11:30 〜 11:45
[SCG46-19] Experimental investigation of the post-antigorite reaction process in cold subducting slabs
キーワード：脱水カイネティクス、X線その場観察、地球深部水、加水軟化、脱水脆性化
Subduction of partially serpentinized peridotite is an important process for the water transportation to the deep earth and the occurrence of intermediate-depth and deep earthquakes. Although there have been many phase equilibrium studies, the post-antigorite reaction has not been well understood, especially under the cold slab conditions where deep earthquakes occur (thermal parameters > 4000 km and ~350-450°C at a depth of 240 km). In addition, because most experiments have been conducted using powders or oxide mixtures as starting materials in metal capsules (i.e., closed system), it is difficult to discuss reaction processes and their effects on rheology and shear instability under actual slab conditions. Here we report results of high-pressure experiments on the mechanisms and kinetics of the post-antigorite reaction, and discuss the possible processes in cold subducting slabs.
High-pressure experiments were conducted by an in-situ X-ray observation method at BL04B1 and BL05XU at SPring-8. Natural antigorite (Atg) core with a diameter of 1.4 mm was used as a starting material and embedded in a NaCl medium. The post-antigorite reactions were observed at ~4-8 GPa and ~450-800°C by time-resolved X-ray diffraction measurements every 0.1-300 sec. Several additional high-pressure experiments were also conducted by a quenching method at Kyushu Univ., in which the same antigorite core was put in a Horoman peridotite capsule to examine reaction processes of NAMs (Nominally Anhydrous Minerals) with dehydrated fluid.
We observed three reaction processes by in-situ X-ray observation depending on pressures: (1) Atg => olivine (Ol) + talc-like phase => Ol + enstatite (En) + H2O at ~4 GPa, (2) Atg => Ol + 10Å phase + H2O => Ol + En + H2O at ~6-7 GPa, (3) Atg => Phase E (PhE) + 10Å phase (10Å) => Ol + high-clinoenstatite (hCEn) + H2O at ~7-8 GPa. Previous studies have also reported reactions (1) and (2), those are expected in relatively warm slab at higher than ~500°C. The reaction (3) is a new process and the first stage (Atg => PhE+10Å) is thought to be a possible candidate for the post-antigorite reaction in cold slabs. Analysis of the kinetic data obtained indicates that this pressure-induced reaction is kinetically possible at ~350-450°C in geological timescales. Thus, water in partially serpentinized peridotite may be transported into the deeper low-temperature seismic regions by this reaction. We also observed in preliminary quenching experiments at ~10 GPa and 500°C that the dehydrated fluid from atg reacts with NAMs in surrounding peridotite to form two layers of reaction rims: Phase A + hCEn and PhE. The hydration reaction proceeds by the infiltration of the outside layer of PhE into Ol cracks and grain boundaries, which possibly affects mechanical properties of surrounding peridotite.
High-pressure experiments were conducted by an in-situ X-ray observation method at BL04B1 and BL05XU at SPring-8. Natural antigorite (Atg) core with a diameter of 1.4 mm was used as a starting material and embedded in a NaCl medium. The post-antigorite reactions were observed at ~4-8 GPa and ~450-800°C by time-resolved X-ray diffraction measurements every 0.1-300 sec. Several additional high-pressure experiments were also conducted by a quenching method at Kyushu Univ., in which the same antigorite core was put in a Horoman peridotite capsule to examine reaction processes of NAMs (Nominally Anhydrous Minerals) with dehydrated fluid.
We observed three reaction processes by in-situ X-ray observation depending on pressures: (1) Atg => olivine (Ol) + talc-like phase => Ol + enstatite (En) + H2O at ~4 GPa, (2) Atg => Ol + 10Å phase + H2O => Ol + En + H2O at ~6-7 GPa, (3) Atg => Phase E (PhE) + 10Å phase (10Å) => Ol + high-clinoenstatite (hCEn) + H2O at ~7-8 GPa. Previous studies have also reported reactions (1) and (2), those are expected in relatively warm slab at higher than ~500°C. The reaction (3) is a new process and the first stage (Atg => PhE+10Å) is thought to be a possible candidate for the post-antigorite reaction in cold slabs. Analysis of the kinetic data obtained indicates that this pressure-induced reaction is kinetically possible at ~350-450°C in geological timescales. Thus, water in partially serpentinized peridotite may be transported into the deeper low-temperature seismic regions by this reaction. We also observed in preliminary quenching experiments at ~10 GPa and 500°C that the dehydrated fluid from atg reacts with NAMs in surrounding peridotite to form two layers of reaction rims: Phase A + hCEn and PhE. The hydration reaction proceeds by the infiltration of the outside layer of PhE into Ol cracks and grain boundaries, which possibly affects mechanical properties of surrounding peridotite.