Activation of dissolution-precipitation creep causes weakening and viscous behaviour in experimentally-deformed antigorite

Antigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring-shear experiments on antigorite and antigorite-quartz mixtures at temperatures between 20 and 500 ◦C and 150 MPa effective normal stress. Pure antigorite 20 is strain hardening, with frictional coefficient (μ) > 0.5, and developed cataclastic microstructures. In contrast, antigorite-quartz mixtures (10 % quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200◦C to 0.22 at 500◦C. Antigorite-quartz mixtures developed foliation similar to natural serpentinite shear zones. Although reaction between antigorite and quartz is expected to form mechanically weak talc, we only find small, localised amounts of talc in our deformed samples, and mechanical data are inconsistent with reaction-dependent weakening. Instead, we propose that the observed weakening primarily results from silica dissolution leading to a lowered pore-fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution-precipitation creep. We suggest that under our experimental conditions, efficient dissolution-precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional-viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution-precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example in faults juxtaposing serpentinite with quartz-bearing rocks. Data is from hydrothermal ring shear experiments on simulated gouges of antigorite and antigorite-quartz. The data is provided in 12 subfolders for 10 experiments/samples. Detailed information about the files in these subfolders as well as information on how the data is processed is given in the explanatory file Data-Documentation_Antigorite.pdf.