Origin of the Lunar Ultramafic Glasses Constrained by Experiments and Models

Abstract

To place further constraints on the origin of the lunar ultramafic glasses and the evolution of the lunar interior, phase equilibrium experiments are carried out on two synthetic compositions which represent hybridized, bulk source compositions of (1) high-titanium and (2) very low-titanium/high-aluminum primary magmas. The compositions are designed to produce liquids compositionally similar to the high-Ti Apollo 14 Black (A14B) glass (16.4 wt.% TiO₂) and the high-Al Apollo 14 Very Low Titanium (A14VLT) glass (0.61 wt.% TiO₂, 9.60 wt.% Al₂O₃). Experiments on the synthetic source composition “HiTi1” at pressures of 1.5-2.0 GPa, temperatures of 1380-1460°C, and degrees of melting ≈ 30% produce the best fitting melts to A14B. Experiments on the synthetic source composition “VLTCum1” at pressures of 1.8-2.0 GPa, temperatures of 1460-1480°C, and degrees of melting ≈ 30-45% produce the best fitting melts to A14VLT. The forward melting experiments performed on both source compositions contain equilibrium mineral assemblages that match those obtained through inverse melting experiments on the glass compositions. Experimental conditions that produced good fits to the target glass compositions overlap with conditions corresponding to previously determined olivineorthopyroxene multiple saturation pressures and temperatures for the glasses. Our experimental results confirm melting from a compositionally heterogeneous lunar mantle source that was hybridized through cumulate mantle overturn. Using a petrogenetic mass balance model, we suggest source components which could have been involved in the production of these hybridized source regions. We also calculate density as a function of pressure for several high-Ti glasses as well as the A14VLT glass and determine if these liquids are positively buoyant at their hypothesized depth of origin relative to their mantle residue. We find that the A14VLT glass is always positively buoyant at relevant depths, while some of the high-Ti glasses are positively buoyant only at depths corresponding to more oxidizing source region compositions.