Subsolidus Phase Relations in the System MgO--SiO2--Gr--O in Equilibrium with Metallic Cr, and their Significance for the Petrochemistry of Chromium |
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| Authors: | LI JIAN-PING; O'NEIL HUGH ST C; SEIFERT FRIEDRICH |
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| Institution: | BAYERISCHES GOEOINSTITUT, UNIVERSIT T BAYREUTH D95440 BAYREUTH, GERMANY |
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| Abstract: | Subsolidus phase relations have been determined in the systemsSiO2-Cr-0 and MgO-SiO2-Cr-O in equilibrium with metallic Cr,at 1100–1500C and 0–288 kbar. There are no ternaryphases in the SiO2-Cr-O system at these conditions, ie. onlythe assemblage eskolaite + Cr-metal + SiO2 is found. However,in the MgO-containing system, extensive substitution of Cr2+for Mg is observed in (Mg,Cr2+)2SiO4 olivine, (Mg,Cr2+)2Si2O6pyroxene, and (Mg,Cr2+)Cr3+204 spinel. Cr 3+ levels in olivineand pyroxene are below detection limits. The pyroxene is orthorhombicat  monoclinic at higher . The structure of the spinels becomestetragonally distorted at is limited by the breakdown of olivine to pyroxene + spinel+ metal. This maximum amount increases strongly with increasingtemperature, reaching  >0.2 at 1500C and 48 kbar. Increasing pressure reduces the maximum. Increasing temperature also increases the maximum amounts of Cr2+ which substitute intopyroxene and spinel, indicating that end-member Cr2Si206 andCr3O4 may become stable above  1650C if melting does not intervene.Powder X-ray diffraction analysis of selected runs has beenused to extract molar volumes of the Mg-Cr2+ solid solutionsas a function of composition, which may be extrapolated to predictmolar volumes for Cr2SiO4 (olivine), Cr2Si2O6 (ortho- and clino-pyroxene)and Cr2O4 (cubic spinel) of 477, 680 and 449 cm3, respectively.The experimental data have been fitted to a thermodynamic model,including free energies of formation for end member Cr2SiO4,Cr2Si2O6 and Cr3O4. This model is then used to predict the amountsof Cr2+ which can be expected in olivine in equilibrium withCr-bearing spinel as a function of T, P and fo2. This amountincreases strongly with temperature along standard T-fo2 buffercurves, and is sufficient to explain the observed high Cr contentsof olivine from komatiites and diamond inclusions at reasonableterrestrial fo, values. The lower fo2 of the lunar environmentresults in significant Cr2+ in olivine being stable to muchlower temperatures. The tendency for the oxidation state ofCr, and hence its geochemical properties, to change with temperaturerelative to other redox reactions makes it a potentially usefulmonitor of the temperatures of uppermantle processes, and isa significant factor in the differing styles of igneous differentiationin the Earth and Moon.
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