Dehydration at subduction zones and the geochemistry of slab fluids

Abstract

Subducting oceanic slabs undergo metamorphic dehydration with the increase of temperature and pressure during subduction. Dehydration is an essential step for element recycling, and slab fluids are critical agents for mediating slab-mantle interaction. Dehydration is mainly controlled by the thermal structure of subduction zones and the stability of hydrous minerals. At fore-arc depths, slab dehydration produces aqueous fluid with dissolved salts such as NaCl. As subduction proceeds deeper, the content of silicate components increases. At sub-arc and post-arc depths, a hydrous silicate melt is likely to form, or a supercritical fluid could arise from complete miscibility between silicates and H2O. The partitioning of elements between slab fluid and the residual solid rock is controlled by the type of fluid, and generally it is the supercritical fluid that is the most capable of mobilizing trace elements, being an effective carrier even for high field strength elements. Understanding the chemistry of slab fluids relies on sophisticated integration of experiments, theoretical computation and investigation of natural rock samples. This contribution focuses on the content and speciation of key volatiles, including carbon, nitrogen and sulfur, in slab fluids as well as important fluid properties such as oxygen fugacity and acidity. The properties of slab fluids show complicated variation under the control of mineral assemblages and T-P conditions. Slab fluids at great depths of subductions have been inferred to be modestly alkaline and not necessarily very oxidizing as often assumed. Further progress in the research of slab dehydration and the chemistry and properties of slab fluids demands urgently the development of innovative experimental and computational technology including in situ analytical methods at high T-P.