Bulk Mg isotopic compositions of Ca-Al-rich inclusions and amoeboid olivine aggregates

Abstract

High-precision Mg isotopic compositions of Ca-Al-rich inclusions (CAIs) from both Ningqiang (ungrouped) and Allende (CV3) carbonaceous chondrites and amoeboid olivine aggregations (AOAs) from Allende were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The CAIs from Allende plot on a line, with an inferred initial 26Al/ 27Al ratio of (4.77 +/- 0.39) 10^(-5) close to the canonical value. This indicates a relatively closed Al-Mg system in the CAIs and no significant Mg isotope exchange with ambient materials, although two of the CAIs are severely altered. The AOAs contain excess 26Mg and plot close to the CAI regression line, which is suggestive of their contemporary formation. The CAIs from Ningqiang define a different line with a lower inferred (26Al/27Al)0 ratio of (3.56 +/- 0.08) 10^(-5). None of the CAIs and AOAs studied in this work shows significant mass fractionation with enrichment of the heavier Mg isotopes, arguing against an evaporation origin.