Abstract
The preatmospheric mass of Angra dos Reis is estimated to be about 80 kg, based on cosmic ray track densities of 1−3 × 10 6 cm −2 in feldspars, olivines, and pyroxenes. The 244Pu-fission track densities in pyroxenes and adjacent olivine-feldspar track detectors, as well as in whitlockites and adjacent pyroxene-olivine detectors are about 20 and 280 × 10 6 cm −2 respectively. These values correspond exactly to the 244Pu-fission xenon contents in the two minerals. This suggests an almost instantaneous cooling (>70 K per m.y.) of the Angra dos Reis material 4.55 b.y. ago. This fast cooling is the reason for a quantitative 4He retention since that time. If the 4He excess found in Angra dos Reis is a real effect, the source of this excess remains unknown.
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