Researchers determine quantitative compositio

University of Science and Technology of China

In a study published in PNAS, Prof. XIAO Yilin's group from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) quantitatively determined, for the first time, the chemical composition of supercritical fluids in deep subduction zones, through 3D imaging modelling of ultrahigh-pressure (UHP) multiphase fluid inclusions, and revealed the important role of supercritical fluids in the cycling of carbon and sulfur in subduction zones, which is of great importance for an in-depth and systematic understanding of the role of supercritical fluids in nature.

Fluids, including oceans, lakes, rivers, and the wide range of geological fluids found in the Earth's interior, are inseparable parts and important media, helping to transfer the matter and energy among different layers of the Earth. Depending on their geochemical properties, they can be further divided into water-rich fluids, water-bearing melts, and supercritical fluids.

Formed amidst high temperature and pressure, supercritical fluids are characterized by relatively low viscosity, high activity, and exceptional elemental mobility. These unconventional physicochemical properties endow them with a vital role in triggering meso-deep seismicity and volcanism, facilitate subduction zone elemental transport, material cycling, and metal enrichment mineralization, and influence the evolution of the Earth's habitability. However, it remains a challenge to identify a supercritical fluid activity from natural samples, especially when the critical and quantitative geochemical indicators are severely scarce.

The researchers studied the UHP metamorphic vein body of the continental ultra-deep subduction zone in the Dabie Mountains, and found a large number of multiphase fluid inclusions coexisting with the iconic UHP metamorphic mineral, Kochnite. The inclusions are preserved with multiple seed minerals with a relatively consistent overall assemblage, consisting mainly of quartz, calcite, anhydrite, and a large amount of water.

Systematic petrographic, laser Raman, and elemental surface sweep studies showed that the multiphase fluid inclusions in omphacite and garnet are primary UHP fluid inclusions, preserving the chemical composition of the deeper high-pressure fluids in the subduction zone intact. 3D laser Raman modeling and quantitative compositional calculations of these inclusions recovered the original vein-forming fluid composition recorded in the multiphase fluid inclusions. The fluids were shown to contain mainly SiO2, CaO, and water, as well as significant amounts of volatile elements such as carbon and sulfur.

Furthermore, the researchers discovered that fluids preserved in the inclusions have supercritical properties (e.g., migration capacity) and compositional characteristics. These supercritical fluids are highly efficient in activating and transporting carbon and sulfur in subduction zone sheets, and may transport them into the mantle wedge and even the deep mantle, thus having a wide and important impact on the efficiency and flux of the carbon-sulfur cycle between the Earth's surface and the deep, as well as the evolution of the habitability of the Earth.

This study provided a detailed characterization of the properties of supercritical fluids in deep subduction zones. It also revealed the critical role the UHP fluids played in terms of carbon and sulfur cycle, which has long been underestimated.

Proceedings of the National Academy of Sciences

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

8-May-2023

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