High intermediate precision rhenium isotopic measurements in geological samples

Abstract

Stable rhenium (Re) isotopes are important tracers for geological processes related to melting and differentiation of planetary interiors, ore mineralization and paleoenvironmental redox conditions. However, methods for the precise determination of Re isotope ratios have received limited investigation to date due to the extremely low Re mass fractions of most geological materials. In this study, we present a new protocol for increased intermediate precision in the measurement results of Re isotope ratios in geological materials using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Preconcentration of Re is an essential first step, involving digestion of 1–2 g of sample powder by concentrated NH₃ solution, followed by coprecipitation of matrix ions other than Re, Mn and Mo. Subsequent purification and extraction of Re from the residual matrix was performed on TEVA resin. This approach provided an efficient purification with a low blank (<0.2 pg) and high yield of Re (93.2 ± 2.4%, 2s, n = 5) for reference materials BCR-2 and TDB-1. Instrumental mass bias was monitored and corrected through a combination of standard-sample bracketing (SSB) and internal normalization of Ir as a calibration standard. This approach yielded about a two-fold improvement in the intermediate precision relative to SSB alone. In addition, a further two-fold improvement in precision was obtained by the use of a membrane desolvation system which can increase the intensity of Re isotopic signals by an order of magnitude. This protocol improved the intermediate precision of measured δ¹⁸⁷Re_SRM3143 for a 1 ng g⁻¹ Re analyte solution to 0.048‰. The δ¹⁸⁷Re_SRM3143 of USGS basalt reference materials were measured using this measurement procedure, yielding δ¹⁸⁷Re_SRM3143 of −0.31 ± 0.04‰ (2s, n = 6), −0.33 ± 0.05‰ (2s, n = 3), −0.35 ± 0.05‰ (2s, n = 3) for BCR-2, BHVO-2, BIR-1a, respectively, in agreement with values obtained in previous studies. Other reference materials were analyzed, yielding δ¹⁸⁷Re_SRM3143 of −0.35 ± 0.05‰ (2s, n = 2), −0.36 ± 0.04‰ (2s, n = 3) and −0.31 ± 0.03‰ (2s, n = 3) for diabase (TDB-1), komatikic basalt (OKUM), and andesite (AGV-2), respectively. The developed procedure was successfully applied to measurement of samples with low Re mass fractions, extending the range of geological materials for which Re isotopic values can be determined.