Mg isotope heterogeneity in the Allende meteorite measured by UV laser ablation-MC-ICPMS and comparisons with O isotopes

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in.situ in meteoritical materials show that there are mass-dependent variations in delta(25) Mg and delta(26)Mg up to 1.5 % per amu in chondrules and 0.3% per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high delta(25)Mg and low Delta(17)O. The laser ablation analyses of the CAI show that high-precision determinations of both delta(25)Mg and delta(26)Mg can be used to discriminate features of the Al-26-Mg-26 isotope system that are relevant to chronology from those that result from element mobility. Copyright (C) 2002 Elsevier Science Ltd.