Abstract
We present criteria for the recognition of relics of the original lunar crust. A small suite of anorthositic, troctolitic, noritic, gabbroic, and dunitic plutonic rocks displays modes, mineral compositions, and bulk major and trace element abundances that strongly suggest these rocks originated as cumulates. Although some retain the texture of plutonic rocks and a very few have relic cumulate textures, most were annealed under conditions of the granulite facies during the cooling of the Moon and/or were crushed during excavation by gigantic meteorite impacts. These plutonic rocks contain low abundances of trace siderophile elements, no petrographic evidence for mixing, and ancient ages, indicating that they have not been chemically modified by meteorite impact events. The petrographic and chemical evidence for a nonimpact cumulate origin suggests that these plutonic rocks are relics of the original lunar crust that presumably formed by fractional crystallization from a planet-wide magma ocean. Mineral chemistry of coexisting plagioclase and olivine or orthopyroxene yield two trends: one is defined by Mg-rich bulk compositions and is interpreted to represent fractional crystallization in the magma ocean; the other [constant An in plagioclase for a range of 0.25 in Mg/(Mg+Fe) of coexisting mafic minerals] is defined by anorthosites. The significance of the anorthosite trend is not fully understood.
