Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle

Recent discoveries of significant variations in stable K isotope ratios ( 41 K/ 39 K or δ 41 K) among various terrestrial samples indicate that K isotopes can be a novel tracer for the global K cycle, but a key observation that seawater δ 41 K is ∼ 0.6 ‰ higher than the bulk silicate Earth remains unexplained. An unconstrained component critical to this puzzle is hydrothermal systems that represent both a major K source and sink in the ocean. Here we report δ 41 K results on mid-ocean ridge (MOR) hydrothermal fluids from the Gorda Ridge and ∼9°N East Pacific Rise (EPR), including time-series samples that recorded major perturbations in fluid chemistry induced by a local volcanic eruption. Fluid δ 41 K values range from -0.46‰ to -0.15‰, falling between those of fresh basalts and seawater. δ 41 K values of “time-zero” fluids collected shortly after the volcanic eruption are shifted towards the seawater value, followed by a return to pre-eruption values within ∼2 years. Fluid δ 41 K variations are largely influenced by water–rock interactions, but they cannot be solely explained by simple mixing of seawater and K leached from basalts at high temperatures. Instead, these data imply small but significant isotope fractionation that enriches heavy K isotopes in basalts, likely caused by low-temperature alteration during the recharge stage of hydrothermal circulation. Our results preclude MOR hydrothermal systems as the cause for the heavy δ 41 K value of seawater. Using fluid δ 41 K data and K isotope fractionation constrained here for hydrothermal systems, a K mass-balance model implies a critical role for a marine sedimentary sink, possibly authigenic clay formation, in the global K cycle. Also, applying the K isotope fractionation constrained here to the published δ 41 K data from ophiolites shows the possibility for significantly lower seawater δ 41 K during the Ordovician, which can be explained by enhanced reverse weathering in response to distinct climate and tectonics at that time. • Potassium isotopes of modern hydrothermal fluids are reported for the first time. • Potassium isotope fractionation in hydrothermal systems is resolved and quantified. • Hydrothermal systems cannot explain the heavy K isotope signature of seawater. • Authigenic clay formation likely has a significant role in the global K cycle.