Fractionation of stable rhenium isotopes in terrestial hydrothermal systems

Rhenium (Re) is a redox-sensitive element. Recent advances in the precision of measurement of the stable isotopic composition of Re (δ¹⁸⁷Re) allow exploration of its potential as a proxy for paleoredox and/or chemical weathering ^[1]. However, as yet, there have been few studies reporting the geochemical cycling of Re and stable Re isotopes in the modern environments ^{[2] [3]}, and processes that regulate the Re isotope behavior in hydrothermal systems remain unexplored.

Here we present results of the analysis of Re concentration and δ¹⁸⁷Re (relative to NIST3143) for water samples collected from hydrothermal and groundwater systems in Iceland. We show that Re in basalt-hosted boiled hydrothermal fluids from Hellisheidi, Nesjavellir, Reykjanes and Svartsengi sites is isotopically heavier (δ¹⁸⁷Re = –0.01 to +0.32‰) than Re in Icelandic basalts (δ¹⁸⁷Re = ~–0.32‰). The direction of fractionation holds regardless of types of fluid reservoir (meteoric vs. seawater), and is consistent with precipitation of isotopically light sulfides in the hydrothermal system and/or kinetic fractionation of Re during degassing. By contrast, Re in cold (< 10°C) groundwaters collected from the Mývatn area is isotopically indistinguishable from host basalt. Natural hot spring waters exhibit variable δ¹⁸⁷Re values (–0.28 to +0.26‰), likely reflecting mixing between hydrothermal and groundwater endmembers. The relatively isotopically heavy δ¹⁸⁷Re from hydrothermal sources has the potential to modify the oceanic budget, which has implications for the isotope mass balance of Re.

[1] Dellinger et al. (2020) JAAS, 35, 377. [2] Dickson et al. (2020) GCA, 287, 221-228. [3] Dellinger et al. (2021) EPSL, 573, 117131.