Upper mantle anisotropy: A preliminary model

Abstract

Seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion. The physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide syst. em of each mineral. The dominant glide systems are assumed to be [100](010) for olivine and [001](100) for the pyroxenes. These yield a predominantly orthorhombic fabric with the olivine [100] and the pyroxene [001] axes aligned in the upper mantle flow direction and the olivine [010] and the pyroxene [100] axes aligned normal to the upper mantle flow plane. These glide systems have a threshold temperature of enhanced mobility of 1100-1200 K, which yields a solid-state, thermally defined lithosphere-asthenosphere boundary in a olivine-pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere. Mantle anisotropy due to mineral alignment is then actively maintained below this boundary (a•thenosphere and mesosphere) and is a fossil state above this boundary (lithosphere). We •.!se the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models (pyrolite and piclogite) for the upper mantle by an extrapolation of single-crystal, anisotropic mineral elastic properties to a •epth of 400 km. The real-earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy (isotropy) from random mineral alignment. The seismic anisotropy of the upper 220 km is best represented by the pyrolite model, which reduces to quasi-hexagonal symmetry with the unique axis in the direction of mantle flow. The Lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy. The piclogite model has orthorhombic symmetry •istinctly different from that of the pyrolite. The piclogite anisotropy model can appear quasi-isotropic, when measured by transverse isotropy parameterization, if the mantle flow is mainly horizontal with a horizontal shear plane (that is, the shear flow gradient dult /dR dominates). 0148-0227/86/004B-5212505.00 tropy in the upper mantle is due to crystallographic alignment of intrinsically anisotropic minerals, mainly olivine and the !•roxenes. Evidence for alignment of these minerals comes from studies of ophiolites [e.g., Christensen and Lundquist, 1982; Christensen, 1984] and upper mantle xenoliths [e.g., Mercier and Nicolas, 1975]. Thus not only is there evidence for substantial alignment of olivine (ol}, but there is also evidence for alignment of orthopyroxene (opx) and weak alignment of clinopyroxene {cpx} (e.g., diopside, see Peselnick et al. [1974]). The petrofabric evidence shows that ol [100] and opx [001] tend to be aligned in the same direction, that ol [010] and opx [100] tend to be aligned in the same direction, and...

Rayleigh-Love wave coupling in an azimuthally anisotropic medium.

“…Hereafter, B-model of Kawasaki (1986) in both the LVZ and the lithospheric plate above the LVZ. This feature is consistent with above-mentioned measurement of ultramafic rocks (e.g., Christensen and Salisbury, 1979), and with petrological modeling of Estey and Douglas (1986). The overall feature of upper mantle anisotropy of KB-Z model is also quite consistent with seismic anisotropy observations in the Pacific Ocean; Pn velocity anisotropy of up to 8 percent (e.g., Shimamura et al 1983), a small azimuthal anisotropy of Sn velocity (e.g., Shearer and Orcutt, 1986), SH-SV polarization anisotropy (e.g., Mitchell and Yu, 1980), azimuthal anisotropy of up to 4 percent of mantle Rayleigh wave velocities (Nishimura and Forsyth, 1988) and a small azimuthal anisotropy of mantle Love wave velocities (Nishimura and Forsyth, 1985).…”

The Seismic anisotropy of the Earth's mantle: From single crystal to polycrystal

“…A major problem still remains as no clinopyroxene temperature derivatives are available. Estey and Douglas (1986) proposed using the orthopyroxene temperature derivatives of Frisillo and Barsch (1972). For completeness garnet has been included in Table 1.…”

“…To allow calculations of seismic properties at mantle pressures, Estey and Douglas (1986) have proposed using 80% of the orthopyroxene pressure derivatives of Frisillo and Barsch (1972). However, comparison of the most recent orthopyroxene pressure derivatives of Chai et al (1997b) with the calculated values for clinopyroxene of Matsui and Busing (1984) shows that they differ by 200% for C 11 , C 33 and C 44 suggesting large errors would occur in using the method proposed by Estey and Douglas (1986). We would recommend using the values given by Matsui and Busing (1984) until an experimental determination has been made.…”

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

“…⁎⁎⁎⁎ Opx values were taken instead of the missing data for cpx, as recommended by Estey and Douglas (1986).…”