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Jussi S Heinonen - One of the best experts on this subject based on the ideXlab platform.
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deep mixing of mantle Melts beneath continental flood basalt provinces constraints from olivine hosted Melt Inclusions in primitive magmas
Geochimica et Cosmochimica Acta, 2017Co-Authors: Eleanor S Jennings, John Maclennan, Jussi S HeinonenAbstract: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 reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Parana–Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood 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 identified in other CFB provinces and contrasts with findings 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, Galapagos, Samoa), may reflect 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.
Babita Rani Choudhary - One of the best experts on this subject based on the ideXlab platform.
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Melt Inclusion evidence for mantle heterogeneity and magma degassing in the deccan large igneous province india
Lithos, 2019Co-Authors: Babita Rani Choudhary, Gajananrao Jadhav, Benedetto De Vivo, Mysore Santosh, E V S S K BabuAbstract:Abstract Silicate Melt Inclusions (MI) trapped in minerals provide direct tools to evaluate source characteristics and magma evolution including magma chamber processes. Here we present results from the study of MIs in basalts from the Western Ghats region of the Deccan Large Igneous Province (LIP) in India. The MIs in plagioclase and clinopyroxene phenocrysts were analyzed. The MIs exhibit post-entrapment modification by in situ crystal fractionation, chemical interaction with the host phenocryst (plagioclase or clinopyroxene), degassing/decrepitation, and crystallization of daughter minerals during cooling. The MIs display variable silica (SiO2 41–68 wt%), low potassium, and high Fe Ti contents, corresponding to differentiation of basaltic to andesitic magma. Our data indicate H2O content of about 2 wt% which is consistent with recent evidence for elevated primary H2O content in the Deccan and other LIPs, corresponding to elevated H2O in the lithospheric mantle and plumes passing through the mid-mantle inheriting the hydrous nature.
Le Zhang - One of the best experts on this subject based on the ideXlab platform.
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the origins of high ti and low ti magmas in large igneous provinces insights from Melt Inclusion trace elements and sr pb isotopes in the emeishan large igneous province
Lithos, 2019Co-Authors: Le Zhang, Monica R Handler, Yadong Wu, Lei Zhang, Shengping Qian, Qing Yang, Yigang XuAbstract:Abstract High-Ti and low-Ti lava series occur simultaneously in many large igneous provinces (LIP); however, their origins remain debated. To address this issue, we performed a detailed geochemical study of olivine and olivine-hosted Melt Inclusions from the Dali picrites in the Emeishan LIP. Although the Dali picrite lavas have a limited range of whole-rock Ti/Y ratios (~356–404; Wu et al., 2018), olivine-hosted Melt Inclusions have Ti/Y ratios that range from low-Ti to high-Ti (213–741). The Sr Pb isotopic compositions and trace element ratios (e.g., Nb/U and Nb/La) of the Melt Inclusions have a restricted range and display little correlation with Ti/Y ratios, indicating that the low-Ti and high-Ti lavas share compositionally similar source. Trace element mass balance modeling of the Emeishan lavas indicates a mantle source comprising ~84% peridotite from the lower mantle, ~15% recycled MORB, and ~1% pelagic sediment. Modeling results for the Emeishan and Karoo LIPs show that partial Melts formed at greater depth in the mantle have higher Ti contents and Ti/Y ratios than those formed at shallower depths. Mixing between Melts formed at different depths with different degrees of partial Melting can produce the continuous range of Ti/Y ratios observed. The results demonstrate the strong influence of partial Melting degree and pressure on the Ti content and Ti/Y ratio of partial Melts.
