The effect of high-temperature alteration of oceanic crust on the potassium isotopic composition of seawater

High-temperature hydrothermal alteration of oceanic crust is one of the two main sources of potassium (K) to the oceans, with modern flux estimates ranging roughly from -8 to 30 % of the K flux from rivers. Despite the role of high-temperature hydrothermal fluids in the global seawater K budget, little is known about its effect on the K isotopic composition of the oceans. Here we present stable K isotope measure-ments (841KSRM3141a) of globally distributed high-temperature hydrothermal fluids from three mid-ocean ridge systems: the East Pacific Rise (n = 21), the Juan de Fuca Ridge (n = 34), and the Mid-Atlantic Ridge (n = 18). We find a strong correlation between 841K and Mg/K ratios, consistent with conservative mixing between a high-temperature hydrothermal fluid (i.e., Mg = 0) and seawater as fluids ascend to the sea -floor and/or due to seawater entrainment during sampling. The delta K-41 of end-member hydrothermal fluids is found to range between -0.80 %. and 0.07 %., with an average value of -0.36 +/- 0.30 %. (2cs, n = 38). Most (-76 %) of the variability in end-member fluid 841K compositions observed here can be explained by high-temperature fluid-rock K exchange, with little (-0.2 %.) to no K isotope fractionation between hydrothermal fluid and altered crust. Larger deviations from the average end-member hydrothermal fluid value are likely to result from processes other than high-temperature fluid-rock exchange, such as (1) low-temperature hydrothermal reactions during fluid recharge, (2) reaction of fluids with local sedimen-tary sources, and (3) phase separation. The K contents of end-member fluids vary considerably, from -1 to 38 mM, thus a K-weighted average of -0.37 +/- 0.24 %. (2cs, n = 38) is estimated to represent the 841K composition of the global hydrothermal K flux. Our results suggest that K sourced from axial hydrother-mal alteration does not contribute to the elevated 41K/39K of seawater compared to bulk silicate Earth (BSE). In addition, subduction of oceanic crust altered under high-temperature conditions is unlikely to be a significant source of K isotopic heterogeneity to Earth's mantle. (C) 2022 Published by Elsevier Ltd.

Automated summary

High-temperature hydrothermal alteration of oceanic crust is an important source of potassium in the oceans, but its impact on potassium isotopic composition is not well understood. This study analyzes stable potassium isotope measurements in globally distributed high-temperature hydrothermal fluids and finds a strong correlation between potassium isotopes and Mg/K ratios. The variability in potassium isotopic composition can be explained mostly by high-temperature fluid-rock potassium exchange.