Dehydration softening of serpentine as a trigger of intermediate-depth earthquakes

A popular hypothesis for the occurrence of double seismic zones in subducting slabs is dehydration embrittlement of serpentinized mantle. Deformation experiments of serpentinites using gas-medium apparatus demonstrated the role of pore pressure in the ductile-to-brittle transition at the dehydration temperature (e.g., Raleigh and Paterson, 1965, JGR). However, it is questionable if the same mechanism could be effective in subducting slabs at the depth. Deformation experiments of serpentinites have been also conducted at higher pressure using multi-anvil and Griggs-type apparatus but little is known about the effects of dehydration reaction on the mechanical behavior of serpentinite. We conducted deformation experiments of antigorite-serpentinite (Oeyama ultramafic body, Japan). Cylindrical samples of serpentinite with the diameter of 10 mm and the length of 15 mm were jacketed in Ag tubes and disks. "Slow" and "fast" experiments were conducted at strain rates of 3.3x10^-5 sec ^-1 and 2x10^-4 sec^-1, respectively. Axial compression tests were conducted at 800 MPa confining pressure using a solid-medium deformation apparatus. The dehydration temperature is about 650 ^oC at this pressure. Antigorite was hard at 500^oC and not yielded up to 900 MPa differential stress. The experimental run at 700^oC without pre-heating is characterized by strain hardening. The sample was deformed by foliation-parallel slip, kinking, and micro-faulting of antigorite. On the contrary, samples deformed at 700^oC after static heating showed drastic weakening and steady creep behaviors. A velocity step test indicated that the flow stress is insensitive to the strain rate. The deformed samples contain forsterite and enstatite in the antigorite matrix. Antigorite changed in color from dark green to pink, possibly due to highly oxidized atmosphere resulting from free water release. Intergranular pores were well developed. No microcracks or microfaults were observed. No evidence for intracrystalline slip was obtained in the SEM-EBSD analysis. Significant volume loss in the samples suggests compaction and escape of water during deformation experiments. Mechanical behaviors and microstructural features of the pre-heated samples both indicated that the dominant deformation mechanism was cataclastic flow and compaction of reaction products. The reacted samples showed "dehydration softening" rather than "embrittlement". In contrast, the same antigorite-serpentinite deformed in a gas-medium apparatus at a low confining pressure (200 MPa) exhibited a semi-brittle behavior. This fact suggests that dehydration embrittlement is only effective at the shallow to middle crustal levels. In the subducting slabs, strain localization at the serpentinized mantle due to dehydration softening and high fluid pressure caused by pore collapse possibly trigger earthquakes in surrounding peridotite mantle.