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

The observation of a large anomaly in the isotopic composition of Mg in a Ca-Al rich chondrule from the Allende meteorite is discussed. A strong correlation is found in this chondrule between the Mg-26 excess and the Al/Mg ratio. The most plausible cause of the anomaly is attributed to the in situ decay of now extinct Al-26. However, the general pattern of Mg isotopic anomalies is not yet shown to be fully consistent with the Al-26 hypothesis, since an apparently negative Mg-26 anomaly is obtained which cannot be explained by Al-26. A large natural fractionation effect is observed for Mg isotopes. The resolution of the O anomaly and its relation with Mg effects require further investigation.