation trends as a consequence of phase changes and transport occurring in disequilibrium. These include a negative correlation between Na2O and FeO in melts generated at the same Tp and the continued increase of the melt’s CaO/Al2O3 after Cpx exhaustion. Our model results also emphasize the role of disequilibrium arising from diffusion for the interpretation of trace-element signatures. The latter is shown to be able to reconcile the major- and trace-element compositions of abyssal peridotites with field evidence indicating extensive reaction between peridotites and melts. The combination of chemical disequilibrium of major elements and sluggish diffusion of trace elements may also result in weakened middle rare earth to heavy rare earth depletion comparable with the effect of residual garnet in mid-ocean ridge basalt, despite its absence in the modelled melts source. We also find that the crystallization of basalts ascending in disequilibrium through the asthenospheric mantle could be responsible for the formation of olivine gabbros and wehrlites that are observed in the deep sections of ophiolites. The presented framework is general and readily extendable to accommodate additional processes of geological relevance (e.g. melting in the presence of volatiles and/or of complex heterogeneous sources, refertilization of the lithospheric mantle, magma channelization and shallow processes) and the implementation of other geochemical and isotopic proxies. Here we illustrate the effect of heterogeneous sources on the thermo-mechanical-chemical evolution of melts and residues using a mixed peridotite-pyroxenite source.
Abstract: According to the existing models of kimberlite origin, free exsolution CO2 may be an important agent in the evolution of primary kimberlite magma and initiation of crack propagation. We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO2 and H2O in experiments at 5.5 GPa and 1200-1450 °C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the contents of molecular CO2 in the melt, which impedes immediate separation of CO2 fluid from melt equilibrated with the peridotite source. The solubility of molecular CO2 in the melt decreases from 20 -25 wt% at 4.5-6.8 wt% SiO2 typical of carbonatite to below 7-12 wt% in more silicic melts with 26-32 wt% SiO2. Interaction of garnet lherzolite with carbonatitic melt (at a weight proportion of 2:1) in the presence of 2-3 wt% H2O and 17-24 wt% of total CO2 at 1200-1450 °C yields low-SiO2 (<10 wt%) alkali?carbonated melts, which shows multiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can originate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM). Excessive volatiles may be present in carbonatitic melts not equilibrated with the peridotitic source due to the formation of metasomatic reaction zones. Having separated from the source, carbonatitic magma enriched in molecular CO2 and H2O can rapidly become more silicic (>25 wt% SiO2) by dissolution and carbonation of entrapped peridotite. Furthermore, interaction of garnet lherzolite with carbonatitic melt rich in K, CO2, and H2O at 1350 °C produces immiscible carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops globules of foam-like vesicular glass. Differentiation of immiscible melts early during their ascent may equalize the compositions of kimberlite magmas generated in different CLM sources. The fluid phase can release explosively from ascending magma at lower pressures as a result of SiO2 increase which reduces the solubility of CO2 and due to the decarbonation reaction of magnesite and orthopyroxene.
Abstract: We present new key paleomagnetic pole at 13°S, 152°E (k = 21, A95 = 7.8°) for recently identified 1864.4 ± 2.7 Ma (weighted mean age of four Pbsingle bondPb ages) mafic magmatic event, based on a detailed paleomagnetic study of dolerite dykes and sills intruding Archean basement rocks and Tadipatri formation of the Cuddapah basin, Dharwar craton respectively. The Pbsingle bondPb baddeleyite geochronology yields a crystallisation age of 1867.1 ± 1.0 Ma (MSWD = 1.02) for N77°E trending dyke in the southern region to Cuddapah basin. This new age obtained, confirms the presence of ~1864 Ma magmatic episode with a spatial extent of ~400 km in the Eastern Dharwar craton, within the brief period of ~5 Ma. The paleomagnetic results in these dykes revealed reverse polarity magnetisation direction with mean D = 107°, I = 24° (N = 13 sites, ?95 = 10°). Here, we also update the normal polarity magnetic directions on ~1.89 Ga swarm, and the corresponding paleopole situated at 21°N, 336°E (N = 79 sites, A95 = 3.6°). The paleoposition of India is constrained around the equator during ~1.89-1.86 Ga time. The paleogeographic reconstructions were also been attempted at ~1.89 Ga and ~ 1.86 Ga with available key poles from other cratons, indicates the possibility of single plume acting as a source for two distinguishable radial emplacement of mafic dyke swarms across India (Dharwar and Bastar craton) and Western Australia (Yilgarn craton) within a time span of ~35 Ma. The individual movement of India, Baltica and Siberia with a drift rate of ~5.55 cm/yr towards the south, whereas Amazonia craton has moved rapidly to the north (~24.9 cm/yr), do not suggest the amalgamation of a supercontinent (Columbia/ Nuna) during ~1.88-1.86 Ga time.
