The importance of melt TiO2 in affecting major and trace element partitioning between Fe–Ti oxides and lunar picritic glass melts

Abstract

Lunar mare basalts and picritic glasses have TiO2 abundances ranging from less than 1wt.% to over 16%. Any high-Ti mare basalt or picritic glass petrogenetic model must include Fe–Ti oxides in the mantle source or invoke Fe–Ti oxide assimilation during magma ascent to the lunar surface. We conducted partitioning experiments to investigate high field strength element (HFSE), rare earth element (REE), and transition metal distribution between Fe–Ti oxides and lunar picritic glass melts over a range of melt compositions. Our results suggest that ilmenite–melt and armalcolite–melt HFSE, Cr, and V partition coefficients (DHFSE, DCr, DV) are strongly dependent on melt TiO2 content, whereas ilmenite–melt REE partition coefficients appear to be insensitive to melt composition. As TiO2 increases in picritic glass melts, HFSE, Cr, and V activities in melt also increase and Fe–Ti oxide–melt DHFSE, DCr and DV decrease. The effect of Ti on partitioning behavior can be attributed to the formation of Fe–O–Ti melt species in high-Ti melts. Ilmenite DHFSE range from compatible in oxides in equilibrium with low-Ti melts to incompatible in oxides in equilibrium with depolymerized high-Ti picritic glass melts. DHFSE are inversely correlated with TiO2 abundance in the melt and become nearly constant for melts with more than 6.8% TiO2. We present simple partitioning models that utilize the solubility of ilmenite and armalcolite in melt to effectively predict HFSE partition coefficients across a wide range of picritic glass melt compositions. The HFSE budget of ilmenite cumulates that crystallize from the lunar magma ocean strongly depends on the composition of the magma ocean. Low-Ti and high-Ti lunar basalts can be produced by an ilmenite or armalcolite bearing hybridized mantle source, or by assimilation of late-stage magma ocean cumulates. The dependence of DHFSE and DCr on melt TiO2 is consistent with the formation of lunar Type 1 armalcolite from high TiO2 picritic glass melt, however, lunar Type 2 and 3 armalcolite have a more complicated (yet undetermined) history of formation.