Effects of silicate liquid composition on mineral‐liquid element partitioning from Soret diffusion studies

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Abstract
Thermal (Soret) diffusion of major, minor, and trace components of naturally occurring silicate melts ranging in composition from tholeiitic basalt to high‐silica rhyolite is examined at 1 GPa, mean temperatures of 1380°–1535°C, and temperature gradients of 50°–80°C mm−1. Soret diffusion in silicate melts is channeled along network former/network modifier compositional vectors indistinguishable from those observed in silicate liquid immiscibilty studies, but in the former case, gradients are continuous between the compositional (and thermal) extremes. Si, the chief network former, invariably segregates to the hot end of the temperature gradient as do Rb, K, and Na in melts of mafic to intermediate bulk composition. Ba, Sr, Ca, Mg, Mn, Fe, La, Ce, Nd, Y, and Ti always show separations antithetic to Si, while Rb, K, and Na fractionate from Si only in silicic melts. Li and Al display little separation in a temperature gradient. Soret separations of alkali and alkaline earth cations are consistent with their participation in the network as charge compensators for trivalent network formers (i.e., Al3+, etc.). These network formers as well as network modifiers show fractionations from Si that depend directly on cation field strength and size, and the polymerization state of the melt. Temperature and absolute concentration have no measurable effect on the fractionation behavior of these components relative to that of SiO2, the principal determinant of melt structure. The melt compositional dependence of element partitioning is characterized using these data. Twofold to fourfold changes in mineral‐liquid partition coefficients for minor and trace elements are expected over the melting range for typical basalts, and similar or greater variabilty is expected during differentiation of silicic magmas. These expectations corroborate reported variability of mineral‐liquid partition coefficients from studies of natural igneous rock samples and experimental phase equilibria.

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