15:30 〜 15:45
[SCG61-07] マントルウェッジ浅部におけるSi-, Mg-, CO2交代作用の重要性
★招待講演
キーワード：元素移動、シリカ交代作用、マグネシウム交代作用、CO2交代作用
The mantle wedge is a unique region where crustal materials interact with mantle rocks. Si metasomatism in mantle rocks is thought to result from the infiltration of Si-rich aqueous fluids from subducting sediments. Recent studies provide natural evidence suggesting that a significant amount of Mg can also migrate from the mantle to the crust, leading to the formation of chlorite rocks. Additionally, more than 60 MtC per year is subducted as carbonates or organic materials. However, the effects of these subducted carbon species on metasomatic reactions and the mechanical properties of the crust-mantle interface remain unclear. In this talk, we will discuss the relative mobilities of elements and metasomatic reactions at sediment-mantle contacts under mantle wedge conditions, with a particular focus on Si, Mg, and CO2, based on high-pressure, high-temperature experiments and geochemical modeling.
We conducted a series of reaction experiments at 500 dgreeC and 1 GPa to simulate metasomatic reactions at the interface between the mantle and subducting sediments. In our experimental setup, a core of pelitic schist was sandwiched between harzburgite and serpentinite. We used either H2O fluid (4 wt percent) or H2O-CO2 fluids (4 wt percent, XCO2 = 0.2). In the peridotite-pelitic schist-serpentinite experiments with H2O fluid, talc formed in both peridotite and serpentinite, while in the pelitic schist, albite was preferentially replaced by Mg-saponite. Mass balance analyses revealed that Mg transport from the mantle to the crust was greater than Si transport from the crust to the mantle. In experiments with H2O-CO2 fluids, a large amount of talc and magnesite formed in both peridotite and serpentinite, with significantly more talc than in the H2O-fluid experiments.
We also conducted geochemical modeling of mantle metasomatism along the geothermal gradient at the subduction interface in NE Japan and the Nankai subduction zone. Our simulations examined the infiltration of aqueous fluids in equilibrium with carbon-bearing sediments into the mantle wedge. We found that thick talc layers can form via carbonic fluids, particularly in warm subduction zones such as the Nankai subduction zone. Our results suggest that Mg metasomatism and CO2 metasomatism have a greater influence on the mechanical properties of the plate interface than previously thought. Variations in XCO2 over time and space may play a crucial role in controlling the rheology and seismic activity along the subduction zone interface.
References Okamoto A., Oyanagi R. (2023) Prog. Earth. Planet. Sci., 10:39.
Oyanagi, R., Okamoto, A., (2024) Nature Communications, 15, 7159.
We conducted a series of reaction experiments at 500 dgreeC and 1 GPa to simulate metasomatic reactions at the interface between the mantle and subducting sediments. In our experimental setup, a core of pelitic schist was sandwiched between harzburgite and serpentinite. We used either H2O fluid (4 wt percent) or H2O-CO2 fluids (4 wt percent, XCO2 = 0.2). In the peridotite-pelitic schist-serpentinite experiments with H2O fluid, talc formed in both peridotite and serpentinite, while in the pelitic schist, albite was preferentially replaced by Mg-saponite. Mass balance analyses revealed that Mg transport from the mantle to the crust was greater than Si transport from the crust to the mantle. In experiments with H2O-CO2 fluids, a large amount of talc and magnesite formed in both peridotite and serpentinite, with significantly more talc than in the H2O-fluid experiments.
We also conducted geochemical modeling of mantle metasomatism along the geothermal gradient at the subduction interface in NE Japan and the Nankai subduction zone. Our simulations examined the infiltration of aqueous fluids in equilibrium with carbon-bearing sediments into the mantle wedge. We found that thick talc layers can form via carbonic fluids, particularly in warm subduction zones such as the Nankai subduction zone. Our results suggest that Mg metasomatism and CO2 metasomatism have a greater influence on the mechanical properties of the plate interface than previously thought. Variations in XCO2 over time and space may play a crucial role in controlling the rheology and seismic activity along the subduction zone interface.
References Okamoto A., Oyanagi R. (2023) Prog. Earth. Planet. Sci., 10:39.
Oyanagi, R., Okamoto, A., (2024) Nature Communications, 15, 7159.