Rhenium elemental and isotopic variations at magmatic temperatures

Abstract

Recent analytical advances in the measurement of rhenium (Re) isotope ratios allow its potential as a palaeoredox and chemical weathering proxy to be explored. However, a successful isotopic proxy must be grounded by an understanding of its composition and behaviour in the solid Earth. Here, we present Re concentrations and Re isotopic (δ¹⁸⁷Re) compositions for a well-characterised sequence of lavas from Hekla volcano, Iceland. The concentration of Re varies from 0.02 to 1.4 ng/g, decreasing from basalt to more evolved lavas. We show that the crystallisation and removal of magnetite is responsible for the Re decrease in this system. By contrast, δ¹⁸⁷Re values for the same suite of samples show a relatively narrow range (−0.45 to −0.22 ‰), suggesting minimal resolvable Re isotope fractionation between magnetite and the silicate melt. Together with other samples, including mid-ocean ridge basalts, these first igneous data can be used to estimate a baseline for terrestrial materials (δ¹⁸⁷Re = −0.33 ± 0.15 ‰, 2 s.d., n = 14), from which low-temperature Re isotope variations in Earth’s surficial environments can be assessed, alongside the global isotope mass balance of Re.