Garnet-spinel-olivine-orthopyroxene equilibria in the FeO-MgO-Al2O3-SiO2-Cr2O3 system: I. Composition and molar volumes of minerals

Abstract

Experiments in the FMASCr system at pressures of 30-50 kbar and temperatures of 1200-1500 degrees C model mineral equilibria in depleted mantle harzburgite. Special emphasis was on the partitioning of chromium between garnet, spinel and orthopyroxene and the influence of chromium on the garnet to spinel peridotite transition. The experiments were carried out using mixtures of synthetic minerals as starting materials with different initial compositions of phases, approaching the conditions of reversed experiments. The equilibrium between minerals was attained rapidly with respect to Fe-Mg exchange, whereas a considerable scatter in Cr/(Cr-+Al) values was observed in all phases studied. The extreme compositions of phases in reversed experiments give insight on the equilibrium phase composition. Cr/(Cr+Al) ratio of garnet coexisting with orthopyroxene, olivine and spinel increases with pressure and temperature, exceeding 0.4 at 50 kbar and 1500 degrees C. At high pressure spinel becomes also more chromian, while orthopyroxene composition approaches the enstatite-ferrosilite join. After quench, the minerals were studied by X-ray diffraction in order to obtain unit-cell parameters of solid solutions and refine their volume properties. Spinels in the system (Mg, Fe)(Cr, Al) 2 O 4 show negative deviation from ideal mixing volume at high Cr contents and positive deviation in Al-rich composition (Margules excess mixing volume parameters: W CrAl = 0.017 and W AlCr = -0.007 J/bar). Small negative excess volume of mixing is related to Fe-Mg mixing in spinel. Cr-Al mixing in garnet is characterised by a small excess mixing volume, which may be approximated by a symmetrical model (W V AlCr = W V CrAl = 0.018 J/bar). Our results are compatible with zero excess mixing volume of orthopyroxene solid solution. All the excess volumes of mixing are small and their influence on equilibria is substantial only at very high pressures (>50 kbar).