Abstract
The He, Ne and A isotopes of 59 and the total K content of 48 stony meteorites have been determined. The isotopic composition of the spallation-produced rare gases is in agreement with production rates, calculated from the cosmic ray spectrum including secondaries and from cross-section measurements of high-energy protons. Different chemical composition of the various types of stony meteorites can account for the large variations of the measured isotopic ratios. In some cases a different effective bombarding energy due to the amount of shielding is indicated. The distribution of the amount of spallation products points to a continuous breaking of the meteorites rather than to a single event. The He and K-A ages range from 0,2 to 5,1 × 10 9y and have a maximum in their distribution around 4,5 × 10 9y. A considerable number of meteorites have ages below 2 × 10 9y and some have large differences between the He and K-A ages. This can only be explained by an additional heating source after their formation. Heating by collisions seems to be more likely than by solar radiation.
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