Ages of the Ca-Rich Achondrites

Abstract

Mass-spectrometric measurements of He, Ne, and Ar in 25 eucrites, in 2 howardites, and in separated mineral fractions of 5 eucrites confirm Megrue’s observation that the feldspar in these meteorites has lost virtually all cosmogenic ³He and variable fractions of ²¹Nec. Eucrites do not contain detectable amounts of trapped He and Ne. The results show further that our small (~ 50 mg) ‘bulk’ samples of eucrites are often considerably richer or poorer in feldspar than the average norm of 36%. This is seen from the (³He/³⁸Ar)_c ratios which range from 2.3 to 19. A eucrite with average feldspar content should have a ratio of about 8. Unfortunately, without any information on the feldspar contents of the samples, no reliable radiation age can be calculated.

We have solved this dilemma by calculating (³He/³⁸Ar)_c as a function of the feldspar content, using the known production rates of ³Hec and ³⁸Arc in the major elements. With this correlation, the apparent feldspar content of any sample can be estimated from its measured (³He/³⁸Ar)_c ratio. The apparent feldspar contents, in turn, are used to adjust ³He. Radiation ages are calculated with adopted production rates of 2.10 and 2.14 × 10⁻⁸ cm³ STPg⁻¹ 10⁶ year⁻¹ for eucrites and howardites.

The radiation ages range from 0.1 to 62 × 10⁶ year with some evidence for grouping at about 5 and 11 − 10⁶ year. The gross age-distribution is similar to that of ordinary chondrites, especially of the hypersthenes and amphoterites. Hence, the Ca-rich achondrites seem to come from the same parent bodies as the great majority of the chondrites, or from bodies with very similar orbital elements.

The gas-retention ages cannot be estimated very accurately. Even so, it appears quite certain that more than one-half of the U-He and K-Ar ages are strongly discordant. Among these cases, roughly one half of the K-Ar ages are 2 − 10⁹ year or more older than the corresponding U, Th-He ages, with most of the K-Ar ages between 3 and 4 − 10⁹ year. This strong discordance seems to speak in favour of a late reheating of the parent body.