Experimental and thermodynamic study of Fe-Mg exchange between olivine and orthopyroxene in the system MgO-FeO-SiO2

Abstract

Equilibrium compositions of ol ss and opx ss were experimentally determined in the fields of ol ss /opx ss /qtz (A) and ol ss /opx ss (B) co-existence in the system MgO-FeO-SiO 2 . The aim was to obtain new data on the effect of composition, pressure and temperature on K DFe−Mg ol-opx and to derive activity-composition relations for ol ss and opx ss . High-pressure runs involving the two reactions : 1/2 Fe 2 SiO 4 (ol)+1/2 SiO 2 (qtz) = FeSiO 3 (opx) ; FeSiO 3 (opx)+1/2 Mg 2 SiO 4 (ol) = MgSiO 3 (opx)+1/2 Fe 2 SiO 4 (ol) have been reversed in NaCl-cells in a piston-cylinder apparatus using different bulk compositions and oxygen fugacities in the range 800°-1000°C / 5-15 kbar. The compositional data on coexisting ol ss and opx ss determined by microprobe and/or by X-ray diffraction were processed in a non-linear least-squares procedure using the simple mixture model for both solid solutions, resulting in the interaction parameter normalized to the one cation basis W g ol = 7124 ± 2000 J. Close agreement between calculated and observed compositions of the coexisting phases is obtained, showing that the simple mixture model is sufficient to describe the thermodynamic behaviour of ol ss and opx ss