Rhenium isotope variations in Icelandic groundwaters and hydrothermal systems

Abstract

The isotopic composition of rhenium (Re) has potential for use as a proxy to infer changes in seafloor redox and/or global oxidative weathering intensity. Despite an emerging dataset on this nascent isotope system in Earth's surficial environments, very little is known about processes that control Re isotope fractionation, nor the isotopic composition of hydrothermal systems. Here we present Re concentrations and Re isotopic compositions (reported as δ¹⁸⁷Re, relative to NIST 3143) of groundwaters and hydrothermal fluids from three Icelandic settings. First, we show that high-temperature fluids that have experienced vapour-phase segregation (boiling) from the Reykjanes peninsula and the Hengill volcanic system have the highest δ¹⁸⁷Re values (−0.01 to +0.34 ‰) observed to date, inferred to result from Re isotope fractionation during incorporation of Re into secondary reduced minerals. Second, we examine the Mývatn area in northern Iceland, which has both cold and warm groundwaters. Cold groundwaters (< 10 °C) have δ¹⁸⁷Re values indistinguishable from Icelandic basalts (−0.36 to −0.32 ‰) whilst warm waters have higher δ¹⁸⁷Re (−0.31 to +0.19 ‰) which increase with increasing temperature (up to 45 °C). The variation of δ¹⁸⁷Re in Mývatn groundwaters is closely mirrored by variations in δ⁹⁸Mo, consistent with mixing between compositionally distinct water end-members. Finally, geothermal waters from the Geysir field have variable Re concentrations and δ¹⁸⁷Re values (−0.23 to +0.34 ‰), likely reflecting multiple physico-chemical processes. Using these results, we show that hydrothermal activity is unlikely to exert a large net impact on the seawater Re budget, nor the secular changes in seawater δ¹⁸⁷Re. These findings also point toward developing a novel Re isotope tracer for redox processes.