Trace Element and Isotopic Evolution of the Lunar Interior During Magma Ocean Solidification, Cumulate Overturn, and Subsequent Melting: Insights Into Lunar Melt Sources

Abstract The solidification of a lunar magma ocean (LMO) after a giant impact event formed the Moon's gravitationally unstable juvenile mantle. Hybridization of the lunar mantle during the overturn of late‐crystallized Fe‐ and Ti‐rich ilmenite‐bearing cumulates (IBC) in the lunar interior is required to explain the variable TiO 2 and Rare Earth Element (REE) abundances of lunar basalts and picritic melts. We model chemical fractionation during LMO solidification, mantle hybridization, and partial melting of hybridized and unhybridized cumulates to evaluate the formation of lunar picritic glasses. Solidification models indicate that >65% of the plagioclase formed during LMO solidification must be removed by flotation to explain negative Eu anomalies exhibited by lunar picritic glasses. Melting models demonstrate that unhybridized cumulates could produce low‐Ti picritic glasses but not Ti enriched glasses. Post‐hybridization melting models demonstrate that hybridized garnet‐free sources can generate elemental ratios of most Ti enriched picritic glasses. The heavy rare earth element depleted compositions of some orange picritic glasses require a ∼0.25–5% garnet component in the downwelling IBC or hybridized sources. If the LMO extends to the core‐mantle boundary, cumulate overturn is likely required for garnet to form in the lunar mantle. Isotopic models indicate that the Nd and Hf trends of lunar basalt sources reflect primary lunar differentiation processes coupled with post differentiation hybridization. The combined elemental and isotopic characteristics of Ti enriched picritic glasses and basalts are strong evidence of cumulate overturn during early lunar differentiation.