Irradiation records in regolith materials, II

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Geochimica et Cosmochimica Acta, ISSN: 0016-7037, Vol: 66, Issue: 16, Page: 2929-2958

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Palma, Russel L.; Becker, Richard H.; Pepin, Robert O.; Schlutter, Dennis J.
Elsevier BV
Earth and Planetary Sciences; Kapoeta howardite; isotopic composition; light noble gases; Stars, Interstellar Medium and the Galaxy
article description
High-resolution stepped heating has been used to extract light noble gases implanted in a suite of 13 individual lunar ilmenite and iron grains and in the Kapoeta howardite by solar wind (SW) and solar energetic particle (SEP) irradiation. Isotopic analyses of gases evolved at low temperatures from the lunar grains confirm the neon and argon compositions obtained by Pepin et al. (Pepin R. O., Becker R. H., and Schlutter D. J., “Irradiation records in regolith materials, I: Isotopic compositions of solar-wind neon and argon in single lunar regolith grains”, Geochim. Cosmochim. Acta 63, 2145–2162, 1999) in an initial study of 11 regolith grains, primarily ilmenites. Combination of the data sets from both investigations yields 20 Ne/ 22 Ne = 13.85 ± 0.04, 21 Ne/ 22 Ne = 0.0334 ± 0.0003, and 36 Ar/ 38 Ar = 5.80 ± 0.06 for the lunar samples; the corresponding 36 Ar/ 38 Ar ratio in Kapoeta is 5.74 ± 0.06. The neon ratios agree well with those measured by Benkert et al. (Benkert J.-P., Baur H., Signer P., and Wieler R., “He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes”, J. Geophys. Res. (Planets) 98, 13147–13162, 1993) in gases extracted from bulk lunar ilmenite samples by stepped acid etching and attributed by them to the SW. The 36 Ar/ 38 Ar ratios, however, are significantly above both Benkert et al.’s (1993) proposed SW value of 5.48 ± 0.05 and a later estimate of 5.58 ± 0.03 from an acid-etch analysis of Kapoeta (Becker R. H., Schlutter D. J., Rider P. E., and Pepin R. O., “An acid-etch study of the Kapoeta achondrite: Implications for the argon-36/argon-38 ratio in the solar wind”, Meteorit. Planet. Sci. 33, 109–113, 1998). We believe, for reasons discussed here and in our earlier report, that 5.80 ± 0.06 ratio most nearly represents the wind composition. The 3 He/ 4 He ratio in low-temperature gas releases, not measured in the first particle suite, is found in several grains to be indistinguishable from Benkert et al.’s (1993) SW estimate. Elemental ratios of He, Ne, and Ar initially released from grain-surface SW implantation zones are solar-like, as found earlier by Pepin et al. (1999). Gases evolved from these reservoirs at higher temperatures show evidence for perturbations from solar elemental compositions by prior He loss, thermal mobilization of excess Ne from fractionated SW components, or both.