Experimental petrology of the Apollo 15 group A green glasses: Melting primordial lunar mantle and magma ocean cumulate assimilation

Abstract

Phase equilibrium experiments have been performed to place constraints on the melting processes in the lunar interior using the Apollo 15 group A ultramafic green glass, the most abundant pyroclastic glass composition found in the polymict regolith breccia (15426) near Hadley Rille at station 7. Experiments on two compositional end members of the 15A green glass trend yield experiments saturated with olivine and orthopyroxene on the liquidus at 1520°C and 2.1GPa. Experiments were also performed on high-Al and -Ca analog compositions and on mixtures of the green glass compositions and natural minerals (olivine, orthopyroxene and garnet) in order to obtain experimental liquids multiply saturated with olivine, orthopyroxene, clinopyroxene and garnet. Three liquid compositions in equilibrium with a garnet-lherzolite residual phase assemblage were produced at 2.35GPa, 1440 and 1420°C and at 2.6GPa, 1460°C.Observed compositional variations in the 15A glasses rule out any simple partial melting process that would involve a olivine+orthopyroxene bearing source region. The results of these experiments are used to test petrogenetic models for the origin of the 15A green glasses. Synergistic evidence from major and trace element geochemistry, geophysical data on the lunar interior structure and results from models of lunar magma ocean differentiation is used to develop a model for the origin of the 15A ultramafic glasses. We propose that a mixing process involving a primary magma, derived by melting primordial lunar mantle, and remelted late-stage magma ocean cumulates led to the origin of the 15A glass. The model uses the 15A green glass multiple saturation point at 1520°C and 2.1GPa to constrain the depth of a cumulate remelting process. Decompression partial melting of undifferentiated lunar mantle, produced by adiabatic upwelling of the deep, hot interior of the moon, produced melts that infiltrated and selectively melted late stage magma ocean cumulates that had sunk to the base of a lunar magma ocean. The compositional variations observed in the 15A glasses are consistent with this model.