A Library of Elastic Tensors for Lowermost Mantle Seismic Anisotropy Studies and Comparison With Seismic Observations

Abstract

The exact mechanism for lowermost mantle seismic anisotropy remains unknown; however, work on the elasticity and deformation of lower mantle materials has constrained a few possible options. The most probable minerals producing anisotropy are bridgmanite, postperovskite, and ferropericlase. While there is an extensive literature on the elasticity and deformation of lower mantle minerals, we create a comprehensive uniform database of D″ anisotropy scenarios. In order to characterize a range of the possible fabrics for D″ anisotropy, we carry out VPSC (visco-plastic self-consistent modeling) to predict textures for each proposed mineral and dominant slip system. We numerically deform each mineral under different geometrical scenarios: simple shear, pure shear, and extension. By using published single crystal elasticity values, we produce a library of 336 candidate elastic tensors. We used the elastic tensor library to revisit previously published D″-associated seismic anisotropy studies for crossing raypaths (Siberia, North America, the Afar region of Africa, and Australia). While we cannot identify a single, unique mechanism that explains all of these data sets, we find that postperovskite (dominant slip on [100](010) or [100](001)) and periclase (dominant slip on {100}<011>) provide the best fit to the observations and suggest reasonable shear directions for each region of interest. Bridgmanite generally provides a poor fit to the observations; however, we cannot completely rule out any particular model. As the number of anisotropy observations for D″ increases, this elastic tensor library will be helpful for observational seismologists in identifying possible mechanisms of anisotropy and shear directions at in the lowermost mantle.Plain-Language Summary At 2,800 km below the Earth's surface, minerals are being deformed under the high pressures, temperatures, and stresses of the deep mantle. A seismic phenomenon (seismic anisotropy) has been observed within this region, likely due to the deformation of some unknown mineral. There have been three proposed minerals based on experimental and theoretical work. As a result, we create a library of proposed mechanisms of this anisotropy by numerically calculating a series of plausible deformed rocks with a code called VPSC (viscoplastic self-consistent modeling). We have established an open-source library of elastic tensors (plausible deformed rocks) for all of the proposed minerals. We compare this library to data that have been previously observed near the core-mantle boundary (Siberia, North America, the Afar region of Africa, and Australia). We find that some of the tensors cannot consistently fit all of the available data sets (such as bridgmanite-the most abundant mineral in the lower mantle). However, two of the minerals can fit all of the data quite well (postperovskite and ferropericlase). Postperovskite is a result of bridgmanite changing its structure due to the high pressures and temperatures, approximately 200 km above the core-mantle boundary, and the lowe...

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

“…The single crystal elastic shear anisotropy of pPv is significantly greater than bridgmanite (Wentzcovitch et al, 2006), and thus pPv has the potential to produce larger bulk anisotropy. Indeed, calculations of texturing and bulk anisotropy for pPv are broadly consistent with seismic observations in the D" (Creasy et al, 2020;Miyagi et al, 2010;Wu et al, 2017; see also Romanowicz and Wenk 2017 for a review) and this further supports its presence in the D" and its importance to lowermost mantle anisotropy. However, variations in the estimations of both core temperatures and the bridgmanite-to-pPv Clapeyron slope will change the depth of the stability field of pPv (Spera et al, 2006;Hernlund and Labrosse, 2007).…”

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

“…While our modeling favors the CPO of post‐perovskite or bridgmanite as the more likely scenarios, we cannot completely rule out CPO of ferropericlase as a possible mechanism, and further testing of more realistic ferropericlase CPO scenarios will be needed. Interestingly, when ferropericlase aggregates are deformed in experiments (e.g., Marquardt et al., 2009), or when ferropericlase textures are simulated using a VPSC approach, the bulk elastic properties are surprisingly similar to those of deformed post‐perovskite aggregates (Creasy et al., 2020), even though the single‐crystal elastic tensors are different (and have different symmetry). While we only examined single‐phase endmember models, the actual lower mantle is made up of multiple phases, and future work on the behavior of polyphase aggregates will be necessary to understand how textures develop in the real Earth.…”

“…The first set of models (Models A-F) were based on single-crystal elastic tensors; we assumed that 12% of the crystals were aligned and obtained a bulk elastic tensor by mixing them linearly with an isotropic equivalent (This approach is described in more detail in Thomas et al [2011] and Creasy et al [2019]). The second set of models (Models G-J) came from a published library of elastic tensors for lowermost mantle seismic anisotropy (Creasy et al, 2020) and were derived from VPSC modeling of post-perovskite deformed under simple shear with 100% strain, with various assumptions about the dominant slip systems.…”

“…This assumption is based on experimental work that has suggested that the [100] or {100} slip direction is preferred for the lowermost mantle (Amodeo et al, 2012;Cordier et al, 2012;Girard et al, 2012;Immoor et al, 2018). Models G-J consider CPO of post-perovskite, similar to Models A-D, but instead of elastic tensors based on single-crystal elasticity we instead use tensors calculated using VPSC for the simple shear of post-perovskite with 100% strain, from Creasy et al (2020). In order to calculate the reflection polarities for PdP and SdS for Models G-J, we removed the lower symmetry monoclinic and triclinic components (∼0.2% of each elastic tensor) from the VPSC-based elastic tensors using the decomposition method of Browaeys and Chevrot (2004), as implemented in the MSAT toolkit (Walker & Wookey, 2012).…”

“…[2019]). The second set of models (Models G–J) came from a published library of elastic tensors for lowermost mantle seismic anisotropy (Creasy et al., 2020) and were derived from VPSC modeling of post‐perovskite deformed under simple shear with 100% strain, with various assumptions about the dominant slip systems.…”