Implications of grain size evolution on the seismic structure of the oceanic upper mantle

Abstract

We construct a 1-D steady-state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion-dislocation creep rheology. Grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth. Applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth, seafloor age, and mantle water content. The resulting grain size structure is used to predict shear wave velocity (VS) and seismic quality factor (Q). For a plate age of 60 Myr and an olivine water content of 1000 H/106Si, we find that grain size reaches a minimum of ~15 mm at ~150 km depth and then increases to ~20–30 mm at a depth of 400 km. This grain size structure produces a good fit to the low seismic shear wave velocity zone (LVZ) in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep. Further it predicts a viscosity of ~1019 Pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle, consistent with geophysical observations. We predict larger grain sizes than proposed in recent studies, in which the LVZ was explained by a dry mantle and a minimum grain size of 1 mm. However, we show that for a 1 mm grain size, diffusion creep is the dominant deformation mechanism above 100– 200 km depth, inconsistent with abundant observations of seismic anisotropy from surface wave studies. We therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle. Our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of ~1 mm.