Abstract:
Grain size is an important material property that has significant effects on the
viscosity, dominant deformation mechanism, attenuation, and shear wave
velocity of the oceanic upper mantle. Several studies have investigated the
kinetics of grain size evolution, but have yet to incorporate these evolution
equations into large-scale flow models of the oceanic upper mantle. We
construct self-consistent 1.5-D steady-state Couette flow models for the oceanic
upper mantle to constrain how grain size evolves with depth assuming a
composite diffusion-dislocation creep rheology. We investigate the importance of
water content by examining end-member models for a dry, wet, and dehydrated
mantle (with dehydration above ~60-70 km depth). We find that grain size
increases with depth, and varies with both plate age and water content.
Specifically, the dehydration model predicts a grain size of ~11 mm at a depth of
150 km for 75 Myr-old oceanic mantle. This results in a viscosity of ~1019 Pa s,
consistent with estimates from geoid and glacial rebound studies. We also find
that deformation is dominated by dislocation creep beneath ~60-70 km depth, in
agreement with observations of seismic anisotropy in the oceanic upper mantle.
The calculated grain size profiles are input into a Burger's model system to
calculate seismic quality factor (Q) and shear wave velocity (Vs). For ages older
than 50 Myrs, we find that Q and Vs predicted by the dehydration case best
match seismic reference models for Q and the low seismic shear wave velocity
zone (LVZ) observed in the oceanic upper mantle.
