EVALUATION OF 26AL/27AL AT CRYSTALLIZATION IN EFREMOVKA CAIS BY HIGH-PRECISION, IN SITU ION MICROPROBE ANALYSES

Abstract

Estimates of the initial “canonical” (~5 10 -5) value of 26Al/27Al in the solar nebula are based primarily on ion microprobe analyses of high Al/Mg phases in CAIs. Some of these minerals (e.g., plagioclase, melilite) are known to be susceptible to alteration and sub-solidus isotopic mobility. Recent high-precision analyses by ICPMS have uncovered evidence for apparently higher (>7 10-5) initial 26Al/ 27Al values characteristic of bulk CAIs [3] or of more magnesian mineral phases in coarse-grained CAIs [4]. These data can be interpreted as indicating an appreciable temporal gap between initial Al/Mg fractionation during formation of proto-CAIs and subsequent melting/crystallization.