The models currently adopted to predict isovalent element partitioning assume that the composition and structure of the solid phase, rather than melt composition or temperature, exerts the greater control on trace-element mineral-liquid partition coefficients. Blundy and Wood (1994) proposed a model that relates crystal-lattice site parameters to element partitioning and allows the optimum ionic radius, ro, the site Young's modulus E of the site and the strain-compensated partition coefficient Do to be calculated. This type of analysis has been proven to be applicable to plagioclase-melt and clinopyroxene-melt (Blundy and Wood, 1994) and has been also used to discuss experimental amphibole and phlogopite partitioning results (LaTourrette et al., 1995; Brenan et al., 1995). However, owing to the complexity of both the amphibole structure and of its consequent cation-ordering patterns, which depends not only on P, T and X, but also on fR2, amphibole-melt partitioning experiments are not yet comprehensive. A study of structural control by means of singlecrystal X-ray structure refinement (SREF) performed on the same crystal on which S/LD have been measured, is presented here for the first time. In this work we investigate the role of the structural and crystal-chemical control on trace-element partitioning in amphiboles with a significant oxy component crystallised under conditions relevant to mantle processes (T and P range, fo2 close to the NNO buffer) from natural and synthetic basanitic and lamproitic melts doped with 33 trace elements. The concentration of each element has been designed to optimize counting statistics in SIMS and LAM-ICPMS analysis. The Na/(Na+K), Mg/(Mg+Fe) and Ti/ (Ti+Si) ratios of the starting material obtained from oxide mixture has been varied in order to verify their influence on trace element Amph/LiqD. In the hypothesis of ordering of Na +, K § and Rb + at the A site, R E E 3+ at the M4 site, and of Ti 4+, Zr 4+, HI 4+ at the M2 site, least-squares fitting procedures have been applied to cation partitioning between amphibole (specifically pargasite, kaersutite, richterite) and silicate melt. The values of ro derived by fitting for the M2, M4 and A sites have been compared with the results of SREF results (Fig. 1). In this figure, a good agreement between measured site dimensions and calculated ro values is observed, thus suggesting that the predictive approach can be extended to amphiboles. However, from Fig. 1 it is also apparent that the r0 values calculated by fitting R E E 3§ at the M4 site deviate from the 1:1 reference line with intercept at 1.38 A (the ionic radius ofO 2 ). Fig. 2 shows representative behaviours of Amph/ LiqDnEe in the hypothesis of ordering at the M4 site for (a) kaersutite with significant (Mg,Fe) occupancy
Read full abstract