Semi-brittle behavior of wet olivine aggregates: the role of aqueous fluid in faulting at upper mantle pressures
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The role of aqueous fluid in fracturing in subducting slabs was investigated through a series of deformation experiments on dunite that was undersaturated (i.e., fluid-free) or saturated with water (i.e., aqueous-fluid bearing) at pressures of 1.0–1.8 GPa and temperatures of 670–1250 K, corresponding to the conditions of the shallower regions of the double seismic zone in slabs. In situ X-ray diffraction, radiography, and acoustic emissions (AEs) monitoring demonstrated that semi-brittle flow associated with AEs was dominant and the creep/failure strength of dunite was insensitive to the dissolved water content in olivine. In contrast, aqueous fluid drastically decreased the creep/failure strength of dunite (up to ~ 1 GPa of weakening) over a wide range of temperatures in the semi-brittle regime. Weakening of the dunite by the aqueous fluid resulted in the reduction of the number of AE events (i.e., suppression of microcracking) and shortening of time to failure. The AE hypocenters were located at the margin of the deforming sample while the interior of the faulted sample was aseismic (i.e., aseismic semi-brittle flow) under water-saturated conditions. A faulting (slip rate of ~ 10−3 to 10−4 s−1) associated with a large drop of stress (Δσ ~ 0.5 to 1 GPa) and/or pressure (ΔP ~ 0.5 GPa) was dominant in fluid-free dunite, while a slow faulting (slip rate < 8 × 10−5 s−1) without any stress/pressure drop was common in water-saturated dunite. Aseismic semi-brittle flow may mimic silent ductile flow under water-saturated conditions in subducting slabs.
KeywordsOlivine Aqueous fluid Acoustic emission Semi-brittle Fault
T.O. conceived the idea, conducted experiments, and wrote the manuscript. X.L. contributed to technical developments on the acoustic emissions measurement. T.S. and K.F. contributed to TEM observations. Y.H. and Y.T. assisted in situ experiments. We thank H. Ohfuji for his technical support for TEM observations and A. Nicolas for discussion. Official review by two anonymous reviewers improved the manuscript. This research was conducted under the approval of SPring-8 (Nos. 2017A0075, 2017B1184, and 2018A1717) and supported by the Grant-in-Aid for Scientific Research (Nos. 25707040, 16H01122, and 16H04077).
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