Absence of Stress‐Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite

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 behavior is elastic up to high stress ( ≳80% of rock strength) and nondilatant. 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 localized 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 behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.

In Situ Quantitative Tensile Testing of Antigorite in a Transmission Electron Microscope

“…The present study involving in situ tensile deformation experiments in a transmission electron microscope emphasizes the prominent role played by GB sliding in the plastic behavior of antigorite at room temperature. This mechanism has not been very much considered so far, but its occurrence seems fully consistent with previous studies on antigorite at room temperature reported by Escartin et al () and David et al (). The structure and properties of grain boundaries are not well known in complex structures like phyllosilicates.…”

“…Although our experiments are conducted under very different conditions (small specimen size, no confining pressure), the mechanical data are very consistent. In particular, the cyclic loading experiments of David et al (), see their Figures 4 and 5, compare very well to ours in terms of yield stress and maximum stress to failure. The added value permitted by our experiments is to observe in situ the evolution of the microstructure in direct relationship with the mechanical response.…”

“…We present what is, to the best of our knowledge, the first characterization of the elastic (E and ν ), and viscoplastic properties of antigorite from strain measurements performed in situ in a TEM during tensile tests. In terms of mechanical properties at low strains, the tests yield to Poisson ratio values from 0.2 to 0.22 (see Table ), slightly lower, but which compare quite well to the value of 0.26 reported in the literature on bulk specimens (Bezacier et al, , Brillouin scattering data; David et al, , static compression data). For reasons clarified hereafter, we call this an “apparent” Poisson ratio.…”

“…Escartin et al () emphasize the importance of nondilatant brittle deformation in this regime by shear microcracking with no strong evidence for intragranular crystal plastic deformation. This conclusion is further supported by the recent study of David et al (). At higher pressure, up to 4 GPa, Hilairet et al () report a power law rheology (stress exponent 3) and microstructures indicative of distributed and, to some extent, intracrystalline deformation.…”

“…Escartin et al () used a gas confining medium and a solid confining medium (Griggs‐type) apparatus. David et al () conducted deformation experiments in an oil medium triaxial apparatus. Although our experiments are conducted under very different conditions (small specimen size, no confining pressure), the mechanical data are very consistent.…”