HR: 09:45h
AN: V41I-08    [Abstracts]
TI: Grain-scale stress and strain patterns in elastically deforming partially molten rock using a FEM: Estimation of effective moduli and seismic velocity
AU: * Hersum, T G
EM: hersum@ldeo.columbia.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: * Hersum, T G
EM: hersum@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Acton, K A
EM: kacton1@jhu.edu
AF: Department of Civil Engineering, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Graham-Brady, L L
EM: lori@jhu.edu
AF: Department of Civil Engineering, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AU: Marsh, B D
EM: bmarsh@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 United States
AB: Interpretation of the structures and processes within partially molten regions of the Earth gained through seismic inversion techniques rely on the accurate determination of seismic properties, including seismic velocity, of partially molten rocks. Estimates of seismic velocity in partially molten rocks have been made both experimentally, but at seismic frequencies up to six orders of magnitude greater than the dominant seismic frequencies that typically occur in nature, and through numerical modeling of idealized igneous microstructures with simplified pore geometry and topology. An alternative method is presented here that uses a finite-element method (FEM) to calculate local stresses and displacements within digitized igneous microstructures in response to edge displacement boundary conditions. The microstructures are assumed here to behave as poly-phase elastic composites and two independent types are tested: 1) a microstructure simulated using a stochastic algorithm for progressive crystallization, and 2) a natural microstructure obtained through X-ray computed tomography of a partially melted basalt. Specific elastic moduli are assigned to elements that represent either the solid or melt phase. The effective anisotropic elastic moduli of the composite microstructure and, hence, anisotropic seismic velocities, are determined by recovering averaged stress and strain tensors over the entire domain of the sample. Seismic velocities are determined over both a wide interval of melt fraction as well as a range in `seismic frequency' by introducing non-zero rigidity of the melt phase. For both types of microstructure, local von Mises stress maxima are concentrated dominantly in the stiffer solid phase particularly in regions at or near percolation. The model, however, ignores possible grain-to-grain debonding and instead assumes material contiguity across the grain boundary. Provided such an assumption and yield strength criteria for the solid phase, local von Mises stress maxima are used to infer locations of failure of the crystalline network and, hence, characterize patterns of damage as a function of melt fraction. Such information is useful in understanding high strain rate deformation and failure of partially molten rock in light of processes driving chemical differentiation of magmas and in volcanic eruption phenomenon.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005