Determination of melt influence on divalent element partitioning between anorthite and CMAS melts
We propose a theory for crystal-melt trace element partitioning that considers the energetic consequences of crystal-lattice strain, of multi-component major-element silicate liquid mixing, and of trace-element activity coefficients in melts. We demonstrate application of the theory using newly determined partition coefficients for Ca, Mg, Sr, and Ba between pure anorthite and seven CMAS liquid compositions at 1330 °C and 1 atm. By selecting a range of melt compositions in equilibrium with a common crystal composition at equal liquidus temperature and pressure, we have isolated the contribution of melt composition to divalent trace element partitioning in this simple system. The partitioning data are fit to Onuma curves with parameterizations that can be thermodynamically rationalized in terms of the melt major element activity product (a_(Al_2O_3))(a_(SiO_2_)^2 and lattice strain theory modeling. Residuals between observed partition coefficients and the lattice strain plus major oxide melt activity model are then attributed to non-ideality of trace constituents in the liquids. The activity coefficients of the trace species in the melt are found to vary systematically with composition. Accounting for the major and trace element thermodynamics in the melt allows a good fit in which the parameters of the crystal-lattice strain model are independent of melt composition.
© 2006 Elsevier Inc. Received 12 January 2006; accepted in revised form 16 June 2006. We thank Ma Chi and Julie Paque for electron microprobe assistance, as well as Ian Hutcheon and Doug Phinney for help with ion microprobe analyses that allowed us to select our analytical course of action in this study. The manuscript benefited from the dedicated reviews of Alexandre Corgne and an anonymous reviewer. This work was supported by NASA grants NAG5-11640 and NNG05GH79G to D. Burnett and NSF EAR-0239513 to P. Asimow. Associate editor: F.J. Ryerson