Some remarks on the origin of seismic anisotropy in the D” layer

Abstract

Physical mechanisms of seismic anisotropy in the D" layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D" layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg,Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D" layer conditions is {100} rather than {110} as observed at lower pressures. Such a change in glide plane in MgO (or (Mg,Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the V SH > V SV (V SV > V SH ) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant V SH > V SV in the D" layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated threedimensional flow presumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.

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

“…The consensus from these studies is that SPO-type models generally do a good job of matching the seismological constraints. Models that invoke the LPO of lowermost mantle minerals can also plausibly explain splitting observations in D 00 if deformation is being accommodated via dislocation creep (e.g., Karato 1998). A recent review of experimental constraints on LPO in lowermost mantle minerals can be found in Yamazaki and Karato (2007) and we provide only a brief summary here.…”

“…In contrast to the overlying lower mantle, observations of seismic anisotropy in the D 00 layer at the base of the mantle are abundant, and both SPO-and LPO-type models have been proposed (e.g., Kendall and Silver 1998;Lay et al 1998;Karato 1998). Models that invoke SPO as a mechanism for generating anisotropy in D 00 rely on the presence of a material in the lowermost mantle with elastic properties that sharply contrast with the surrounding matrix.…”

Development of finite strain in the convecting lower mantle and its implications for seismic anisotropy

“…Not only does (Mg, Fe)SiO 3 perovskite have a lower intrinsic anisotropy than (Mg, Fe)O [Stixrude, 1998], but it is likely that (Mg, Fe)O is the weaker phase [Yamazaki and Karato, 2001] and undergoes significantly more strain than (Mg, Fe)SiO 3 perovskite in the two-phase system. Moreover, fabric experiments on perovskite analog materials indicate a lower degree of anisotropy exhibiting V SV > V SH anisotropy for horizontal shear [Karato, 1998a;Yamazaki and Karato, 2001].…”

“…[4] Possible causes of D 00 anisotropy are discussed by Karato [1998aKarato [ , 1998b, Lay et al [1998aLay et al [ , 1998b, and Kendall [2000]. On one hand, SPO can result from oriented melt inclusions or heterogeneous material with a large contrast in elastic constants.…”

“…A better understanding of the cause of this anisotropy may lead to a clearer picture of mantle chemistry and dynamics. Two of the mechanisms for the formation of anisotropy that have been proposed are shape-preferred orientation (SPO) [Kendall and Silver, 1996] and lattice-preferred orientation (LPO) [Karato, 1998a[Karato, , 1998bStixrude, 1998]. It is of considerable interest to determine which of these potential mechanisms lead to the observed anisotropy because SPO requires elastically heterogeneous material, which implies chemical heterogeneity or melting, whereas LPO can occur in a homogeneous material by deformation under high stress.…”