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Published June 2018 | Published + Supplemental Material
Journal Article Open

Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016

Abstract

Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm‐sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO_2 laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic ^(21)Ne, ^(38)Ar, ^(83)Kr, and ^(126)Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for ^3He and ^(21)Ne, and to a lesser extent for ^(36)Ar (Signer et al. 1977), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.

Additional Information

© 2018 The Authors. Meteoritics & Planetary Science published by Wiley Periodicals, Inc. on behalf of The Meteoritical Society. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Received 22 December 2016; revision accepted 05 February 2018. We are grateful to NASA and CAPTEM for allocation of these precious Apollo samples. Evelyn Füri, Michael Broadley, and Laurent Zimmermann are thanked for fruitful discussions. Reto Trappitsch kindly provided Excel files that allowed us to make calculations of the Ne and Ar SCR components. We thank R. Wieler, P. Heck, and an anonymous reviewer for their constructive comments as well as M. Caffee for his careful editorial handling. This study was supported by the European Research Council (grants no. 267255 and 695618). This is CRPG contribution #2528.

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August 19, 2023
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