4:45 PM - 5:00 PM
[SCG45-36] Weakening of forearc mantle wedge at the subduction interface in a warm subduction zone: Insights from the phase equilibria modeling on the fluid-rock reaction
Keywords:Metasomatism, Slow slip, Phase equilibria modeling, Fluid-rock interaction
Slow earthquakes are commonly observed between the shallow seismogenic zone and the deep stable slip (non-earthquake slip) zone in subduction zones. For example, in the Nankai subduction zone in SW Japan, slow slip occurs at a depth of 30-40 km, and >40 km is the stable slip zone. In these depths, fluids derived from subducting slab (oceanic crust and sediments) may infiltrate into forearc mantle wedge, modifying mineral component at the base of mantle wedge. Accurate prediction of the products of fluid–rock interactions is important for understanding the variation of seismic activity at the subduction interface.
Most studies fluids in the subduction zone assumed to be pure H2O. However, geological evidence indicates that fluids in subduction zone is not pure H2O and contains multiple major rock components (e.g., Si, Al, and Ca). Moreover, thermal structures along the slab–mantle interface and the chemical compositions of subducting sediment (including organic and inorganic carbon species) are variable in subduction zones. As a result, the composition and volume of fluids entering the mantle wedge differ with depth and across various arcs. These differences lead corresponding variations in the mineral assemblage of the mantle wedge, which may be related to variations in seismic activity at the subduction interface. However, the nature of the depth-related variations in the mineral assemblage of the forearc mantle and their influence on seismic activity remain poorly understood.
In this study, we present numerical modeling to predict the characteristics of the fluid (chemical composition and fluid flux) and the mineral assemblages at the subduction interface from shallow (~20 km) to deep (~80 km). We took into account the variation in the variation of subducting sediments and the thermal structure of the typical cold subduction zone of NE Japan and the warm subduction zone of SW Japan. The calculations showed that amount and composition of fluids generated in the two subduction zones differ, causing significant changes in the types and amounts of altered minerals in the forearc mantle at the subduction interface. In the forearc mantle of NE Japan, carbonate minerals are not formed, and talc is formed only at a depth of about 70 km. In contrast, in SW Japan, both talc and magnesite are produced over a wide range of depths from 35 to 70 km, and the amount of these minerals increases from a depth of 40 km, with thicker layers existing at greater depths.
Interestingly, the depth at which the thick talc layer was predicted in the SW Japan subduction zone (40 km or deeper) coincides with the down-dip limit of the seismic zone containing slow earthquakes. Talc has a very low coefficient of friction, is known to cause stable sliding (e.g., Hirauchi et al. 2013), and is widely observed at metapelite–ultramafic rock contact in the metamorphic terranes (Okamoto et al. 2021; Okamoto and Oyanagi 2023; Oyanagi et al. 2023). The present study proposes that in the SW Japan subduction zone, carbon cycling from the surface to the Earth's interior increases the amount of talc and carbonate with depth, facilitating the mechanical weakening of the plate boundary and the transition from a seismogenic to a stable sliding zone.
