5:15 PM - 7:15 PM
[SCG45-P12] The time-dependent contrasting slip behavior induced by H2O-CO2 fluid within slab-mantle interface and implication to slow earthquakes at shallow mantle wedge
Keywords:Mantle wedge, Carbonation, Episodic tremor and slip
More than 60 Mt per year of carbon is brought to interior of the earth through subduction zone. Part of the subducted carbon potentially mobilized into mantle wedge, which produces a various types of carbonated mantle rocks such as listvenite (quartz + carbonate rock) and soapstone (talc + carbonate rock; Okamoto et al., 2021; Sieber et al., 2020; 2022). Talc, a byproduct of carbonated mantle rock, is the weakest mineral in the mantle rocks, influencing the mechanical properties and seismic activities within the subduction zone megathrust (Oyanagi and Okamoto, 2024; Lindquist et al., 2023). Lindquist et al. (2023) suggests the linkage of talc layer formation and slow earthquakes at the slab-mantle interface, however, the effect of carbonation to slow earthquakes is poorly understood.
In this study, we conducted hydrostatic and deformation experiments using sedimentary and mantle rock with H2O-CO2 fluids to understand coupled process of reactions and deformation at a slab-mantle interface. Our experiments were conducted at 500°C and 1 GPa using core samples of quartzite for sedimentary rock and serpentinite or harzburgite for mantle rocks. In the hydrostatic experiments, quartzite is sandwiched by serpentinite (wet mantle) and harzburgite (dry mantle) changing duration (13h to 70h), as an analogue of slab-mantle interface. For deformation experiments, we prepared the precut samples of cores and contact quartzite and serpentinite/harzburgite at 45° to the maximum compression direction. The duration for the deformation and displacement rate were 9 – 13h and 1.0-2.0×10-4 mm/s, respectively. In all experiments, 4 wt% of H2O-CO2 fluid (XCO2 = 0.20-0.25) is enclosed between lithological contact. The decomposition of Oxalic Acid Dihydrate (OAD) provides the source of CO2, which demonstrates the CO2 fluid derived from organic materials.
In experiments, various products were observed at lithological contacts. The hydrostatic experiments with short duration (13h) produced Qz+Mgs at the boundary and Tlc +Mgs at the interiors of both serpentinite and harzburgite. In contrast, the long-duration experiment revealed the formation of the monomineralic talc layer (60 – 80 µm thickness) at the boundary, whereas Tlc + Mgs were formed inside of the mantle rocks.
The deformation experiments using harzburgite for mantle rock produced Qz+Mgs including olivine adhering to quartzite, which was peeled from harzburgite. In addition, talc layer (20 µm thickness) was formed at the lithological boundary. In case of serpentinite-quartzite deformation experiment, listvenite mainly formed at the boundary and less talc formed. In addition, a distinct shear zone was developed between unaltered (serpentinite) and altered serpentinite (Qz+Mgs), in which Qz and Mgs grains occurred without deformation. The stress-displacement curves revealed the contrasting behavior between the products of Qz+Mgs (harzburgite-quartzite experiments) and Tlc+Mgs (serpentinite-quartzite experiments); the former showed strain- weakening behavior, whereas the latter showed strain-strengthening behavior.
The series of reaction experiments revealed that spatio-temporal change of precipitated minerals on the slab-mantle interface. Our experiments and thermodynamic calculation suggest that (i) at the initial stage of carbonation, listvenite would be formed at the boundary due to the high XCO2 and fluid-rock ratio, and (ii) as the reaction proceeds, the decrease in XCO2 in the fluid and high silica potential by the presence of quartzite leads listvenite to decompose to talc. The deformation experiment revealed that frictional behavior is strongly dependent on the products of carbonation; listvenite or soapstone.
The present study suggests the spatio-temporal heterogeneity in mechanical properties can be produced at slab-mantle interface due to the infiltration of H2O-CO2 fluid. Subducted metasediments show large variety of XCO2 (<0.01 to 0.6) in the fluid from thin-section scale (Enami et al., 2021) to metamorphic zonation scale (Goto et al., 2007). Such distribution of CO2-rich fluid and reaction-induced temporal change of minerals leads to the formation of asperity and barrier, which may explain wide spectrum of slow slips (episodic tremor and slip) occurring at mantle wedge.
In this study, we conducted hydrostatic and deformation experiments using sedimentary and mantle rock with H2O-CO2 fluids to understand coupled process of reactions and deformation at a slab-mantle interface. Our experiments were conducted at 500°C and 1 GPa using core samples of quartzite for sedimentary rock and serpentinite or harzburgite for mantle rocks. In the hydrostatic experiments, quartzite is sandwiched by serpentinite (wet mantle) and harzburgite (dry mantle) changing duration (13h to 70h), as an analogue of slab-mantle interface. For deformation experiments, we prepared the precut samples of cores and contact quartzite and serpentinite/harzburgite at 45° to the maximum compression direction. The duration for the deformation and displacement rate were 9 – 13h and 1.0-2.0×10-4 mm/s, respectively. In all experiments, 4 wt% of H2O-CO2 fluid (XCO2 = 0.20-0.25) is enclosed between lithological contact. The decomposition of Oxalic Acid Dihydrate (OAD) provides the source of CO2, which demonstrates the CO2 fluid derived from organic materials.
In experiments, various products were observed at lithological contacts. The hydrostatic experiments with short duration (13h) produced Qz+Mgs at the boundary and Tlc +Mgs at the interiors of both serpentinite and harzburgite. In contrast, the long-duration experiment revealed the formation of the monomineralic talc layer (60 – 80 µm thickness) at the boundary, whereas Tlc + Mgs were formed inside of the mantle rocks.
The deformation experiments using harzburgite for mantle rock produced Qz+Mgs including olivine adhering to quartzite, which was peeled from harzburgite. In addition, talc layer (20 µm thickness) was formed at the lithological boundary. In case of serpentinite-quartzite deformation experiment, listvenite mainly formed at the boundary and less talc formed. In addition, a distinct shear zone was developed between unaltered (serpentinite) and altered serpentinite (Qz+Mgs), in which Qz and Mgs grains occurred without deformation. The stress-displacement curves revealed the contrasting behavior between the products of Qz+Mgs (harzburgite-quartzite experiments) and Tlc+Mgs (serpentinite-quartzite experiments); the former showed strain- weakening behavior, whereas the latter showed strain-strengthening behavior.
The series of reaction experiments revealed that spatio-temporal change of precipitated minerals on the slab-mantle interface. Our experiments and thermodynamic calculation suggest that (i) at the initial stage of carbonation, listvenite would be formed at the boundary due to the high XCO2 and fluid-rock ratio, and (ii) as the reaction proceeds, the decrease in XCO2 in the fluid and high silica potential by the presence of quartzite leads listvenite to decompose to talc. The deformation experiment revealed that frictional behavior is strongly dependent on the products of carbonation; listvenite or soapstone.
The present study suggests the spatio-temporal heterogeneity in mechanical properties can be produced at slab-mantle interface due to the infiltration of H2O-CO2 fluid. Subducted metasediments show large variety of XCO2 (<0.01 to 0.6) in the fluid from thin-section scale (Enami et al., 2021) to metamorphic zonation scale (Goto et al., 2007). Such distribution of CO2-rich fluid and reaction-induced temporal change of minerals leads to the formation of asperity and barrier, which may explain wide spectrum of slow slips (episodic tremor and slip) occurring at mantle wedge.