Multiphase inclusions in HP rocks (Western Mongolia): Remnants of high saline–CO2 fluids released during deep subduction

Subduction zone fluids play a critical role in the global element cycle. During subduction and progressive heating of the subducting slab, hydrous minerals become unstable, which releases water (Schmidt and Poli 1998). The released water is the trigger for element transport within the subducting slab, along the plate interface, and in the overlying mantle wedge (Bebout 2007). Such released fluid composition has been changed due to fluid-rock interaction during the metamorphism, which has been provided by fluid inclusion behavior in high-pressure (HP) minerals in eclogite facies rock (Bayarbold et al., 2023). Moreover, element mobility along the subduction might be highly dependent on volatile compounds in the metamorphic fluid (Tanis et al. 2016). Related to the size of the trapped fluid and fluid-mineral interaction after being trapped in HP minerals, the reconstructed fluid composition from eclogite facies has not yet been properly quantified. In this study, we report the results of our investigation of fluid inclusions (FIs) in the Khungui eclogite, found in western Mongolia.

Based on the main mineral assemblages of the Khungui eclogite, it can be classified into two types: (i) dry eclogite (<15 vol.% for hydrous minerals) and (ii) wet eclogite (>45 vol.% for hydrous minerals). Numerous primary FIs are present in the omphacite (Omp) from dry eclogite. Omp in dry eclogite is clearly distinguished into two types (Omp1 and Omp2) by their textures. Omp1 occurs within large (up to 100 µm) elongated quartz crystals in the matrix. The fractured Omp1 contains mineral inclusions (amphibole and quartz) and abundant primary FIs. In contrast to Omp1, Omp2 shows an equilibrium boundary with barroisite and garnet in the matrix. Furthermore, no mineral inclusion or FIs occur in Omp2, which is partially replaced by hornblende and plagioclase, whereas Omp1 is not.  FIs in Omp1 consist of variable phases such as liquid, vapor, and several solid phases with various phase ratios. The solid phases in FI within Omp consist of halite (Hl), amphibole, and calcite. These observations reveal that (1) the peak pressure-temperature (P-T) of dry eclogite is higher than the eclogite facies P-T condition of wet eclogite (2.1–2.2 GPa, 580–610°C); (2) high-saline and CO₂ saturated fluid interacted with rock at eclogite facies; and (3) Hl in primary FIs in Omp suggests that the salinity of fluid might be up to 26.3 wt.% NaCl equivalent.