Gerontology of the Allende meteorite

Abstract

IN the Allende meteorite several elements are found to have an isotopic composition that cannot be due to radioactive or spallation or fractionation processes1. These isotope anomalies are mostly confined to white inclusions enriched in refractory elements2 (Ca–Al-rich inclusions) and are thought to be introduced into the Solar System by precondensed grains3. We have previously reported an anomalously high K–Ar age of a composite sample of five white inclusions from the Allende meteorite4; no direct verification of this lone-standing result of 4,800 Myr was possible because, with the exception of a small chip of one of the inclusions used for a thin section, the material was fused in the 40Ar–39Ar experiment. We therefore began a search for old material in other Ca–Al-rich inclusions. Three different samples from a new type of inclusion proved to be no older than most of Allende's constituents, 4,510 Myr (ref. 5). A small white inclusion gave a well developed 40Ar/39Ar plateau over almost the entire 39Ar release, with an age of 4,500±20 Myr (ref. 6). We report here on the 40Ar–39Ar analysis of two further coarsegrained Allende inclusions that showed ages in excess of 4,550 Myr.