Cosmic ray produced noble gases in mesosiderite meteorites

Abstract

Mass spectrometric analyses of the noble gas isotopes were performed on the mesosiderites, Clover Springs, Crab Orchard, Patwar, and Hainholtz with comparative analyses also conducted on the achondrites, Binda and Pavlovka. Gases were released in incremental temperature steps so that variation of the spallogenic compositions could be studied. All samples were found to be rich in cosmogenic neon and argon. The ²¹Ne/²²Ne ratio showed a large variation, between 0.83 and 1.00, indicating a definite dependence on the combined effects of target element abundances and cosmic ray shielding. The shielding effect seems to be especially pronounced in the mesosiderites. The ³⁶Ar/³⁸Ar ratio for spallation was found to be ~ 0.65 in good agreement with previous work. Cosmic ray exposure ages determined from cosmogenic argon abundances fall in the range 40 to 87 myr for mesosiderites. Crab Orchard gave a K-Ar age of 3.2 x 10⁹yr. The Binda achondrite gave 44 nyr and 3.2 x 10⁹ yr respectively for cosmic ray exposure and K-Ar ages. The xenon cosmogenic isotope patterns were calculated independently for each temperature fraction from Patwar and also for the Estherville mesosiderite using previously published data. A distinct correlation with temperature was observed which is comparable to other results on achondrites and lunar materials. These spallation product compositional variations appear to depend on target and energy variations. Cosmogenic yields of ¹²⁹Xe are also distinctly correlated, suggesting a lack of radiogenically produced contributions, i.e. from extinct decay. The ¹²⁹Xe/¹²⁶Xe spallation ratio varied from about 1.8 in the low temperature fractions from Patwar to 0.7 in the high temperature Estherville fractions.