PX xenotime: a new reference material for SIMS oxygen isotopic microanalysis and potential applications†

Xenotime is a common accessory mineral found in igneous, metamorphic, and sedimentary rocks, as well as hydrothermal ore deposits. Although the U–Th–Pb isotopic compositions of xenotime have been used widely to constrain host rock ages, O isotopic compositions have rarely been exploited. A major challenge is that microanalysis techniques, such as secondary ion mass spectrometry (SIMS), are required to analyze small and zoned xenotime crystals, and these techniques require matrix-matched reference materials to correct for instrumental mass fractionation and to monitor analytical accuracy. Currently, there are no reference materials for SIMS O isotopic analysis of xenotime, which impedes its application in geological studies. We present PX xenotime, a sample collected from Zagi Mountain, Pakistan, that exhibits a homogeneous O isotopic composition as confirmed through SIMS analyses. Laser fluorination–isotopic ratio mass spectrometry yielded a δ¹⁸O value of 6.25‰ ± 0.13‰ (2SD, n = 6). Thus, we establish PX xenotime as the first reference material for SIMS xenotime O isotopic analysis. We also explore the potential applications of SIMS xenotime O isotopic analysis through O isotopic fractionation between quartz and xenotime and between xenotime and water. Our results show that the quartz–xenotime O isotope thermometer is promising for low-temperature hydrothermal systems (<400 °C). Moreover, fluid δ¹⁸O compositions can be directly determined by SIMS quartz–xenotime δ¹⁸O analysis in a single session.