Über die leichten Edelgase in dem Chondriten Kirin / About the Light-weight Noble Gases in the Kirin H-chondrites

Abstract

Results are reported for the contents of He, Ne, and Ar of three different specimens from the Kirin H-chondrite which, with a recovered weight of about 4000 kg, is the largest known stone meteorite. The concentrations of spallogenic gases cover a range of more than a factor of two; bulk samples with ratios 3He/21Ne ≲ 2 and a FeNi nugget with 3He/38Ar = 8.8 ± 0.6 indicate strong diffusion losses of 3He from the silicates and of tritium from the metal. - Radiogenic 4He and 40Ar have been affected by diffusion, too, resulting in discordant U/Th-4He- and 40K-40Argas retention ages as well as distinctly different ages for different samples. Stepwise heating experiments show the main release of 4He and 40Ar to occur at around 800 °C and the difference in the gas contents to be due to differences in the low-temperature part of the gas release curve. - An attempt is made to account for the observed positive correlation between the concentrations of spallogenic and radiogenic noble gas nuclides. Either the diffusion losses of both have occured at the same time which requires a (quasi-)continuous loss due to a small perihelion distance or a catastrophic event late during the cosmic ray exposure history, but more than about 105 years before the fall of the meteorite. As an alternative model it is suggested that the Kirin meteoroid was hot upon the ejection from its parent body. As the subsequent cooling rate of the meteoroid is smallest in the interior the diffusion losses of radiogenic 4He and 40Ar will be largest where the production rate of the cosmogenic nuclides is smallest.