Experimental and thermodynamic study of Fe-Mg exchange between olivine and orthopyroxene in the system MgO-FeO-SiO2
Koch-Müller, Monika; Cemic, Lado; Langer, Klaus
European Journal of Mineralogy Volume 4 Number 1 (1992), p. 115 - 136
published: Feb 13, 1992
manuscript accepted: Jul 16, 1991
manuscript received: Nov 9, 1990
ArtNo. ESP147050401010, Price: 29.00 €
Abstract Equilibrium compositions of olss and opxss were experimentally determined in the fields of olss/opxss/qtz (A) and olss/opxss (B) co-existence in the system MgO-FeO-SiO2. The aim was to obtain new data on the effect of composition, pressure and temperature on KDFe-Mg Ol-opx and to derive activity-composition relations for olss and opxss. High-pressure runs involving the two reactions: 1/2 Fe2SiO4(ol) + 1/2 SiO2(qtz) - FeSiO3(opx) FeSiO3(opx) + 1/2 Mg2SiO4(ol) = MgSiO3(opx) + 1/2 Fe2SiO4(ol) have been reversed in Nad-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 olss and opxss 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 WGol = 7124 ± 1800 J and WGopx = 3417 ± 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 olss and opxss.