Seismic Anisotropy in Subduction Zones: Evaluating the Role of Chloritoid

Abstract

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs), are often invoked to explain these observations. However, the elasticity and the LPO of chloritoid, which is one of such hydrous phases relevant in subduction zone settings, are poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples, obtained the LPO-induced seismic anisotropy, and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy with a maximum AVS of 18% and a P-wave anisotropy with an AVP of 10%. The role of chloritoid LPO in seismic anisotropy was evaluated in natural rock samples and a hypothetical blueschist. Our results indicate that the strong LPO of chloritoid along the subduction interface and in subducting slabs can influence the trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.

Pore and Mineral Fabrics Control the Elastic Wave Velocities of Metapelite With Implications for Subduction Zone Tomography

“…The Orocopia schist is composed of 53% phyllosilicates, which is a class of minerals that have anisotropic wave speeds and low wave speeds normal to their basal planes (Lee et al., 2021; Mookherjee & Mainprice, 2014; Sayers & den Boer, 2018). The Orocopia schist has a well‐defined foliation due to aligned phyllosilicates that contribute to anisotropic velocities and low wave speeds normal to foliation.…”

“…Although there are no measurements of metasedimentary rock velocities at the relevant conditions, previous experimental and theoretical work indicates that a number of features in these rocks may lead to distinct velocity signatures. The lattice preferred orientation of phyllosilicates often causes anisotropy with fast velocities parallel to the foliation plane and slow velocity normal to foliation (Christensen, 1965(Christensen, , 1966Kern & Wenk, 1990;Lee et al, 2021;Mainprice & Ildefonse, 2009;Mookherjee & Mainprice, 2014;Song & Suh, 2014;C. Wang et al, 1975).…”

“…Although there are no measurements of metasedimentary rock velocities at the relevant conditions, previous experimental and theoretical work indicates that a number of features in these rocks may lead to distinct velocity signatures. The lattice preferred orientation of phyllosilicates often causes anisotropy with fast velocities parallel to the foliation plane and slow velocity normal to foliation (Christensen, 1965, 1966; Kern & Wenk, 1990; Lee et al., 2021; Mainprice & Ildefonse, 2009; Mookherjee & Mainprice, 2014; Song & Suh, 2014; C. Wang et al., 1975). We also know that pore space has a significant influence on , , and , through not only the magnitude of porosity, but also the shape and preferred orientations of the pores (Berryman et al., 2002; Birch, 1960, 1961; Brantut & David, 2018; O’Connell & Budiansky, 1974; Walsh, 1965; X. Q. Wang et al., 2012).…”

Predictive modelling of seismic properties in single-foliated slates

“…Specifically, we considered the Crystallographic Preferred Orientations (CPO) and the relative volumetric contributions of major slate-forming mineral phases, which typically include quartz, muscovite, chlorite, and plagioclase, and other phases that constitute more than 2% of the volume. The used single-crystal elastic properties are listed in Table S2 in the Supplementary Material (Brown et al, 2006;Lee et al, 2021;Mao et al, 2015;Militzer et al, 2011;Mookherjee and Mainprice, 2014;Sang and Bass, 2014;Satta et al, 2022;Waeselmann et al, 2016;Wang et al, 2015).…”

Anomalous Sound Velocities of Talc at High Pressure and Implications for Estimating Water Content in Mantle Wedge

“…However, talc has a very strong anisotropy due to its special crystal structure (Mainprice et al., 2008; Peng et al., 2022), which is the largest compared with other layered hydrous silicates, such as chlorite and serpentine (Peng et al., 2022). Moreover, a very strong crystallographic preferred orientation (CPO) is formed in the boundary between the subducting slab and the mantle wedge (Lee et al., 2021). This makes it more difficult to explain the results of seismic tomography based on the elastic properties of the talc, which needs further research.…”