Mg ISOTOPIC ANOMALIES in the ALLENDE METEORITE and CORRELATION with O and Sr EFFECTS

Mg isotopic anomalies were determined on inclusions in the Allende meteorite. Measured Mg isotopic ratios were corrected for mass fractionation by normalizing to the terrestrial(normal) Mg-25/Mg-24. Relative to normal Mg, Allende, samples show distinct isotopic anomalies which correspond to enrichment of Mg-24 or Mg-26 of up to 3 per mil or depletion of Mg-25 of up to 1.5 per mil. One sample shows a depletion of Mg-24 or Mg-26 by 1.7 per mil or an enrichment of Mg-25 by 0.85 per mil. These anomalies are smaller by a factor of ten than found for the oxygen isotopes. Samples with large oxygen effects (up to 5%) yield normal Mg indicating a lack of correlation between the O and Mg effects. A Ca-Al rich chondrule with primitive Sr-87/Sr-86 and normal Sr-84/Sr-86 has a significant Mg anomaly. The Mg anomalies do not correlate with Al/Mg and do not appear to be due to Al-26 decay. The anomaly in the only sample depleted in Mg-26 cannot be due to Al-26 decay. The isotopic composition of Mg in a sample of the total meteorite is normal. These results accentuate the important character of Allende inclusions but do not uniquely identify the nuclear processes responsible for the Mg and O effects.