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

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 ²⁶Mg while the abundances of ²⁵Mg and ²⁴Mg are terrestrial in value. There is a strong correlation in this chondrule between the ²⁶Mg excess and the Al/Mg ratio so that the most plausible cause of the anomaly is the in situ decay of now extinct ²⁶Al (τ_½ = 0.72 × 10⁶ 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 (δ²⁶Mg = 0.3%). These data indicate that this aggregate was isotopically homogenized in a high Al/Mg environment after the decay of ²⁶Al had occurred or that some of the Mg anomalies are due to effects other than in situ decay of ²⁶Al.