B. Dominik

40Ar39Ar ages of Allende

KAr and/or 40Ar39Ar plateau ages of Allende samples—whole rock, matrix, chondrules, white inclusions–range from 3.8 AE for matrix of ≳5 AE for some white inclusions, but cluster strongly near 4.53 AE. This age marks the dominant KAr resetting of Allende materials. Age spectra show disturbances due to 39Ar recoil or some other argon redistribution processes. Possible explanations for the apparent presolar ages (>4.6 AE) include: ≳20% loss of 39Ar; ≳40% loss of 40K ∼3.8 AE ago with no loss of 40Arl trapped argon of unique 40Ar/36Ar isotopic composition; admixture of “very old” presolar grains.

Early lunar differentiation: 4.42-AE-old plagioclase clasts in Apollo 16 breccia 67435

Abstract We report on a40Ar-39Ar study of the Apollo 16 breccia 67435 and present ages of five samples representing matrix, lithic clasts and plagioclase clasts. While the matrix age spectrum does not have a well-defined plateau, the two lithic clasts gave plateau ages of 3.96 and 4.04 AE. Since all samples had apparent ages of ∼1 AE in the fractions ≤600°C extraction temperature, the breccia might have been assembled in a rather mild process at about that time or even more recently out of material with different metamorphic ages. The two plagioclase samples, of which one was a single 9-mg mineral clast and the other a 15-mg composite of several clasts, also have ages of ∼1 AE in the low-temperature release fractions, but are apparently undisturbed by any ∼4-AE events since they both have well-defined plateaux at 4.42 AE. The age of these strongly calcic plagioclase clasts, believed to be remnants of the anorthositic lunar crust, establishes a lower age limit to the end of the early lunar differentiation and thus places a strong constraint to the lunar evolution.

Gerontology of the Allende meteorite

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 Allendes 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.