Potassium isotopic heterogeneity in subducting oceanic plates

Oceanic crust and sediments are the primary K sinks for seawater, and they deliver considerable amounts of K to the mantle via subduction. Historically, these crustal components were not studied for K isotopes because of the lack of analytical precision to differentiate terrestrial variations. Here, we report a high-precision dataset that reveals substantial variability in oceanic plates and provides further insights into the oceanic K cycle. Sixty-nine sediments worldwide yield a broad δ⁴¹K range from -1.3 to -0.02‰. The unusually low values are indicative of release of heavy K during continental weathering and uptake of light K during submarine diagenetic alteration. Twenty samples of altered western Pacific crust from ODP Site 801 display δ⁴¹K from -0.60 to -0.05‰, averaging at -0.32‰. Our results indicate that submarine alteration of oceanic plates is essential for generating the high-δ⁴¹K signature of seawater. These regionally varying subducting components are heterogeneous K inputs to the mantle.