References
Clift, P. D. (2017). A revised budget for Cenozoic sedimentary carbon subduction. Reviews of Geophysics, 55(1), 97–125. https://doi.org/10.1002/2016RG000531
Hirauchi, K. I., den Hartog, S. A. M., & Spiers, C. J. (2013). Weakening of the slab-mantle wedge interface induced by metasomatic growth of talc. Geology, 41(1), 75–78. https://doi.org/10.1130/G33552.1
Okamoto, A., Oyanagi, R., Yoshida, K., Uno, M., Shimizu, H., & Satish-Kumar, M. (2021). Rupture of wet mantle wedge by self-promoting carbonation. Communications Earth & Environment, 2(1), 1–10. https://doi.org/10.1038/s43247-021-00224-5
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, 10(1), 39. https://doi.org/10.1186/s40645-023-00568-w
Oyanagi, R., & Okamoto, A. (2024). Subducted carbon weakens the forearc mantle wedge in a warm subduction zone. Nature Communications, 15(1), 7159. https://doi.org/10.1038/s41467-024-51476-6
Oyanagi, R., Uno, M., & Okamoto, A. (2023). Metasomatism at a metapelite–ultramafic rock contact at the subduction interface: Insights into mass transfer and fluid flow at the mantle wedge corner. Contributions to Mineralogy and Petrology, 178(5), 27. https://doi.org/10.1007/s00410-023-02011-1
Most studies fluids in the subduction zone assumed to be pure H2O. However, geological evidence indicates that fluids in subduction zone is not pure H2O and contains multiple major rock components (e.g., Si, Al, and Ca). Moreover, thermal structures along the slab–mantle interface and the chemical compositions of subducting sediment (including organic and inorganic carbon species) are variable in subduction zones. As a result, the composition and volume of fluids entering the mantle wedge differ with depth and across various arcs. These differences lead corresponding variations in the mineral assemblage of the mantle wedge, which may be related to variations in seismic activity at the subduction interface. However, the nature of the depth-related variations in the mineral assemblage of the forearc mantle and their influence on seismic activity remain poorly understood.
In this study, we present numerical modeling to predict the characteristics of the fluid (chemical composition and fluid flux) and the mineral assemblages at the subduction interface from shallow (~20 km) to deep (~80 km). We took into account the variation in the variation of subducting sediments and the thermal structure of the typical cold subduction zone of NE Japan and the warm subduction zone of SW Japan. The calculations showed that amount and composition of fluids generated in the two subduction zones differ, causing significant changes in the types and amounts of altered minerals in the forearc mantle at the subduction interface. In the forearc mantle of NE Japan, carbonate minerals are not formed, and talc is formed only at a depth of about 70 km. In contrast, in SW Japan, both talc and magnesite are produced over a wide range of depths from 35 to 70 km, and the amount of these minerals increases from a depth of 40 km, with thicker layers existing at greater depths.
Interestingly, the depth at which the thick talc layer was predicted in the SW Japan subduction zone (40 km or deeper) coincides with the down-dip limit of the seismic zone containing slow earthquakes. Talc has a very low coefficient of friction, is known to cause stable sliding (e.g., Hirauchi et al. 2013), and is widely observed at metapelite–ultramafic rock contact in the metamorphic terranes (Okamoto et al. 2021; Okamoto and Oyanagi 2023; Oyanagi et al. 2023). The present study proposes that in the SW Japan subduction zone, carbon cycling from the surface to the Earth's interior increases the amount of talc and carbonate with depth, facilitating the mechanical weakening of the plate boundary and the transition from a seismogenic to a stable sliding zone.
References
Clift, P. D. (2017). A revised budget for Cenozoic sedimentary carbon subduction. Reviews of Geophysics, 55(1), 97–125. https://doi.org/10.1002/2016RG000531
Hirauchi, K. I., den Hartog, S. A. M., & Spiers, C. J. (2013). Weakening of the slab-mantle wedge interface induced by metasomatic growth of talc. Geology, 41(1), 75–78. https://doi.org/10.1130/G33552.1
Okamoto, A., Oyanagi, R., Yoshida, K., Uno, M., Shimizu, H., & Satish-Kumar, M. (2021). Rupture of wet mantle wedge by self-promoting carbonation. Communications Earth & Environment, 2(1), 1–10. https://doi.org/10.1038/s43247-021-00224-5
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, 10(1), 39. https://doi.org/10.1186/s40645-023-00568-w
Oyanagi, R., & Okamoto, A. (2024). Subducted carbon weakens the forearc mantle wedge in a warm subduction zone. Nature Communications, 15(1), 7159. https://doi.org/10.1038/s41467-024-51476-6
Oyanagi, R., Uno, M., & Okamoto, A. (2023). Metasomatism at a metapelite–ultramafic rock contact at the subduction interface: Insights into mass transfer and fluid flow at the mantle wedge corner. Contributions to Mineralogy and Petrology, 178(5), 27. https://doi.org/10.1007/s00410-023-02011-1