Matrix bias calibrations for ion microprobe oxygen isotope analysis of wolframite

Abstract

Wolframite, the principal ore mineral of tungsten, is a continuous solid solution between ferberite (FeWO₄) and hübnerite (MnWO₄). Its oxygen isotope composition (δ¹⁸OVSMOW) serves as a powerful tracer of hydrothermal fluid sources and ore-forming processes in tungsten deposits. However, accurate in situ δ¹⁸O analysis of wolframite by ion microprobe or secondary ion mass spectrometry (SIMS) is complicated by matrix-dependent instrumental mass fractionation (IMF) related to its variable Fe/Mn ratio. This study establishes the first systematic matrix bias calibration for SIMS wolframite oxygen isotope analysis. A suite of 16 wolframite reference materials and/or samples spanning the ferberite-hübnerite solid solution series [XFe = Fe/(Fe+Mn) from 0.0 to 0.9] were analysed by SIMS and laser fluorination isotope ratio mass spectrometry. Based on the systematic dependence of IMF on XFe, with bias δ¹⁸O varying monotonically across the solid solution, a compositional calibration model is proposed that allows SIMS-derived δ¹⁸O values to be corrected based on XFe measured by electron microprobe. Failure to account for these matrix effects may introduce systematic errors of up to ±13.8‰, significantly affecting interpretations of fluid source and temperature. This study thus provides an important methodological foundation for the investigation of tungsten ore genesis and hydrothermal fluid evolution, and briefly explores the possibility of constructing a universal matrix bias δ¹⁸O correction model across minerals.