Does Heterogeneous Strain Act as a Control on Seismic Anisotropy in Earth’s Lower Mantle?

Abstract

Plastic deformation and texture development in minerals of the lower mantle can result in seismic anisotropy, and studying deformation of lower mantle materials is therefore important for interpreting lower mantle flow. Most previous deformation experiments documenting texture development at lower mantle pressures have been conducted on single-phase samples and/or at room temperature. However, real rocks deform at high temperature and are poly-phase and deformation is therefore likely different from that of a single phase. Here we report on high temperature diamond anvil cell deformation experiments on a multiphase assemblage of bridgmanite, ferropericlase, and ringwoodite compressed from ∼28 to ∼39 GPa and resistively heated at a constant temperature of 1,000 K. We employ the elasto-viscoplastic self-consistent method to model both texture and lattice strain of bridgmanite as a function of deformation mechanisms. Simulations indicate deformation of bridgmanite is accommodated by about half of slip activity on (100)[010] with the remainder split between (100)[001] and/or (100)〈011〉. Texture in bridgmanite is consistent with most seismic observations in the lowermost mantle. Although there is texture development in both bridgmanite and ringwoodite, ferropericlase does not develop coherent texture throughout the course of the experiment. Analysis of lattice strains suggests that the lack of coherent texture development in ferropericlase is due to heterogeneous plastic deformation resulting from microstructural interactions imposed by other phases. Variations in texturing of bridgmanite and ferropericlase could therefore cause laterally varying, complex anisotropy. Our models for binary mantle-like mixtures of bridgmanite and ferropericlase show that changes in strain and texture partitioning can explain the range of observed lower mantle anisotropies.

Deformation of NaCoF<sub>3</sub> perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism

“…It would be interesting to study the effect of temperatures at these lower pressures to constrain the effect of temperature on the plasticity of perovskite phases. Further studies on analogues might also include the investigation of post-perovskite deformation textures in NaCoF 3 and other analogues, as well as potential post-post-perovskite phases (Crichton et al, 2016;Umemoto and Wentzcovitch, 2019). A general understanding of plasticity in perovskite and post-perovskite will help with improving the interpretation of seismic models in terms of mantle dynamics such as heat transfer through convection.…”

Viscosity of bridgmanite determined by in situ stress and strain measurements in uniaxial deformation experiments

“…On the other hand, stress from (002) and (110), which is also useful to estimate the dominant slip system by the elastic-viscoplastic self-consistent (EVPSC) ( 19 ), cannot be calculated in this study because (002) + (110) doublet cannot be separated as shown in fig S5. The EVPSC models of a single dominantly active slip system with (100) slip plane up to sample strain of 20% ( 19 ) suggested that stress from (200) was twice larger than that from (111) in (100)[010] slip system and that the stress difference between (200) and (111) by (100)[001] slip system is smaller than that by (100)[010] slip system. In this study, obvious stress difference along (111), (112), and (200) was not observed at steady-state creep.…”

“…To determine the viscosity of bridgmanite in the dislocation creep region, we carried out in situ stress and strain measurements of MgSiO 3 bridgmanite during uniaxial deformation at temperatures of 1473 to 1673 K and pressures of 23 to 27 GPa using the Kawai-type cell assembly with both the deformation DIA–type apparatus (D-DIA), hereafter we called as “KATD” ( 17 , 18 ), and the deformation-111 (D111) apparatus ( 19 , 20 ) at the synchrotron x-ray radiation facility (Materials and Methods).…”

“…The viscosity-depth models of Earth's mantle proposed from many geophysical observations (1)(2)(3)(4) indicate that the lower mantle has the largest viscosity among all the mantle layers. Viscosities of the transition zone and that of the top of the lower mantle have been assessed 10 [19][20][21] and 10 [21][22] Pa•s, respectively, generating a viscosity contrast of one to two orders of magnitude. To interpret the observed viscosity profiles of Earth's mantle in terms of mineral physics, it is indispensable to constrain the rheological properties of the constituent minerals, especially those of bridgmanite, which is the most dominant mineral in the lower mantle.…”

Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

“…For Model E, we used single-crystal elastic constants appropriate for bridgmanite (Stackhouse et al, 2005); when interpreting our models in terms of deformation geometry, we assumed dominant slip on [010](100) (Couper et al, 2020;Tsujino et al, 2016). Model F invoked ferropericlase as the cause for lowermost mantle seismic anisotropy, using single-crystal elastic constants from Karki et al (1999) and assuming a dominant slip system [100](001).…”