Investigation of High Pressure Faulting Associated with the Olivine Spinel Transition

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The underlying failure mechanism responsible for deep-focus earthquakes has remained unknown since their discovery almost 70 years ago. High pressure faulting was discovered in specimens of Mg2GeO4 olivine deformed in the spinel stability field under conditions where the kinetics of the transformation become just possible on the time-scale of the experiment. Very fine-grained spinel is found in the fault zones. The PIs pose four major questions: (i) Are microanticracks unstable? (ii) What is the process by which mode -I anticrack link-up leading to through- going faults? (iii) Which transformation mechanism operates at the tips of anticracks? (iv) Does the anticrack mechanism operate in the x - B transformation? They will answer question (i) by instrumenting the high pressure apparatus and determining quantitatively the abundance, magnitude, time-dependence and wave form of acoustic emissions before and during failure. Questions (ii) and (iii) will be pursued by electron channeling scanning electron microscopy and transmission electron microscopy to determine the size, shape, orientation and spatial distribution of microanticracks and the grain-size and crystallographic orientation of spinel crystals within them with respect to the host olivine crystals. Question (iv) will be investigated by conducting deformation experiments on Mn2Geo4 olivine within the B stability field in HWG's unique 5GPa deformation apparatus.