Radiogene, spallogene und primordiale Edelgase in Steinmeteoriten

Abstract

The total content as well as the isotopic composition of helium and neon of 47 stone meteorites have been determined. The concentrations of 3He, 4He, 20Ne, 21Ne and 22Ne are included in the tables. For meteorite samples without 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 radiogenic 4He the U-Th-He ages of the investigated meteorites have been calculated using an average uranium content of 1.1·10-8 g/g (except those cases were uranium determinations existed) and the 3.5 fold amount for thorium.The cosmic ray exposure ages were calculated using the average production rates for 3He and 21Ne derived from the decay rate of tritium and 22Na measured in some meteorites. The calculated values for the exposure ages lie between 0.5·106 years and 40· 106 years.Some of the meteorites show very low 3He/21Ne ratios which indicate diffusion loss of the spallogenic rare gases.Our results indicate differences in the distribution of the U-Th-He ages as well as of the cosmic ray exposure ages between the chondrites of the low iron group (L) and the chondrites of the high iron group (H). On the average the U-Th-He ages of the H group chondrites are considerably higher than those of the L group. In fact, there are only a few H group chondrites with low U-Th-He ages, and for most of these exceptions we show that they lost their radiogenic 4He during the time of cosmic ray exposure. The cosmic ray exposure ages of the H group chondrites are below 10 million years in 74% of the cases, while for those of the L group only 40% are below this value.Investigations on the 47 meteorites also disclosed two additional meteorites (Pultusk and Vigarano) which exhibit a high content of light primordial rare gases, as was earlier found by us for Pantar, Breitscheid and Tabor.