Supercritical fluids at subduction zones: Evidence, formation condition, and physicochemical properties

There is a general consensus that at subduction zones, mass transfer from the subducted slab to the overlying mantle wedge is mediated by a hydrous mobile phase. However, it is under intense debate whether this phase is an aqueous fluid, hydrous silicate melt, or supercritical fluid with intermediate composition (H2O concentration in the range of 30 wt%-70 wt%). Supercritical fluids, with fluid-like viscosity and melt-like wetting and element-carrying capability, are an ideal agent for chemical transport at subduction zones. After clarifying the phase relations of silicate-H2O systems and the definition of supercritical fluids, this contribution reviews existing evidence for the presence of supercritical fluids at subduction zones, mostly from experimental investigation of phase relations of mineral-H2O and rock-H2O systems and the atomic structure and physicochemical properties of supercritical fluids. H2O-rich multi-phase solid inclusions from continental subduction zones, once their bulk water contents can be confirmed, may provide a direct record of supercritical fluids. Experimental results generally indicate that supercritical fluids can derive from the slab at 160 km depth, but there is still significant discrepancy between different studies on H2O-saturated solidus, fluid-melt critical curve, and the second critical end point (SCEP). Some novel experimental methods are proposed to resolve the controversies over the formation condition of supercritical fluids. The special physicochemical properties of supercritical fluids arise fundamentally from their intermediate composition and intermediate degree of polymerization that are distinctive from aqueous fluids or silicate melts. Probing the structure and properties of supercritical fluids, which are required for understanding of their petrological and chemical behavior and geophysical characteristics, relies mainly on in situ spectroscopy, but first principles molecular dynamics is becoming a potentially powerful alternative approach. (C) 2017 Elsevier B.V. All rights reserved.