Partitioning of calcium, magnesium, and transition metals between olivine and melt governed by the structure of the silicate melt at ambient pressure

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Olivine/melt partitioning of the transition metal cations, Fe2+, Mn2+, Co2+, and Ni2+, together with Mg2+ and Ca2+, has been examined experimentally as a function of melt composition at ambient pressure. Melt structure was inferred from bulk-chemical composition, existing structural data, and 57Fe resonant absorption Mossbauer spectroscopy. Under isothermal conditions, K D(i-Mg)olivine/melt = ( C i/ C Mg)olivine/( C i/ C Mg)melt, is an exponential function of melt NBO/T for i = Ca2+, Mn2+, Co2+, and Ni2+. For i = Fe2+, the relationship is parabolic with maximum K D(Fe2+-Mg)olivine/melt -values at NBO/T near 1. At constant melt NBO/T, K D(i-Mg)olivine/melt increases systematically with decreasing cation radius, an effect that is more pronounced the more polymerized the melt. The K D(i-Mg)olivine/melt is also a positive and linear function of Na/(Na + Ca) of Al-free melts. This latter effect results from changes in Qn-species abundance governed by Na/(Na + Ca) of the melts. The enthalpy of the exchange equilibrium, iolivine + Mgmelt = imelt + Mgolivine, derived from the temperature-dependence of K D(i-Mg)olivine/melt, is also a positive function of the ionic radius of the cation. The relationship of enthalpy to melt polymerization also depends on cation radius. The K D(Fe2+-Mg)olivine/melt does not, however, follow this trend possibly because the bond distance, d Fe2+-O, in the melts depends on melt composition. The cations examined in this study are network-modifiers in silicate melts at ambient pressure. The solution behavior of network-modifying cations in melts is governed by the extent of steric hindrance near nonbridging oxygen, which in turn affects the energetics of metal-nonbridging oxygen bonds. Those structure effects, in turn, are related to the type of Qn-species, their abundance, and on the Al-distribution between the Qn-species. It is suggested, therefore, that the observed variations of mineral/melt partition coefficients with melt composition can be understood by considering bulk polymerization (NBO/T), the distribution of Al3+ among coexisting Qn-species, and the distribution of network-modifying cations among nonbridging in these Qn-species.