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Melt Inclusions in the olivine from the nantianwan intrusion implications for the parental magma of ni cu pge sulfide bearing mafic ultramafic intrusions of the 260 ma emeishan large igneous province sw china
Journal of Asian Earth Sciences, 2017Co-Authors: Le Zhang, Christina Yan WangAbstract:Abstract Olivine-hosted Melt Inclusions provide an archive of the parental magma and early magma history that is unavailable from bulk-rock analyses of cumulates. For those olivine-bearing mafic-ultramafic intrusions, a combined in situ analysis of major elements and Pb isotopic compositions for the Melt Inclusions and host olivine crystals may provide an effective way to understand the nature of the parental magma of the intrusions. In this study, we take the Nantianwan intrusion in the Emeishan large igneous province (SW China) as an example to analyze the Melt Inclusions and the host olivine. The Nantianwan intrusion is mainly composed of gabbronorite, with minor olivine gabbro. The olivine crystals in the olivine gabbro have Fo contents varying from 81.1 to 89.2 and Ni from 0.05 to 0.30 wt.%. The Melt Inclusion hosted in the most Mg-rich olivine has 50.9 wt.% SiO2, 1.0 wt.% TiO2, 15.1 wt.% MgO and 2.9 wt.% Na2O + K2O, indicating that the parental magma of the intrusion was of high-Mg basaltic composition. The Melt Inclusions overall have 208Pb/206Pb ratios of 2.0567–2.1032 and 207Pb/206Pb of 0.8287–0.8481, similar to the Pb isotopic compositions of the Emeishan flood basalts and consistent with insignificant crustal contamination. Given that the Nantianwan intrusion contains the most Mg-rich olivine among the Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions in the Emeishan LIP, we infer that the composition of the Melt Inclusion in the most Mg-rich olivine from the Nantianwan intrusion may represent the least evolved parental magma of the Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions in the Emeishan LIP. This can be further used to constrain the magma process related to Ni-Cu-(PGE) sulfide mineralization.
John Maclennan - One of the best experts on this subject based on the ideXlab platform.
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Melt Inclusion constraints on petrogenesis of the 2014 2015 holuhraun eruption iceland
Contributions to Mineralogy and Petrology, 2018Co-Authors: Margaret E Hartley, John Maclennan, Eniko Bali, David A Neave, S A HalldorssonAbstract:The 2014–2015 Holuhraun eruption, on the Barðarbunga volcanic system in central Iceland, was one of the best-monitored basaltic fissure eruptions that has ever occurred, and presents a unique opportunity to link petrological and geochemical data with geophysical observations during a major rifting episode. We present major and trace element analyses of Melt Inclusions and matrix glasses from a suite of ten samples collected over the course of the Holuhraun eruption. The diversity of trace element ratios such as La/Yb in Holuhraun Melt Inclusions reveals that the magma evolved via concurrent mixing and crystallization of diverse primary Melts in the mid-crust. Using olivine–plagioclase–augite–Melt (OPAM) barometry, we calculate that the Holuhraun carrier Melt equilibrated at 2.1 ± 0.7 kbar (7.5 ± 2.5 km), which is in agreement with the depths of earthquakes (6 ± 1 km) between Barðarbunga central volcano and the eruption site in the days preceding eruption onset. Using the same approach, Melt Inclusions equilibrated at pressures between 0.5 and 8.0 kbar, with the most probable pressure being 3.2 kbar. Diffusion chronometry reveals minimum residence timescales of 1–12 days for Melt Inclusion-bearing macrocrysts in the Holuhraun carrier Melt. By combining timescales of diffusive dehydration of Melt Inclusions with the calculated pressure of H2O saturation for the Holuhraun magma, we calculate indicative magma ascent rates of 0.12–0.29 m s−1. Our petrological and geochemical data are consistent with lateral magma transport from Barðarbunga volcano to the eruption site in a shallow- to mid-crustal dyke, as has been suggested on the basis of seismic and geodetic datasets. This result is a significant step forward in reconciling petrological and geophysical interpretations of magma transport during volcano-tectonic episodes, and provides a critical framework for the interpretation of premonitory seismic and geodetic data in volcanically active regions.
