Experimental study of trace element partitioning between lunar orthopyroxene and anhydrous silicate melt: effects of lithium and iron

Orthopyroxene (Opx) was present during a significant portion of crystallisation of the lunar magma ocean, but its influence on co-existing melt trace element contents is not well quantified. We performed high-pressure (P, 1.1 to 3.2 GPa), high-temperature (T, 1400 to 1600 °C) experiments on synthetic Fe-rich compositions at reducing conditions relevant to the lunar mantle to constrain trace element partitioning between Opx and anhydrous silicate melts. Opx-melt partition coefficients ( melt Opx D ) for a wide range of trace elements (LILE: Li, Ba; REE: La, Ce, Nd, Sm, Dy, Eu, Er, Tm Y, Yb, Lu; HFSE: Zr, Nb, Hf, Ta, Th, U; and transition metals: Sc, V, Mn, Co, Mo show only very minor variations across the considered P-T range. REE partition coefficients increase from melt opx La D = 0.0014 ± 0.0008 to melt opx Lu D = 0.051 ± 0.007. D values for highly charged elements vary from melt opx Th D = 0.0013 ± 0.0008 through melt opx Nb D = 0.0018 ± 0.0006 and melt opx U D = 0.0015 ± 0.0006 to melt opx Ti D = 0.068 ± 0.0010. melt opx Lu D / melt opx Hf D values of 6.3 ± 2.4 are at the high end of reported values for minerals that played a role during crystallisation of the lunar magma ocean, and higher than previously reported for Opx under identical oxygen fugacity conditions, implying Opx-rich cumulates in the lunar mantle have highly superchondritic Lu-Hf ratios. Lattice strain modelling of our REE partitioning data suggest that varying the concentration of divalent Fe in Opx very slightly decreases the ideal cation radius for M elements entering the M2 site, r0 , whereas the partitioning of M elements entering the M1 site is unaffected. A subtle increase in the maximum partition coefficient for M elements entering both the M1 and M2 sites with decreasing T is identified, when experiments carried out at similar reducing oxygen fugacities are considered. The presence of Li at concentrations of up to ~350 ppm does not have a measureable effect on the Opx-melt partitioning behaviour of REE or any other element, showing that chargebalancing of M, M and M elements in Opx is likely dominated by a vacancy mechanism.