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 H 2 O. 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.

The "Global Salt Cycle": Formation of Giant Salt Accumulations, a Result of Subduction, Mantle Upwelling, and Rifting

“…Several authors have documented the role of sediments, mid-ocean basalts and different minerals in relation to water-carrying capacity (and, therefore, also salts) into the subduction zone (Hacker et al Part 1 and 2 (2003), Li and Ni (2020)).…”

Numerical Simulation of the Effects of Wedge Subduction on the Lithospheric Thermal Structure and the Seismogenic Zone South of Chile Triple Junction

“…Several processes that occur in the subduction zone, such as shear heating, partial melting, hydrothermal cooling, and dehydration, were not covered in this study. Dehydration and partial melting are related to element recycling, intermediate and deep earthquakes, and double seismic zones (Seno et al, 2001;Li and Ni, 2020). Dehydration is essentially a result of the slab responding to the temperature and pressure change during subduction.…”

“…Dehydration is essentially a result of the slab responding to the temperature and pressure change during subduction. Dehydration has significantly different effects in cold and hot subduction zones (Li and Ni, 2020). The subduction zone to the north of the CTJ is cold and that to the south is hot.…”

Effect of Terrigenous Sediment Addition on the Generation of Arc Silicic Magma: Constraints From the Comparative Partial Melting Experiment at 1.5 GPa

“…The oceanic plate is dehydrated during subduction at these margins. During this subduction, the oceanic plate releases fluids, and these fluids join the overlying mantle wedge, causing partial melting of the mantle wedge and generating island arc basaltic magma (Sun et al, 2014;Zheng et al, 2015;Li and Ni, 2020;Liu et al, 2019;Zheng et al, 2019;Wei and Zheng, 2020;Xiong et al, 2020). Besides the basalt, there are a number of silicic magmas located on convergent plate margins (even typical oceanic island arcs, such as Tonga and the Kuril Islands), as shown in Figure 1.…”