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Geoffrey H Howarth - One of the best experts on this subject based on the ideXlab platform.
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olivine Megacryst chemistry monastery kimberlite constraints on the mineralogy of the himu mantle reservoir in southern africa
Lithos, 2018Co-Authors: Geoffrey H HowarthAbstract:Abstract Olivine phenocryst chemistry is a useful tracer of mantle source lithology as olivine is typically the first mineral to crystallize in a range of mafic to alkaline magma types and contains geochemical information about the primary parent magma composition. It is particularly useful in discriminating between pyroxenite (i.e.,recycled crustal component) and peridotite mantle source lithologies. Radiogenic isotope studies of HIMU basalts have shown the important role of a recycled crustal component in the source. However, olivine chemistry of HIMU basalts suggests a dominantly peridotitic mantle source with a subduction-derived metasomatic carbonate component. To further investigate how olivine chemistry can be used to understand the source of HIMU magmas, I present major and trace element data for olivine Megacrysts related to the 89 Ma Cr-poor Megacryst suite from the Monastery kimberlite (South Africa), which have previously been interpreted to crystallize at high P-T conditions of ~1400 °C and ~5 GPa from a magma sourced from a HIMU reservoir. Olivine Megacrysts have high-Ni concentrations at a given Fo (forsterite) content, overlapping ocean island basalts (OIB) interpreted to have formed from pyroxenite-dominated sources but are distinct from typical olivine in HIMU basalts. However, they have low 100*Mn/Fe (0.8–1.1) and no correlation is observed between Ni and trace elements indicative of recycled components such as Co, Li, or Zn. The olivine Megacryst chemistry is similar to that of aillikite olivine interpreted to be controlled by phlogopite in the source rather than pyroxenite. Comparison with olivine chemistry from orangeites/Group II kimberlites (sourced from phlogopite-rich mantle lithologies) supports a phlogopite controlon low 100*Mn/Fe in olivine. Further comparison with olivine phenocryst chemistry of HIMU melilitites (76–58 Ma) in southern Africa suggests a mineralogically heterogeneous HIMU reservoir formed by metasomatic modification of the lithospheric mantle in southern Africa. Thus, olivine Megacryst chemistry supports interpretations for the recycled component of the HIMU reservoir as mineralogically complex metasomatic lithologies formed by the infiltration of subduction-derived melts into the base of the lithospheric mantle. In addition to carbonate-richHIMU mantle lithologies in southern Africa and worldwide, Monastery kimberlite olivine Megacrysts and olivine in melilitites suggest that a phlogopite-richHIMU lithology is present in southern Africa. An important question arising from this study is in reconciling the homogenous isotopic ratios of HIMU basalts worldwide with an apparent heterogeneous lithological source indicated by olivine chemistry.
D S Westerman - One of the best experts on this subject based on the ideXlab platform.
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rapid incremental assembly of the monte capanne pluton elba island tuscany by downward stacking of magma sheets
Geological Society of America Bulletin, 2010Co-Authors: Federico Farina, Andrea Dini, Fabrizio Innocenti, Sergio Rocchi, D S WestermanAbstract:The late Miocene Monte Capanne pluton (Elba Island, Italy) is characterized by the widespread occurrence of euhedral K-feldspar Megacrysts, for which variations in size and abundance have been determined at 392 stations. The variability of Megacryst distribution defines three main facies characterized by low (San Piero facies), high (Sant9Andrea facies), and intermediate (San Francesco facies) Megacryst abundance. The three facies show minor yet systematic differences in whole-rock major- and trace-element contents, isotopic composition, and biotite mineral chemistry, with no detectable link between chemical variabilities and Megacryst abundance. These results, along with the reconstruction of the crystallization sequence, suggest that the facies formed at depth as distinct magma batches with their own peculiar geochemical features, which were preserved after ascent and emplacement. The new geological map based on K-feldspar Megacryst distribution thus reveals the composite structure of the pluton, which was built up incrementally by downward stacking of three slightly different magma batches, resulting in a sheeted pluton in the intermediate-shallow crust. The three magma batches were emplaced in a short time sequence in order to have internal magmatic contacts preserved, thus giving a new perspective to the ongoing geochronological efforts to unravel age differences between internal plutonic facies.
