Implications of grain size evolution on the seismic structure of the oceanic upper mantle
Date
2009-03-04Metadata
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https://hdl.handle.net/1912/2859As published
https://doi.org/10.1016/j.epsl.2009.03.014Abstract
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.
Description
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 282 (2009): 178-189, doi:10.1016/j.epsl.2009.03.014.
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Suggested Citation
Preprint: Behn, Mark D., Hirth, Greg, Elsenbeck, James R., "Implications of grain size evolution on the seismic structure of the oceanic upper mantle", 2009-03-04, https://doi.org/10.1016/j.epsl.2009.03.014, https://hdl.handle.net/1912/2859Related items
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