In situ measurement of Os isotopic ratios in sulfides calibrated against ultra-fine particle standards using LA-MC-ICP-MS

Abstract

In this study, an improved method is developed to determine the Os isotopic ratio in sulfides using LA-MC-ICP-MS. Ultra-fine chalcopyrite and pyrite powders are first produced via an ultrafine process to make a pressed tablet and a fused pellet. Analyses indicate that the pressed tablets CR-1P (high-Re and extremely low-Os) and CO-1P (high-Os and low-Re) are homogeneous enough to be used as in-house matrix-matched standards for Re–Os isotopic analyses by LA-MC-ICP-MS. Compared to the fused pellet of pyrite (POR-1F), the fused pellet of chalcopyrite (COR-1F) shows a higher degree of phase separation, implying that the Fe–S system may be more suitable than the Cu–Fe–S system for making sulfide fused pellets. The mass fractionation coefficients of Re (β_Re) for isobaric interference correction are then calculated according to the real-time analysis of CR-1P and applied to the real samples via linear interpolation. Mass fractionation of Os is corrected by normalizing ¹⁸⁹Os/¹⁸⁸Os to 1.21978. To estimate the upper limit of reliable interference correction of ¹⁸⁷Re on ¹⁸⁷Os, critical lines are obtained by the analyses of Re-bearing samples. It is demonstrated that the uncertainty of the calculated ¹⁸⁷Os/¹⁸⁸Os ratio can be limited to within 0.8% (relative error) for samples with ¹⁸⁷Re/¹⁸⁸Os <1.4 and 0.5% (relative error) for samples with ¹⁸⁷Re/¹⁸⁸Os <0.6.