Microstructural Heterogeneity May Explain Missing Seismic Signature in Upper-Mantle Grain-Boundary Sliding
A new computational study suggests that a broad distribution of grain-boundary viscosities in olivine can suppress and spread a predicted seismic attenuation peak into an undetectable background signal. Classical theory predicts a sharp, localized Debye-like peak from elastically accommodated grain-boundary sliding (EAGBS), but this peak is absent or weak in dry olivine laboratory experiments. The findings imply that EAGBS may still operate in Earth's upper mantle even when it leaves no clear experimental fingerprint, with implications for interpreting seismic wave attenuation and velocity.
Researchers have submitted a study to the Journal of Geophysical Research: Solid Earth proposing a resolution to a long-standing discrepancy between theory and experiment in upper-mantle seismology. Elastically accommodated grain-boundary sliding (EAGBS) is a candidate mechanism for seismic attenuation and dispersion in the upper mantle, but classical models predict a narrow Debye-like loss peak that is not clearly observed in dry olivine experiments. Using 2-D finite-element simulations on periodic Voronoi tessellations — which mimic realistic polycrystalline microstructures — the authors tested whether microstructural heterogeneity could account for this mismatch. They found that irregular grain geometry alone alters the baseline EAGBS response modestly, and increasing grain-size variance produces only small changes in peak height or spectral width. However, introducing a broad distribution of grain-boundary viscosities progressively suppresses and broadens the loss peak into a weak, wide-frequency background, arising from the superposition of many relaxation processes with different characteristic timescales. The authors conclude that the absence of a pronounced EAGBS peak in experiments does not rule out the mechanism itself; if grain boundaries span a wide viscosity range, the EAGBS contribution may be experimentally invisible while still influencing upper-mantle seismic properties.
What's missing
The study is a preprint submitted but not yet peer-reviewed. The simulations are 2-D and use idealized Voronoi geometries; the authors do not address whether 3-D effects or more realistic grain-boundary chemistry would alter the conclusions. The physical origin and plausible natural range of grain-boundary viscosity distributions in actual mantle olivine are not constrained by the model itself, leaving open whether the broad viscosity distributions invoked are geologically realistic. The study also does not directly compare simulated spectra against specific experimental datasets to quantitatively test the proposed explanation.
What different sources said
- arXiv physicsCenter
Effects of microstructural heterogeneity on the macroscopic spectrum of elastically accommodated grain-boundary sliding
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