9:15 AM - 9:30 AM
[MIS23-02] Dynamic interplay of reaction-fracturing-mass transport at the crust-mantle boundary induced by CO2 fluids
★Invited Papers
Keywords:CO2 fluids, mantle, subduction zone, geochemical modeling
Huge amounts of carbon are brought into the Earth's interior from subduction zones. However, compared to the well-studied H2O fluids, the generation of carbon-bearing fluids (CO2, CH4), their reactions with rocks, and their effects on deformation are poorly understood. It is known that mantle rocks are highly reactive with CO2 fluids subsurface environments, such as listvenite mountain in Oman. On the other hand, the extent of carbonation in mantle wedges in subduction zones is considered to be the missing link in the global carbon cycle. The CO2 consuming reaction causes a volume expansion of rocks, but is it possible in mantle wedges. In this talk, we discuss the process of carbonation at mantle wedge depths in subduction zones, especially focusing on the coupling of mass transport-volume change-fracturing, based on the occurrence of carbonated serpentinite bodies in high pressure metamorphic belt, as well as in-situ reaction experiments and thermodynamic modeling.
Many ultrabasic rock bodies and blocks in the higher-grade zones of the Sanbagawa Belt, and they are thought to be of mantle wedge origin. One of them, the Higuchi serpentinite body in Nagatoro, Kanto Mountains, is a small serpentinite body composed of antigorite with various carbonate + talc veins; magnesite + talc veins, dolomite + talc veins, and calcite + dolomite veins [1]. A reaction zone composed of actinolite + chlorite schist (serpentinite side) and chlorite rock (pelitic schist side) was developed at the lithological boundary. Mass balance calculations indicate that CO2, Si, and Ca were transported from the crust to the mantle, causing dehydration reactions by carbonation of the serpentinite body. In contrast, H2O and Mg moved from the mantle to the crust. Calculations of fluid compositions in equilibrium with crust and mantle rocks show that, in warm subduction zones such as the Sanbagawa Belt, the Mg concentration in solution in equilibrium with mantle rocks can be higher than the Si concentration in solution in equilibrium with crustal rocks [2]. As the result of bi-directional mass transfer, carbonation of serpentinite at depth does not involve an increase in mantle rock volume, but rather a decrease, resulting in self-promoting carbonation with forming branching fracture networks. Similar Mg, Si, and CO2 behaviors were confirmed in the reaction experiments at 500degreeC, 1 GPa, which simulated the crust-mantle boundary under mantle wedge conditions. In the experiments with H2O-CO2 fluids, the formation of talc in the crust-mantle interface is more likely due to CO2 metasomatism than to Si metasomatism. Geochemical modeling along the subduction zone geotherms indicates that significant carbonation of the mantle wedge does not occur along the cold geotherm of the northeastern Japan, whereas it is more pronounced in the subduction zone of warm southwestern Japan. Carbonation of the serpentinized mantle is a dehydration reaction accompanied by an increase in pore fluid pressure, and talc that forms with it is the mantle mineral with the lowest frictional strength; therefore carbonation may be responsible for the transition from the seismogenic zone to aseismic steady-state creep in the subduction zone and the deep slow earthquakes that occur in the transition zone.
[1] Okamoto, A., Oyanagi, R., Yoshida, K., Uno, M., Shimizu, H., Satishkumar, M., 2021. Rupture of wet mantle wedge by self-promoting carbonation. Communications Earth & Environment, 2, 151.
[2] Okamoto, A., Oyanagi, R., 2023. Si- versus Mg-metasomatism at the crust–mantle interface: Insights from experiments, natural observations and geochemical modeling. Progress in Earth and Planetary Science (PEPS), 10, no. 39.
Many ultrabasic rock bodies and blocks in the higher-grade zones of the Sanbagawa Belt, and they are thought to be of mantle wedge origin. One of them, the Higuchi serpentinite body in Nagatoro, Kanto Mountains, is a small serpentinite body composed of antigorite with various carbonate + talc veins; magnesite + talc veins, dolomite + talc veins, and calcite + dolomite veins [1]. A reaction zone composed of actinolite + chlorite schist (serpentinite side) and chlorite rock (pelitic schist side) was developed at the lithological boundary. Mass balance calculations indicate that CO2, Si, and Ca were transported from the crust to the mantle, causing dehydration reactions by carbonation of the serpentinite body. In contrast, H2O and Mg moved from the mantle to the crust. Calculations of fluid compositions in equilibrium with crust and mantle rocks show that, in warm subduction zones such as the Sanbagawa Belt, the Mg concentration in solution in equilibrium with mantle rocks can be higher than the Si concentration in solution in equilibrium with crustal rocks [2]. As the result of bi-directional mass transfer, carbonation of serpentinite at depth does not involve an increase in mantle rock volume, but rather a decrease, resulting in self-promoting carbonation with forming branching fracture networks. Similar Mg, Si, and CO2 behaviors were confirmed in the reaction experiments at 500degreeC, 1 GPa, which simulated the crust-mantle boundary under mantle wedge conditions. In the experiments with H2O-CO2 fluids, the formation of talc in the crust-mantle interface is more likely due to CO2 metasomatism than to Si metasomatism. Geochemical modeling along the subduction zone geotherms indicates that significant carbonation of the mantle wedge does not occur along the cold geotherm of the northeastern Japan, whereas it is more pronounced in the subduction zone of warm southwestern Japan. Carbonation of the serpentinized mantle is a dehydration reaction accompanied by an increase in pore fluid pressure, and talc that forms with it is the mantle mineral with the lowest frictional strength; therefore carbonation may be responsible for the transition from the seismogenic zone to aseismic steady-state creep in the subduction zone and the deep slow earthquakes that occur in the transition zone.
[1] Okamoto, A., Oyanagi, R., Yoshida, K., Uno, M., Shimizu, H., Satishkumar, M., 2021. Rupture of wet mantle wedge by self-promoting carbonation. Communications Earth & Environment, 2, 151.
[2] Okamoto, A., Oyanagi, R., 2023. Si- versus Mg-metasomatism at the crust–mantle interface: Insights from experiments, natural observations and geochemical modeling. Progress in Earth and Planetary Science (PEPS), 10, no. 39.