11:00 〜 13:00
[SCG44-P07] Antigorite grain boundary sliding accommodated by micro-cracking and dissolution-precipitation creep in subduction zone
キーワード：Serpentinite、Deformation mechanism、Grain boundary sliding、Accommodated process、Micro-cracking、Dissolution- precipitation creep
Geophysical and geological studies indicate subduction boundaries are associated with high fluid pressure and associated with hydration of the forearc mantle. Hydration of the relatively cold forearc mantle results in the formation of serpentinite composed mainly of antigorite and it is important to include understanding of the deformation mechanism of this mineral in discussions of the rheological properties of the subduction boundaries under the forearc mantle domain.
Proposed antigorite deformation mechanisms include dissolution-precipitation creep under lower P-T conditions; (e.g., Wassmann et al., 2011) and dislocation creep (e.g., Katayama et al., 2009; Padròn-Navarta et al., 2012). Other studies have emphasized the importance of the presence of a minor phase of weaker minerals such as talc and brucite, which are both formed by metasomatic reactions (e.g., Boschi et al, 2006; Moor & Lockner, 2013). Recent experimental and natural studies propose grain boundary sliding (GBS) of antigorite grains is also a potentially important deformation mechanism for antigorite serpentinite (Nagaya et al., 2018; Idrissi et al., 2020). To allow for grain boundary sliding to occur some accommodation processes is required to fill the gaps that are inevitably opened as irregular grains slide past one another. The presence even in very small amounts of minor minerals with the low coefficients of friction such as talc may also facilitate antigorite GBS.
In this study detailed EBSD studies were made of centimeter-scale antigorite shear zones from the antigorite schist reported by Nagaya et al. (2018), where the antigorite deformation and the shear zone formation due to antigorite GBS has been proposed. Some of the grains within the shear zones show micro-cracking.
EPMA analyses of antigorite shows micro-mixing of talc within antigorite grains in the shear zones. However, talc grains are not present at the grain boundaries of antigorite grains. This suggests that changes in the mineral chemical compositions of antigorite grains within the shear zones are related to chemical changes on the atomic scale. The presence of talc within the antigorite grains can be interpreted as the result of interaction with aqueous SiO2 and is indirect support for the operation of dissolution-precipitation creep. Such solution precipitation can be an effective accommodation process to allow GBS.
In this study, we propose that antigorite GBS is an important deformation mechanism under a wide range of P-T conditions, and that micro-cracking and dissolution-precipitation creep allow the initially interlocking grains of antigorite to slide past one another and undergo rigid-body rotation. Further constraints on the accommodation mechanisms and the P-T conditions under which different deformation mechanisms occur are important still unresolved issues that need to be addressed to quantitatively evaluate the deformation of antigorite serpentinite from natural and experimental studies.
Proposed antigorite deformation mechanisms include dissolution-precipitation creep under lower P-T conditions; (e.g., Wassmann et al., 2011) and dislocation creep (e.g., Katayama et al., 2009; Padròn-Navarta et al., 2012). Other studies have emphasized the importance of the presence of a minor phase of weaker minerals such as talc and brucite, which are both formed by metasomatic reactions (e.g., Boschi et al, 2006; Moor & Lockner, 2013). Recent experimental and natural studies propose grain boundary sliding (GBS) of antigorite grains is also a potentially important deformation mechanism for antigorite serpentinite (Nagaya et al., 2018; Idrissi et al., 2020). To allow for grain boundary sliding to occur some accommodation processes is required to fill the gaps that are inevitably opened as irregular grains slide past one another. The presence even in very small amounts of minor minerals with the low coefficients of friction such as talc may also facilitate antigorite GBS.
In this study detailed EBSD studies were made of centimeter-scale antigorite shear zones from the antigorite schist reported by Nagaya et al. (2018), where the antigorite deformation and the shear zone formation due to antigorite GBS has been proposed. Some of the grains within the shear zones show micro-cracking.
EPMA analyses of antigorite shows micro-mixing of talc within antigorite grains in the shear zones. However, talc grains are not present at the grain boundaries of antigorite grains. This suggests that changes in the mineral chemical compositions of antigorite grains within the shear zones are related to chemical changes on the atomic scale. The presence of talc within the antigorite grains can be interpreted as the result of interaction with aqueous SiO2 and is indirect support for the operation of dissolution-precipitation creep. Such solution precipitation can be an effective accommodation process to allow GBS.
In this study, we propose that antigorite GBS is an important deformation mechanism under a wide range of P-T conditions, and that micro-cracking and dissolution-precipitation creep allow the initially interlocking grains of antigorite to slide past one another and undergo rigid-body rotation. Further constraints on the accommodation mechanisms and the P-T conditions under which different deformation mechanisms occur are important still unresolved issues that need to be addressed to quantitatively evaluate the deformation of antigorite serpentinite from natural and experimental studies.