[S-CG43] Shallow and intermediate depth intraslab earthquakes: seismogenesis and rheology of the slab

[SCG43-P05] Deformation of metastable olivine under the conditions of Earth’s mantle transition zone: the role of olivine transformation on deep earthquakes

*Kohei Matsuda¹, Tomohiro Ohuchi¹, Yuji Higo², Yoshinori Tange² (1.Geodynamics Research Center, Ehime University, 2.Japan Synchrotron Radiation Research Institute)

Keywords:deep earthquakes, olivine phase transformation , in-situ measurements

The subduction zone produces a quite many of earthquakes which are divided into shallow (<40 km depth) earthquakes, intermediate-depth (>40 km) earthquakes and deep-focus (>300 km) earthquakes. The frequency of seismic activity becomes lower at greater depths due to the positive pressure dependency of frictional strength. Surprisingly, the frequency increases with depths at the depths in the mantle transition zone (e.g., Florich, 1989), suggesting that the mechanism of deep earthquakes is significantly different from that of shallow earthquakes. To investigate the mechanism of deep earthquakes, many experimental studies have been conducted using an analog material of germinate olivine at 1-2 GPa (Mg₂GeO₄: Green and Burnley, 1989) or fayalite at 4-9 GPa (Fe₂SiO₄: Officer and Secco, 2020). They reported that the olivine-spinel transformation under metastable condition could induce a faulting as a result of superplasticity due to grain size reduction along the fault. However, both germinate olivine and fayalite may not be an adequate analogue, because they directly transform to spinel phase and the amount of latent heat produced by their transformation reactions is much small than that for mantle olivine. Thus, we performed deformation experiments on metastable olivine at the pressures and temperatures of the mantle transition zone.
We conducted in-situ uniaxial deformation experiments on as-is olivine aggregates at pressures of 14-16 GPa and temperatures of 1050-1250 K with a constant displacement rate using a D-DIA apparatus at BL04B1/SPring-8. Pressure, stress, and strain were measured by using x-ray diffraction patterns and radiographs. Acoustic emissions (AEs) were also recorded by using six sensors during deformation, and the hypocenters of AEs were determined. Some off-line annealing experiments were also conducted at 15.5 GPa and temperatures of 950-1250 K using a D-DIA apparatus at GRC, Ehime Univ. Based on the results on annealing experiments, we estimated the lower limit temperature for nucleation of the high-pressure phases in ultrafine-grained domain such as fault gouge and damage zones.
We observed aseismic plastic deformation followed by faulting at temperatures ~1100 K. The faulting were terminated by an unstable slip, resulting in a blow-out. AEs were radiated around the timing of faulting. Microstructures of the recovered sample revealed that the fault gouge consists of nanoparticles of olivine, wadsleyite and a trace amount of platinum blobs, which are products of melting of the strain marker due to temperature ramping above 2200 K during the faulting. Off-line annealing experiments showed that the lower limit temperature for the nucleation of wadsleyite/ringwoodite in ultrafine-grained olivine aggregates is about 970 K, suggesting that the damage zone and gouge along the fault (i.e., pulverized domains) could be a preferential site for nucleation of wadsleyite/ringwoodite. Nucleation of such high-pressure phases may promote further grain size reduction of old olivine grains.