Effect of smectite dehydration on pore water geochemistry in the shallow subduction zone: An experimental approach

Abstract

The diagenetic smectite to illite transition is widely accepted to cause ubiquitous pore water freshening at convergent margins. However, experimental consolidation tests show that smectite also dehydrates partially with increasing effective stress. To shed light on this process three hydrothermal consolidation tests on a silty smectite-rich claystone were conducted with stresses (P) up to 70 MPa and at constant temperatures (T) of 20°C, 60°C and 100°C. Fluids expelled during the tests were analyzed for major and trace elements to evaluate dehydration and fluid-rock interaction with increasing PT conditions. The results document that fluid freshening starts when the effective stress exceeds 1.3 MPa. The smectite interlayer water content decreases from 27 wt-% to ∼20 wt-% during the experiments, which is equivalent to an interlayer collapse from 18.5 to ∼15.4 Å. The released interlayer H 2 0 accounts for up to 17% of the total fluid volume released from the consolidating sediment. Major and volatile element geochemistry is influenced by temperature and smectite interlayer collapse. The interlayer collapse is characterized by K and Ca uptake while is B released from smectite. Application of experimental results to the Barbados accretionary margin shows that stress dependent smectite dehydration is an important fluid source mechanism and accounts for substantial fluid freshening. Modeled smectite dehydration suggests a decrease of smectite interlayer water content from 27 wt-% at the surface down to ∼22 wt-% at 2 km depth. This leads to chlorinity values as low as 460 mM. © 2012. American Geophysical Union. All Rights Reserved.