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

Abstract

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 1,420°C in the system CaO-MgO-Al2O3-SiO2 and subsystem CaO-MgO-SiO2. 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 DLaOpx-melt~0.0005 to DLuOpx-melt~ 0.109, D values for highly charged elements vary from DThOpx-melt~ 0.0026 through DNbOpx-melt ~ 0.0033 and DUOpx-melt~ 0.0066 to DTiOpx-melt~ 0.058, 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 (Mg2Si2O6) and ferrosilite (Fe2Si2O6). Calculated solution energies show that R2+ cations are more soluble in Opx than R3+ cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R3+ 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 r0, maximum partition coefficients D0, and apparent Young's moduli E for substitutions onto the Opx M1 and M2 sites. Experimental r0 values for R3+ substitutions are 0.66-0.67 Å for M1 and 0.82-0.87 Å for M2. Simulations for enstatite result in r0 = 0.71-0.73 Å for M1 and ~0.79-0.87 Å for M2. Ferrosilite r0 values are systematically larger by ~0.05 Å for both M1 and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in r0M2 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. © The Author(s) 2009.