Abstract DGP2026-47

The amount and distribution of water within the Moon and the related accessibility of water or hydrogen at or near the lunar surface are of central interest for future lunar missions as well as our understanding of the Moon’s formation and evolution. Yet, the water budget of the Moon and its evolution over time are still poorly understood.
Combining models of lunar magma ocean (LMO) solidification, water (H2 and H2O) solubility, degassing and water partitioning, we simulate the evolution of the lunar water budget and assess the relative importance of key parameters including magma ocean depth, initial water content, oxygen fugacity, water degassing efficiency and water partitioning between the magma ocean liquid and crystallizing minerals.
A plausible parameter space can be identified by comparing modeled mineral water contents with observed water contents in lunar materials, in particular pristine ferroan anothosites (FANs), which might constitute primary products of the lunar magma ocean. Using FAN plagioclase water contents as primary constraint, we find that the current experimental uncertainties in the plagioclase water partitioning coefficient constitute a key source of uncertainty for estimates of the initial lunar water content and the distribution of water in the lunar interior, e.g. in the source regions of lunar mare basalts and basaltic glasses.
Considering the range of published water partitioning data for plagioclase [1, 2], modeled initial LMO water contents span several orders of magnitude. High water partition coefficients between magma and plagioclase [1] imply initial LMO water contents as low as ~0.1 ppm. Such a dry magma ocean is expected to experience only little degassing, because it is largely water undersaturated during most of its evolution, and requires trapped liquid in the mantle cumulate or an additional water source to explain the high water contents observed in mantle-derived volcanic products.
If low  water partition coefficients between magma and plagioclase [2] are assumed, initial LMO water contents in the order of several 100 or 1000 ppm are possible. For such high initial water contents, the degassing efficiency is controlling the water budget, so that tighter constraints on plausible initial water contents are only possible by consulting more detailed magma ocean mixing and degassing models that can constrain the degassing efficiency in a convecting magma ocean.
In conclusion, better constraints on the water partition coefficient between magma and plagioclase at lunar conditions are crucial for precise estimates of the initial lunar water budget and our understanding of water abundance and distribution in the Moon.
References:
[1] Xu, Y., Lin, Y., Zheng, H., & van Westrenen, W. (2024). Non-Henrian behavior of hydrogen in plagioclase–basaltic melt partitioning. Chemical Geology, 661, 122153.
[2] Hamada, M., Ushioda, M., Fujii, T., & Takahashi, E. (2013). Hydrogen concentration in plagioclase as a hygrometer of arc basaltic melts: Approaches from melt inclusion analyses and hydrous melting experiments. Earth and Planetary Science Letters, 365, 253-262.