Water solubility in evolved lunar melts with implications for lunar degassing history

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2023-12

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Abstract

Constraint of lunar water budgets is vital to understanding the evolution of the rocky body. A wide variety of water abundances have been measured in lunar samples, leading to a large range of initial water budget estimates. This study targets evolved late-stage Lunar Magma Ocean (LMO) melts trapped at the base of the crust, which are enriched in volatiles. Synthesized samples are saturated, experimentally placed at relevant P-T conditions, and quenched into glass. The measured water contents represent the water capacity of this composition as a function of pressure. Comparing measurements against six previous models, I evaluate their ability to reproduce the observed data and suggest a best fit model for this unique composition. Furthermore, at reduced conditions H₂ becomes a more dominant fluid species, reducing the partial pressure and consequently the solubility of H₂O. Previous models do not include the effects of oxygen fugacity or H-volatile speciation. I provide a thermodynamic framework to calculate H₂O/H₂ speciation as a function of oxygen fugacity. With the best fit model and thermodynamic framework, the last 1% of LMO liquid (urKREEP) is found to have a maximum capacity of 0.43 wt.% water at IW-2 and a crustal thickness of 40 km. Using previous bulk LMO water contents, it is concluded that urKREEEP could exceed the saturation limit in many scenarios. Excess water was therefore likely degassed through the lunar crust, effectively reducing the observable water budget of the lunar mantle.

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