Phase Transformation Volume Change Control of Strain and Instability Mechanisms
Abstract
Phase transformations that are accompanied by significant change in volume self-organize the transformation process under stress to enhance strain and/or decrease stress. When the kinetics of the reaction are reasonably rapid, this occurs by nucleation and growth of phase(s) on grain boundaries of appropriate orientation to facilitate strain. When the kinetics of reaction are sluggish (ratio of nucleation rate to growth rate >> 1) and the reaction is polymorphic and exothermic, under specific conditions this process can lead to shearing instability in which the volume change of reaction drives runaway nucleation at stress concentrations leading to macroscopic faulting. Under these conditions, the detailed processes and microstructures produced depend on the sign of volume change but the failure is very similar whether volume change is positive or negative. The microstructures confirm that the mesoscale physics of this process (self-organization of primary features and nucleation/propagation of a fault) is the same as for brittle shear fracture, despite the large difference in the fundamental microscopic physics (formation of nanocrystalline microlenses instead of tensile cracks, and fault propagation by grain-boundary sliding rather than frictional sliding). When volume change is negative and the kinetics of reaction are less sluggish (ratio of nucleation rate to growth rate of order 1), nucleation of the new (denser) phase occurs preferentially on grain boundaries normal to maximum compression and new crystals nucleated on any orientation of grain boundary grow parallel to maximum compression. The result is stylolite-like volume loss normal to maximum compression. Observations of polyphase metamorphic rocks suggest that similar creep can occur during prograde metamorphism by nucleation of more-dense phases and less-dense phases on different populations of phase-boundary orientations such that the overall pattern achieves a macroscopic strain by volume transfer. The presence of metastable olivine in the mantle transition zone (MTZ; ~350-700km) of cold subducting slabs leads to a faulting instability that explains deep-focus earthquakes, including the upturn in earthquake frequency at the top of the MTZ and earthquake termination prior to slab entry into the lower mantle. To date metastable olivine has been found in 4 subduction zones: Izu-Bonin, Tonga, Marianas and Japan.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T41B..01G
- Keywords:
-
- 3612 MINERALOGY AND PETROLOGY / Reactions and phase equilibria;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures;
- 8004 STRUCTURAL GEOLOGY / Dynamics and mechanics of faulting;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones