Ages of the Ca-Rich Achondrites

Abstract

Mass-spectrometric measurements were made of He, Ne, and Ar in 25 eucrites, 2 howardites, and in separated mineral fractions of 5 eucrites. [Techniques are described.] Radiation ages range from 0.1 to 62 X 10 (super 6) yr with some evidence for grouping at 5 and 11 X 10 (super 6) yr. The gross age-distribution is similar to that of ordinary chondrites, especially hypersthenes and amphoterites. Hence, the Ca-rich achondrites seem to come from the same parent bodies as most chondrites or from bodies with very similar orbital elements. It appears certain that more than one half of the U-He and K-Ar ages are strongly discordant. Roughly one half of the K-Ar ages are 2 X 10 (super 9) yr or more older than the corresponding U, Th-He ages, with most between 3 and 4 X 10 (super 9) yr. This strong discordance favors late reheating of parent body.