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olivine hosted Melt Inclusions as an archive of redox heterogeneity in magmatic systems
Earth and Planetary Science Letters, 2017Co-Authors: Margaret E Hartley, John Maclennan, Oliver Shorttle, Yves Moussallam, Marie EdmondsAbstract:Abstract The redox state of volcanic products determines their leverage on the oxidation of Earth's oceans and atmosphere, providing a long-term feedback on oxygen accumulation at the planet's surface. An archive of redox conditions in volcanic plumbing systems from a magma's mantle source, through crustal storage, to eruption, is carried in pockets of Melt trapped within crystals. While Melt Inclusions have long been exploited for their capacity to retain information on a magma's history, their permeability to fast-diffusing elements such as hydrogen is now well documented and their retention of initial oxygen fugacities ( f O 2 ) could be similarly diffusion-limited. To test this, we have measured Fe 3+ /ΣFe by micro-XANES spectroscopy in a suite of 65 olivine-hosted Melt Inclusions and 9 matrix glasses from the AD 1783 Laki eruption, Iceland. This eruption experienced pre-eruptive mixing of chemically diverse magmas, syn-eruptive degassing at the vent, and post-eruptive degassing during lava flow up to 60 km over land, providing an ideal test of whether changes in the f O 2 of a magma may be communicated through to its cargo of crystal-hosted Melt Inclusions. Melt Inclusions from rapidly quenched tephra samples have Fe 3+ /ΣFe of 0.206 ± 0.008 (ΔQFM of +0.7 ± 0.1), with no correlation between their f O 2 and degree of trace element enrichment or differentiation. These Inclusions preserve the redox conditions of the mixed pre-eruptive Laki magma. When corrected for fractional crystallisation to 10 wt.% MgO, these Inclusions record a parental magma [Fe 3+ /ΣFe] (10) of 0.18 (ΔQFM of +0.4), significantly more oxidised than the Fe 3+ /ΣFe of 0.10 that is often assumed for Icelandic basalt magmas. Melt Inclusions from quenched lava selvages are more reduced than those from the tephra, having Fe 3+ /ΣFe between 0.133 and 0.177 (ΔQFM from −0.4 to +0.4). These Inclusions have approached equilibrium with their carrier lava, which has been reduced by sulfur degassing. The progressive re-equilibration of f O 2 between Inclusions and carrier Melts occurs on timescales of hours to days, causing a drop in the sulfur content at sulfide saturation (SCSS) and driving the exsolution of immiscible sulfide globules in the Inclusions. Our data demonstrate the roles of magma mixing, progressive re-equilibration, and degassing in redox evolution within magmatic systems, and the open-system nature of Melt Inclusions to f O 2 during these processes. Redox heterogeneity present at the time of Inclusion trapping may be overprinted by rapid re-equilibration of Melt Inclusion f O 2 with the external environment, both in the magma chamber and during slow cooling in lava at the surface. This can decouple the Melt Inclusion archives of f O 2 , major and trace element chemistry, and mask associations between f O 2 , magmatic differentiation and mantle source heterogeneity unless the assembly of diverse magmas is rapidly followed by eruption. Our tools for understanding the redox conditions of magmas are thus limited; however, careful reconstruction of pre- and post-eruptive magmatic history has enabled us to confirm the relatively oxidised nature of ocean island-type mantle compared to that of mid-ocean ridge mantle.
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deep mixing of mantle Melts beneath continental flood basalt provinces constraints from olivine hosted Melt Inclusions in primitive magmas
Geochimica et Cosmochimica Acta, 2017Co-Authors: Eleanor S Jennings, John Maclennan, Jussi S HeinonenAbstract: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 reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Parana–Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood 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 identified in other CFB provinces and contrasts with findings 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, Galapagos, Samoa), may reflect 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.
Vadim S Kamenetsky - One of the best experts on this subject based on the ideXlab platform.
