Experimental versus Natural Two-Mineral Partition Coefficients: A “High-Tech” Controversy

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Abstract
Development of several different microbeam techniques permits in situ analysis of trace elements in natural and synthetic mineral assemblages, which in turn makes possible determination of two-mineral partitioning behavior for trace elements (D-values). With the experimental approach, control of compositional and physical variables is possible, but equilibrium needs to be carefully assessed, and in some instances compositions and optimal experimental conditions are different from the natural situation. With the natural mineral approach, physical conditions must be assessed independently, accepting the uncertainties involved in geothermobarometric determinations based on coexisting mineral compositions. A potentially complex history of formation must be unravelled as well, with the attendant possibility of non-equilibrium. Comparison of D-values determined for coexisting amphibole and Ca-clinopyroxene indicates overall good agreement for Sr, Zr, Hf, Y, and REE, but significant discrepancy for Rb, Ba, Nb, and Ta (experimental values much lower than natural values). For coexisting Ca-clinopyroxene and garnet, the spread of data is greater and fewer elements can be compared (Rb, Ba, Nb, and Ta data are not adequate). However Sr, Y, Zr, Hf, and Nd-Lu (of the rare-earth elements) agree reasonably well, but La and Ce experimental values for clinopyroxene/garnet are much lower than the natural values. These differences for both mineral pairs may be attributed in part to critical compositional differences between synthetic and natural minerals, where compositional factors play a key role in controlling accommodation of trace elements in the mineral structure. A second factor is the possibile existence of minute inclusions rich in trace elements trapped in the natural minerals. These inclusions may be submicroscopic and unavoidable, even by microbeam analysis. More work on both experimental and natural systems should resolve the discrepancies and help to realize the full potential for using trace elements to assess petrological processes in the mantle, where knowledge of D-value variation as a function of composition, pressure, and temperature is essential.