SUBSOLIDUS PHASE-RELATIONS IN THE SYSTEM MGO-SIO2-CR-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-O and MgO-SiO2-Cr-O in equilibrium with metallic Cr, at 1100-1500 degrees C and 0-28.8 kbar. There are no ternary phases in the SiO2-Cr-O system at these conditions, i.e. 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+)(2)SiO4 olivine, (Mg,Cr2+)(2)Si2O6 pyroxene, and (Mg,Cr2+)Cr23+O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthorhombic at X(Cr2+)(px) < 0.2, monoclinic at higher X(Cr2+)(px). The structure ofthe spinels becomes tetragonally distorted at X(Cr2+)(sp) > 0.2. The maximum X(Cr2+)(ol) is limited by the breakdown of olivine to pyroxene + spinel + metal. This maximum amount increases strongly with increasing temperature, reaching X(Cr2+)(ol) = 0.25 at 1500 degrees C and 4.8 kbar. Increasing pressure reduces the maximum X(Cr2+)(ol) Increasing temperature also increases the maximum amounts of Cr2+ which substitute into pyroxene and spinel, indicating that end-member Cr2Si2O6 and Cr3O4 mag became stable above similar to 1650 degrees 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 Cr3O4 (cubic spinel) of 47.7, 68.0 and 44.9 cm(3), 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 f(O2). This amount increases strongly with temperature along standard T-f(O2) buffer curves, and is sufficient to explain the observed high Cr contents of olivine from komatiites and diamond inclusions at reasonable terrestrial f(O2) values. The lower f(O2) 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 upper-mantle processes, and is a significant factor in the differing styles of igneous differentiation in the Earth and Moon.