William L. Griffin - One of the best experts on this subject based on the ideXlab platform.
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Chemical abrasion of zircon and ilmenite Megacrysts in the Monastery kimberlite: Implications for the composition of kimberlite melts
Chemical Geology, 2014Co-Authors: Vadim S. Kamenetsky, Elena Belousova, Andrea Giuliani, Maya B. Kamenetsky, Karsten Goemann, William L. GriffinAbstract:Abstract Ilmenite and zircon Megacrysts, among other minerals representing the subcontinental lithospheric mantle, are exclusively delivered to the surface by kimberlite magmas. The intimate association of ilmenite and zircon with their transporting kimberlite melts still remains perplexing, as these minerals do not belong to the kimberlite liquidus assemblage at crustal pressures. The ilmenite and zircon Megacrysts from the Monastery kimberlite (South Africa) represent a textbook example of the Megacryst suite. The Megacrysts show substantial chemical modification along contacts with the host kimberlite. Fine-grained “reaction” assemblages, comprising minerals rich in Zr (baddeleyite and sodium–zirconium silicates) and Ti (Ti–Fe oxides, perovskite, sphene, kassite), are present around zircon and ilmenite, respectively. At the zircon–ilmenite contact, chemical contributions from both minerals are recorded in Zr–Ti-rich phases such as calzirtite and zirkelite. The Megacrysts contain crystallised melt pools and secondary melt inclusions in healed fractures; their mineral assemblage is dominated by alkali-bearing phases, including silicates (nepheline, kalsilite, sodalite, phlogopite–tetraferriphlogopite), titanates (priderite, freudenbergite), zirconium silicates (khibinskite, parakeldyshite), carbonates (zemkorite, eitelite), phosphates (apatite, bradleyite, nahpoite), sulfates (aphthitalite) and chlorides (halite, sylvite). These inclusions and melt pools are interpreted to be produced by reaction between the Megacrysts and the transporting kimberlite melt, which infiltrated fractures in the Megacrysts. Most secondary minerals at contacts with kimberlite require a supply of Ca, which is readily available in the carbonatite component of the kimberlite magma. The enrichment of the encapsulated mineral assemblages in alkali and volatile elements (Na, K, S, Cl) also appears to originate from the kimberlite melt. The similar U–Pb ages and identical Hf-isotope compositions of the Megacryst assemblage (89.2 ± 2.8 Ma; eHf − 0.4 to + 1.3), the reaction assemblage (98 ± 7 Ma) and the host kimberlite (90 ± 4 Ma; eHf − 0.6 to + 1.7), imply their close genetic affinity. Although the Megacrysts and kimberlite magma originated from the same source at the same time, the chemical disequilibrium recorded in the alteration of Megacrysts precludes a simple “parental melt–cognate crystal” relationships. This apparent paradox can be resolved by considering the unmixing of a protokimberlite melt into silicate-oxide and carbonate liquids at mantle conditions.
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Trace element geochemistry of ilmenite Megacrysts from the Monastery kimberlite, South Africa
Lithos, 1992Co-Authors: Rory O. Moore, William L. Griffin, John J. Gurney, Chris Ryan, D.r. Cousens, Soey H. Sie, G.f. SuterAbstract:Abstract Ilmenite Megacrysts in the Monastery kimberlite occur both as discrete monomineralic crystals and intergrown with all the other phases of the Cr-poor Megacryst suite (cpx, opx, garnet, Fe-rich olivine, phlogopite and zircon). The ilmenites show systematic variations in trace element content which are interpreted in terms of a fractional crystallization model. Covariation of major and trace elements in the ilmenites with respect to their Nb content defines smooth curves, with breaks in the trends corresponding to changes in the inferred cumulate assemblage. Nb apparently behaved as an incompatible element in the Megacryst magma throughout its crystallization history, and the Nb content of ilmenite serves as a useful fractionation index. After the appearance of ilmenite, the crystallization sequence of the Monastery Megacryst suit is ilmenite+cpx+garnet+opx, followed by ilmenite+phlogopite, then ilmenite+zircon±phlogopite and finally by ilmenite+zircon+olivine+phlogopite. The incompatible behaviour of Nb indicates that ilmenite was overall never a predominant phase in the cumulate assemblage. Elevated Cr contents in late-stage ilmenites cannot be explained by the simple fractional crystallization model, and may require another process such as magma mixing or magma reaction with wall rock. The parent Megacryst magma must have been highly magnesian and enriched in incompatible trace elements and in this respect may have been similar to meimechite. The late differentiate of this magma cannot be kimberlite, but must be undersaturated and high in iron, potassium, titanium and incompatible elements.
