Strain‐induced seismic anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone

Abstract

[1] We use forward models based on recent high-pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite, the dominant mineral in the upper transition zone. These models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition (60% wadsleyite, 40% garnet) at transition zone conditions: $2% for P and $1% for S waves for a shear strain of 1. Both P and S wave anisotropy patterns show an orthorhombic symmetry. P waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane. S wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction. Horizontal shearing results therefore in higher velocities for horizontally propagating P waves (PH ) and horizontally polarized S waves (SH ), as well as in weak azimuthal variation of SV and SH velocities. On the other hand, vertical flow leads to higher velocities for vertically propagating P waves (PV ) and vertically polarized S waves (SV) and to a weak azimuthal variation of SV velocity but to a roughly constant SH velocity. Analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone, in agreement with predictions of geodynamical models that explicitly introduce phase transitions.

Seismic anisotropy changes across upper mantle phase transitions

“…LPO of wadsleyite and/or ringwoodite could indeed explain the observed signal in the upper and lower transition zone, respectively, as they have been shown to be intrinsically anisotropic. Wadsleyite single crystal anisotropy is about 14% (Zha et al, 1997), and recent modeling showed that a polycrystal of pyrolytic composition at MTZ conditions can have ∼ 1% seismic anisotropy (Tommasi et al, 2004;Kawazoe et al, 2013), compatible with our model. The intrinsic elastic anisotropy of ringwoodite is more ambiguous.…”

“…According to mineral physics analyses by Tommasi et al (2004) horizontal shear has to be dominant in the MTZ in order to be compatible with radial anisotropy models showing V SH > V SV (Montagner and Kennett, 1996;Beghein et al, 2006;Panning and Romanowicz, 2006) and with V SV az-imuthal anisotropy stronger than V SH anisotropy as found by Trampert and van Heijst (2002). Models of radial anisotropy have however large uncertainties at these depths (Beghein and Trampert, 2004b;Beghein et al, 2006) and other results show V SH < V SV in the MTZ (Visser et al, 2008b).…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…There is some observational evidence for anisotropy in the transition zone, mainly from surface wave and normal mode observations (e.g., Trampert and van Heijst 2002;Beghein and Surv Geophys (2009) Fouch and Fischer 1996;Iidaka and Niu 1998). The development of LPO in wadsleyite, which has an intrinsic shear wave anisotropy of *13% (Mainprice 2007) has been simulated using polycrystalline plasticity modeling by Tommasi et al (2004). Overall, however, there is very little experimental data on LPO development in transition zone materials (Karato 2008).…”

First principles investigation of the structural and elastic properties of hydrous wadsleyite under pressure

“…Our calculated polarization anisotropy indicates that the observed polarization anisotropy can be explained by the transversely anisotropic aggregate of both dry and hydrous wadsleyite with the vertical c principal axis. Since dry and hydrous wadsleyites show relatively similar azimuthal and polarization anisotropy, the difference of the rheological properties such as crystal-preferred orientation [e.g., Tommasi et al, 2004] between dry and hydrous wadsleyite may hold the key to whether hydrous wadsleyite is detectable by assessing seismic anisotropy.…”