Origin of lunar high-titanium ultramafic glasses: A hybridized source?

Abstract

Two new hypotheses for the origin of the lunar high-Ti ultramafic glasses are considered in this study. These new models are motivated by experimental results on a model hybridized lunar magma ocean cumulate composition and by the failure of current models to adequately account for the processes that lead to the origin of these unique lunar ultramafic magmas. In the first model we propose that the observed compositional variability at the high-Ti end of the glass spectrum is created by melting of compositionally heterogeneous source materials produced during the late stages of magma ocean crystallization. The lunar hybridized source composition that we have investigated can broadly reproduce the major element compositional characteristics of the Apollo 17 orange glass and is saturated with olivine and orthopyroxene as residual phases in the source. Models of high-Ti glass generation that include fractional crystallization and/or melting involving olivine and orthopyroxene are attractive because all lunar ultramafic melts show evidence of high-pressure multiple saturation with these phases. Crystallization of olivine and orthopyroxene in the proportions indicated from high-pressure experimental results however, will not produce the entire spectrum of high-Ti glasses. Perhaps the compositional variability is caused by heterogeneity in the proportions of phases stored as late stage cumulate residues. In the second model, low-degree partial melts of the hybridized magma ocean source are segregated during partial melting and sink into and interact with underlying hotter olivine+orthopyroxene cumulates by reactive porous flow, giving rise to the compositional spectrum observed in the high-Ti ultramafic glasses. In this model, the Apollo 17 high-Ti orange glass is produced by the highest degree of melting of the hybridized source. Higher-Ti ultramafic glasses (e.g. Apollo 15 red and Apollo 14 black glasses) are produced by smaller degrees of melting of the hybridized source when olivine+orthopyroxene+clinopyroxene are still present as saturating phases. In this model, the depth of segregation of the high-Ti ultramafic magma is determined by the change in buoyancy brought about by the reactive dissolution of olivine+orthopyroxene cumulates. As the high-Ti melt dissolved these phases, its density decreases, until it becomes neutrally or positively buoyant.