Abstract: The differentiation of Earth ~4.5 billion years (Ga) ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation, and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate because of the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (?57Fe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7-Ga metabasalts from the Isua Supracrustal Belt (Greenland). The ?57Fe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (?182W). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the ?57Fe and ?182W heterogeneity of modern oceanic basalts.
Annual Review of Earth Planetary Sciences, Vol. 49, pp. 465-484.
Mantle
Magmatism
Abstract: Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H2O and CO2 that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work.
Abstract: Pre-magmatic continental extension often precedes the major magmatic expulsion of large igneous provinces (LIPs). However, the cause-and-effect relationship between pre-magmatic rifting and the extrusion of large amount of magma is controversial. It remains unclear whether magmatism arises as a consequence of passive rifting or whether it is related to active upwelling of the mantle. In addition, the relationship between the pre-magmatic stages and the final breakup, with the onset of conjugate passive margins, is ambiguous. In this study, we compiled available data from six LIPs (Central Atlantic, Karoo, Parana-Etendeka, Deccan, North Atlantic, and Afar igneous provinces) that successively occurred during the fragmentation of Pangea and found that pre-magmatic rift trends may show a high obliquity or even be orthogonal with respect to the future passive margins. We conclude that syn-magmatic rifts should not be directly correlated, both structurally and dynamically, to the ancient pre-magmatic rift phase. Furthermore, following the breakup of a supercontinent, seafloor spreading usually initiates within volcanic passive margins (VPMs) and then propagates away to create non-volcanic passive margins (NVPMs) as a consequence of the consumption and cooling of a sub-lithospheric positive thermal anomaly. Major transform faults often exist between VPMs and NVPMs, acting as a mechanical barrier to mantle melting and magmatism transportation.
Earth & Environment Communications, 10.1038/s43247-021-00145-3 16p. Pdf
United States, Hawaii
magmatism
Abstract: Volcanoes represent one of the largest natural sources of metals to the Earth’s surface. Emissions of these metals can have important impacts on the biosphere as pollutants or nutrients. Here we use ground- and drone-based direct measurements to compare the gas and particulate chemistry of the magmatic and lava-seawater interaction (laze) plumes from the 2018 eruption of K?lauea, Hawai’i. We find that the magmatic plume contains abundant volatile metals and metalloids whereas the laze plume is further enriched in copper and seawater components, like chlorine, with volatile metals also elevated above seawater concentrations. Speciation modelling of magmatic gas mixtures highlights the importance of the S2? ligand in highly volatile metal/metalloid degassing at the magmatic vent. In contrast, volatile metal enrichments in the laze plume can be explained by affinity for chloride complexation during late-stage degassing of distal lavas, which is potentially facilitated by the HCl gas formed as seawater boils.
Annual Review of Earth and Planetary Sciences, Vol. 49, pp. 465-494.
Mantle
magmatism
Abstract: Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H2O and CO2 that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work. Inclusions of silicate melt (magma) trapped inside of crystals formed by magma crystallization provide a rich, detailed record of what happens beneath volcanoes. These inclusions record information ranging from how magma forms deep inside Earth to its final hours as it ascends to the surface and erupts. The melt inclusion record, however, is complex and hazy because of many processes that modify the inclusions after they become trapped in crystals. Melt inclusions provide a primary archive of dissolved gases in magma, which are the key ingredients that make volcanoes erupt explosively.
