Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater

Low-temperature (<100°C) alteration of oceanic crust plays an important role in determining the chemical composition of the oceans. Although a major sink of seawater potassium, little is known about the effects of low-temperature basalt alteration on the potassium isotopic composition of seawater (δ41K∼0‰), which is ∼0.50‰ enriched relative to bulk silicate Earth (BSE, δ41K= -0.54‰). Here, we present a suite of isotopic systems (δ41K, δ26Mg, δ7Li, 87Sr/86Sr) and major/minor elements in bulk rock, veins and mineral separates from the upper volcanic section of Cretaceous (Troodos ophiolite) and Jurassic (Ocean Drilling Program Hole 801C) oceanic crust. We use these data to estimate the K isotopic fractionation associated with low-temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of Mg and K in seawater. We find that hydrothermally altered basalts from the Troodos ophiolite and ODP Hole 801C, most of which are enriched in K relative to the unaltered glass compositions, have δ41K values both higher and lower than BSE, ranging from +0.01‰ to -1.07‰ (n=83) and +0.04‰ to -0.88‰ (n=17), respectively. Average δ41K values of bulk-rock samples from Troodos and Hole 801C are indistinguishable from each other at ∼-0.50‰, indicating that low-temperature basalt alteration is a sink of 39K from seawater, and explaining, in part, why seawater has a higher 41K/39K than BSE. In contrast to K, average δ26Mg values for both Troodos (∼0.00‰) and Hole 801C (∼0.20‰) indicate that altered oceanic crust (AOC) is a sink of 26Mg from seawater, likely contributing to the light δ26Mg composition of seawater (∼-0.8‰) relative to BSE (∼-0.2‰). We observe isotopically heavy δ26Mg values in basalt samples characterized by small to no changes in bulk Mg content, consistent with extensive isotopic exchange of Mg between seawater and oceanic crust during low-temperature oceanic crust alteration. Finally, we find that variability in δ7Li and δ41K across three sites in the Troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust.