Measurements of rhenium isotopic composition in low-abundance samples

Abstract

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. In particular, fractionation of its isotopes ¹⁸⁷Re (62.6% abundance) and ¹⁸⁵Re (37.4%) may be used to improve our understanding of redox reactions during weathering, both in the modern day and in geological archives. Published methods for measurement of Re isotopic composition are limited by the requirements of Re mass to reach a desirable precision, making the analysis of many geological materials unfeasible at present. Here we develop new methods which allow us to measure Re isotope ratios (reported as δ¹⁸⁷Re) with improved precision: ±0.10‰ (2σ) for a mass of Re of ∼1 ng to ±0.03‰ (2σ) for a mass of Re of >10 ng. This is possible due to the combination of a modified column chemistry procedure and the use of 10¹³ Ω amplifiers for measurement via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). For river water samples (with Re concentrations typically ∼10⁻¹² g g⁻¹) we design a field-based pre-concentration of Re that can be used with large volumes of filtered water (5–20 L) shortly after sample collection to provide abundant Re for isotope analysis. As a result of these developments we provide new measurements of δ¹⁸⁷Re in standards reference materials (δ¹⁸⁷Re values range from −0.06 ± 0.07‰ to +0.19 ± 0.05‰) and a seawater standard (δ¹⁸⁷Re = +0.10 ± 0.04‰), providing impetus for further exploration of the Re isotope system.