5:15 PM - 6:45 PM
[SCG40-P35] Salinity and pressure recorded in fluid inclusions in carbonate-silicate veins in pelitic schists of the Kii Shimanto belts and in serpentinite of Kanto Sambagawa belts
Keywords:Serpentinization, Carbonation, Fluid inclusion, Seawater, pore-fluids, Slab-derived fluids
Carbonates are virtually insoluble in fresh water, but soluble in saline fluids. Carbonate-bearing veins are formed when rocks react with carbonate ions in saline fluids. One of the mechanisms of slow earthquakes may be related to open crack veins (Okamoto et al., 2021, Comm Earth Environ). By analyzing fluid inclusions in veins, the chemical composition of the fluid and its density can be determined. Once the density is known, it is possible to calculate an isochoric line and estimate changes in temperature and pressure.
The Sambagawa metamorphic belt, low-temperature, high-pressure metamorphic rocks, and the Shimanto belt, Cretaceous to Neogene accretionary complexes, are located in the outer zone of southwestern Japan. These were formed in the deep and shallow subduction zones since the Cretaceous.
The Mugitani complex is a geological body classified as the Shimanto belt in the Yoshino area of the Kii Peninsula. The Mugitani complex contains blocks or slabs of sandstone, chert, and basalt in a predominantly muddy matrix. It also shows block-in-matrix structures and overprinting schist, elongation line structures and folds (Shimura et al., 2021, JAES). This indicates that the Mugitani complex is deformed by both the Shimanto belt and the Sambagawa metamorphic belt. Peak temperatures are estimated to be 280-290°C (Shimura et al., 2021, JAES). The salinity of fluid inclusions in veins composed of quartz and calcite in the pelitic schists of the Mugitani complex is 3.0 wt.% NaCl eq. which is close to that of seawater (3.5 wt.%). This indicates that sedimentary pore-fluids removed by consolidation in the shallow part of the subduction zone is involved in the formation of the veins. The homogenization temperature of the fluid inclusion is 138±26°C. Assuming a peak temperature of 285°C, the peak depth is estimated to be 296 MPa and at 11 km, assuming a density of 2.75 g/cm3. Fluid inclusions may be formed at this depth or shallower.
Fluid inclusions in magnesite and dolomite veins in the Higuchi serpentine body, one of the serpentine bodies in the Sambagawa metamorphic belt, Kanto mountains, were analyzed. The homogenization temperature and salinity of the inclusions in the magnesite vein are 152±4°C and 3.9±0.2%, respectively, whereas the homogenization temperature and salinity of the fluid inclusions in the dolomite vein that were formed later are 154±14°C and 2.8±0.6%, respectively. Assuming a temperature of 450°C (Okamoto et al., 2022, Comm Earth Environ), a pressure of 572 MPa is calculated, which corresponds to a depth of about 21 km, assuming a density of 2.75 g/cm3. If the fluid inclusions were formed in the mantle wedge, the Moho can be relatively shallow.
The Sambagawa metamorphic belt, low-temperature, high-pressure metamorphic rocks, and the Shimanto belt, Cretaceous to Neogene accretionary complexes, are located in the outer zone of southwestern Japan. These were formed in the deep and shallow subduction zones since the Cretaceous.
The Mugitani complex is a geological body classified as the Shimanto belt in the Yoshino area of the Kii Peninsula. The Mugitani complex contains blocks or slabs of sandstone, chert, and basalt in a predominantly muddy matrix. It also shows block-in-matrix structures and overprinting schist, elongation line structures and folds (Shimura et al., 2021, JAES). This indicates that the Mugitani complex is deformed by both the Shimanto belt and the Sambagawa metamorphic belt. Peak temperatures are estimated to be 280-290°C (Shimura et al., 2021, JAES). The salinity of fluid inclusions in veins composed of quartz and calcite in the pelitic schists of the Mugitani complex is 3.0 wt.% NaCl eq. which is close to that of seawater (3.5 wt.%). This indicates that sedimentary pore-fluids removed by consolidation in the shallow part of the subduction zone is involved in the formation of the veins. The homogenization temperature of the fluid inclusion is 138±26°C. Assuming a peak temperature of 285°C, the peak depth is estimated to be 296 MPa and at 11 km, assuming a density of 2.75 g/cm3. Fluid inclusions may be formed at this depth or shallower.
Fluid inclusions in magnesite and dolomite veins in the Higuchi serpentine body, one of the serpentine bodies in the Sambagawa metamorphic belt, Kanto mountains, were analyzed. The homogenization temperature and salinity of the inclusions in the magnesite vein are 152±4°C and 3.9±0.2%, respectively, whereas the homogenization temperature and salinity of the fluid inclusions in the dolomite vein that were formed later are 154±14°C and 2.8±0.6%, respectively. Assuming a temperature of 450°C (Okamoto et al., 2022, Comm Earth Environ), a pressure of 572 MPa is calculated, which corresponds to a depth of about 21 km, assuming a density of 2.75 g/cm3. If the fluid inclusions were formed in the mantle wedge, the Moho can be relatively shallow.