Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melt: substitution mechanisms and the effect of iron

Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420A degrees C in the system CaO-MgO-Al(2)O(3)-SiO(2) and subsystem CaO-MgO-SiO(2). We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, Ce, Nd, Sm, Y, Yb, Lu; high field strength: Zr, Nb, Hf, Ta, Ti; transition metals: Sc, V, Cr, Co) for use in petrogenetic modelling. REE partition coefficients increase from alpha, and are all virtually independent of temperature. Cr and Co are the only compatible trace elements at the studied conditions. To elucidate charge-balancing mechanisms for incorporation of REE into Opx and to assess the possible influence of Fe on Opx-melt partitioning, we compare our experimental results with computer simulations. In these simulations, we examine major and minor trace element incorporation into the end-members enstatite (Mg(2)Si(2)O(6)) and ferrosilite (Fe(2)Si(2)O(6)). Calculated solution energies show that R(2+) cations are more soluble in Opx than R(3+) cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R(3+) cations by coupled substitution with Li(+) on the M1 site that is energetically favoured over coupled substitution involving Al-Si exchange on the tetrahedrally coordinated site. We derived best-fit values for ideal ionic radii r (0), maximum partition coefficients D (0), and apparent Young's moduli E for substitutions onto the Opx M1 and M2 sites. Experimental r (0) values for R(3+) substitutions are 0.66-0.67 for M1 and 0.82-0.87 for M2. Simulations for enstatite result in r (0) = 0.71-0.73 for M1 and similar to 0.79-0.87 for M2. Ferrosilite r (0) values are systematically larger by similar to 0.05 for both M1 and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in 3/4 in the presence of Fe. Additional systematic studies in Fe-bearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.