Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites

Abstract

Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured 22Ne 21Ne . Exposure ages calculated from 22Na 22Ne and 26Al 21Ne ratios and constant relative production rates show a bias between the two ages due to variations in 22Na 26Al . Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the 22Na 22Ne and 26Al 21Ne production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.