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

Mineral/melt trace element partition coefficients, Dimineral/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, Dimineral/melt, and the degree of polymerization of the silicate melt, NBO/T. Linear relationships between Dimineral/melt and NBO/T sometimes exist over restricted NBO/T-ranges where the type of Qn-species in the melt does not change and their abundance does not vary greatly. Other experimental Dimineral/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 Dimineral/melt and NBO/T of the melt. elating mineral/melt partition coefficients to degree of melt polymerization, NBO/T, it is assumed that a principal melt structural control on Dimineral/melt is the activity of nonbridging oxygen in the melt, aNBO. The aNBO is related to NBO/T. The NBO/T is not, however, a quantitative measure of aNBO because the nonbridging oxygens in coexisting Qn-species in melts are energetically non-equivalent. This is evident in relationships between activity coefficient ratios of melt network-modifying cations, γi/γj, 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, XQ3/XQ2, vs. NBO/T of the melt. Interestingly, this NBO/T-value is near that of natural basalt melt. These observations suggest that i-NBO(Q3) and j-NBO(Q2) 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.