Cat–87 clinopyroxene: a new reference material for oxygen isotope microanalysis, with applicability tested across a range of Ca–Mg–Fe clinopyroxene compositions

Abstract

Clinopyroxene oxygen isotope compositions provide critical insights into diverse geological processes. In situ oxygen isotope analysis is preferred over bulk methods because of complications such as inclusions, alteration, and isotope heterogeneity, all of which are common in natural clinopyroxene samples. Secondary ion mass spectrometry (SIMS) enables precise in situ oxygen isotope analysis at a spatial resolution of 10–15 µm. However, accurate SIMS results require homogeneous reference materials (RMs) that match the chemistry and crystal structure of the target minerals. Despite the wide compositional variability of natural clinopyroxenes, the number of available oxygen isotope RMs is limited. In this study, we characterize Cat–87 clinopyroxene as a new RM for oxygen isotope microanalysis. The major element composition of Cat–87 (Wo_36.0En_55.2Fs_8.8; Wo = wollastonite, En = enstatite, Fs = ferrosilite) expands the compositional coverage of available clinopyroxene RMs, and multiple SIMS measurements demonstrate its oxygen isotope homogeneity. A laser fluorination δ¹⁸O value of 5.53 ± 0.14‰ (2SD) is recommended as the reference value for Cat–87. Evaluation using eight Ca–Mg–Fe clinopyroxenes with variable chemistry demonstrates that Cat–87 is suitable for SIMS clinopyroxene oxygen isotope analysis in the compositional range Wo_33.8–37.6En_51.6–58.0Fs_8.2–11.0. In contrast, Cat–87 is not suitable for clinopyroxenes with higher Ca contents (Wo_44.6–47.5En_48.3–50.2Fs_3.7–7.2).