The Role of Antigorite’s Anisotropic Thermal Conductivity in Slab Thermal Evolution

Antigorite is a phyllosilicate exhibits a marked mechanical anisotropy: the crystals are relatively stiff along the foliation plane (001), while being very compressible along the [001] crystallographic direction. Moreover, under shear deformation, antigorite develops a strong crystallographic preferred orientation (CPO) with the [001] direction oriented perpendicularly to the shear plane. Given its mechanical anisotropy, antigorite crystals hosted in subducting slabs are predicted to align their [001] direction perpendicular to the slab dip. Other physical properties of antigorite might show similar anisotropic behavior. Lattice thermal conductivity Λ, for example, is closely related to the elastic stiffness of a mineral. For this reason, we measured the in-plane Λ^[010], and cross-plane Λ^[001] thermal conductivities of antigorite using the Time-Domain Thermo-Reflectance (TDTR) technique^[1]. We found that Λ^[001] is ~2-3 times lower than Λ^[010], even at the high-P,T conditions of subducting slabs. To investigate the large-scale effects of antigorite’s anisotropic Λ we designed a finite difference heat diffusion model of a 2D vertically subducting slab, in which we prescribed the presence of a 3-km-thick layer of serpentinite. This insulating layer hinders the propagation of thermal energy toward the cold core of the slab. This effect would reduce the stability field of hydrous phases in the external part of the slab, thus promoting dehydration embrittlement. Furthermore, the presence of antigorite in pre-existing faults can trap the frictional heating inside shear zone, thus favouring the onset of thermal runaway. Potentially antigorite is a key mineral for the development of double seismic zones (DSZs).