Cosmic-ray production rates of helium, neon and argon isotopes in H chondrites based on chlorine-36/argon-36 ages

We present the concentrations and isotopic compositions of He, Ne, and Ar f or nonmagnetic fractions and bulk samples of 17 H chondrites which were rec ently investigated for their Cl-36-Ar-36 cosmic-ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic-ray expo sure ages close to the 7 Ma peak. The rare gas data are consistent with Be- 10 and Cl-36 production rates in the metal phase. Remarkably, only I out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and Cl-36-Ar-36 exposure ages, Ne -21 production rates as a function of Ne-22/Ne-21 and a mean Ar-38 producti on rate are determined. The results confirm model calculations which predic t that the relationship between Ne-21 production rates and (NC)-N-22/Ne-21 is ambiguous for high shielding. Besides the mean 38Ar production rate we a lso give production rate ratios p(Ar-38 from Ca)/P(Ar-38 from Fe). They var y between 10 and 77, showing no significant correlation with 38Ar concentra tions or Ne-22/Ne-21. By investigating the metal separates, Graf et al. (20 01) found significant He-3 deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a He-3/Ne-21 vs. Ne-22/Ne-21 diagram plot in the area defined by most of the H chondrites. This means that He-3 deficits in the metal phase are muc h more pronounced than in silicate minerals and we will argue that H-3 diff usive losses in meteorites should be the rule rather than the exception. Th e Ne-21 exposure ages, calculated on the basis of modeled Ne-21 production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low He-3/Ar-38 in the metal formed in a separate event than those with normal 3He/38Ar 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.