Comparative determination of rhenium isotopes in geological reference materials with and without HF-desilicification

Abstract

The utilization of rhenium (Re) mass fractions or isotopes (δ¹⁸⁷Re, reported relative to SRM 3143) as a high-sensitivity geochemical tracer has advanced substantially in the past decade, driven by its unique redox-controlled mass-dependent fractionation behavior spanning low- to high-temperature geological processes and systems. Current analytical protocols mainly employ HCl–HNO₃ digestion without desilicification for Re mass fraction analysis and HF–HNO₃ digestion with complete desilicification for δ¹⁸⁷Re_{SRM 3143} analysis, the latter procedure ensuring quantitative Re liberation but concurrently introducing potential interfering elements. In this study, we developed a novel and simple chromatographic separation protocol making use of HCl–HNO₃ and HF–HNO₃ mixtures to obtain pure Re fractions for analysis of silicate-hosted Re contributions to bulk δ¹⁸⁷Re_{SRM 3143} signatures. The Re isotope ratio of the purified fraction was determined by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) employing a calibration model combining sample-standard bracketing with internal normalization (C-SSBIN). The robustness and reliability of the newly developed separation procedure was validated through analysis of certified reference materials digested by HF–HNO₃, including BCR-2, TDB-1 and OKUM, showing good agreement with literature data. Comparative analysis of non-desilicified reference materials (WPR-1a, OKUM, TDB-1, BHVO-2, AGV-2) revealed δ¹⁸⁷Re_{SRM 3143} discrepancies ≤ 0.07‰ (intermediate precision) despite the presence of >10% silicate-bound Re, demonstrating that HF-desilicification is not a necessary operation for Re isotope analysis. This finding suggests that sample mass can be increased to obtain sufficient Re for high-precision isotope measurement. Our separation procedure can be applied to various types of samples regardless of their Re mass fractions. Additionally, the δ¹⁸⁷Re_{SRM 3143} of the sulfur-rich peridotite WPR-1a (first reported here) shows relatively lower values compared to other silicate reference materials, implying that redox processes can induce Re isotope fractionation.