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-Al 2 O 3 -SiO 2 and subsystem CaOMgO-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, $ 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 (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 Å for M1 and 0.82-0.87 Å for M2. Simulations for enstatite result in r 0 = 0.71-0.73 Å for M1 and *0.79-0.87 Å for M2. Ferrosilite r 0 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 r M2 0 in the presence of Fe. Additional systematic studies in Febearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.

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We examine the structure of the hydrated Ti(IV) complex under both ambient and supercritical conditions using first-principles molecular dynamics. We find that an unanticipated fivefold coordination of Ti(IV) is favoured under ambient conditions, with rapid interconversions between square pyramidal and trigonal bipyramidal structures. At supercritical conditions the Ti coordination increases from five to six, adopting both octahedral and trigonal prismatic geometries. At 1000 K, the magnitude of the increase in the Ti to oxygen coordination number with increasing water density is similar to that of Li-O under comparable conditions. We present a detailed picture of the bonding in the hydrated Ti(IV) complex under both ambient and supercritical conditions.