PETROGENESIS OF LUNAR TROCTOLITES

Models of petrogenesis for lunar troctolites are severely constrained by the high Mg* (>88) values of the most primitive members of the Mg-rich suite. Melts derived by partial melting of olivine cumulates to the lunar magma ocean have high Mg* but are strongly undersaturated with respect to plagioclase. Assimilation of lunar crust cannot bring these melts to the plagioclase liquidus without unduly reducing the Mg* values of olivine. Partial melts derived by melting a Iherzolite primitive interior have higher normative plagioclase contents but require sources with Mg* >90. More terrestrial-like Mg* (similar to 88) values are permitted if rising thermal plumes are capable of entraining the most Mg-rich cumulates of the magma ocean. The Mg* values of lunar troctolites may, therefore; reflect a hybrid source composed of primitive mantle and Mg-rich cumulates of the magma ocean. Higher Mg* values may also arise from the reduction of small amounts of FeO to Fe. Alternatively, the parent magmas to lunar troctolites may be impact melts of the anorthositic crust and the Mg-rich dunite cumulates of the magma ocean; the latter were brought to the upper mantle during the overturn of the unstable cumulate pile of the magma ocean.