Search for ^(26)Al effects in the Allende Fun Inclusion C1

Abstract

The Mg isotopic composition in plagioclase from the Allende inclusion C1 has been measured to correlate excess ^(26)Mg^* with isotopic anomalies for elements of higher atomic number. No large ^(26)Mg^* excesses were observed in samples with ^(27)Al/^(24)Mg up to 165. The inferred initial ^(26)Al/^(27)Al is 5 × 10^(−6) which is a factor of ten less than previously observed in other Allende inclusions. The low ^(26)Al abundance may be the result of late formation of C1 (by 2.5 m.y.) or due to the non-uniform addition of ^(26)Al to the solar system. The major mineral phases in C1 which are Mg rich and the bulk alteration products all have a uniform Mg isotopic composition with 30‰ per mass unit fractionation and δ^(26)Mg≈ −1.6‰. The plagioclase data show variable Mg isotopic fractionation which ranges from 0 to 30‰ per mass unit. Isotopic homogeneity was demonstrated for the major mineral phases rich in Mg both in macroscopic samples and in microscopic individual crystals. The detailed correlation of ^(26)Al with the more general isotopic anomalies and its use as a chronometer marking the production and introduction of isotopically anomalous material into the solar system remains an open question.