Supercritical fluid in deep subduction zones as revealed by multiphase fluid inclusions in an ultrahigh-pressure metamorphic vein

Due to their low viscosity, high mobility, and high element contents, supercritical fluids are important agents in the cycling of elements. However, the chemical composition of supercritical fluids in natural rocks is poorly understood. Here, we investigate well-preserved primary multiphase fluid inclusions (MFIs) from an ultrahigh-pressure (UHP) metamorphic vein of the Bixiling eclogite in Dabieshan, China, thus providing direct evidence for the components of supercritical fluid occurring in a natural system. Via the 3D modeling of MFIs by Raman scanning, we quantitatively determined the major composition of the fluid trapped in the MFIs. Combined with the peak-metamorphic pressure-temperature conditions and the cooccurrence of coesite, rutile, and garnet, we suggest that the trapped fluids in the MFIs represent supercritical fluids in a deep subduction zone. The strong mobility of the supercritical fluids with respect to carbon and sulfur suggests that such fluids have profound effects on global carbon and sulfur cycling.

Automated summary

Supercritical fluids play a crucial role in the cycling of elements due to their low viscosity, high mobility, and high element contents. However, the chemical composition of supercritical fluids in natural rocks remains poorly understood. In this study, well-preserved primary multiphase fluid inclusions from an ultrahigh-pressure metamorphic vein in China provide direct evidence for the components of supercritical fluids in a natural system. By using 3D modeling and Raman scanning, we were able to quantitatively determine the major composition of the fluid trapped in the inclusions, indicating that these fluids represent supercritical fluids in a deep subduction zone. The high mobility of these fluids suggests significant effects on global carbon and sulfur cycling.