H. Y. Mcsween - One of the best experts on this subject based on the ideXlab platform.
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petrology and geochemistry of olivine phyric shergottites lar 12095 and lar 12240 implications for their petrogenetic history on mars
Meteoritics & Planetary Science, 2019Co-Authors: E T Dunham, Brian J Balta, M Wadhwa, T G Sharp, H. Y. McsweenAbstract:Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite lnds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine Megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these Megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine Megacrysts have Mg# ~70, whereas Megacryst rims and groundmass olivines typically have Mg# ~58-60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.
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crystallization kinetics of olivine phyric shergottites
Meteoritics & Planetary Science, 2014Co-Authors: Megan Elizabeth Ennis, H. Y. McsweenAbstract:Crystal size distribution (CSD) and spatial distribution pattern (SDP) analyses are applied to the early crystallizing phases, olivine and pyroxene, in olivine-phyric shergottites (Elephant moraine [EET] 79001A, Dar al Gani [DaG] 476, and dhofar [Dho] 019) from each sampling locality inferred from Mars ejection ages. Trace element zonation patterns (P and Cr) in olivine are also used to characterize the crystallization history of these Martian basalts. Previously reported 2-D CSDs for these meteorites are re-evaluated using a newer stereographically corrected methodology. Kinks in the olivine CSD plots suggest several populations that crystallized under different conditions. CSDs for pyroxene in DaG 476 and EET 79001A reveal single populations that grew under steady-state conditions; pyroxenes in Dho 019 were too intergrown for CSD analysis. Magma chamber residence times of several days for small grains to several months for olivine Megacrysts are calculated using the CSD slopes and growth rates inferred from previous experimental data. Phosphorus imaging in olivines in DaG 476 and Dho 019 indicate rapid growth of skeletal, sector-zoned, or patchy cores, probably in response to delayed nucleation, followed by slow growth, and finally rapid dendritic growth with back-filling to form oscillatory zoning in rims. SPD analyses indicate that olivine and pyroxene crystals grew or accumulated in clusters rather than as randomly distributed grains. These data reveal complex solidification histories for Martian basalts, and are generally consistent with the formation at depth of olivine Megacryst cores, which were entrained in ascending magmas that crystallized pyroxenes, small olivines, and oscillatory rims on Megacrysts.
Federico Farina - One of the best experts on this subject based on the ideXlab platform.
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rapid incremental assembly of the monte capanne pluton elba island tuscany by downward stacking of magma sheets
Geological Society of America Bulletin, 2010Co-Authors: Federico Farina, Andrea Dini, Fabrizio Innocenti, Sergio Rocchi, D S WestermanAbstract:The late Miocene Monte Capanne pluton (Elba Island, Italy) is characterized by the widespread occurrence of euhedral K-feldspar Megacrysts, for which variations in size and abundance have been determined at 392 stations. The variability of Megacryst distribution defines three main facies characterized by low (San Piero facies), high (Sant9Andrea facies), and intermediate (San Francesco facies) Megacryst abundance. The three facies show minor yet systematic differences in whole-rock major- and trace-element contents, isotopic composition, and biotite mineral chemistry, with no detectable link between chemical variabilities and Megacryst abundance. These results, along with the reconstruction of the crystallization sequence, suggest that the facies formed at depth as distinct magma batches with their own peculiar geochemical features, which were preserved after ascent and emplacement. The new geological map based on K-feldspar Megacryst distribution thus reveals the composite structure of the pluton, which was built up incrementally by downward stacking of three slightly different magma batches, resulting in a sheeted pluton in the intermediate-shallow crust. The three magma batches were emplaced in a short time sequence in order to have internal magmatic contacts preserved, thus giving a new perspective to the ongoing geochronological efforts to unravel age differences between internal plutonic facies.