Structural changes of potassium-saturated smectite at high pressures and high temperatures: Application for subduction zones

In a subduction zone, water-rich pelagic sediments overlying the subducting oceanic plate are accumulated, forming an accretionary wedge. A portion of these sediments, primarily smectites, can subduct and lose the water present in their pores, but the mechanism of the clay structural transformation and its composition are not well-defined. Smectite clay minerals can remain stable, carrying water at pressures corresponding to the upper mantle and thus contributing to the occurrence of melting and metasomatic processes in the mantle. In this work, we performed several high-pressure and high-temperature experiments to verify the structural behavior of a potassium-saturated smectite under various pressure and temperature conditions: (1) atmospheric pressure at two different temperatures (100 °C to 700 °C); (2) room temperature (25 °C) and pressure up to 11.5 GPa - diamond anvil cell (DAC) experiments; (3) under pressures of 2.5 GPa and 4.0 GPa and temperatures from 200 °C to 700 °C. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and infrared spectroscopy (FTIR) analysis on all samples studied suggest that, under elevated pressures (2.5 GPa, which corresponds to a depth of approximately 75 km, and 4.0 GPa, equivalent to a depth of approximately 120 km), smectite remains stable until 200 °C. With increasing temperature, the initial structure partially loses its interlayer H2O and transforms into a mixed layer of illite/smectite, which is maintained until 400 °C under 2.5 GPa and until 300 °C under 4.0 GPa. Beyond these conditions, with increasing temperature, the smectite loses all of its interlayer H2O and transforms into muscovite. Additionally, at room temperature, smectite is stable under nearly 12 GPa. These results contribute significantly to the understanding of how pelagic sediment dehydration and transformation occur in the subduction process, as well as the behavior of smectite under the influence of increasing pressure and temperature.