Low-Frequency Attenuation in Polycrystalline Silicates and Silicate Partial Melts

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Cooper 0106620 The experimental measurement of the low-frequency attenuation behavior of polycrystalline olivine, of olivine-orthopyroxene and of olivine-based partial melts is the practical emphasis of this research. Of theoretical focus is a new hypothesis concerning the physical mechanism(s) and significance of the "high-temperature background" mechanical absorption, specifically, that this absorption relates to the diffusion-effected relaxations of grain and solid-state phase boundaries and particularly of (deformation-induced) subgrain boundaries. The physical implication, then, is that a small-amplitude, oscillatory variation in stress, one that is added to a nominally constant differential stress that is effecting dislocation creep in a material, is "sampling" the deformation-induced microstructure, specifically the network of subgrains. If this hypothesis is correct--and discerning this correctness is the long-range goal of the experimental work--one can envision an effective way to do attenuation-based "geophysical prospecting" of active tectonic regions, e.g., for levels of differential stress and, perhaps, strain, with a calibration based on laboratory experiments. Experiments in this (one-year) funding period will emphasize dynamic ("sub-resonant" oscillatory loading) and quasi-static (constant loading, i.e., creep), high-resolution torsional mechanical responses of carefully engineered aggregates of (ferromagnesian) olivine and of two-phase solid olivine-orthopyroxene. These will be studied as functions of (i) temperature (1150-1350 degC), (ii) frequency (10^[-4]-1 Hz) and (iii) grain size (5-20 micrometers; fine enough that [a] thermal-expansion-anisotropy effected grain/phase boundary cracking cannot occur and [b] [for the levels of differential stress employed] lattice dislocations are not nucleated in the course of mechanical testing). In addition, benchmarking deformation experiments, comparing the high-resolution ambient-pressure measurements with the mechanical response of the same material under a confining pressure of ~300 MPa will be performed.