Abstract: A large compilation of quality-curated major and trace element data has been assembled to investigate how trace element patterns of mafic and ultramafic magmas have varied with time through particular settings from the Archean to the Phanerozoic, the primary objective being to recognise at what times particular patterns of variation emerge, and how similar these are to baseline data sets representing tectonic settings in the modern Earth. The most informative element combinations involve Nb, Th and the REE, where REE are represented by ‘lambda’ parameters describing slope and shape of patterns. Combinations of the ratios of Th, Nb, La and lambda values from Archean and early Proterozoic basalts and komatiites reveal a distinctive pattern that is common in most well-sampled terranes, defining a roughly linear trend in multi-dimensional space from compositions intermediate between modern n-MORB and primitive mantle at one end, towards compositions approximating middle-to-upper continental crust at the other. We ascribe this ‘Variable Th/Nb’ trend in most instances to varying degrees of crustal contamination of magmas with similar compositions to modern oceanic plateau basalts. Komatiites had slightly more depleted sources than basalts, consistent with the hypothesis of derivation from plume tails and heads, respectively. The most significant difference between Precambrian and Phanerozoic plume-derived basalts is that the distinctive OIB-like enriched source component appears to be largely missing from the Archean and Proterozoic geologic record, although isolated examples of OIB-like trace element characteristics are evident in datasets from even the oldest preserved greenstones. Phanerozoic intra-cratonic LIPs, such as the 260?Ma Emeishan LIP in China, have fundamentally different geochemical characteristics to Archean and Paleoproterozoic assemblages; the oldest Proterozoic LIP we have identified that has this type of ‘modern’ signal is the Midcontinent Rift at 1100?Ma. The data are consistent with plume tail sources having changed from being dominantly depleted in the Archean Earth to dominantly enriched in the Phanerozoic Earth, while plume head sources have hardly changed at all. Trace element patterns considered to be diagnostic of subduction are locally present but rare in Archean terranes and become more prevalent through the Proterozoic, although this conclusion is tempered by the large degree of overlap in compositional space between continental arc magmas and continental flood basalts. This overlap reflects the difficulty of distinguishing the effects of supra-subduction metasomatizm and flux melting from those of crustal contamination. Additional factors must also be borne in mind, particularly that trace element partitioning systematics may have been different in all environments in a hotter planet, and large-scale asthenospheric overturns might have been predominant over modern-style plumes in the Archean Earth. Some basaltic suites in particular Archean terranes, notably the western parts of both the Yilgarn and Pilbara cratons in Western Australia and parts of the Superior Craton, have restricted, but locally predominant, suites of basalts with characteristics akin to modern oceanic arcs, suggesting that some process similar to modern subduction was preserved in these particular belts. Ferropicrite magmas with distinctive characteristics typical of modern OIBs and some continental LIPs (notably Emeishan) are rare but locally predominant in some Archean and early Proterozoic terranes, implying that plume sources were beginning to be fertilised by enriched, probably subducted, components as far back as the Mesoarchean. We see no evidence for discontinuous secular changes in mantle-derived magmatism with time that could be ascribed to major mantle reorganisation events. The Archean-Proterozoic transition appears to be entirely gradational from this standpoint. The transition from Archean-style to Phanerozoic-style plume magmatism took place somewhere between 1900?Ma (age of the Circum-Superior komatiitic basalt suites) and 1100?Ma (the age of the Midcontinent Rift LIP).
South African Journal of Geology, Vol. 124, pp. 421-442.
Africa, South Africa
magmatism
Abstract: At least four spatially overlapping Large Igneous Provinces, each of which generated ~1 x 106 km3 or more of basaltic magmas over short time intervals (<5 m.y.), were emplaced onto and into the Kaapvaal Craton between 2.7 and 0.18 Ga: Ventersdorp (2 720 Ma, ~0.7 x 106 km3), Bushveld (2 056 Ma, ~1.5 x 106 km3), Umkondo (1 105 Ma, ~2 x 106 km3) and Karoo (182 Ma, ~3 x 106 km3). Each of these has been suggested to have been derived from melting of sub-continental lithospheric mantle (SCLM) sources, but this is precluded because: (1) each widespread heating event sufficient to generate 1 to 2 x 106 km3 of basalt from the Kaapvaal SCLM (volume = 122 to 152 x 106 km3) would increase residual Mg# by 0.5 to 2 units, depending on degree of melting, and source and melt composition, causing significant depletion in already-depleted mantle, (2) repeated refertilization of the Kaapvaal SCLM would necessarily increase its bulk density, compromising its long-term buoyancy and stability, and (3) raising SCLM temperatures to the peridotite solidus would also have repeatedly destroyed lithospheric diamonds by heating and oxidation, which clearly did not happen. It is far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of continental crustal components. Combined Sr and Nd isotopic data (n = 641) for the vast volumetric majority of Karoo low-Ti tholeiitic magmatic products can be successfully modelled as an AFC mixing array between a plume-derived parental basalt, with <10% of a granitic component derived from 1.1 Ga Namaqua-Natal crust. Archaean crustal materials are far too evolved (?Nd ~ -35) to represent viable contaminants. However, a very minor volume of geographically-restricted (and over-analysed) Karoo magmas, including picrites, nephelinites, meimechites and other unusual rocks may represent low-degree melting products of small, ancient, enriched domains in the Kaapvaal SCLM, generated locally during the ascent of large-volume, plume-derived melts. The SCLM-derived rocks comprise the well-known high-Ti (>2 to 3 wt.% TiO2) magma group, have ?Nd, 182 values between +10.5 and -20.9, and are characteristically enriched in Sr (up to 1 500 ppm), suggesting a possible connection to kimberlite, lamproite and carbonatite magmatism. These arguments may apply to continental LIPs in general, although at present, there are insufficient combined Sr + Nd isotopic data with which to robustly assess the genesis of other southern African LIPs, including Ventersdorp (n = 0), Bushveld (n = 55) and Umkondo (n = 18).
