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Dean C Presnall - One of the best experts on this subject based on the ideXlab platform.
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melting behaviour of model Lherzolite in the system cao mgo al2o3 sio2 feo at 0 7 2 8 gpa
Journal of Petrology, 2000Co-Authors: Gudmundur H Gudfinnsson, Dean C PresnallAbstract:We have determined the isobarically univariant melting relations of the Lherzolite phase assemblage in the CMASF system in the only at small degrees of melting. 0·7–2·8 GPa pressure range. Isobarically, for every 1 wt % increase in the FeO content of the melt in equilibrium with the Lherzolite phase assemblage, the equilibrium temperature is lower KEY WORDS: CMASF; Lherzolite solidus; mantle melting by about 3–5°C. Relative to the solidus of model Lherzolite in the CaO–MgO–Al2O3–SiO2 system, melt compositions in the CMASF system are displaced slightly towards the alkalic side of the basalt tetrahedron. The transition on the solidus from spinel to plagioclase
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melting relations of model Lherzolite in the system cao mgo al2o3 sio2 at 2 4 3 4 gpa and the generation of komatiites
Journal of Geophysical Research, 1996Co-Authors: Gudmundur H Gudfinnsson, Dean C PresnallAbstract:Isobarically invariant phase relations in the CaO-MgO-Al2O3-SiO2 system (CMAS) involving the Lherzolite phase assemblage in equilibrium with liquid have been determined at 2.4–3.4 GPa. These phase relations form the solidus of model Lherzolite in the CMAS system. Our data, which include determinations of all phase compositions, are in excellent agreement with the 3.0 and 4.0 GPa points of Milholland and Presnall [1991] and Davis and Schairer [1965], respectively. The invariant transition on the P-T solidus curve from spinel- to garnet-Lherzolite at 3.0 GPa, 1575°C [Milholland and Presnall, 1991], is confirmed, but we observe that the theoretically required temperature depression on the solidus curve at this point is not experimentally detectable. Composition trends along the solidus take a sharp turn at the transition. In the spinel-Lherzolite stability field, melt compositions become increasingly Fo-normative and less En-normative with increasing pressure, but become less Fo-normative and more pyroxenitic as pressure increases in the garnet-Lherzolite stability field. Calculated melting reactions indicate that forsterite is in reaction relationship with the melt up to 3.0 GPa. Orthopyroxene is also in reaction relationship at pressures higher than just over 2.8 GPa and is the only phase in reaction relationship with the melt in the garnet-Lherzolite stability field. Comparison of the normative compositions and the CaO/Al2O3 values of the komatiites of Gorgona Island and of the Reliance Formation in Zimbabwe with the compositions of liquids along the solidus of model Lherzolite in the CMAS system indicates that the former komatiites were generated at pressures close to 3.7 GPa and the latter at close to 4.5 GPa, assuming that the melt generation occurred in the presence of the complete garnet-Lherzolite assemblage.
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melting behavior of simplified Lherzolite in the system cao mgo al2o3 sio2 na2o from 7 to 35 kbar
Journal of Petrology, 1994Co-Authors: Michael J Walter, Dean C PresnallAbstract:Phase equilibrium data have been collected for isobarically univariant melting of simplified Lherzolite compositions in the system CaO-MgO-Al2O3-SiO2-Na2O over a pressure range of 7-35 kbar. These data permit the melting behavior of a wide variety of model Lherzolite compositions to be determined quantitatively by algebraic methods. Two P-T univariant melting reactions, corresponding to plagioclase to spinel Lherzolite and spinel to garnet Lherzolite, are identified as peritectic-ty pe transitions and have positive Clapeyron slopes. The univariant curves move to higher pressures and temperatures with increasing Na2O in the liquid. The effect of the univariant curves on melting is to produce low-temperature regions and isobarically invariant melting intervals along Lherzolite solidi. In the plagioclase Lherzolite stability field, melting of four-phase model Lherzolite is pseudo-invari ant, occurring over small temperature intervals (~ 5 °C) and producing liquids that are quartz tholeiites at 8 kbar. Calculated equilibrium constants for plagioclase-liquid equilibria show both temperature and pressure dependence. Plagioclase with anorthite content (AN) >90 mol%, as observed in some oceanic basalts, can crystallize from liquids with 10%). On the basis of limited data in the garnet Lherzolite field, melts from garnet Lherzolite are more silica rich for a given degree of melting than melts from spinel Lherzolite, and liquid compositions trend toward enstatite with increase in pressure. Source fertility (especially Na2O content) has a strong control on the temperature of melting and liquid composition. Less fertile sources produce smaller amounts of liquids richer in normative silica. For certain bulk compositions (high SiO2 and low A12O3), spinel is not a stable phase along the Lherzolite solidus.
