He, Ne and Ar in four mesosiderites and the Binda achondrite

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Mass spectrometric analyses of He, Ne and Ar were performed on the mesosiderites, Clover Springs, Crab Orchard, Patwar, and Hainholtz with comparative analyses also conducted on the achondrite Binda. Gases were released in incremental temperature steps so that variation of spallogenic compositions could be studied. All samples were found to be rich in cosmogenic Ne and Ar. The ²¹Ne/²²Ne ratio showed a 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 agreement with previous work. Cosmic ray exposure ages determined from cosmogenic Ar abundances fall in the range 40 to 87myr for mesosiderites. Crab Orchard gave a K-Ar age of 3.2 × 10⁹yr. The Binda achondrite gave 44myr and 3.2 × 10⁹yr respectively for cosmic ray exposure and K-Ar ages.