First matrix bias calibration for ion microprobe oxygen isotope analysis of Ti-bearing garnet

Abstract

Garnet, commonly found in eclogites, granulites and skarns, is a geological “archival mineral”. In situ microanalysis of garnet provides insights into rock formation, mineralisation, tectonic environments and the Earth's evolution. It has long been recognised that, due to the complex mineralogical chemistry of garnet, significant instrumental mass (or isotope) fractionation can occur when measuring its oxygen isotope ratio using secondary ion mass spectrometry (SIMS). However, calibration for matrix effects (or matrix-induced isotopic mass fractionation) has not been documented for certain endmembers, such as Ca–Ti garnet (CTG). This study, for the first time, measured 44 garnet reference materials and/or samples using SIMS and/or laser fluorination isotope ratio mass spectrometry to determine the matrix bias calibration for Ti-bearing garnets in addition to grossular, spessartine and andradite. Matrix biases as large as 5.7‰ were observed for garnets with X_CTG = 0.4. Therefore, caution is required when performing SIMS oxygen isotope analysis of Ti-bearing garnets along with grossular, spessartine and andradite.