Preliminary matrix bias calibrations for ion microprobe oxygen isotope analysis of pyroxene

Abstract

Similar to garnet or olivine, pyroxene commonly occurs in diverse igneous and metamorphic rocks such as peridotites, eclogites, skarns and meteorites. Its oxygen isotope compositions can be analysed in situ using secondary ion mass spectrometry (SIMS) or an ion microprobe to help constrain the formation of rocks. However, unlike garnet and olivine, calibration of matrix-induced oxygen isotope (¹⁸O/¹⁶O) discrimination in pyroxene is not fully established, limiting its wide application in Earth and planetary sciences. This study provides preliminary matrix bias calibrations using 51 reference materials and/or samples of pyroxene, either low-Ca or high-Ca, including omphacite and jadeite, some (18) of which have also been analysed here by laser fluorination isotope ratio mass spectrometry. Both conventional offline (bias δ¹⁸O vs. mole fractions of ferrosilite or jadeite endmembers: X_Fs or X_Jd) and online (bias δ¹⁸O vs.⁵⁶Fe¹⁶O/¹⁶O) calibration methods can be applicable, including the first matrix bias estimation for jadeitic clinopyroxene or omphacite as well as Mn-rich pyroxene. Investigation is needed for intermediate-Ca, particularly Ti-rich pyroxene.