09:15 〜 09:30
[SCG54-02] Deformation-induced antigorite dehydration and intermediate-depth earthquakes
キーワード：脱水脆性化、流体放出、スラブカンラン岩、高圧変形、AE活動
The subduction of partially serpentinized slab peridotite is crucial for understanding water transport into the deep Earth and the occurrence of deep-focus earthquakes. The distribution of intermediate-depth earthquakes (IDEQs, with depths of ~60-300 km and pressures ranging from ~2-10 GPa) is thought to coincide with the dehydration reaction boundary of antigorite, which has long supported the hypothesis of dehydration embrittlement as a cause of IDEQs. However, in medium- and low-temperature slabs, the distribution of IDEQs extends significantly to the lower-temperature side of the closed-system dehydration boundary (e.g., Abers+, EPSL13; Wei+, Sci.Adv.17), raising the question of whether antigorite dehydration truly plays a role in the occurrence of IDEQs.
To investigate this, we studied the dehydration process of antigorite under slab peridotite conditions up to ~10 GPa using high-pressure in-situ X-ray observation methods at the beamlines of BL04B1, BL05XU, and BL15XU, SPring-8. While numerous phase equilibrium experiments in closed systems using metal capsules have been conducted, the conditions inside slab peridotite are not a closed system. In our previous study, we performed dehydration experiments in an open system using NaCl media. Compared to closed systems, open systems exhibit lower dehydration temperatures and faster kinetics, with the notable feature of a high fluid production rate despite lower temperatures. However, the water activity in slab peridotite environments, while not necessarily 1 as in closed systems, cannot be easily reproduced by NaCl media. In reality, it is likely influenced by reactions between the surrounding peridotite and fluids, as well as the permeability of the peridotite.
Therefore, we conducted new dehydration reaction experiments of antigorite using peridotite capsules and found that, at least on laboratory timescales, the kinetic behavior was close to that of a closed system. Interestingly, when conducting similar dehydration experiments under uniaxial deformation, we observed that the permeability of the peridotite capsules significantly increased, accelerating dehydration. It appears that, within the peridotite capsules, fluid infiltration leads to the formation of Mode I cracks, dynamic recrystallization, and, at pressures above 7-8 GPa, the formation of DHMS (Dense Hydrous Magnesium Silicate). This increase in permeability results in a decrease in dehydration temperature compared to when no deformation is applied.
These results imply that antigorite in deforming slab peridotite can undergo dehydration under conditions where IDEQs occur. We also detected some acoustic emission activities associated with the antigorite dehydration during the deformation runs. This likely occurs not in the antigorite region but in the surrounding peridotite, consisting of olivine and pyroxenes. Thus, although further investigations are necessary, deformation-induced fluid infiltration into peridotite lowers the dehydration temperature and embrittles the surrounding nominally anhydrous minerals, which could be an important process for IDEQs across a wide range of pressures.
To investigate this, we studied the dehydration process of antigorite under slab peridotite conditions up to ~10 GPa using high-pressure in-situ X-ray observation methods at the beamlines of BL04B1, BL05XU, and BL15XU, SPring-8. While numerous phase equilibrium experiments in closed systems using metal capsules have been conducted, the conditions inside slab peridotite are not a closed system. In our previous study, we performed dehydration experiments in an open system using NaCl media. Compared to closed systems, open systems exhibit lower dehydration temperatures and faster kinetics, with the notable feature of a high fluid production rate despite lower temperatures. However, the water activity in slab peridotite environments, while not necessarily 1 as in closed systems, cannot be easily reproduced by NaCl media. In reality, it is likely influenced by reactions between the surrounding peridotite and fluids, as well as the permeability of the peridotite.
Therefore, we conducted new dehydration reaction experiments of antigorite using peridotite capsules and found that, at least on laboratory timescales, the kinetic behavior was close to that of a closed system. Interestingly, when conducting similar dehydration experiments under uniaxial deformation, we observed that the permeability of the peridotite capsules significantly increased, accelerating dehydration. It appears that, within the peridotite capsules, fluid infiltration leads to the formation of Mode I cracks, dynamic recrystallization, and, at pressures above 7-8 GPa, the formation of DHMS (Dense Hydrous Magnesium Silicate). This increase in permeability results in a decrease in dehydration temperature compared to when no deformation is applied.
These results imply that antigorite in deforming slab peridotite can undergo dehydration under conditions where IDEQs occur. We also detected some acoustic emission activities associated with the antigorite dehydration during the deformation runs. This likely occurs not in the antigorite region but in the surrounding peridotite, consisting of olivine and pyroxenes. Thus, although further investigations are necessary, deformation-induced fluid infiltration into peridotite lowers the dehydration temperature and embrittles the surrounding nominally anhydrous minerals, which could be an important process for IDEQs across a wide range of pressures.