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Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016
Author(s) -
Bekaert David V.,
Avice Guillaume,
Marty Bernard
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13069
Subject(s) - basalt , isotope , meteorite , geology , krypton , noble gas , regolith , isotope fractionation , geochemistry , chemistry , radiochemistry , mineralogy , xenon , astrobiology , nuclear physics , physics , organic chemistry
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 3 He and 21 Ne, and to a lesser extent for 36 Ar (Signer et al. [Signer P., 1977]), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.