David H. Green - One of the best experts on this subject based on the ideXlab platform.
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melting of plagioclase spinel Lherzolite at low pressures 0 5gpa an experimental approach to the evolution of basaltic melt during mantle refertilisation at shallow depths
Lithos, 2013Co-Authors: Francoise Chalotprat, David H. Green, Trevor J Falloon, William O HibbersonAbstract:The presence of plagioclase + spinel Lherzolites among ocean floor samples and in some ophiolite complexes invites speculation on their origin and relationships to processes of magmatism and lithosphere refertilisation beneath mid-ocean ridges. In an experimental approach to their petrogenesis, we have determined the compositions of liquids and co-existing minerals in the six phase assemblage [liquid+olivine+orthopyroxene+clinopyroxene+plagioclase+spinel] at 0.5GPa and 1100°C to 1200°C. In our experimental approach we maintained the olivine Mg# [Mg/(Mg+Fe)] close to 90 (i.e., 88.8-95.5) but varied plagioclase from anorthite to albite. The major variations in liquid compositions are related to plagioclase composition. Liquids have much lower MgO and FeO and higher SiO2 and Al2O3 than liquids in the 6-phase plagioclase+spinel Lherzolite at 0.75GPa and 1GPa. Liquids are quartz-normative (silica-oversaturated) for plagioclase that are more calcic than An40 but nepheline-normative (critically silica-undersaturated) for plagioclase that are more sodic than An25. Liquid compositions are quite unlike natural MORB glasses with similar Mg# (i.e., compatible with parental magmas from lherzolitic mantle with Mg#≈90). Our study provides no support for models of MORB petrogenesis which suggest extraction of near-solidus melts from plagioclase Lherzolite at low pressure. Similarly, referring to numerical models of melting volumes beneath mid-ocean ridges (Langmuir et al., 1992; McKenzie and Bickle, 1988) in which melt increments are calculated for different sites and these increments pooled to form MORB, our data argue that melts equilibrated with plagioclase±spinel Lherzolite at <1GPa cannot be significant components of such 'pooled melt' focussed from within the melting volume. The compositions of minerals from plagioclase±spinel Lherzolite at Lanzo (northern Italy; Piccardo et al., 2007) are compared with our experimental assemblages at 0.5, 0.75 and 1GPa, leading to the conclusion that the Lanzo plagioclase±spinel Lherzolites equilibrated at pressures between 0.75 and 1GPa, at temperatures ~100-200°C below the solidus. Field, petrological and geochemical studies argue that the Lanzo plagioclase±spinel Lherzolites are 'refertilised' by the reaction of residual harzburgite or Lherzolite with percolating intergranular basaltic magma (Piccardo et al., 2007). The experimental study suggests that the process of refertilisation took place at depths of 25-30km. Our experimental data also define the co-variance of Na2O in coexisting plagioclase (An25 to An94) and clinopyroxene at 0.5 and 0.75GPa. From these data, the Na2O content of clinopyroxene can be used as a predictor for the co-existing plagioclase composition in the very common occurrences of partially serpentinised peridotite in which plagioclase is completely saussuritised. © 2013 Elsevier B.V..
