Origin of lunar ultramafic green glasses: constraints from phase equilibrium studies

Abstract

Phase equilibria experiments on an Apollo 14B green glass composition are used to develop a petrogenetic model for the Apollo 14B green glasses. New compositional data for the Apollo 14B green glasses are presented, which replace the original data set of Delano (1986). Near-liquidus phase relations of the 14B green glass are determined from 1.3 to 2.7 GPa and 1380 to 1570°C. The liquidus is saturated with olivine to 2.4 GPa and orthopyroxene above 2.4 GPa. A multiple saturation point with olivine and orthopyroxene occurs at 1560°C and 2.4 GPa. Crystallization of the ol + opx saturation boundary involves 40 wt% ol and 60 wt% opx, and occurs over a 140°C interval before cpx appears at a reaction boundary. Crystallization/melting models are developed to reproduce the compositional variations displayed by the Apollo 14B glasses using these phase equilibrium constraints. Fractional crystallization alone cannot account for the observed variation in the major element abundance of the Apollo 14B green glasses. Assimilation is also necessary, in particular, to fit variations in TiO 2, FeO, Na 2O and Al 2O 3. The 14B glasses show evidence of simultaneous high-Ti assimilation and fractional crystallization of olivine, followed by simultaneous KREEP assimilation and olivine fractionation. The data requires the high-Ti assimilant to be at a greater depth than the KREEP assimilant, but only constrains the total assimilation/fractional crystallization process to occur between approximately 2.3 and 0.5 GPa. Assimilation of a high-Ti cumulate within this depth range is consistent with an overturn/hybrid mantle remelting model. Highlands crust assimilation is precluded for the Apollo 14B green glasses.