Subsolidus phase relations in the system MgO - SiO₂ - Gr - O in equilibrium with metallic Cr, and their significance for the petrochemistry of chromium

Subsolidus phase relations have been determined in the systems SiO₂-Cr-0 and MgO-SiO₂-Cr-O in equilibrium with metallic Cr, at 1100-1500°C and 0-28·8 kbar. There are no ternary phases in the SiO₂-Cr-O system at these conditions, ie. only the 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⁺)₂Si₂O₆ pyroxene, and (Mg,Cr²⁺)Cr³⁺₂0₄ spinel. Cr ³⁺ levels in olivine and pyroxene are below detection limits. The pyroxene is orthorhombic at XpxCr2+ monoclinic at higher XpxCr2+. The structure of the spinels becomes tetragonally distorted at XpxCr2+ is limited by the breakdown of olivine to pyroxene + spinel + metal. This maximum amount increases strongly with increasing temperature, reaching XolCr2+ >0.2 at 1500°C and 4·8 kbar. Increasing pressure reduces the maximum XolCr2+. Increasing temperature also increases the maximum amounts of Cr²⁺ which substitute into pyroxene and spinel, indicating that end-member Cr₂Si₂0₆ and Cr₃O₄ may become stable above {reversed tilde}1650°C if melting does not intervene. Powder X-ray diffraction analysis of selected runs has been used to extract molar volumes of the Mg-Cr²⁺ solid solutions as a function of composition, which may be extrapolated to predict molar volumes for Cr₂SiO₄ (olivine), Cr₂Si₂O₆ (ortho- and clino-pyroxene) and Cr₂O₄ (cubic spinel) of 47·7, 68·0 and 44·9 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 amounts of Cr²⁺ which can be expected in olivine in equilibrium with Cr-bearing spinel as a function of T, P and fo₂. This amount increases strongly with temperature along standard T-fo₂ buffer curves, and is sufficient to explain the observed high Cr contents of olivine from komatiites and diamond inclusions at reasonable terrestrial fo, values. The lower fo₂ of the lunar environment results in significant Cr²⁺ in olivine being stable to much lower temperatures. The tendency for the oxidation state of Cr, and hence its geochemical properties, to change with temperature relative to other redox reactions makes it a potentially useful monitor of the temperatures of uppermantle processes, and is a significant factor in the differing styles of igneous differentiation in the Earth and Moon.