Volatile loss during homogenization of lunar melt inclusions

Abstract

Volatile abundances in lunar mantle are critical factors to consider for constraining the model of Moon formation. Recently, the earlier understanding of a "dry" Moon has shifted to a fairly "wet" Moon due to the detection of measurable amount of H_2O in lunar volcanic glass beads, mineral grains, and olivine-hosted melt inclusions. The ongoing debate on a "dry" or "wet" Moon requires further studies on lunar melt inclusions to obtain a broader understanding of volatile abundances in the lunar mantle. One important uncertainty for lunar melt inclusion studies, however, is whether the homogenization of melt inclusions would cause volatile loss. In this study, a series of homogenization experiments were conducted on olivine-hosted melt inclusions from the sample 74220 to evaluate the possible loss of volatiles during homogenization of lunar melt inclusions. Our results suggest that significant loss of H_2O could occur even during minutes of homogenization, while F, Cl and S in the inclusions remain unaffected. We model the trend of H_2O loss in homogenized melt inclusions by a diffusive hydrogen loss model. The model can reconcile the observed experimental data well, with a best-fit H diffusivity in accordance with diffusion data explained by the "slow" mechanism for hydrogen diffusion in olivine. Surprisingly, no significant effect for the low oxygen fugacity on the Moon is observed on the diffusive loss of hydrogen during homogenization of lunar melt inclusions under reducing conditions. Our experimental and modeling results show that diffusive H loss is negligible for melt inclusions of >25 μm radius. As our results mitigate the concern of H_2O loss during homogenization for crystalline lunar melt inclusions, we found that H_2O/Ce ratios in melt inclusions from different lunar samples vary with degree of crystallization. Such a variation is more likely due to H_2O loss on the lunar surface, while heterogeneity in their lunar mantle source is also a possibility. A similar size-dependence trend of H_2O concentrations was also observed in natural unheated melt inclusions in 74220. By comparing the trend of diffusive H loss in the natural MIs and in our homogenized MIs, the cooling rate for 74220 was estimated to be ∼1 °C/s or slower.

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