Experimental deformation and recrystallization of olivine – processes and timescales of damage healing during postseismic relaxation at mantle depths
Abstract. Experiments comprising sequences of deformation (at 300 or 600 °C) and annealing at varying temperature (700 to 1100 °C), time (up to 144 h) and stress (up to 1.5 GPa) were carried out in a Griggs-type apparatus on natural olivine-rich peridotite samples to simulate deformation and recrystallization processes in deep shear zones that reach mantle depth as continuations of seismically active faults. The resulting olivine microfabrics were analysed by polarization and electron microscopy (SEM/EBSD, TEM). Core-and-mantle-like microstructures are the predominant result of our experiments simulating rapid stress relaxation (without or with minor creep) after a high-stress deformation event: porphyroclasts (> 100 μm) are surrounded by new grains comprising fragments and recrystallized grains with a wide range in size (2 to 40 μm). Areas with small grains (≤ 10 μm) trace former high-strain zones generated during initial high-stress deformation even after annealing at a temperature of 1100 °C for 70 h. A weak crystallographic preferred orientation (CPO) of new olivine grains is related to the orientation of the original host crystals but appears unrelated to the strain field. Based on these findings, we propose that olivine microstructures in natural shear-zone peridotites with a large range in grain size, localized fine-grained zones, and a weak CPO not related to the strain field are diagnostic for a sequence of high-stress deformation followed by recrystallization at low stresses, as to be expected in areas of seismic activity. We extended the classic Avrami-kinetics equation by accounting for time-dependent growth kinetics and constrained the involved parameters relying on our results and previous studies devoted to the kinetics of defect processes in olivine. Extrapolation to natural conditions suggests that the observed characteristic microstructure may develop within as little as tens of years and less than ten thousands of years. These recrystallization microstructures have a great diagnostic potential for past seismic activity because they are expected to be stable over geological timescales, since driving forces for further modification are not sufficient to erase the characteristic heterogeneities.