Antigorite rheology? Experimental challenges and complicating observations

Abstract:

Characterizing the rheology of antigorite remains a challenge due to its strongly anisotropic structure and relatively low dehydration temperature. The efficacy of trading off temperature for strain-rate is limited and to date it remains unclear if antigorite can deform by fully plastic flow and, if so, by what rate-limiting mechanism(s).

We present results from general shear experiments on antigorite gouge conducted in a Griggs-rig at 1-2 GPa confining pressure, 300-500 °C and strain-rates from 10^-5/s to 10^-7/s. We observe a peculiar behavior in which strain becomes increasingly localized at higher temperatures. At 300 °C gouge samples are macroscopically ductile with strain accommodated by a dense network of small fractures. At 500 °C strain is fully localized to a single ~10 μm wide fracture. Fractures contained finer-grained foliated antigorite. Increasing pressure from 1-2 GPa had no effect on localization. The apparent friction associated with slip along the fractures decreased from 0.23 to 0.07 at 300 °C and 500 °C respectively. The rate-dependence was relatively low (with an effective stress exponent of ~36) and decreased with increasing temperature. These findings are consistent with previous observations on talc (Escartin et al., 2008) and further complicate to the proposed Peierls creep and power law creep flow laws for antigorite (e.g., Amiguet et al., 2012). The ‘thermal’ embrittlement observed in our experiments may explain some occurrences of seismicity in serpentinized rock at temperatures too cold for dehydration.