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an experimental study of liquid compositions in equilibrium with plagioclase spinel Lherzolite at low pressures 0 75 gpa
Journal of Petrology, 2010Co-Authors: Francoise Chalotprat, David H. Green, Trevor J Falloon, William O HibbersonAbstract:Models of formation of basaltic crust at mid-ocean ridges by adiabatic upwelling of fertile mantle Lherzolite require knowledge of phase relations and phase compositions during melting at appropriate pressures. Spinelþplagioclase Lherzolites are found among peridotite samples from the ocean floor and ophiolitic exposures. At low pressure (51·2 GPa) the five-phase assemblage (olivineþorthopyroxeneþclinopyroxeneþplagioclaseþspinel) is present at the anhydrous Lherzolite solidus. New experimental data on mineral and melt compositions, at 0·75 GPa and 1140^12608C in the (CrþNaþFeþCaþMgþAlþSi) system, demonstrate smooth covariant relationships between oxides for melt compositions and in partition relationships both between mineral pairs and between minerals and melts. Molecular normative projections demonstrate that liquids on the five-phaseþliquid cotectic occupy a narrow compositional range. Of the mineral solid solutions that control the liquid composition, the [Ca/(CaþNa)] or anorthite/ albite content of the plagioclase is dominant and liquids vary from silica undersaturated and nepheline-normative at the sodic (oligoclase) end to orthopyroxene and quartz-normative at the calcic (anorthite) end of the cotectic. Spinel (Cr^Al) solid solution has limited variation on the five-phaseþliquid cotectic. It is very Cr-rich at the sodic end and has limited compositional variation from50 to 20 in Cr/(CrþAl) at the anorthitic end.With fixed plagioclase composition on the cotectic, melt compositions show small compositional shifts with Fe^Mg (at Mg# between 85 and 95) and with Cr/(CrþAl). The compositional vectors are consistent with effects observed in the end-member simple systems (FCMAS) and (CrCMAS). In comparing liquids at the anorthite end of the five-phaseþliquid cotectic with those on the Cr-free CMAS four-phaseþliquid cotectic at 0·75 GPa, it is evident that the presence of Al-rich Cr^Al spinel shifts liquid compositions to more silica-rich and silica-oversaturated composition.These experimentally defined melt compositions in equilibrium with plagioclaseþspinel Lherzolite are unlike quenched glasses from mid-ocean ridge settings.The data do not support models of mantle upwelling at low potential temperature (12808C) that produces low melt fractions at low pressures, leaving residual plagioclaseþspinel Lherzolite. The detailed mineral compositional data at the solidus provide a template for comparison with natural plagioclaseþspinel Lherzolites refertilized by porous reactive flow.
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derivation of potassic shoshonitic magmas by decompression melting of phlogopite pargasite Lherzolite
Lithos, 2004Co-Authors: Rommulo Vieira Conceicao, David H. GreenAbstract:A model metasomatized Lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet Lherzolite mineralogy at 1195 °C and with garnet Lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+pargasite Lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite Lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel Lherzolite mineralogy are silica-saturated trachyandesite (∼5% melt fraction, ∼3% H2O) to silica-oversaturated basaltic andesite (∼8% melt fraction, 4.5% H2O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric Lherzolite with minor phlogopite and pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at ∼1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.
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Carbonatite metasomatism in the southeastern Australian lithosphere
Journal of Petrology, 1998Co-Authors: Gregory M. Yaxley, David H. Green, Vadim S. KamenetskyAbstract:New mineralogical and geochemical data from a suite of glass +/- apatite +/- amphibole +/- phlogopite +/- carbonate-bearing spinel wehrlite, Lherzolite and harzburgite xenoliths from the Newer Volcanics, southeastern Australia, are consistent with metasomatic interactions between harzburgitic or refractory lherzolitic lithosphere, and penetrative sodic dolomitic carbonatite melts. Metasomatism occurred when ascending dolomitic carbonatites crossed the reaction enstatite + dolomite = forsterite + diopside + CO2 at similar to 1.5-2.0 GPa, resulting in partial to complete replacement of primary orthopyroxene by sodic clinopyroxene, together with crystallization of apatite, amphibole and phlogopite, and release of CO2-rich fluid. In the sample suite examined, the minimum amount of carbonatite melt may be estimated on the assumption that metasomatism occurred in a closed system, and that the precursor lithology was clinopyroxene-poor harzburgite. The derivative wehrlite compositions require 6-12% carbonatite addition, the Lherzolites require similar to 8% or less, and the harzburgites require minimal addition of carbonatite. However, metasomatism probably also involved an open system component, during which by partitioning relationships with the reacting carbonatite, resulting in loss from the metasomatized volume of a fugitive, siliceous, aluminous, alkali- and LILE-enriched silicate melt.
