Absence of Stress-induced Anisotropy during Brittle Deformation in Antigorite Serpentinite

TL;DR

This study reveals that antigorite serpentinite exhibits no stress-induced seismic anisotropy during brittle deformation, with microcracking occurring only near faults and primarily through shear along crystal cleavage planes.

Contribution

It provides the first experimental evidence that antigorite's brittle deformation involves shear microcracking without stress-induced anisotropy, contrasting with other crystalline rocks.

Findings

No stress-induced anisotropy observed in antigorite during deformation

Microcracking is localized and occurs along cleavage planes

Antigorite's seismic response during deformation is unique among crystalline rocks

Abstract

Knowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress-induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behaviour is elastic up to high stress ($\gtrsim80$\% of rock strength) and non-dilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at $90-95$\% of their compressive strength. Microcrack damage is extremely localised near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by "mode II" shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite is comparable to that in granite. The mechanical behaviour and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.