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SUMMARY The presence of seismic anisotropy at the base of the Earth's mantle is well established, but there is no consensus on the deformation mechanisms in lower mantle minerals that could explain it. Strong anisotropy in magnesium post-perovskite (pPv) has been invoked, but different studies disagree on the dominant slip systems at play. Here, we aim to further constrain this by implementing the most recent results from atomistic models and high-pressure deformation experiments, coupled with a realistic composition and a 3-D geodynamic model, to compare the resulting deformation-induced anisotropy with seismic observations of the lowermost mantle. We account for forward and reverse phase transitions from bridgmanite (Pv) to pPv. We find that pPv with either dominant (001) or (010) slip can both explain the seismically observed anisotropy in colder regions where downwellings turn to horizontal flow, but only a model with dominant (001) slip matches seismic observations at the root of hotter large-scale upwellings. Allowing for partial melt does not change these conclusions, while it significantly increases the strength of anisotropy and reduces shear and compressional velocities at the base of upwellings.
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Forward modeling seismic anisotropy in the lower mantle through 3-phase aggregate deformation
In recent years there have been several attempts to make the link between mineral properties and seismic anisotropy in the D’’ region but have yet to reach consensus with regards to the dynamics in lower mantle minerals that could give rise to the observed seismic anisotropy. Here, we aim to provide further constraints on the observed long wavelength shear velocity patterns seen in seismic tomography studies. We introduce a forward model of deformation in a subducting slab as it impacts the core mantle boundary (D’’ layer) and proceeds to upwelling at the edge of a simulated LLSVP. By implementing the most recent results from atomistic modeling and high-pressure deformation experiments coupled with a 3-dimensional geodynamic model, we compare the microstructural evolution of an aggregate with a pyrolytic composition to the macroscopically observed seismic anisotropy of the lowermost mantle. We account for topotaxial relations in the forward and reverse phase transitions of MgSiO3-perovskite (Pv) to post-perovskite (pPv) within the slab as well as explore the effects introduced by partial melting near the CMB. Comparisons in the two leading candidate deformation mechanisms in the post-perovskite phase, (001) and (010), are compared. In this study we find that the reverse transition (pPv to Pv) occurs at a depth which is ~ 150 km deeper than that of the forward transition due to increasing temperature near the CMB providing a varying topography of the D’’ discontinuity. Our model also produces good fits with the isotropic velocities of PREM for the bulk lower mantle. When coupled with temperature and pressure dependent forward and reverse phase transitions, a pPv system with dominant (001) slip provides good correlation with the currently observed VSH fast horizontal (~ 1 – 6%) in D’’ and with VSV consistently fast in upwelling areas. Azimuthal variations along the streamline are also investigated showing a symmetry lower than that of the assumed VTI in D’’ introduced by ‘rolling’ effects near the slab’s edge. The addition of 1% partial melting at the CMB is shown to increase S and P wave anisotropy beneath the slab at the base of upwelling with up to ~2.5 & 4.0% P and S wave reductions respectively compared to the global reference.
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- Award ID(s):
- 2054951
- PAR ID:
- 10329114
- Date Published:
- Journal Name:
- Geophysical journal international
- Volume:
- 227
- Issue:
- 3
- ISSN:
- 0956-540X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi.org/10.1093/gji/ggab278