Earth Science Reviews , Vol. 215, 103509, 27p. Pdf
Mantle
magmatism
Abstract: Carbonatites are formed from volatile-rich melts, commonly associated with a characteristic hydrothermal footprint. However, studies of their fluid inclusions are relatively scarce and heterogeneous in terms of detail and completeness of the data presented. Here, we review and discuss comprehensively the results of previous studies and derive a general model for the formation and properties of fluids associated with carbonatitic magmatism. Worldwide, four types of fluid inclusion occur in carbonatites: (type I/HS) vapour-poor H2O-NaCl fluids with up to 50 wt% salinity; (type II/HC) vapour-rich H2O-NaCl-CO2 fluids with <5 wt% salinity; (type III/DS) multi-component fluids with high salinity and without CO2; and (type IV/CDS) multi-component fluids with high salinity and high CO2. This global dataset suggests continuous fluid release from deep to shallow-level intrusions. Modelling of the liquidus surface indicates that carbonatite magmas generally exsolve a saline brine (type I/HS). This brine separates/evolves into a Na-K-sulfate-carbonate/bicarbonate-chloride brine with or without CO2 (types III/DS and IV/CDS), trapped together with low salinity CO2-rich fluids produced by immiscibility. Fluid immiscibility is related to rapid pressure release during fast, forceful and discontinuous magma ascent, which we envisage as a "pneumatic jackhammer" model for carbonatite ascent and emplacement. In this model, cyclic and progressive fluid flux via pressure build-up and subsequent catastrophic pressure release results in a self-sustaining crustal ascent of the buoyant, low-viscosity magma. This process allows for rapid and efficient magma ascent, in particular above the brittle-ductile transition zone, where pressures that prevailed during apatite crystallization have been estimated in numerous complexes. Moreover, this model provides an explanation for the apparent absence of shallow carbonatite magma chambers (in a classical sense) and identifies fenitization as a phenomenon induced by both fluids released during magma ascent and residual fluids.
Abstract: The cratonic lithosphere-asthenosphere boundary is commonly invoked as the site of sheared peridotite and megacryst formation, a well-recognized petrological assemblage whose genetic relationships—if any—remain poorly understood. We have undertaken a comprehensive petrology and Sr-Nd-Hf-Ca isotope study of sheared peridotite xenoliths and clinopyroxene megacrysts from the c. 1150 Ma Premier kimberlite pipe on the central Kaapvaal craton in South Africa. New textural and mineral trace element evidence suggests that strong tectonic and magmatic overprinting affected the lower cratonic mantle over a vertical distance of ?50 km from the lithosphere-asthenosphere boundary located at ?200-225 km depth. Although modification of the central Kaapvaal cratonic mantle is commonly linked to the c. 2056 Ma Bushveld large igneous event, our thermobarometry, mantle redox, and Sr-Nd-Hf-Ca isotope data support a model in which volatile-rich low-volume melts and associated high-density fluids refertilized the lithosphere base shortly before or during asthenosphere-derived kimberlite and carbonatite magmatism at around 1150 Ma. This episode of lithospheric mantle enrichment was facilitated by exceptionally strong shear movements, as are recorded in the plastically deformed peridotites. We argue that stress-driven segregation of percolating carbonated melts contributed to megacryst formation along, or in close proximity to, shear zones within the cratonic mantle lithosphere. Integration of our results from the Kaapvaal craton and modern petrological concepts allows for the identification of a lithosphere-asthenosphere transition zone between ?150 and 225 km depth. This horizon is defined by intersections of the ?40-42 mW m-2 Premier paleogeotherm with (1) CO2-H2O-present solidus curves for peridotite (upper bound), and (2) typical mantle adiabats with potential temperatures between 1315 and 1420 °C (lower bound). At Premier, the most strongly deformed sheared peridotites occur mainly between ?160 and 185 km depth, firmly within the lithosphere-asthenosphere transition zone. Contrary to many previous models, we suggest that sheared peridotite formation occurs in localized deformation zones spaced out across the entire width of the lithosphere-asthenosphere transition zone, rather than being restricted to a single thin layer at the craton base where mantle flow causes viscous drag. Hence, plate-tectonic stresses acting on the lower cratonic lithosphere may be accommodated by extensive networks of shear zones, which provide transient pathways and sinks for percolating volatile-rich melts, linking the formation of megacrysts and sheared peridotites.
