Krypton anomalies in achondritic meteorites

Abstract

The isotopic composition of krypton was measured in seven high-calcium achondrites, in the high-calcium, stony phase of one mesosiderite, and in five low-calcium achondrites. The eight high-calcium meteorites contained large excesses of the krypton isotopes 80, 82, 83, and 84, when compared with quantities in the atmosphere. A relation appears to exist between this krypton excess and the cosmic-ray exposure age of the meteorite, whereby a longer exposure age correlates with a large krypton excess. When the krypton excesses for these meteorites are normalized to Kr82 = 1.00, a clear pattern emerges. The average relative excesses of cosmogenic krypton observed on the eight high-calcium achondritic samples for masses 80, 82, 83, and 84 were 0.61±0.08, 1.00, 1.04±0.12, and 0.78±0.23. These values agree rather well with the results just obtained in two other laboratories on two other achondritic-type meteorites. The low-calcium meteorites revealed lower krypton excesses, and two of these meteorites appeared to have excess Kr80 and Kr82 from n, γ reactions on bromine. Small amounts of fission-produced Kr86 may be present in the achondrites with high uranium content.