Global Average Potassium Isotope Composition of Modern Seawater

Abstract

The potassium isotope system was proposed as a new tracer in continental weathering and global K cycling. The largest K isotope fractionation observed among major reservoirs is between the ocean and bulk silicate Earth (BSE). Seawater is significantly enriched in heavy isotopes compared to BSE, and seawater represents the heaviest reservoir on Earth. Because of limited analyses, it is still unknown whether seawater is homogeneous in terms of K isotopes vertically, laterally, and globally. In addition, what processes (e.g., hydrothermal inputs) and to what degrees these processes have contributed to this heavy isotope enrichment in seawater are still not well constrained. To better understand the K isotopic compositions of modern seawater and to examine the possible influence of seafloor hydrothermal vents on the K isotope composition of seawater, we analyzed the K isotope composition of 46 seawater samples collected as two pairs of depth profiles in two locations from the Atlantic and Pacific oceans, including one near an active hydrothermal vent field (ASHES, Axial Seamount, Juan de Fuca Ridge). We found that within the current analytical uncertainty, all seawater samples have the same K isotope composition regardless of their location, depth, [K] concentration, and salinity. Combining our new analyses with data from previous studies, we define the best representative K isotope composition of modern seawater as +0.12 ± 0.07‰ (2SD). The seawater is significantly higher (0.55 ± 0.18‰) than BSE, which requires large K isotopic fractionation during continental weathering and reverse weathering.

Potassium isotopic heterogeneity in subducting oceanic plates

“…The Hawaiian seawater standard has the highest δ 41 K value of 0.14‰. All these values agree with published data ( 18 , 27 , 30 , 31 , 55 ). In addition, δ 41 K values for duplicates (repeat instrumental measurement) and replicates (repeat dissolution and column chemistry) are reproducible within ±0.06‰ at 95% confidence interval (CI).…”

“…The δ 41 K values measured in altered crust vary from −0.60 to −0.05‰ and in subducting sediments from −1.3 to −0.02‰ (tables S1 and S2); these values span a range of 20 times greater than the long-term precision of our analytical methods. The δ 41 K data straddle the average mantle value of −0.43‰ ( 21 ); however, they are all lower than the seawater value of 0.12‰ (see Materials and Methods for sample details) ( 30 , 31 ).…”

“…These data are reported relative to an in-house seawater standard measured at −0.02 ± 0.17‰, and uncertainties are only available as 2SD ( 34 ). These data are converted to the international K isotopic standard [National Institute of Standards and Technology (NIST) SRM 3141a], to which seawater has a δ 41 K value of 0.12 ± 0.07‰ ( 30 , 31 ).…”

“…Typical δ 41 K ranges for the mantle (fig. S1), upper continental crust ( 23 ), and seawater ( 30 , 31 ) are plotted for comparison. Sediment δ 41 K data are reported in table S2.…”

Potassium Isotope Fractionation During Magmatic Differentiation and the Composition of the Mantle

“…10.1029/2020JB021543 7 of 13 Savage, et al, 2019) and granitoids (mostly granites) (Chen, Tian, et al, 2019;Huang et al, 2020;Teng, Hu, et al, 2020;Xu et al, 2019) are plotted for comparison. SRM 3141a, Hille et al, 2019;Wang et al, 2020). Therefore, feldspar is on average isotopically heavier than mafic magmas as represented by the Kilauea basalts (−0.40‰ ± 0.03‰), and crystallization of the isotopically heavy feldspar results in variable Δ 41 K mineral-melt between 0.01‰ and 0.32‰.…”