An official website of the United States government
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.
more »
« less
Deformation and Transformation Textures in the NaMgF3 Neighborite—Post-Perovskite System
The D″ region of the lower mantle, which lies just above the core–mantle boundary, is distinct from the bulk of the lower mantle in that it exhibits complex seismic heterogeneity and seismic anisotropy. Seismic anisotropy in this region is likely to be largely due to the deformation-induced texture (crystallographic preferred orientation) development of the constituent mineral phases. Thus, seismic anisotropy can provide a marker for deformation processes occurring in this dynamic region of the Earth. Post-perovskite-structured (Mg,Fe)SiO3 is believed to be the dominant mineral phase in many regions of the D”. As such, understanding deformation mechanisms and texture development in post-perovskite is important for the interpretation of observed seismic anisotropy. Here, we report on high-pressure diamond anvil cell deformation experiments on NaMgF3 neighborite (perovskite structure) and post-perovskite. During deformation, neighborite develops a 100 texture, as has been previously observed, both in NaMgF3 and MgSiO3 perovskite. Upon transformation to the post-perovskite phase, an initial texture of {130} at high angles to compression is observed, indicating that the {100} planes of perovskite become the ~{130} planes of post-perovskite. Further compression results in the development of a shoulder towards (001) in the inverse pole figure. Plasticity modeling using the elasto-viscoplastic self-consistent code shows this texture evolution to be most consistent with deformation on (001)[100] with some contribution of glide on (100)[010] and (001)<110> in NaMgF3 post-perovskite. The transformation and deformation mechanisms observed in this study in the NaMgF3 system are consistent with the behavior generally observed in other perovskite–post-perovskite systems, including the MgSiO3 system. This shows that NaMgF3 is a good analog for the mantle bridgmanite and MgSiO3 post-perovskite.
more »
« less
- PAR ID:
- 10534911
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Minerals
- Volume:
- 14
- Issue:
- 3
- ISSN:
- 2075-163X
- Page Range / eLocation ID:
- 250
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Seismic anisotropy is observed above the core-mantle boundary in regions of slab subduction and near the margins of Large Low Shear Velocity Provinces (LLSVPs). Ferropericlase is believed to be the second most abundant phase in the lower mantle. As it is rheologically weak, it may be a dominant source for anisotropy in the lowermost mantle. Understanding deformation mechanisms in ferropericlase over a range of pressure and temperature conditions is crucial to interpret seismic anisotropy. The effect of temperature on deformation mechanisms of ferropericlase has been established, but the effects of pressure are still controversial. With the aim to clarify and quantify the effect of pressure on deformation mechanisms, we perform room temperature compression experiments on polycrystalline periclase to 50 GPa. Lattice strains and texture development are modeled using the Elasto-ViscoPlastic Self Consistent method (EVPSC). Based on modeling results, we find that { 110 } ⟨ 1 1 ¯ 0 ⟩ slip is increasingly activated with higher pressure and is fully activated at ~50 GPa. Pressure and temperature have a competing effect on activities of dominant slip systems. An increasing { 100 } ⟨ 011 ⟩ : { 110 } ⟨ 1 1 ¯ 0 ⟩ ratio of slip activity is expected as material moves from cold subduction regions towards hot upwelling region adjacent to LLSVPs. This could explain observed seismic anisotropy in the circum-Pacific region that appears to weaken near margins of LLVSPs.more » « less
-
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.more » « less
-
Abstract Seismic anisotropy of the Earth's mantle has been mostly attributed to crystallographic preferred orientation (CPO) generated during subduction and convection of an anhydrous mantle. But some hydrous phases are also stable at mantle conditions. Here we present results from diamond‐anvil cell deformation experiments at high pressure and temperature on hydrous phases Egg [AlSiO3(OH)], δ [AlO(OH)] and hydrous stishovite [Si1‐nH4nO2], transformed from the clay mineral kaolinite. They develop strong CPO during axial compression, suggesting that they likely contribute to seismic anisotropy and heterogeneity in the mantle. Comparing experimental results with viscoplastic polycrystal plasticity models suggest that phase Egg deforms dominantly by (001) slip, δ by (010) slip and stishovite by {100} slip which could be incorporated in future models of mantle geodynamics.more » « less
-
Olivine lattice preferred orientation (LPO), or texture, forms in relation to deformation mechanisms such as dislocation creep and can be observed in the upper mantle as seismic anisotropy. Olivine is also mechanically anisotropic, meaning that it responds to stresses differently depending on the direction of the stress. Understanding the interplay between anisotropic viscosity (AV) and LPO, and their role in deformation, is necessary for relating seismic anisotropy to mantle flow patterns. In this study, we employ three methods to predict olivine texture (D-Rex, MDM, and MDM+AV) in a shear box model and a subduction model. D-Rex and MDM are two representative texture development methods that have been compared before, and our results are in line with previous studies showing that textures computed by D-Rex develop faster and are stronger and more point-like than textures calculated with MDM. MDM+AV uses the same isotropic mantle stresses and particle paths as D-Rex and MDM but includes the effect of AV for texture predictions. MDM+AV predicts a texture similar to MDM with a distinct girdle-like orientation for simple shear deformation or at low strain in the subduction model. At larger strains, MDM+AV’s textures are more point-like and stronger compared to the other two methods. The effective viscosity for MDM+AV drops by up to 60% in a shear box model and can be either strengthened or weakened relative to isotropic viscosity for different regions of the subduction model experiencing different patterns of deformation. Our results emphasize the significant role of AV in olivine texture development, which could substantially affect geodynamic processes in the upper mantle.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/min14030250
