The collisional history of the HED parent body inferred from 40Ar 39Ar ages of eucrites

Abstract

In the framework of a consortium studying small solar system bodies we analysed matrix samples and individual clasts from five eucrites—Stannern, Pasamonte, ALHA-76005, ALHA-78132 and ALHA-80102—by high resolution stepwise heating 40Ar 39Ar technique. Our aim was to determine the duration of secondary processes like brecciation and recrystallization caused by collisional events, which were accompanied by rather weak reheating and thus by rather small effects on the KAr system. For this reason all age spectra are complex. The apparent ages increase with increasing degassing temperatures and, in some cases, are additionally disturbed by 39Ar recoil. Thus, the low temperature ages provide estimates of the time of the last thermal disturbing event while the high temperature ages give lower limits for the last total degassing of the rocks. Only a granulitic breccia clast from polymict eucrite Pasamonte yields a plateau age of 4.245 ± 0.021 Ga, which probably dates the formation of its equigranular texture. The K Ca spectra for all samples are constant for the first 70% of the 39Ar release and then level off by about one order of magnitude. Finally, we are not able to prove or dismiss a cataclysmic bombardment of the HED parent body comparable to that of the Earth's moon. Nevertheless, a rather short duration of impact metamorphism seems unlikely. Completed by the data from other studies, our results support a period of intense bombardment of the HED parent body totally resetting the KAr system of the rocks ∼ 4.4-3.9 Ga ago. It was followed by a time of less intense impact induced metamorphism causing only partial loss of radiogenic 40Ar ∼ 3.9-2.8 Ga ago.