AN EXPERIMENTAL-STUDY OF FE-MG PARTITIONING BETWEEN OLIVINE AND ORTHO-PYROXENE AT 1173-K, 1273-K AND 1423-K AND 1.6-GPA

The partitioning of Mg and Fe2+ between co-existing olivines and orthopyroxenes in the system MgO-FeO-SiO2 has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of +/- 0.003 to 0.004 in molar Fe/(Mg + Fe), Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low K(D)) and vice versa (high K(D)), which produced indistinguishable results. The distribution coefficient, K(D), depends on composition and on temperature, but near Fe/(Mg + Fe) = 0.1 (i.e. mantle compositions) these effects cancel out, and K(D) is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg - Fe olivine solid solutions to show small near-symmetric deviations from ideality, with W(G)Mg-Fe(ol), between 2000 nd 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with W(G)Mg-Fe(ol) = 5625 +/- 574 J/mol (implying 5450 J/mol at 1 bar) and W(G)Mg-Fe(opx) = 2145 +/- 614 J/mol, but the two W(G) parameters are so highly correlated with each other that our data are almost equally well fit with W(G)Mg-Fe(ol) = 3700 +/- 800 J/mol, as obtained by Wiser and Wood. This value implies W(G)Mg-Fe(opx) = 280 +/- 900 J/mol, apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.