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 3.2GPa), 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 (Dᴼᵖˣ⁻ᵐᵉˡᵗ) 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 DLₐ ᵒᵖˣ⁻ᵐᵉˡᵗ=0.0014±0.0008 to DLᵤ ᵒᵖˣ⁻ᵐᵉˡᵗ=0.051±0.007. D values for highly charged elements vary from DTₕ ᵒᵖˣ⁻ᵐᵉˡᵗ=0.0013±0.0008 through DNb ᵒᵖˣ⁻ᵐᵉˡᵗ=0.0018±0.0006 and DU ᵒᵖˣ⁻ᵐᵉˡᵗ=0.0015±0.0006 to DTᵢ ᵒᵖˣ⁻ᵐᵉˡᵗ=0.068±0.0010. DLᵤ ᵒᵖˣ⁻ᵐᵉˡᵗ/DHf ᵒᵖˣ⁻ᵐᵉˡᵗ 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 suggests that varying the concentration of divalent Fe in Opx very slightly decreases the ideal cation radius for M³⁺ elements entering the M2 site, r₀ ᴹ², whereas the partitioning of M³⁺ elements entering the M1 site is unaffected. A subtle increase in the maximum partition coefficient for M³⁺ 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 ~350ppm does not have a measureable effect on the Opx–melt partitioning behaviour of REE or any other element, showing that charge-balancing of M³⁺, M⁴⁺ and M⁵⁺ elements in Opx is likely dominated by a vacancy mechanism.