Search for26 Al effects in the Allende Fun Inclusion Cl

Abstract

The Mg isotopic composition in plagioclase from the Allende inclusion Cl has been measured to correlate excess 26Mg* with isotopic anomalies for elements of higher atomic number. No large 26Mg* excesses were observed in samples with 27Al/24Mg up to 165. The inferred initial 26Al/27Al is 5 × 10−6 which is a factor of ten less than previously observed in other Allende inclusions. The low 26Al abundance may be the result of late formation of Cl (by 2.5 m.y.) or due to the non‐uniform addition of 26Al to the solar system. The major mineral phases in Cl which are Mg rich and the bulk alteration products all have a uniform Mg isotopic composition with 30‰ per mass unit fractionation and δ26Mg≈ −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 26Al 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.