Ages of calcium-rich achondrites—I. Eucrites

Abstract

The noble gases He, Ne and Ar were measured in 26 eucrites and 2 howardites, and in separated mineral fractions of 6 eucrites. The feldspar has lost nearly all its cosmogenic He 3, as shown previously by Megrue (1966). Radiation ages were therefore calculated by a new method, using the He 3 Ar 38 ratio to infer the feldspar content and hence the correction for He 3 diffusion losses. Bulk samples as small as 50 mg can thus be reliably dated. Earlier measurements from the literature were recalculated in this manner. The resulting age distribution resembles that of chondrites, ranging from ≤0.1 m.y. to 62 m.y. Two distinct clusters were found at 5 and 11 m.y.; two others may be present at ~18 and ~30 m.y. Two howardites are present in the first 2 clusters. Apparently eucrites and howardites are produced in the same impacts, and hence originated from within a few kilometers of each other. K-Ar ages of eucrites lie between 2.4 and 4.4 AE; U-He ages tend to be systematically shorter, with more than one-third lying below 1 AE. The three eucrites with shortest U-He ages ( <0.2 AE) show pronounced shock effects. Presumably the short ages are due to a recent collision that heated the parent body of these meteorites. This would seem to imply that all eucrites and howardites originated in a single body. The two shergottites have virtually identical radiation ages (2.5 m.y.) and K-Ar ages (0.5–0.6 AE). Perhaps they were involved in the postulated 0.52 AE collision that fragmented the parent body of the hypersthene chondrites and produced the majority of iron meteorites now falling on Earth. Howardites and eucrites differ strikingly in the proportion of gas-rich meteorites. Not a single one of the 30 eucrites and shergottites investigated to date contains light primordial gases, while 3 out of 6 howardites do. Most of the properties of eucrites are equally consistent with a lunar or an asteroidal origin. Two factors favoring an asteroidal origin are the existence of unbrecciated eucrites and of mesosiderites with eucritic silicate inclusions.