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Cosmic‐ray prdocution rates of helium, neon and argon isotopes in H chondrites based on chlorine‐36/argon‐36 ages
Author(s) -
LEYA I.,
GRAF Th.,
NISHIIZUMI K.,
WIELER R.
Publication year - 2001
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/j.1945-5100.2001.tb01933.x
Subject(s) - chondrite , neon , meteorite , isotopes of argon , argon , analytical chemistry (journal) , chemistry , isotope , cosmic ray , mineralogy , astrophysics , astrobiology , physics , nuclear physics , environmental chemistry , organic chemistry
— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their 36 Cl‐ 36 Ar cosmic‐ray exposure ages (Graf et al. , 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with 10 Be and 36 C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and 36 Cl‐ 36 Ar exposure ages, 21 Ne production rates as a function of 22 Ne/ 21 Ne and a mean 38 Ar production rate are determined. The results confirm model calculations which predict that the relationship between 21 Ne production rates and 22 Ne/ 21 Ne is ambiguous for high shielding. Besides the mean 38 Ar production rate we also give production rate ratios P ( 38 Ar from Ca)/ P ( 38 Ar from Fe). They vary between 10 and 77, showing no significant correlation with 38 Ar concentrations or 22 Ne/ 21 Ne. By investigating the metal separates, Graf et al. (2001) found significant 3 He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a 3 He/ 21 Ne vs. 22 Ne/ 21 Ne diagram plot in the area defined by most of the H chondrites. This means that 3 He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that 3 H diffusive losses in meteorites should be the rule rather than the exception. The 21 Ne exposure ages, calculated on the basis of modeled 21 Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low 3 He/ 38 Ar in the metal formed in a separate event than those with normal 3 He/ 38 Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.