Demonstration of ²⁶ Mg excess in Allende and evidence for ²⁶ Al

Abstract

We report the discovery of a large anomaly in the isotopic composition of Mg in a Ca‐Al rich chondrule from the Allende meteorite. This anomaly is manifest independently of instrumental fractionation and is due to an enrichment of about 1.3 percent in 26 Mg while the abundances of 25 Mg and 24 Mg are terrestrial in value. There is a strong correlation in this chondrule between the 26 Mg excess and the Al/Mg ratio so that the most plausible cause of the anomaly is the in situ decay of now extinct 26 Al (τ ½ = 0.72 × 10 6 yr). Mineral phases extracted from a Ca‐Al‐rich aggregate have distinct Al/Mg but show identical, small Mg anomalies which are apparent after correction for fractionation (δ 26 Mg = 0.3%). These data indicate that this aggregate was isotopically homogenized in a high Al/Mg environment after the decay of 26 Al had occurred or that some of the Mg anomalies are due to effects other than in situ decay of 26 Al.