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J.a. Davies - One of the best experts on this subject based on the ideXlab platform.
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The transition from granite to banded aplite-pegmatite sheet complexes: An example from Megiliggar Rocks, Tregonning topaz granite, Cornwall
Lithos, 2018Co-Authors: Karel Breiter, Axel H. E. Müller, Jana Ďurišová, Tomas Hrstka, Zuzana Korbelová, M. Vašinová Galiová, Beth Simons, Robin K. Shail, Ben J. Williamson, J.a. DaviesAbstract:Abstract The genetic relationship between a granite pluton and adjacent complex of rare-metal pegmatite-aplite-banded sheets (Megiliggar Sheet Complex - MSC) has been studied at the border of the Tregonning topaz granite at Megiliggar Rocks, Cornwall, SW England. Similarities in whole-Rock chemical and mineralogical compositions, together with a gradual change in textures away from the granite margin, provide strong evidence for a genetic link between the Tregonning Granite and MSC. The sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton. The escaping melt was peraluminous, had a composition near the F, B, Li slightly enriched granite minimum, and, in comparison with other Cornish granites, was enriched in F, Li, Rb, Cs, Sn, W, Nb, Ta, and U, and depleted in Fe, Mg, Ca, Sr, Th, Zr, and REE. With increasing distance from the Tregonning Granite, the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets (i.e. combinations of bands with granitic, aplitic and pegmatitic textures), then layered aplite-pegmatites; this sequence becoming progressively more depleted in the fluxing and volatile elements F, Li, Rb, and Cs, but showing no change in Zr/Hf ratios. The fixed Zr/Hf ratio is interpreted as indicating a direct genetic link (parental melt) between all Rock types, however the melt progressively lost fluxing and volatile elements with distance from the granite pluton, probably due to wall-Rock Reaction or fluid exsolution and migration via fractures. Differentiation of the primary melt into Na-Li-F-rich and separate K-B-rich domains was the dominant chemical process responsible for the textural and mineral diversity of the MSC. On a large (cliff-section) scale, the proximal Na-Li-F-rich leucogranite passes through complex sheets into K-B-rich aplite-pegmatites, whilst at a smaller (
Jens Fiebig - One of the best experts on this subject based on the ideXlab platform.
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the origin of the fumaroles of la solfatara campi flegrei south italy
Geochimica et Cosmochimica Acta, 2007Co-Authors: Stefano Caliro, Giovanni Chiodini, Roberto Moretti, R Avino, D Granieri, M Russo, Jens FiebigAbstract:Abstract The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly reveals a double thermobarometric signature. A first signature at temperatures of about 360 °C was inferred by methane-based chemical–isotopic geoindicators and by the H2/Ar geothermometer. These high temperatures, close to the critical point of water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A second signature was found to be at around 200–240 °C. At these temperatures, the kinetically fast reactive species (H2 and CO) re-equilibrate in a pure vapor phase during the rise of the plume. A combination of these observations with an original interpretation of the oxygen isotopic composition of the two dominant species, i.e. H2O and CO2, shed light on the origin of fumarolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at about 360 °C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (δD ∼ −20‰, δ18O ∼10‰) and a CO2-rich composition ( X CO 2 ∼ 0.4 ) has been inferred for the magmatic fluids, while for the hydrothermal component a meteoric origin and a CO2 fugacity fixed by fluid-Rock Reaction at high temperatures have been estimated. In the time the fraction of magmatic fluids in the fumaroles increased (up to ∼0.5) at each seismic and ground uplift crisis (bradyseism) which occurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO2-rich magmatic fluids at the bottom of the hydrothermal system.
Henry J B Dick - One of the best experts on this subject based on the ideXlab platform.
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melt Rock Reaction in the mantle mantle troctolites from the parece vela ancient back arc spreading center
Journal of Petrology, 2013Co-Authors: Henry J B Dick, Alessio Sanfilippo, Yasuhiko OharaAbstract:The Godzilla Megamullion is a giant oceanic core complex located in an extinct slow-spreading segment of the Parece Vela Basin (Philippine Sea). It exposes lower crust and mantle, offering an opportunity to unravel the architecture, composition, and magmatic processes operating in the shallow mantle of a back-arc basin. Here we present data on primitive troctolites and associated olivinegabbros close to the breakaway zone of the megamullion. The troctolites are subdivided into olivine (olivine460 vol. %) and plagioclase (plagioclase460 vol. %) sub-types. Both have textures and mineral compositions suggesting formation through melt^Rock Reaction.We suggest that the Ol-troctolites formed by dissolution and disaggregation of dunite with crystallization of new plagioclase and diopside by a mid-ocean ridge basalt (MORB)-like melt, producing a high-Mg melt.The Pl-troctolites probably formed by local dissolution of an anorthositic cumulate and crystallization of olivine, diopside and new plagioclase owing to mixing the high-Mg melt residual to the formation of the Ol-troctolites with melt crystallizing an anorthositic cumulate. We show that the Oland Pl-troctolites and associated Ol-gabbros were strongly influenced by melt^Rock Reaction with the enclosing peridotite, reflecting solidification by Reaction and conductive cooling of ascending melts in the mantle. The effects of the melt^Rock Reaction account for a sharp decrease in plagioclase anorthite content with respect to diopside Mg# and olivine forsterite content. Contrasting olivine nickel contents can constrain the environment of formation of olivine-rich Rocks in the lower crust and mantle, the extent and nature of the processes by which they form, and the melt compositions involved in the Reaction process. Comparison of olivine nickel contents in troctolite and gabbro occurrences in oceanic and ophiolite settings reveals a set of relationships consistent with this. Olivine gabbros from the 1508 m Hole 735B Atlantis Bank gabbro section have low nickel contents, whereas those from the 1400 m Hole U1309D Atlantis Massif gabbro section have high nickel. This is consistent with less-reacted infiltrating melt at the latter, and suggests that U1309D represents a deeper section of the lower crust near the crust^mantle transition, whereas the former has been shown to have crystallized near the dike^gabbro transition. Godzilla troctolites and gabbros bear clear similarities to East Pacific Rise (EPR) troctolites and gabbroic segregations drilled in dunites crosscutting residual mantle peridotite at Hess Deep Site 895.The latter represent multi-stage crystallization of stagnant melts in conduits beneath the EPR.Thus, it appears that the processes that formed the Godzilla troctolites occur beneath ocean ridges across the spreading rate spectrum.
