Ideal mixing of divalent cations in mafic magma and its effect on the solution of ferrous oxide

Abstract

In this study, three sets of aluminosilicate liquids are equilibrated with metallic iron under controlled oxygen fugacity to determine the compositional dependence of the activity of ferrous oxide in petrologically interesting melts. The liquids lie in three ternary systems, of the type Matrix-FeO-MgO, that contain synthetic analogs of standard diabase W-1, MORB and high-TiO2 mare basalt respectively. Using 218 runs with both high- and low-iron starting materials, the ternaries are contoured at aFeO = 0.1 intervals by bracketing each isoactivity curve. The results demonstrate that the relation aternaryFeO = aMg-freeFeO × Fe(Fe + Mg) accounts for the variation of aFeO at constant mole fraction of Matrix (XMatrix = 1 − XFeO − XMgO) in these ternaries. This suggests that ferrous iron and magnesium mix ideally on the sites they occupy in mafic magmas. Extending this approach demonstrates that the compositional dependence of aFeO in a wide range of mafic melts is temperature insensitive and is consistent with anorthite (as in a NORM calculation) being a component of the Matrix and the remaining Ca behaving like Fe, Mn and Mg. Importantly, these findings greatly facilitate determination of the effect of melt chemistry variations on solidliquid distribution coefficients.