Trace element partitioning between ilmenite, armalcolite and anhydrous silicate melt: implications for the formation of lunar high-Ti mare basalts

We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite, armalcolite and anhydrous lunar silicate melt, to constrain geochemical models of the evolution of the interior of the Moon. Experiments yielding partition coefficients were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3 GPa and 1300-1400 °C using a synthetic Tienriched Apollo ‘black glass’ composition in the CaO-FeO-MgO-Al2O3-TiO2-SiO2 system. Trace element partition coefficients (D) for both ilmenite and armalcolite show highly incompatible values for the rare earth elements (REE, melt Ilm La D 0.0020 ± 0.0010 to melt Ilm Lu D 0.069 ± 0.010 for ilmenite and melt Arm La D 0.0048 ± 0.0023 to melt Arm Lu D 0.041 ± 0.008 for armalcolite) with the light REE slightly more incompatible compared to the heavy REE. The high field strength elements vary from highly incompatible for Th, U and to a lesser extent W (for ilmenite: melt Ilm Th D 0.0013 ± 0.0008, melt Ilm U D 0.0035 ± 0.0015 and melt Ilm W D 0.039 ± 0.005, and for armalcolite melt Arm Th D 0.008 ± 0.003, melt Arm U D 0.0048 ± 0.0022 and melt Arm W D 0.062 ± 0.03), to mildly incompatible for Nb, Ta, Zr, and Hf (e.g. melt Ilm Hf D 0.28 ± 0.05 and melt Arm Hf D 0.76 ± 0.07). Both ilmenite and armalcolite fractionate the high field strength elements as DTa > DNb and DHf >DZr. Values for DTa/DNb and DHf /DZr are comparable for ilmenite between 1.3 and 1.6 and for armalcolite 1.3 and 1.4. Armalcolite is slightly more efficient at fractionating Hf from W during lunar magma ocean crystallisation, with DHf/DW = 11.9-12.9 compared to 6.7-7.5 for ilmenite. The transition metals vary from mildly incompatible to compatible, with the highest compatibilities for Cr in ilmenite (D ~7.5) and V in armalcolite (D ~8.1). D values show no clear variation with pressure in the small range covered. Crystal lattice strain modelling of D values for di-, triand tetravalent trace elements shows that in ilmenite, divalent elements prefer to substitute for Fe while armalcolite data suggest REE replacing Mg. Tetravalent cations appear to preferentially substitute for Ti in both minerals, with the exception of Th and U that likely substitute for the larger Fe or Mg cations. Lattice strain models of