Evidence for an early wet Moon from experimental crystallization of the lunar magma ocean

Journal name:
Nature Geoscience
Volume:
10,
Pages:
14–18
Year published:
DOI:
doi:10.1038/ngeo2845
Received
Accepted
Published online

Abstract

The Moon is thought to have been covered initially by a deep magma ocean, its gradual solidification leading to the formation of the plagioclase-rich highland crust. We performed a high-pressure, high-temperature experimental study of lunar mineralogical and geochemical evolution during magma ocean solidification that yields constraints on the presence of water in the earliest lunar interior. In the experiments, a deep layer containing both olivine and pyroxene is formed in the first ~50% of crystallization, β-quartz forms towards the end of crystallization, and the last per cent of magma remaining is extremely iron rich. In dry experiments, plagioclase appears after 68 vol.% solidification and yields a floatation crust with a thickness of ~68km, far above the observed average of 34–43km based on lunar gravity. The volume of plagioclase formed during crystallization is significantly less in water-bearing experiments. Using the relationship between magma water content and the resulting crustal thickness in the experiments, and considering uncertainties in initial lunar magma ocean depth, we estimate that the Moon may have contained at least 270 to 1,650ppm water at the time of magma ocean crystallization, suggesting the Earth–Moon system was water-rich from the start.

At a glance

Figures

  1. Comparison of LMO crystallization sequences from this study and the literature.
    Figure 1: Comparison of LMO crystallization sequences from this study and the literature.

    a, Numerically determined crystallization sequence2. bd, Experimentally determined crystallization sequences up to 78 PCS (per cent solid by volume) for the LPUM bulk Moon compositions at 1, 2 and 4GPa (ref. 10). e,f, Experiments for a dry system (e) and a system containing 3,150 ppm water (f) in a LMO with initial depth of 700km. Cpx, clinopyroxene; Opx, orthopyroxene.

  2. Average thickness of the lunar crust as a function of initial LMO water content.
    Figure 2: Average thickness of the lunar crust as a function of initial LMO water content.

    Filled circles and solid line show experimental results from this study; trianglesand squares connected by dashed/dotted lines show crustal thickness curves assuming different LMO depths calculated using thermodynamic modelling25. Grey bar shows observed average crustal thickness of 34–43km based on GRAIL data24.

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Affiliations

  1. Faculty of Earth and Life Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

    • Yanhao Lin,
    • Elodie J. Tronche,
    • Edgar S. Steenstra &
    • Wim van Westrenen

Contributions

W.v.W., E.J.T. and Y.L. designed the project. Y.L. and E.J.T. performed the experiments. Y.L., E.J.T. and E.S.S. performed sample analyses. All authors discussed the results. Y.L. and W.v.W. wrote the paper with input from all authors.

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The authors declare no competing financial interests.

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