Potassium isotopic composition of low-temperature altered oceanic crust and its impact on the global K cycle

To constrain the behavior of K isotopes during the low-temperature alteration of oceanic crust and reveal its impact on the global K cycle, we measured the K isotopic compositions of 49 fresh and altered basalts recovered from the Integrated Ocean Drilling Program (IODP) Sites U1365 (similar to 100 Ma) and U1368 (similar to 13.5 Ma) in the South Pacific Ocean. The measured basalts representing the uppermost oceanic crust have been subjected to low-temperature (<150 degrees C) alteration of different types and intensities as reflected by different secondary mineral modal abundances and element mobility. Both Sites have remarkably low sedimentation rates (0.71 and 1.1 m/Myr, respectively). The altered basalts from both Sites show higher K2O contents and K/Nb ratios than fresh basalts, suggesting the addition of K during low-temperature alteration. The measured delta K-41 values of the altered basalts vary over a large range (-0.76 parts per thousand to -0.17 parts per thousand) compared to unaltered basalts, indicating significant K isotopic fractionation during low-temperature alteration of the oceanic crust. We propose that the K isotopic characteristics of the altered basalts were caused by the combined effects of equilibrium and kinetic isotopic fractionations between seawater and alteration minerals during the incorporation and adsorption of K into alteration minerals. A weighted average delta K-41 value of -0.40 +/- 0.33 parts per thousand (2sd) is given for the low-temperature altered oceanic crust (AOC) at Sites U1365 and U1368, which is indistinguishable from the fresh N-MORB value (-0.44 +/- 0.17 parts per thousand), but significantly lower than its K source, i.e., seawater {delta K-41 = +0.12 +/- 0.07 parts per thousand, 2sd). Therefore, low-temperature AOC serves as an important sink for K and is a cause for the heavy K isotopic composition of seawater. In addition, the heterogeneous K isotopic compositions of the low-temperature AOC, together with large scale fractionation during various crustal processes, further highlights the utility of the K isotope system to trace subducted crustal materials. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

By analyzing fresh and altered basalts from the IODP Sites U1365 and U1368, it was found that K isotopes undergo significant fractionation during low-temperature alteration of oceanic crust. This altered oceanic crust serves as an important sink for K and contributes to the heavy K isotopic composition of seawater, highlighting the utility of the K isotope system in tracing subducted crustal materials.