Decoupling of Ree in Plagioclase and Orthopyroxene in the Lunar Ferroan Anorthosite

Introduction: According to the canonical lunar magma ocean (LMO) model, ferroan anorthosites (FANs) were part of the lunar anorthosite crust formed via plagioclase flotation and subsequent solidification in a globally distributed LMO [1-4]. The solidification time for a globally distributed LMO is model dependent and ranges from 10 4-10 8 yrs [3,5,6]. At a given time during lunar crust formation, a highly simplified lunar crust would consist of a protocrust with perhaps higher mafic mineral abundance than typical FANs at the top, a growing plagioclase-rich crust largely free of intercumulus melt in the middle, and a mush layer composed of plagioclase, smaller quantities of pyrox-ene ± olivine and a relatively dense, FeO-enriched melt of the LMO at the bottom. Due to their negative buoyancy relative to the plagioclase the intercumulus melts tend to " lift " the plagioclase matrix, forming " rock-bergs " and perhaps even forcing the matrix to deform (compact) against the completely solidified crust [7,8]. The ability to segregate and expel significant amount of intercumulus melts from the plagioclase mush is a necessary condition for the formation of an anorthosite crust that is characterized by small mafic mineral fraction and low incompatible trace element abundance. Segregation of intercumulus melts from their crystalline matrix due to compaction and solidification will inevitably result in chemical fractionations in the minerals and melt [9,10]. Consider two rare earth elements Sm and Nd. Nd is more compatible than Sm in anorthite (k Nd An = 0.12 and k Sm An = 0.67, [11]), whereas