Isotopic analyses of krypton and xenon in fourteen stone meteorites

The concentrations and isotopic compositions of Kr and Xe in twelve chondrites and two achondrites have been determined. In addition to the trapped component, Kr and Xe produced by spallation and (n, γβ) reactions as well as fission and radiogenic Xe have been found. Because of the chemical uniformity of the ordinary chondrites the concentrations of spallation-produced Kr and Xe found in these meteorites correlate with the concentrations of the spallation-produced light rare gases. It is shown that variations in the Kr spallation spectrum are caused by differences in the hardness of the energy spectrum, which are also responsible for the variations in the spallation ratio Ne21/He3 in ordinary chondrites. Excesses of Kr80, Kr82, and Xe128 produced by capture of secondary neutrons in Br and I, respectively, are sensitive indicators for shielding and preatmospheric meteorite sizes. Neutron slowing down densities and minimal preatmospheric radii and masses are calculated on this basis for some of the investigated chondrites. Kr81 radiation ages are obtained for three chondrites and for the achondrites. For the chondrites and for a Ca-poor achondrite the Kr81 ages agree with the ages calculated from the He3 concentrations. However, the Ca-rich achondrites show Kr81 ages up to one order of magnitude higher, indicating He3 diffusion losses. A remarkable correlation between the concentrations of Br and trapped Ar, Kr, and Xe in chondrites has been found. The isotopic compositions of the trapped components in Kr- and Xe-rich ordinary chondrites are found to agree with those known for carbonaceous chondrites; however, the Khor Temiki achondrite seems to differ in this respect. For the Mezö Madaras chondrite a sample of unequilibrated host material and a sample of an inclusion of equilibrated material have been analyzed for all five rare gas elements. The spallation components of both samples are in excellent agreement, whereas the concentrations of trapped Ne, Ar, Kr, and Xe are about twenty times higher in the unequilibrated sample.