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experimental tests of low degree peridotite partial melt compositions implications for the nature of anhydrous near solidus peridotite melts at 1 gpa
Earth and Planetary Science Letters, 1997Co-Authors: Trevor J Falloon, David H. Green, H St C Oneill, W HibbersonAbstract:We present results of an experimental study to determine the nature of minimum to near-minimum melt compositions in equilibrium with upper mantle peridotite mineralogy at 1 GPa. We confirm earlier conclusions that anhydrous melts of Lherzolite at 1 GPa are basaltic with ; 15-20% normative diopside, ) 10% normative olivine and at low degrees of melting are Na O and K O-rich and nepheline-normative in 'fertile' mantle. The most extreme Na O-rich minimum melt 22 2 composition is in equilibrium with an albite-bearing harzburgite residue at 1220 8C. This melt composition is nepheline- wx normative with ; 64% SiO and about ; 12% Na O. Our results disagree with recent reports 1,2 that peridotitic 22 minimum melt compositions have an 'andesitic' character at 1 GPa. We present reversal experiments showing that these latter melts are not in equilibrium with a spinel or plagioclase Lherzolite upper mantle assemblage. We use our new data and data from the literature to define minimum melts i.e. melts in equilibrium with olivine q orthopyroxeneq clinopyroxeneq . plagioclase " spinel for fertile or enriched to refractory Lherzolite at 1 GPa. The minimum melt compositions are nepheline q olivine-normative for sodium-rich sources and hypersthene q olivine-normative for refractory or depleted compositions with very calcic plagioclase or high CarCaq Na ratios in spinel Lherzolite. It is not possible to derive quartz-normative basaltic or 'andesitic' melt compositions by partial melting of anhydrous Lherzolite at 1 GPa. q 1997 Elsevier Science B.V.
J G Liou - One of the best experts on this subject based on the ideXlab platform.
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u pb shrimp geochronology of zircon in garnet peridotite from the sulu uhp terrane china implications for mantle metasomatism and subduction zone uhp metamorphism
Earth and Planetary Science Letters, 2005Co-Authors: R Y Zhang, Jingsui Yang, Joseph L Wooden, J G LiouAbstract:Abstract We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ± magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo91–92), enstatite (En91–92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al2O3 contents of only 0.04–0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb / 238U zircon age for garnet Lherzolite (C24) is 221 ± 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ± 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ± 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism.
Stephen F. Foley - One of the best experts on this subject based on the ideXlab platform.
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Petrology of spinel Lherzolite xenoliths from Youkou volcano, Adamawa Massif, Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Merlin Patrick Wagsong Njombie, Robert Temdjim, Stephen F. FoleyAbstract:The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing Lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo_89.4−90.5), enstatite (En_89 − 91Fs_8.7−9.8Wo_0.82−1.13), clinopyroxene, spinel (Cr#_Sp = 9.4–13.8), and in some cases amphibole (Mg_# = 88.5–89.1). Mineral equilibration temperatures in the Lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al_2O_3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel Lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al_2O_3 = 6.07–6.56 wt% in clinopyroxene) and the low CaO/Al_2O_3 ratios in the spinel Lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.
Min Sun - One of the best experts on this subject based on the ideXlab platform.
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multistage metamorphism of orogenic garnet Lherzolite from zhimafang sulu uhp terrane e china implications for mantle wedge convection during progressive oceanic and continental subduction
Lithos, 2009Co-Authors: Yan Ru Song, Yi Chen, Jingbo Liu, Min SunAbstract:Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet Lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet Lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel Lherzolite during continental subduction. The Lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet Lherzolite (M4) and UHP garnet Lherzolite (M5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–Lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet Lherzolite (M6) and chlorite-amphibole-bearing spinel Lherzolite (M7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation.