16:00 〜 16:15
[SCG45-29] Effects of fluid chemistry on metasomatic reactions at slab-mantle interfaces within the subduction zones: Significance of Si vs. Mg and CO2
キーワード：交代作用、マグネシウム vs シリカ、滑石、緑泥石、二酸化炭素、地殻ーマントル境界
The slab-mantle interface within the subduction zone is one of the sites with largest chemical potential gradient within the earth’s interiors. The aqueous fluids derived by the dehydration of subducting slab is thought to cause hydration of mantle wedge (serpentinization). In addition, silica is thought to be supplied from mantle to crust, resulting in Si-metasomatism to form talc and amphiboles. In particular, the metasomatic reactions at the mantle wedge corner potentially related to the slow earthquakes (Tarling et al., 2019). However, it is still poorly understood how multi-component fluid chemistry evolves within the subduction zone interfaces, and whether a variation in fluid chemistry causes the impacts on seismic/aseismic properties. In particular, recent studies revealed that chlorite forms instead of talc at the crust-mantle interface (Collido et al., 2022), and that talc also forms in relation to carbonation of serpentinites (Okamoto et al. 2021). In this presentation, we show the results of the geochemical modeling on the fluid-rock interactions along the typical subduction zones. Then, we compare the natural occurrences and recent results on the reaction experiments of the metasomatic reactions at the mantle wedge conditions.
We modeled the fluid-rock interaction at the slab-mantle interfaces by (1) incremental addition of mantle peridotite into aqueous fluid in aqueous fluids in equilibrium with pelite and (2) incremental addition of metasediment into aqueous fluid in aqueous fluids in equilibrium with peridotite. We calculated along three subduction zones; NE Japan, Cascadia and Nankai. Geochemical modeling reveals that in the shallow part of a subduction zone, the dissolved Si content of fluids in equilibrium with pelitic schist (CSi,crust) is significantly higher than the dissolved Mg content of fluids in equilibrium with mantle peridotite (CMg,mantle); however, CMg,mantle becomes dominant at depth, resulting in the Mg-metasomatism of crustal rocks to form chlorite rocks. This Mg-metasomatism is more widespread in warmer subduction zones (e.g., the Nankai and Cascadia subduction zones) than in colder subduction zones (e.g., in Northeast Japan). Such results of Mg- and Si-metasomatism are consistent with the natural observations that chlorite rocks are developed within the crustal rocks, whereas talc is only formed within serpentinite of the high-pressure metamorphic terranes such as Sanbagawa belt. In addition, the model predicts that subduction of the sediments produces the carbonates (dolomite and magnesite) as well as talc. The preliminary results of the experiments on the metasomatic reactions at the pelitic schist and peridotite/serpentinite indicates that at 500 degreeC and 1.0 GPa, the formation of talc can more occur with intensive fracturing by CO2-metasomatism rather than Si-metasomatism, which potentially makes drastic weakening of mantle wedge.
Tarling et al., (2019) Nature Geosci, 12:1034–1042.
Collido et al., (2022) Geochem Geophys Geosys 23:e2021GC10206.
Okamoto et al., (2021) Com Earth Env, 2:151. doi:10.1038/s43247-021-00224-5
We modeled the fluid-rock interaction at the slab-mantle interfaces by (1) incremental addition of mantle peridotite into aqueous fluid in aqueous fluids in equilibrium with pelite and (2) incremental addition of metasediment into aqueous fluid in aqueous fluids in equilibrium with peridotite. We calculated along three subduction zones; NE Japan, Cascadia and Nankai. Geochemical modeling reveals that in the shallow part of a subduction zone, the dissolved Si content of fluids in equilibrium with pelitic schist (CSi,crust) is significantly higher than the dissolved Mg content of fluids in equilibrium with mantle peridotite (CMg,mantle); however, CMg,mantle becomes dominant at depth, resulting in the Mg-metasomatism of crustal rocks to form chlorite rocks. This Mg-metasomatism is more widespread in warmer subduction zones (e.g., the Nankai and Cascadia subduction zones) than in colder subduction zones (e.g., in Northeast Japan). Such results of Mg- and Si-metasomatism are consistent with the natural observations that chlorite rocks are developed within the crustal rocks, whereas talc is only formed within serpentinite of the high-pressure metamorphic terranes such as Sanbagawa belt. In addition, the model predicts that subduction of the sediments produces the carbonates (dolomite and magnesite) as well as talc. The preliminary results of the experiments on the metasomatic reactions at the pelitic schist and peridotite/serpentinite indicates that at 500 degreeC and 1.0 GPa, the formation of talc can more occur with intensive fracturing by CO2-metasomatism rather than Si-metasomatism, which potentially makes drastic weakening of mantle wedge.
Tarling et al., (2019) Nature Geosci, 12:1034–1042.
Collido et al., (2022) Geochem Geophys Geosys 23:e2021GC10206.
Okamoto et al., (2021) Com Earth Env, 2:151. doi:10.1038/s43247-021-00224-5