Understanding Potassium Isotope Fractionation During Authigenic Clay Formation in Pore-fluid Systems: Implications for the δ41K of Seawater

Abstract:

Improvements in analytical precision on the latest generation multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS) have revealed a ~2‰ range in the ratios of stable potassium isotopes (⁴¹K/³⁹K) in terrestrial materials (Morgan et al., in prep). Preliminary measurements of δ⁴¹K values indicate that seawater and silicate rocks are isotopically distinct reservoirs, with seawater having a δ⁴¹K value that is ~0.5‰ heavier than the silicate average (-0.5‰; Morgan et al., in prep). The heavy δ⁴¹K character of seawater might be related to 1) an isotopically enriched input flux (rivers and high-temperature hydrothermal reactions); or 2) a ⁴¹K-depleted sink associated with authigenic clay formation during low-temperature alteration of volcanic rocks. Here we present measurements of the δ⁴¹K values of pore-fluids from ODP site 1052 in order to constrain potassium isotope fractionation during secondary clay formation. We find that δ⁴¹K values and K concentrations both decline systematically with depth. Results from 1-D diffusion-advection-reaction modeling of potassium concentrations and isotopic compositions indicate that fractionation of K isotopes during diffusion (Bourg et al., 2010) can explain all of the change in δ⁴¹K values of the pore-fluid with depth. Although the size of the K sink at site 1052 is a trivial fraction of the global K sink in clay minerals, our results suggest that diffusive fractionation of K isotopes in shallow pore-fluids may be, in part, responsible for the elevated δ⁴¹K value of seawater.