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dissolution of mantle orthopyroxene in kimberlitic Melts petrographic geochemical and Melt Inclusion constraints from an orthopyroxenite xenolith from the udachnaya east kimberlite siberian craton russia
Lithos, 2021Co-Authors: Vadim S Kamenetsky, Adam Abersteiner, Alexander V Golovin, K Goemann, Kathy EhrigAbstract:Abstract Reconstructing the original composition of kimberlite Melts in the mantle and delineating the processes that modify them during magmatic ascent and emplacement in the crust remains a significant challenge in kimberlite petrology. One of the most significant processes commonly cited to drive initial kimberlite Melts towards more Si-Mg-rich compositions and decrease the solubility of CO2 is the assimilation of mantle orthopyroxene. However, there is limited direct evidence to show the types of reactions that may occur between mantle orthopyroxene and the host kimberlite Melt. To provide new constraints on the interaction between orthopyroxene and parental kimberlite Melts, we examined a fresh (i.e. unmodified by secondary/post-magmatic alteration) orthopyroxenite xenolith, which was recovered from the serpentine-free units of the Udachnaya-East kimberlite (Siberian Craton, Russia). This xenolith is composed largely of orthopyroxene (~ 90%), along with lesser olivine and clinopyroxene and rare aluminous magnesian chromite. We can show that this xenolith was invaded by the host kimberlite Melt along grain interstices and fractures, where it partially reacted with orthopyroxene along the grain boundaries and replaced it with aggregates of compositionally distinct clinopyroxene, olivine and phlogopite, along with subordinate Fe-Cr-Mg spinel, Fe Ni sulphides and djerfisherite (K6(Fe,Ni,Cu)25S26Cl). Primary Melt Inclusions in clinopyroxene replacing xenolith-forming orthopyroxene, as well as secondary Melt Inclusion trails in xenolith orthopyroxene, clinopyroxene and olivine are composed of similar daughter mineral assemblages that consist largely of: Na K chlorides, along with varying proportions of phlogopite, Fe-Cu-Ni sulphides, djerfisherite, rasvumite (KFe2S3), Cr-Fe-Mg spinel, nepheline and apatite, and rare rutile, sodalite, barite, olivine, Ca-K-Na carbonates and Na K sulphates. The Melt entrapped by these Inclusions likely represent the hybrid products produced by the invading kimberlite Melt reacting with orthopyroxene in the xenolith. The mechanism that could explain the partial replacement of orthopyroxene in this xenolith by clinopyroxene, olivine and phlogopite could be attributed to the following reaction: Orthopyroxene + Carbonatitic (Melt) ➔ Olivine + Clinopyroxene + Phlogopite + CO2. This reaction is supported by theoretical and experimental studies that advocate the dissolution of mantle orthopyroxene within an initially silica-poor and carbonate-rich kimberlite Melt. The mineral assemblages replacing orthopyroxene in the xenolith, together with hosted Melt Inclusions, suggests that the kimberlitic Melt prior to reaction with orthopyroxene was likely carbonate-rich and Na-K-Cl-S bearing. The paucity of carbonate in the reaction zones around orthopyroxene and in Melt Inclusions in clinopyroxene replacing xenolith-forming orthopyroxene and xenolith minerals (orthopyroxene, clinopyroxene and olivine) is attributed to the consumption of carbonates and subsequent exsolution of CO2 by the proposed decarbonation reaction. Concluding, we propose that this orthopyroxenite xenolith provides a rare example of the types of reactions that can occur between mantle orthopyroxene and the host kimberlite Melt. The preservation of this xenolith and zones around orthopyroxene present new insights into the composition and evolution of parental kimberlite Melts and CO2 exsolution.
