Ion microprobe analysis of oxygen isotopes in garnets of complex chemistry

Abstract

Accurate ion microprobe analysis of oxygen isotope ratios in garnet is possible if appropriate standards are employed to correct for instrumental bias, a component of which depends on the cation chemistry of the analyzed mineral. In this study, 26 garnet standards (including 14 new standards) that span the compositional range of pyrope, almandine, grossular, spessartine, and andradite were analyzed repeatedly by ion microprobe to develop a new method of correcting for instrumental bias in garnets. All analyses were normalized to a single master garnet standard (UWG-2) before bias from cation composition was considered. Bias due to cation composition in garnet was found to correlate with grossular content in pyralspite garnets and with andradite in ugrandite garnets. Bias is correlated with molar volume in garnets of all compositions in this study. Although this correlation is suitable as a correction scheme for bias, a more accurate correction scheme based on the grossular and andradite compositions of garnet is proposed. This method reproduces the bias of all but one standard to within a range of 0.4‰, an accuracy that is on the same order as the reproducibility (± 0.3‰, 2 S.D.) of the master garnet standard UWG-2, but that remains an independent source of error. The new correction scheme is used to successfully reproduce laser fluorination analyses along a traverse of a polymetamorphic, zoned skarn garnet from the Adirondack Mountains. While previous analyses were at the mm-scale, the new data resolve a gradient of δ 18O of 2.1‰ over 16 µm. If experimentally derived diffusion coefficients are correct, these new results show that granulite-facies metamorphism was significantly faster than previously assumed and the thermal peak was less than 5 Myr.