09:30 〜 09:45
[SMP24-03] Positive buoyancy of partially carbonated serpentinite in eclogite exhumation mechanisms at oceanic subduction zones: 2D-geodynamic modeling and petrological validation
キーワード：2次元岩石熱力学モデリング、高圧蛇紋岩メランジュ、炭酸塩化した蛇紋岩、青海蛇紋岩メランジュ、エクロジャイト
Serpentinite plays a critical role at convergent plate boundary, significantly influencing mas-flow and -circulation in solid-Earth. Notably, serpentinization in the sub-forearc wedge mantle can alter the rheological properties of the wedge mantle peridotite, thereby affecting subduction zone dynamics, especially the slab exhumation pathways and mechanisms. This study employs geodynamic and petrological approaches to assess the impact of forearc mantle wedge serpentinization on the slab exhumation mechanisms dueing oceanic plate subduction at active continental plate margin.
We calculated a 2D-geodynamic model using the Gerya and Yuen (2003)'s I2VIS code to simulate the subduction of an oceanic plate into a serpentinized mantle wedge. Numerical simulations conducted with three varying convergence rate parameters showed that serpentinization in the forearc mantle leads to reduced density and viscosity in the subduction channel, facilitating the ascent of high-pressure metamorphic complexes. These simulations accurately reproduced the prograde pressure-temperature (P–T) path with a geothermal gradient of 10°C/km, pinpointing peak metamorphic conditions consistent with eclogite-facies at ∼1.8 GPa and ∼600°C for a plate convergence rate of 2 cm/yr. In the calculated P–T paths, following this peak condition, the metamorphic slabs are uplifted isothermally to just below the Moho level at ∼1.1 GPa and ∼600°C, before stagnating in the subduction channel. We did confirm that a slower plate convergence rate (2 cm/yr) causes the subducted plate to drag the subduction channel into deeper mantle regions (100 km), whereas a faster convergence rate (10 cm/yr) keeps the subduction channel at relatively shallower depths (65 km). To validate the reliability of this 2D-geodynamic model, the we investigated antigorite-bearing serpentinite and carbonated serpentinite associated with eclogites in the Omi serpentinite mélange (OmSM), Japan. We newly discovered veinlets of metamorphic olivine accompanied with antigorite, Na-rich tremolite, and chlorite cutting porphyroblastic olivines in the serpentinite. Phase equilibrium modeling, coupled with geological contexts, indicates that the serpentinites attained pressure-temperature (P-T) conditions ranging from 1.1 to 1.9 GPa and 580 to 620°C. Although serpentinite exhibits a density of approximately 3.1 g/cm3 at a depth corresponding to 600℃ and 1.8 GPa, the process of carbonation reduces the density of serpentinite to ∼2.85–2.9 g/cm3, providing it with increased buoyancy relative to the surrounding mantle peridotite. This strongly suggests that partially carbonated serpentinite plays a crucial role as a low-density material within the subduction zones of oceanic plates. Considering our new findings and the metamorphic records of eclogites and associated ultramafic rocks of OmSM, we propose a scenario in which eclogite-facies rocks and dense meta-serpentinites were coupled in a deep subduction zone interface, and the development of buoyant partially carbonated serpentinite might have significantly enhanced the initial exhumation toward the Moho level.
We calculated a 2D-geodynamic model using the Gerya and Yuen (2003)'s I2VIS code to simulate the subduction of an oceanic plate into a serpentinized mantle wedge. Numerical simulations conducted with three varying convergence rate parameters showed that serpentinization in the forearc mantle leads to reduced density and viscosity in the subduction channel, facilitating the ascent of high-pressure metamorphic complexes. These simulations accurately reproduced the prograde pressure-temperature (P–T) path with a geothermal gradient of 10°C/km, pinpointing peak metamorphic conditions consistent with eclogite-facies at ∼1.8 GPa and ∼600°C for a plate convergence rate of 2 cm/yr. In the calculated P–T paths, following this peak condition, the metamorphic slabs are uplifted isothermally to just below the Moho level at ∼1.1 GPa and ∼600°C, before stagnating in the subduction channel. We did confirm that a slower plate convergence rate (2 cm/yr) causes the subducted plate to drag the subduction channel into deeper mantle regions (100 km), whereas a faster convergence rate (10 cm/yr) keeps the subduction channel at relatively shallower depths (65 km). To validate the reliability of this 2D-geodynamic model, the we investigated antigorite-bearing serpentinite and carbonated serpentinite associated with eclogites in the Omi serpentinite mélange (OmSM), Japan. We newly discovered veinlets of metamorphic olivine accompanied with antigorite, Na-rich tremolite, and chlorite cutting porphyroblastic olivines in the serpentinite. Phase equilibrium modeling, coupled with geological contexts, indicates that the serpentinites attained pressure-temperature (P-T) conditions ranging from 1.1 to 1.9 GPa and 580 to 620°C. Although serpentinite exhibits a density of approximately 3.1 g/cm3 at a depth corresponding to 600℃ and 1.8 GPa, the process of carbonation reduces the density of serpentinite to ∼2.85–2.9 g/cm3, providing it with increased buoyancy relative to the surrounding mantle peridotite. This strongly suggests that partially carbonated serpentinite plays a crucial role as a low-density material within the subduction zones of oceanic plates. Considering our new findings and the metamorphic records of eclogites and associated ultramafic rocks of OmSM, we propose a scenario in which eclogite-facies rocks and dense meta-serpentinites were coupled in a deep subduction zone interface, and the development of buoyant partially carbonated serpentinite might have significantly enhanced the initial exhumation toward the Moho level.