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petrographic and Melt Inclusion constraints on the petrogenesis of a magmaclast from the venetia kimberlite cluster south africa
Chemical Geology, 2017Co-Authors: Vadim S Kamenetsky, Adam Abersteiner, Andrea Giuliani, David PhillipsAbstract:Abstract Kimberlitic magmaclasts are discrete ovoid magmatic fragments that formed prior to emplacement from disrupted kimberlite magma. To provide new constraints on the origin and evolution of the kimberlite Melts, we document the mineralogy and petrography of a magmaclast recovered from one of the ca. 520 Ma Venetia kimberlites, South Africa. The sample (BI9883) has a sub-spherical shape and consists of a ~ 10 mm diameter central olivine macrocryst, surrounded by porphyritic kimberlite. The kimberlitic material consists of concentrically aligned, altered olivine phenocrysts, set in a crystalline groundmass of calcite, chromite, perovskite, phlogopite, apatite, ilmenite, titanite, sulphides, rutile and magnetite along with abundant alteration phases (i.e. serpentine, talc and secondary calcite). These features are typical of archetypal hypabyssal kimberlites. We examined primary fluid/Melt Inclusions in chromite, perovskite and apatite containing a diversity of daughter phases. Chromite and perovskite host polycrystalline Inclusions containing abundant alkali-carbonates (i.e. enriched in K, Na, Ba, Sr), phosphates, Na-K chlorides, sulphides and equal to lesser quantities of olivine, phlogopite and pleonaste. In contrast, apatite hosts polycrystalline assemblages with abundant alkali-carbonates and Na-K chlorides and lesser amounts of olivine, monticellite and phlogopite. Numerous solid Inclusions of shortite (Na 2 Ca 2 (CO 3 ) 3 ), Na-Sr-carbonates and apatite occur in groundmass calcite along with fluid Inclusions containing daughter crystals of Na-carbonates and Na-chlorides. The primary Inclusions in chromite, perovskite and apatite are considered to represent remnants of fluid(s)/Melt(s) trapped during crystallisation of the host minerals, whereas the fluid Inclusions in calcite are probably secondary in origin. The component proportions of these primary fluid/Melt Inclusions were estimated in an effort to constrain the composition of the evolving kimberlite Melt. These estimates suggest Melt evolution from a silicate-carbonate kimberlite Melt that became increasingly enriched in carbonates, phosphates, alkalis and chlorides, in response to the fractional crystallisation of constituent minerals (i.e. olivine to apatite). The concentric alignment of crystals around the olivine kernel and ovoid shape of the magmaclast can be ascribed to the low viscosity of the kimberlite Melt and rapid rotation whilst in a liquid or partial crystalline state, or to progressive layer-by-layer growth of the magmaclast. Although the mineralogy of our sample is similar to hypabyssal kimberlites worldwide, it differs from hypabyssal kimberlite units in the main Venetia pipes, which contain monticellite-phlogopite rich assemblages and segregationary matrix textures. Therefore magmaclast BI9883 probably originated from a batch of magma distinct from those that produced known hypabyssal units within the Venetia kimberlite cluster.
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silicate natrocarbonatite liquid immiscibility in 1917 eruption combeite wollastonite nephelinite oldoinyo lengai volcano tanzania Melt Inclusion study
Lithos, 2012Co-Authors: Victor V. Sharygin, Anatoly N. Zaitsev, Vadim S Kamenetsky, Maya B KamenetskyAbstract:Primary silicate–Melt and carbonate–salt Inclusions occur in the phenocrysts (nepheline, fluorapatite, wollastonite, clinopyroxene) in the 1917 eruption combeite–wollastonite nephelinite at Oldoinyo Lengai. Silicate– Melt Inclusions in nepheline clearly show liquid immiscibility phenomena expressed in the presence of carbonate globules in silicate glass. The coexistence of Inclusions with markedly different proportions of silicate glass+vapor-carbonate globule in the core of nepheline phenocrysts, the presence of carbonate– salt Inclusions in fluorapatite and our heating experiments strongly suggest that their entrapment began at temperatures higher than 1130 °C in an intermediate chamber when initial carbonated nephelinite Melt was heterogeneous and represented a mixture of immiscible liquids. Silicate–natrocarbonatite Melt immiscibility took place at high temperature