Element partitioning between minerals and melt, melt composition, and melt structure

Abstract

Mineral/melt trace element partition coefficients, D i mineral/melt (=concentration ratio, mineral/melt), can vary by up to two orders of magnitude in the melt composition range of natural magmatic liquids. As most geochemically interesting minor and trace elements are network-modifiers in silicate melts, one would expect, as observed, positive correlation between mineral/melt partition coefficients, D i mineral/melt, and the degree of polymerization of the silicate melt, NBO/T. Linear relationships between D i mineral/melt and NBO/T sometimes exist over restricted NBO/T-ranges where the type of Q n -species in the melt does not change and their abundance does not vary greatly. Other experimental D i mineral/melt vs. NBO/T data suggest log-linear or log–log relations in some cases, whereas in other studies, there are no simple relationships between D i mineral/melt and NBO/T of the melt. When relating mineral/melt partition coefficients to degree of melt polymerization, NBO/T, it is assumed that a principal melt structural control on D i mineral/melt is the activity of nonbridging oxygen in the melt, a NBO. The a NBO is related to NBO/T. The NBO/T is not, however, a quantitative measure of a NBO because the nonbridging oxygens in coexisting Q n -species in melts are energetically non-equivalent. This is evident in relationships between activity coefficient ratios of melt network-modifying cations, γ i / γ j , where the ionization potentials of i and j differ, and NBO/T of the melt. Such relationships are parabolic with minima or maxima at NBO/T near 1 and resemble the relations of abundance ratio, X Q3/ X Q2, vs. NBO/T of the melt. Interestingly, this NBO/T-value is near that of natural basalt melt. These observations suggest that i-NBO(Q 3) and j-NBO(Q 2) bond characteristics differ from one another, govern trace element solution behavior in silicate melts and, therefore, control the effect of melt composition on mineral/melt partitioning in silicate systems.