Tin isotopes indicative of liquid–vapour equilibration and separation in the Moon-forming disk

Abstract

The depletion in moderately volatile elements in the Moon relative to Earth and comparison of the isotope compositions of the Moon and Earth have placed important constraints on models of lunar formation. A liquid–vapour protolunar disk from a high-energy giant impact has been proposed to explain some of these constraints. Here we present high-precision tin isotope data for lunar rocks, measured by double-spike MC-ICP-MS (multicollector inductively coupled plasma mass spectrometry). The lunar rocks are enriched in light tin isotopes compared to the Earth (Δ124/116SnMoon–Earth = −0.48 ± 0.15‰). On the basis of our data and constraints on tin speciation, we show that this tin isotope fractionation is inconsistent with volatile loss from a lunar magma ocean. Instead, we propose a scenario with vigorous mixing between the protolunar disk and the Earth in high-energy conditions during the impact, followed by liquid–vapour equilibration and phase separation at around 2,500 K while the disk was cooling. This scenario is consistent with the depletion in moderately volatile elements and isotope composition of the Moon.

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Fig. 1: Sn isotope composition of lunar samples.
Fig. 2: T50% condensation curves for K, Sn and Zn as a function of total pressure and temperature in a disk wet BSE composition.
Fig. 3: Cartoon depicting the processes that could lead to volatile element depletion in the protolunar disk.
Fig. 4: Evolution of Δ124SnMoon–BSE as a function of Sn concentration in the condensate.

Data availability

All data that support the findings of this study are available in the manuscript or the supplementary materials.

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Acknowledgements

We thank the CAPTEEM of NASA for providing the lunar rocks. This project was funded by ANR project ISOVOL and by LABEX LIO (Lyon Institute of Origins). B.F. was funded by grants AST-1517541 and AST-1412175 from the NSF Astronomy Program and NASA grant NNX17AC02G (XRP Program), and thanks N. S. Jacobson for useful discussions. We thank J. Creech for insightful comments that helped improve this manuscript.

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X.W. and C.F. developed the chemical and isotopic methods. X.W. performed Sn isotope and abundance analysis of all samples. C.F. and B.B. conceived the study, organized sample acquisition, contributed to the modelling and wrote significant parts of the manuscript. B.F. performed the calculations of condensation temperatures and the speciation in the condensed phase. S.C. carried out the equilibration timescale between liquid and vapour and associated modelling. All authors contributed to the interpretation of results and the writing of the manuscript.

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Correspondence to Caroline Fitoussi.

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Supplementary Text, Figs. 1–9 and Tables 1–4.

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Wang, X., Fitoussi, C., Bourdon, B. et al. Tin isotopes indicative of liquid–vapour equilibration and separation in the Moon-forming disk. Nat. Geosci. 12, 707–711 (2019). https://doi.org/10.1038/s41561-019-0433-4

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