Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geophysics

Supervisor

Dr. Richard Secco

Abstract

In subduction zones, earthquakes are thought to be associated with faulting that arises from phase transformations. In order to test the viability of this mechanism experimentally, it was necessary to make microseismic measurements while the mineral under investigation was subjected to the pressure and temperature (P,T) conditions corresponding to their environment at depth. A system has been developed capable of making in situ acoustic emission (AE) measurements on samples under P,T conditions representative of the upper mantle and transition zone. Experiments were performed in a 3000-ton multi-anvil press using an 18/11 octahedral cell with 6 piezoelectric transducers mounted on the rear side of the anvils. 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 microseismic array allowed for events to be located within the sample through manual or automatic arrival time picking and least squares inversion techniques. Uncertainty in location estimates was ~1mm. The multi-anvil apparatus constitutes an inherently noisy environment both acoustically and electrically, therefore methods of noise reduction were developed. This technique has been used to measure acoustic signals generated from the fracturing of quartz beads during high pressure deformation and to investigate the possibility that the phase transformation from olivine to spinel, known to occur in subduction zones, is associated with deep-focus earthquakes (300 – 690 km depth). The analog material fayalite (Fe2SiO4) was examined. Information about its synthesis and sintering is discussed. Results of AE experiments on samples under high pressure (P = 4-9 GPa) and high temperature (T = 773-1273 K) conditions in the spinel stability field, while experiencing deviatoric stress, showed acoustic events that locate within the sample in multiple experiments defined by the P,T envelope P = 3.8 – 8.4 GPa and T = 650 – 950 K. This is the first time an olivine→spinel structured transition in a silicate mineral has demonstrated macroscopic faulting and associated microstructures, as well acoustic activity, under conditions that would normally promote plastic deformation. The system was also used to detect liquid↔solid phase transformations in Hg by measuring the abrupt change in sound velocity due to the intrinsic change in velocity between phases, and a change in delay between the triggering of an amplitude threshold and the arrival of the waveform.

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