OLIVINE-LIQUID EQUILIBRIA AND THE CHEMICAL ACTIVITIES OF FEO, NIO, FE2O3, AND MGO IN NATURAL BASIC MELTS

The distribution of the cations Fe+2, Mg+2, Ni12, Mn+2, and Ca12 between olivine and natural basic liquid is experimentally explored at one atmosphere as functions of oxygen fugacity, temperature, and composition. By performing the experiments in equilibrium with NiFe alloys, the chemical activities of FeO, Fe2O3, and NiO were monitored in the melt and from knowledge of the thermodynamics of olivine mixing, the activity of MgO was also calculated. This is the first time that the activity of Fe2O3 has been monitored in a natural melt and also the first observations of the variation of a(FcO) with oxygen fugacity. Empirical models for the variation of the activities of these components as functions of oxygen fugacity, temperature, and composition are presented and the models for FeO and MgO are shown to be able to predict the olivine composition. The partition coefficients for FeO, MgO, NiO, and MnO for olivine were found to be invariant over more than 9 orders of magnitude in oxygen fugacity. The calcium partition coefficient decreases with increasing fo2, but this is due to the chaning olivine composition with oxygen fugacity. Nickel, manganese, and calcium partitioning are functions of composition and temperature, although thermodynamic analysis shows that the temperature variation for NiO and MnO is primarily due to the temperature dependence of the activity coefficients rather than a large enthalpy of the exchange reactions. Empirical predictive models are presented for the partition coefficients of all of the divalent cations.