A new view of lunar ferroan anorthosites: Postmagma ocean petrogenesis

Abstract

New melting experiments in the range of 0.1 MPa to 4.7 GPa provide the basis for calculations of polybaric crystallization which show that both a >300 km deep magma ocean (MO) enriched in refractory elements and a >600 km deep magma ocean with refractory element concentrations similar to the Earth's upper mantle produce sufficient amounts of differentiated, residual liquid, capable of crystallizing the troctolitic-noritic mineral assemblages that are prevalent in lunar ferroan anorthosites (LFA). However, the both oceans become saturated with augite after ∼20-30% crystallization of the plagioclase-saturated residuum and thereafter are no longer capable of producing noritic assemblages. A second set of melting experiments reveals the existence of a previously unrecognized equilibrium (augite + liquid = pigeonite + plagioclase) and a thermal minimum on the pigeonite + plagioclase liquidus surface at 0.6 GPa. These equilibria will produce melts of mafic cumulates with lower Wo content than those from which the cumulates originally formed in a postmagma ocean environment of overturning mafic cumulates. Melt thus formed and entrained in a buoyant segregation of plagioclase would then form diapiric masses that would crystallize troctolitic-noritic assemblages after intruding upward. Diapiric intrusions could also provide the local deformational environment needed to produce the apparently high (≥95%) plagioclase modes of the LFA. An unstable density profile in the cumulate pile following crystallization of most of the MO should provide the impetus for overturn and the heat for partially remelting mafic cumulates.