SUBSOLIDUS PHASE RELATIONS IN THE SYSTEM MGO–SIO2–GR–O IN EQUILIBRIUM WITH METALLIC CR, AND THEIR SIGNIFICANCE FOR THE PETROCHEMISTRY OF CHROMIUM

Abstract

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 f$${{\displaystyle \hbox{ X }}}_{px}^{C{r}^{2+}}$$/f monoclinic at higher <f>$${{\displaystyle \hbox{ X }}}_{px}^{C{r}^{2+}}$$</f>. The structure of the spinels becomes tetragonally distorted at <f>$${{\displaystyle \hbox{ X }}}_{px}^{C{r}^{2+}}$$</f> is limited by the breakdown of olivine to pyroxene + spinel + metal. This maximum amount increases strongly with increasing temperature, reaching <f>$${{\displaystyle \hbox{ X }}}_{ol}^{C{r}^{2+}}$$</f> >0.2 at 1500°C and 4?8 kbar. Increasing pressure reduces the maximum <f>$${{\displaystyle \hbox{ X }}}_{ol}^{C{r}^{2+}}$$</f>. 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 ~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.