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Published July 2016 | Supplemental Material
Journal Article Open

D-poor hydrogen in lunar mare basalts assimilated from lunar regolith


Apatite grains in lunar mare basalts contain hydrogen that ranges in D/H ratio by more than a factor of two. For most of these basalts, the D/H ratios in their apatite grains decrease with measures of the host basalts' time spent at elevated temperature, specifically the Fe-Mg homogenization of their pyroxenes. Most basalts with homogeneous pyroxenes (i.e., with constant Fe/Mg ratio) have apatite grains with low D/H (δD ≈ −100‰), whereas most basalts with heterogeneous pyroxenes (i.e., varying or zoned Fe/Mg) have apatite with high D/H (δD up to ~ +1100‰). This relationship suggests that low D/H values were acquired during thermal processing, i.e., during Fe-Mg chemical equilibration, during or after emplacement. This light hydrogen is likely derived from solar wind implanted into the lunar regolith (with δD from −125‰ to −800‰), and could enter basalts either by assimilation of regolith or by vapor transport from regolith heated by the flow. If a basalt could not interact with regolith rich in solar wind (e.g., it was emplaced onto other fresh basalts), its apatite could retain a magmatic D/H signature. The high D/H component (in the apatites of unequilibrated basalts) is most reasonably that indigenous magmatic hydrogen, i.e., representing hydrogen in the basalt's source mantles, or magmatic hydrogen that was residual after partial degassing of H_2.

Additional Information

© 2016 Mineralogical Society of America. Manuscript received October 6, 2015; Manuscript accepted March 9, 2016; First Published on July 01, 2016. Manuscript handled by Ian Swainson. All data generated for this study are available in the online deposit material2. The second author acknowledges support received from coauthors and colleagues during this project, which was considerably lengthened by a near-fatal illness. We are grateful to the institutions and people who made samples available for his research, including the Lunar Sample and Meteorite curators at Johnson Space Center; F. McCubbin and J. Mosenfelder, who provided apatite and other mineral standard materials; and K. Joy for valuable discussions. The manuscript was improved substantially by reviews, especially that by R. Tartése, to whom we are particularly grateful; we also thank five anonymous reviewers. This research was supported by NASA Early Career Fellowships to Boyce (NNX13AG40G), as well as NASA grants to Treiman and Gross (NNX12AH64G) and Greenwood (NNX11AB29G). The authors have no competing interests. LPI Contribution 1913.

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