Olivine and enstatite formation during seismic faulting in serpentinite: an experimental approach

Olivine and enstatite, formed by dehydroxylation of serpentine, have been naturally and experimentally documented as evidence of paleo-earthquake rupture propagation within natural serpentinite-bearing slip zones. To investigate the rheological and textural evolution during dehydroxylation of serpentinite, we performed rotary-shear friction experiments on water-saturated serpentinite powders using drained and undrained conditions (where drained conditions allow for excess fluid pressure to escape the gouge holder). Using a purpose-built gouge sample holder containing a thermocouple 1.5 mm from the eventual principal slip zone (PSZ), the experiments were performed at a seismic slip rate (1 m/s) and 10 MPa normal stress. Mechanical results show that in undrained experiments, the apparent friction coefficient (μ) initially reaches a peak value of ~0.21-0.24, followed by dramatic weakening to a steady-state value of 0.12-0.09, associated with gouge compaction, while the temperature at the thermocouple steadily increased reaching a maximum of ~180°C. In drained experiments, a plateau-like friction coefficient with a value of ~0.42 was reached, associated with gouge compaction at a steady temperature of ~250°C at the thermocouple, followed by a drop to a steady-state value of ~0.19, associated with gouge dilation at a temperature of ~450°C. The friction coefficient then gradually increased, reaching a value of ~0.3 (i.e. restrengthening) with gouge compaction and a max. temperature at the thermocouple of ~635°C by the end of the experiment. The PSZ of the products were examined by scanning electron microscope, in-situ synchrotron X-ray diffraction, and focused ion beam transmission electron microscope. Microanalysis showed no mineral phase changes in undrained experiments, which we interpret to indicate that fluid vaporization and pressurization buffered the temperature to below that required for serpentinite dehydroxylation. However, the PSZ in drained experiments contains well-developed aggregates of nanometric, rounded to polygonal forsterite + enstatite, which provide evidence for serpentinite dehydroxylation at temperatures of >600°C within the PSZ. Our observations indicate that fluid drainage facilitates a significant temperature increase within the gouge layer at seismic slip rates. We conclude that dehydroxylation of natural serpentinite gouges may occur under relatively dry conditions or when co-seismic permeability increases (e.g. due to fracturing) allow for efficient fluid drainage and decrease the efficiency of thermal pressurization.