and immiscible nephelinite and carbonatite liquids coexisted over a wide temperature range from ≥1130 °C to 600 °C. Homogenization of a carbonate globule (dissolution of the gas bubble in carbonate Melt) at 900–940 °C indicates that after separation from silicate magma the natrocarbonatite represented homogeneous liquid in the 900–1130 °C temperature range, whereas below these temperatures immiscible Melts of different composition and fluid phase have separated from it. The bulk composition of homogeneous natrocarbonatite Melt may be estimated as ≈20% CaF2, 40–60% (Na, K)2CO3 and 20–40% CaCO3 based on the coexistence of nyerereite, calcite and fluorite and the rapid phase transition (carbonate aggregate→carbonate liquid) at 550–570 °C observed in vapor-carbonate globules of nepheline-hosted silicate–Melt Inclusions and on the Na2CO3–CaCO3–CaF2 phase diagram. Silicate glasses of nepheline-hosted immiscible Inclusions drastically differ from host nephelinite in the abundance of major and trace elements. They are high peralkaline ((Na+K)/Al — up to 9.5) and virtually free of water (H2O<0.6 wt.%). Their very high Zr/Hf and Nb/Ta ratios and Li contents indicate that these silicate glasses represent the most evolved compositions at Oldoinyo Lengai. The peralkaline character of nephelinite Melt is expressed in the composition of the daughter mineral assemblage within silicate–Melt Inclusions in nepheline (delhayelite, leucite, mica, clinopyroxene). These minerals show strong deficiency in Al and enrichment in Fe3+ that is also common to the groundmass of the Oldoinyo Lengai combeite–wollastonite nephelinites.
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the origin of medium k ankaramitic arc magmas from lombok sunda arc indonesia mineral and Melt Inclusion evidence
Chemical Geology, 2007Co-Authors: Marlina Elburg, Vadim S Kamenetsky, John Foden, A V SobolevAbstract:High-calcium, nepheline-normative ankaramitic basalts (MgO>10 wt.%, CaO/Al2O3>1) from Rinjam volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo(85-92)) with Inclusions of spinel (Cr# 58-77) and crystallised Melt. Olivine crystals have variable but on average low NiO (0.10-0.23 wt.%) and high CaO (0.22-0.35 wt.%) contents for their forsterite number. The CaO content of Fo(89-91) olivine is negatively correlated with the Al2O3 content of enclosed spinel (9-15 wt.%) and positively correlated with the CaO/Al2O3 ratios of Melt Inclusions (0.9-1.5). Major and trace element patterns of Melt Inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of Melt, with compositions similar to the Melt Inclusions. The liquidus temperature of the magma was similar to 1275 degrees C, and its oxygen fugacity <= FMQ + 2.5. Correlations between KO, Zr, Th and LREE in the Melt Inclusions are interpreted to reflect variable degrees of Melting of the source; correlations between Al2O3, Na2O, Y and HREE are influenced by variations in the mineralogy of the source. The Melts probably formed from a water-poor, clinopyroxene-rich mantle source.
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primary aqueous fluids in rhyolitic magmas Melt Inclusion evidence for pre and post trapping exsolution
Chemical Geology, 2007Co-Authors: Paul Davidson, Vadim S KamenetskyAbstract:This study examines Melt Inclusions containing bubbles of aqueous fluid (L, V, and L + V), occurring in rhyolites from the Okataina Volcanic Centre, New Zealand, and the Rio Blanco Cu-Mo deposit, Chile. We demonstrate that these aqueous fluids coexisted with silicate Melts (magmas) and represent either post-trapping exsolution (in the case of Okataina), or co-trapping of phases already coexisting in the magma (in the case of Rio Blanco). Microthermometry proves that some of the bubbles are a single-phase aqueous liquid, and all are shown by PIXE analysis to be metal rich saline solutions. As such, these aqueous fluids provide the closest approximation to direct testing of the proposition that cooling magmas exsolve metal-rich aqueous fluid. In the case of pre-trapping exsolution at Rio Blanco we show that some Inclusions record and preserve magmatic emulsions (Melt + aqueous fluid) that are the first stage in the evolution of hydrothermal fluids. We demonstrate that heating experiments on volatilerich Melt Inclusions can produce in-situ exsolution of hypersaline metal-rich aqueous fluid bubbles, potentially permitting magmachamber processes to be experimentally modelled.