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-r ich suite. Melts derived by partial melting of olivine cumulates to th e lunar magma ocean have high Mg but are strongly undersaturated with respect to plagioclase. Assimilation of lunar crust cannot bring thes e melts to the plagioclase liquidus without unduly reducing the Mg va lues of olivine. Partial melts derived by melting a Iherzolite primiti ve interior have higher normative plagioclase contents but require sou rces with Mg >90. More terrestrial-like Mg* (similar to 88) values ar e permitted if rising thermal plumes are capable of entraining the mos t Mg-rich cumulates of the magma ocean. The Mg values of lunar trocto lites may, therefore; reflect a hybrid source composed of primitive ma ntle and Mg-rich cumulates of the magma ocean. Higher Mg values may a lso arise from the reduction of small amounts of FeO to Fe. Alternativ ely, the parent magmas to lunar troctolites may be impact melts of the anorthositic crust and the Mg-rich dunite cumulates of the magma ocea n; the latter were brought to the upper mantle during the overturn of the unstable cumulate pile of the magma ocean.