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

David V. BEKAERT1, Guillaume AVICE1,2, and Bernard MARTY1
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13069]
1Centre de Recherches Petrographiques et Geochimiques, UMR 7358 CNRS—Universite de Lorraine, 15 rue Notre Dame desPauvres, BP 20, 54501 Vandoeuvre-les-Nancy, France
2Present address: Division of Geology and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd,Pasadena, California 91125, USA
Published by arrangement with John Wiley & Sons

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 CO2 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 21Ne, 38Ar, 83Kr, and 126Xe, 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 21Ne, and to a lesser extent for 36Ar (Signer et al. 1977), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.

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