Radiogene, spallogene und primordiale Edelgase in Steinmeteoriten III

Abstract

The total content as well as the isotopic composition of helium and neon of 36 stone meteorites have been determined. Except for meteorites with primordial rare gas content, the radiogenic component of 4He has been calculated by subtracting the fourfold amount of 3He from the total 4He in order to allow for the spallation fraction of 4He. From the content of radiogenic 4He the U-Th-He-ages of the investigated meteorites have been calculated. From the content of 3He and 21Ne cosmic ray exposure ages were calculated for all meteorites also. With the new data of the 36 stone meteorites, presented in this paper, the total number of stone meteorites, of which rare gas measurements were carried out so far, amounts to 70 bronzitechondrites and 93 hypersthene-chondrites. The earlier indications for marked differences in the distribution of the U-Th-He-ages as well as for the cosmic ray exposure ages between the bronzitechondrites and the hypersthene-chondrites proved to be undoubtedly correct. During our investigations, we found Elm Creek as one additional bronzite-diondrite with high amounts of primordial rare gases. In two other bronzite-chondrites (Cavour and Dimmit) primordial rare gases were also detected, but in rather small amounts.