Abstract: Some geochemical models for basaltic and more primitive rocks suggest that their parental magmas have assimilated tens of weight percent of crustal silicate wall rock. But what are the thermodynamic limits for assimilation in primitive magmas? We pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus on modeling assimilation of wall-rock partial melts, which is thermodynamically more efficient compared to bulk assimilation of stoped wall-rock blocks in primitive igneous systems. In the simulations, diverse komatiitic, picritic, and basaltic parental magmas assimilate progressive partial melts of preheated average lower, middle, and upper crust in amounts allowed by thermodynamics. Our results indicate that it is difficult for any subalkaline primitive magma to assimilate more than 20?30 wt% of upper or middle crust before evolving to compositions with higher SiO2 than a basaltic magma (52 wt%). On the other hand, typical komatiitic magmas have thermodynamic potential to assimilate as much as their own mass (59?102 wt%) of lower crust and retain a basaltic composition. The compositions of the parental melt and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition given typical temperatures. These findings have important implications for the role of assimilation in the generation and evolution of, e.g., ultramafic to mafic trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type anorthosites.
Abstract: Over the last 20 years, new concepts have emerged into understanding the processes that lead to build up to large silicic explosive eruptions based on integration of geophysical, geochemical, petrological, geochronological and dynamical modelling. Silicic melts are generated within magma systems extending throughout the crust by segregation from mushy zones. Segregated melt layers become unstable and can assemble into ephemeral upper crustal magma chambers rapidly prior to eruption. In the next 10 years, we can expect major advances in dynamical models as well as in analytical and geophysical methods, which need to be underpinned in field research.
Abstract: Available data suggest that the breakup of the Neoarchean Kenorland supercontinent at 2.5-2.4 Ga was likely triggered by a large mantle plume upwelling that caused significant magmatism. Here, we present 2D high-resolution magmatic-thermomechanical numerical models of extension of the continental crust underplated by a hot mantle plume material. Using this model, it is demonstrated that mantle plume underplating generates a large amount of mafic melt by decompression melting. This melt penetrates into the extending continental crust along normal faults thereby forming multiple generations of mafic dyke-like intrusions along normal faults. In case of extension velocity of 0.2-1 cm/yr, lower crustal heating and hot mafic melt emplacement may cause partial melting of the continental crust that can generate significant volume of felsic melts. This in turn triggers emplacement of felsic intrusions that temporarily and spatially associate with the mafic dyke-like intrusions. The modeling results agree well with geological data from the Karelian Craton and provide possible explanation for the observed association of Paleoproterozoic mafic dykes and felsic intrusions which formed in a relatively short time interval (up to 20 Myrs) in the early stages of the supercontinent breakup.
Abstract: The mineralogy of carbonatites reflects both the diversity of the sources of their parent magmas and their unusual chemistry. Carbonatites contain diverse suites of both primary magmatic minerals and later hydrothermal products. We present a summary of the variety of minerals found in carbon-atites, and note the economic importance of some of them, particularly those that are major sources of "critical elements", such as Nb and rare earth elements (REEs), which are essential for modern technological applications. Selected mineral groups are then discussed in detail: the REE carbonates, the alkali-rich ephemeral minerals that are rarely preserved but that may be important in the petrogenesis of carbonatites and their metasomatic haloes in adjacent rocks, and the Nb-rich oxides of the pyrochlore supergroup.-
Nature Communications, 10.1038/s41467-022-28633-w 12p. Pdf
Mantle
magmatism
Abstract: Volcanism on Earth and on other planets and satellites is dominated by the eruption of low viscosity magmas. During explosive eruption, high melt temperatures and the inherent low viscosity of the fluidal pyroclasts allow for substantial post-fragmentation modification during transport obscuring the record of primary, magmatic fragmentation processes. Here, we show these syn-eruption modifications, in the form of melt stripping and agglutination, to be advantageous for providing fundamental insights into lava fountain and jet dynamics, including eruption velocities, grain size distributions and melt physical properties. We show how enigmatic, complex pyroclasts termed pelletal lapilli form by a two-stage process operating above the magmatic fragmentation surface. Melt stripping from pyroclast surfaces creates a spray of fine melt droplets whilst sustained transport in the fountain allows for agglutination and droplet scavenging, thereby coarsening the grain size distribution. We conclude with a set of universal regime diagrams, applicable for all fluidal fountain products, that link fundamental physical processes to eruption conditions and melt physical properties.