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an assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions
Journal of Geophysical Research, 2009Co-Authors: Jessica M Warren, Nobumichi Shimizu, Chie Sakaguchi, Henry J B DickAbstract:[1] Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-Rock Reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.
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melt Rock Reaction in the lower oceanic crust and its implications for the genesis of mid ocean ridge basalt
Earth and Planetary Science Letters, 2008Co-Authors: Johan C Lissenberg, Henry J B DickAbstract:Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host Rock, and olivine is notably absent as a chadacryst despite being abundant in the host Rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of Reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such Reactions are enriched in Al2O3 and MgO and depleted in CaO and SiO2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al2O3–MgO–SiO2 relationships of MORB may result from melt–Rock Reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–Rock Reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB.
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formation of harzburgite by pervasive melt Rock Reaction in the upper mantle
Nature, 1992Co-Authors: Peter B Kelemen, Henry J B Dick, James E QuickAbstract:Many mantle peridotite samples are too rich in Si02 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/Rock Reaction zones in the Trinity peridotite provide evidence for Reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.
Karel Breiter - One of the best experts on this subject based on the ideXlab platform.
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The transition from granite to banded aplite-pegmatite sheet complexes: An example from Megiliggar Rocks, Tregonning topaz granite, Cornwall
Lithos, 2018Co-Authors: Karel Breiter, Axel H. E. Müller, Jana Ďurišová, Tomas Hrstka, Zuzana Korbelová, M. Vašinová Galiová, Beth Simons, Robin K. Shail, Ben J. Williamson, J.a. DaviesAbstract:Abstract The genetic relationship between a granite pluton and adjacent complex of rare-metal pegmatite-aplite-banded sheets (Megiliggar Sheet Complex - MSC) has been studied at the border of the Tregonning topaz granite at Megiliggar Rocks, Cornwall, SW England. Similarities in whole-Rock chemical and mineralogical compositions, together with a gradual change in textures away from the granite margin, provide strong evidence for a genetic link between the Tregonning Granite and MSC. The sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton. The escaping melt was peraluminous, had a composition near the F, B, Li slightly enriched granite minimum, and, in comparison with other Cornish granites, was enriched in F, Li, Rb, Cs, Sn, W, Nb, Ta, and U, and depleted in Fe, Mg, Ca, Sr, Th, Zr, and REE. With increasing distance from the Tregonning Granite, the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets (i.e. combinations of bands with granitic, aplitic and pegmatitic textures), then layered aplite-pegmatites; this sequence becoming progressively more depleted in the fluxing and volatile elements F, Li, Rb, and Cs, but showing no change in Zr/Hf ratios. The fixed Zr/Hf ratio is interpreted as indicating a direct genetic link (parental melt) between all Rock types, however the melt progressively lost fluxing and volatile elements with distance from the granite pluton, probably due to wall-Rock Reaction or fluid exsolution and migration via fractures. Differentiation of the primary melt into Na-Li-F-rich and separate K-B-rich domains was the dominant chemical process responsible for the textural and mineral diversity of the MSC. On a large (cliff-section) scale, the proximal Na-Li-F-rich leucogranite passes through complex sheets into K-B-rich aplite-pegmatites, whilst at a smaller (
Stefano Caliro - One of the best experts on this subject based on the ideXlab platform.
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the origin of the fumaroles of la solfatara campi flegrei south italy
Geochimica et Cosmochimica Acta, 2007Co-Authors: Stefano Caliro, Giovanni Chiodini, Roberto Moretti, R Avino, D Granieri, M Russo, Jens FiebigAbstract:Abstract The analysis of gaseous compositions from Solfatara (Campi Flegrei, South Italy) fumaroles since the early 1980s, clearly reveals a double thermobarometric signature. A first signature at temperatures of about 360 °C was inferred by methane-based chemical–isotopic geoindicators and by the H2/Ar geothermometer. These high temperatures, close to the critical point of water, are representative of a deep zone where magmatic gases flash the hydrothermal liquid, forming a gas plume. A second signature was found to be at around 200–240 °C. At these temperatures, the kinetically fast reactive species (H2 and CO) re-equilibrate in a pure vapor phase during the rise of the plume. A combination of these observations with an original interpretation of the oxygen isotopic composition of the two dominant species, i.e. H2O and CO2, shed light on the origin of fumarolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at about 360 °C by the vaporization of hydrothermal liquids. A typical ‘andesitic’ water type (δD ∼ −20‰, δ18O ∼10‰) and a CO2-rich composition ( X CO 2 ∼ 0.4 ) has been inferred for the magmatic fluids, while for the hydrothermal component a meteoric origin and a CO2 fugacity fixed by fluid-Rock Reaction at high temperatures have been estimated. In the time the fraction of magmatic fluids in the fumaroles increased (up to ∼0.5) at each seismic and ground uplift crisis (bradyseism) which occurred at Campi Flegrei, suggesting that bradyseismic crises are triggered by periodic injections of CO2-rich magmatic fluids at the bottom of the hydrothermal system.
