Trace element evolution during crystallisation and remelting of the lunar magma ocean

Newly determined ilmenite-melt and orthopyroxene-melt partition coefficients are used to construct quantitative forward models of the evolution of high field strength element (HFSE, Zr, Nb, Hf, Ta, Th, U) and rare earth element (REE: Nd, Sm, Lu) concentrations during crystallisation and subsequent remelting of the Lunar Magma Ocean (LMO). Crystallisation models suggest that the initial LMO must have been enriched in these elements with respect to the bulk silicate earth. This is required to account for the trace element concentrations of KREEP samples, which are enriched in potassium, rare earth elements and phosphorus. The trace element signatures and ratios found in the low-Ti and high-Ti mare basalts can be explained by partial melting of a mixture of early-formed olivine and orthopyroxene cumulates, as long as these contain 1-3 wt% of trapped residual liquid (TRL), which fully equilibrated with the cumulates prior to remelting. The observed Nb/Ta, Zr/Hf, Sm/Nd and Lu/Hf systematics of the high-Ti basalts can be explained by mixing partial melts of early olivine and orthopyroxene cumulates with clinopyroxene and ilmenite (ratio 1:3) or with ilmenite only. The Zr/Hf, Sm/Nd and Lu/Hf ratios of low Ti-basalts are best explained by a mix of low-degree (2%) partial melts of Ol and Opx cumulates, that include a 3 wt% TRL. However, modelled Nb/Ta ratios are elevated with respect to ratios measured in the low-Ti basalts in this case, and thus dissolution of a mineral or late stage cumulate possessing subchondritic Nb/Ta ratios seems required. The subchondritic Nb/Ta ratios in lunar samples can be reproduced from an initial lunar magma ocean with a chondritic Nb/Ta ratio.