Detection of high P,T transformational faulting in Fe2SiO4 via in-situ acoustic emission: Relevance to deep-focus earthquakes

At transition zone depths in subduction zones, deep-focus earthquakes (300-690 km depth) are thought to be associated with faulting that arises from phase transformations. In order to test the viability of this mechanism experimentally, an investigation was conducted on fayalite at high pressure, P, and high temperature, T, under deviatoric stress in order to initiate transformational faulting. Experiments were performed in a 3000-ton multianvil press using an 18/11 octahedral cell with 6 piezoelectric transducers mounted on the rear side of the anvils to monitor acoustic activity in situ. Acoustic emission (AE) signals were collected at a sampling rate of 40 MHz using a triggered system and a data buffer for capturing full waveforms of AE events. The use of multiple transducers distributed in a micro-seismic array allowed for events to be located within the sample based on the arrival time of signals and non-linear least squares inversion techniques. Uncertainty in location estimates were on the order of similar to 1 mm. The system was tested by comparing the contrasting mechanical properties of quartz beads and AgCl samples. The multi-anvil apparatus constitutes an inherently noisy environment both acoustically and electrically, therefore methods of noise reduction were developed. Results of AE experiments on Fe2SiO4 under high pressure (3.7-9.2 GPa) and high temperature (673-1273 K) conditions in the spinel stability field showed acoustic events that locate within or within lo of the sample in five experiments defined by the P,T envelope P = 3.9-8.4 GPa and T = 748-923 K. Optical and scanning electron microscopy of the recovered specimens displayed conjugated faulting associated with transition from fayalite (olivine structure) to ahrensite (spinel structure) and microstructural analysis revealed microstructural morphology identical to that found in similar faulting experiments on Mg2GeO4. This is the first time an olivine- spinel structured transition in a silicate mineral has demonstrated macroscopic faulting and associated microstructures, as well as acoustic activity, under conditions that nominally promote plastic deformation.