Role of pore fluid pressure on transient strength changes and fabric development during serpentine dehydration at mantle conditions: Implications for subduction-zone seismicity

- Method presented to drain pore fluid in sold-confining media apparatus at mantle conditions.
- Pore-pressure dependent transient slip and weakening occurred in dehydrating serpentinite.
- Dehydrating serpentine deforms via semibrittle flow.
- Intermediate-depth earthquakes in serpentinite are not caused by dehydration embrittlement.

To further investigate the dehydration embrittlement hypothesis and its possible link to subduction-zone seismicity, we conducted deformation experiments on antigorite serpentinite in a Griggs-type apparatus at conditions below and above antigorite stability. Temperature ramps (crossing the antigorite thermal stability) were used in conjunction with a new experimental method that allows fluid produced during dehydration reactions to be drained, partially drained or undrained. During temperature ramps, weakening coupled with transient slip initiated at ∼650°C, coincident with the predicted phase transition of antigorite to olivine and talc at ∼1GPa. The weakening-rate and steady-state strength were dependent on drainage conditions; undrained samples weakened over a few minutes and supported the lowest shear stress (∼50MPa), while drained samples weakened over a few hours and supported the highest shear stress (∼210MPa). The coefficient of friction (shear stress over normal stress) in drained samples decreased from ∼0.4 to ∼0.16 after the temperature ramp. The strengths of samples that were first annealed at 700 °C for ∼12h, then deformed, were similar to those observed in the temperature ramp experiments. Strain localization along fractures occurred in all samples during temperature ramping, regardless of the drainage conditions. However, microstructural observations indicate deformation by ductile mechanisms at higher strain under both undrained and drained conditions. The rheology and microstructures suggest dehydrating serpentinite deforms via semibrittle flow with grain-scale ductile deformation more active at high pore fluid pressures. Our results suggest that earthquakes in serpentinized mantle do not nucleate as a direct result of unstable frictional sliding along fractures generated at the onset of dehydration reactions.