The isotopic composition of lithium in chondrules

Abstract

Trace quantities of Li in chondrules from the Bruderheim and Bjurbole meteorites were isolated by chemical procedures that yielded more than 90% of the Li in the sample. The isotopic composition of Li in two samples of Bruderheim chondrules and one sample of Bjurbole chondrules was measured on a 9-inch-radius-of-curvature, single-focusing mass spectrometer. The problem of mass fractionation which occurs during the mass analysis was solved by an integration procedure. The isotopic composition of Li from two samples of ‘whole-rock’ Bruderheim and one sample of the granite G-1 was also measured for comparison. The Li7/Li6 ratios for the chondrules, whole-rock Bruderheim, and G-1 were found to be identical within 3.3% (at the 95% confidence level). The Li content of all the samples was determined by isotope dilution analyses of aliquots of each sample. The contents found were 0.404 ppm for whole-rock Bruderheim, 0.267 ppm for Bruderheim chondrules, 0.089 ppm for a large lath chondrule from Bruderheim, 0.134 ppm for Bjurbole chondrules, and 20.8 ppm for Gr-1. Blanks were less than 0.3% of the sample sizes in our experiments. We are thus unable to confirm a previous report of a 15% excess of Li6 in the Bruderheim meteorite.