Hydrostatic Dehydration Fabrics of Antigorite at High Pressure and High Temperature: Implications for Trench Parallel Seismic Anisotropy at Convergent Plate Boundaries

Abstract

Antigorite dehydration is well known as a key process at convergent boundaries for the genesis of mantle wedge partial melting and intermediate‐depth earthquakes. However, the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and their effects on the seismic anisotropy of subducting slabs remain ambiguous and controversial. Here we report antigorite dehydration experiments on a foliated serpentinized peridotite at pressures of 0.3–6 GPa and temperatures of 700–900°C. Our results show that olivine CPO evolves from a fiber‐[100] type in fine grains to a type‐C like in coarse grains. We propose that the fiber‐[100] olivine CPO is developed by topotactic growth during incipient hydrostatic dehydration, while the orthorhombic type‐C like olivine CPO is developed by anisotropic growth resulting from an anisotropic fluid flow during later dehydration. The type‐C like olivine CPO in the antigorite‐rich layers after antigorite dehydration could contribute to the trench parallel fast shear wave seismic anisotropy at convergent plate boundaries.

Type‐B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Regions

“…Additionally, we observed an alignment of olivine [001] axes parallel to the lineation in the fine‐grained olivine in our current study (Figure 4). This fabric was also noted in the coarse‐grained olivine in our previous quasi‐static, drained dehydration experiments (Liu et al., 2021). During the dehydration of antigorite under drained conditions, the anisotropy in permeability results in the slowest fluid migration direction being normal to the foliation (Kawano et al., 2011) and the fastest flow occurring along the lineation (Gualtieri et al., 2012; Liu et al., 2021).…”

“…This fabric was also noted in the coarse‐grained olivine in our previous quasi‐static, drained dehydration experiments (Liu et al., 2021). During the dehydration of antigorite under drained conditions, the anisotropy in permeability results in the slowest fluid migration direction being normal to the foliation (Kawano et al., 2011) and the fastest flow occurring along the lineation (Gualtieri et al., 2012; Liu et al., 2021). Furthermore, the deviatoric stress, which distinguishes this study from our previous research (Liu et al., 2021), facilitates anisotropic mass transfer through a fluid phase (Karato & Masuda, 1989).…”

“…Further detailed EBSD analysis of their partially dehydrated natural antigorite‐olivine aggregate revealed that olivine grows topotactically with antigorite, following the crystallographic relationship of (100) ol ∥(001) atg . In our recent quasi‐static, drained dehydration experiments on a foliated antigorite schist under high‐temperature and high‐pressure conditions (700–900°C and 0.3–5 GPa), we also observed the clustering of [100] ol axes oriented normal to the foliation in the fine‐grained olivines (Liu et al., 2021). Consequently, according to this topotaxial relationship, it is expected that olivine [100] axes would cluster in a manner perpendicular to the foliation after the foliated antigorites, which initially have [001] axes concentrated normal to the foliation prior to dehydration (Figure 7a).…”

“…During the dehydration of antigorite under drained conditions, the anisotropy in permeability results in the slowest fluid migration direction being normal to the foliation (Kawano et al., 2011) and the fastest flow occurring along the lineation (Gualtieri et al., 2012; Liu et al., 2021). Furthermore, the deviatoric stress, which distinguishes this study from our previous research (Liu et al., 2021), facilitates anisotropic mass transfer through a fluid phase (Karato & Masuda, 1989). Consequently, [001] axis, which is the fastest growth axis in olivine in the presence of fluids (Miyazaki et al., 2013; Wen et al., 2018), experiences accelerated growth and clusters along the lineation due to the higher fluid pressure gradient in this direction, resulting from anisotropic permeability (Connolly & Podladchikov, 1998; Llana‐Fúnez et al., 2012) and deviatoric stress (Figure 7a).…”

“…While it has been documented that antigorite dehydration can induce the type‐B olivine CPO in naturally deformed serpentinized peridotites (Clément et al., 2019; Dilissen et al., 2018; Nagaya et al., 2014), this process has not been experimentally demonstrated under the pressure‐temperature conditions relevant to the forearc mantle. Experimental investigations into olivine CPO formation by antigorite dehydration, conducted under (un)drained, near‐hydrostatic conditions, reported either a robust [100]‐fiber olivine fabric (Padrón‐Navarta et al., 2015) or a strong type‐C‐like olivine fabric (Liu et al., 2021). However, in dynamic environments such as active subduction zones, where intense antigorite dehydration occurs (Peacock, 2001; Plümper et al., 2017), deformation and fluid migration are expected to be critical factors influencing the nature of the dehydration reaction.…”