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Published September 2015 | Published
Journal Article Open

The chlorine isotope fingerprint of the lunar magma ocean


The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because ^(37)Cl/^(35)Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, ^(37)Cl/^(35)Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high ^(37)Cl/^(35)Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon's history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

Additional Information

© 2015 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited. Submitted 23 March 2015; Accepted 31 July 2015; Published 25 September 2015. We are grateful to the institutions and people who made samples available for this research, including the Lunar Sample and Meteorite curators at the Johnson Space Center, and T. Bunch of Northern Arizona University. We are also grateful to F. McCubbin and J. Mosenfelder, who graciously provided apatite and other mineral standard materials, and M. Le Voyer, who was instrumental in developing Cl isotope capabilities at Caltech. The first author would like to acknowledge the support that he received from his coauthors and colleagues during the duration of this project, which was considerably lengthened by a near-fatal illness. The manuscript benefitted from detailed, thoughtful, and constructive criticism from the editor, two anonymous reviewers, and C. Neal - who is perhaps thanked twice in this sentence. Funding: This research was supported by NASA Early Career Fellowships to J.W.B. (NNX13AG40G) and J.G. (NNX13AF54G), as well as NASA grants to A.H.T. (NNX12AH64G) and J.P.G. (NNX11AB29G). Author contributions: J.W.B., A.H.T., J.M.E., and E.M.S. conceived the project. C.M., A.H.T., and J.G. performed the petrographic and petrologic descriptions. Y.G., C.M., A.H.T., and J.W.B. performed the analyses. All authors participated in the data reduction and writing of the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All new data generated for this paper are found in Table 1. Samples analyzed were borrowed from several sources (see above) and must be requested directly to those sources.

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