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.