Natural records of supercritical fluids in subduction zones

Abstract

A supercritical fluid (SCF) in a silicate-H2O system was generally regarded as a homogeneous phase formed under pressure and temperature (P-T) conditions higher than the second critical endpoint of the system. It evolves into a hydrous melt and aqueous fluid with decreasing P-T conditions or after interactions with wall rocks during fluid migration. Subduction zones are preferable sites for seeking records of SCFs in the natural systems, particularly when the P-T paths of the rocks cross through the stability area of the SCFs. This contribution first defines SCF by considering the homogeneous fluid above the critical curve of the corresponding rock–H2O system as a generalized SCF and then reviews the recent advances about the natural records of SCFs in subduction zones. Specifically, multiphase inclusions are the most direct proxy for SCF with both fluid-bearing and fluid-free ones containing complex mineral associations being probably linked to SCFs. The major element composition of the SCF recovered from multiphase inclusions is broadly consistent with the experimental data, showing an intermediate composition between the aqueous fluid and hydrous melt. The SCF-associated element transportation can be determined in ultrahigh pressure veins, accessory minerals, and mantle wedges, mostly based on the strong capability of SCFs to transport high field strength elements and heavy rare earth elements. The phase separation of SCF is widespread, including both microscale evidence of inclusions and macroscale evidence of composite veins as well as concurrent signals of fluid and melt metasomatism in the mantle wedge. Isotopic fractionations associated with SCFs have been reported intermittently. However, it mainly depends on the isotope composition of source rock and the dissolving capacity of the SCF. Finally, we propose certain identification criteria of SCF relative to aqueous fluid and hydrous melt by integrating the published data, including specific multiphase inclusion signatures; major element ratios of CaO/Al2O3 (fluid/source rock) ≥ 1.15, FeO/Al2O3 (fluid/source rock) ≥ 0.5, and MgO/Al2O3 (fluid/source rock) ≥ 0.6; and large NbTa fractionation. Other signatures of SCFs, such as high sulfur content and abnormal Fe-Mg-Cr-O-S isotope compositions, also display potential. However, further studies are required to validate these.