Open AccessMineralogy and elasticity of the oceanic upper mantle: Origin of the low‐velocity zone
Author(s) -
Stixrude Lars,
LithgowBertelloni Carolina
Publication year - 2005
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2004jb002965
Subject(s) - geology , phase velocity , mantle (geology) , low velocity zone , group velocity , transition zone , shear velocity , isotropy , attenuation , geophysics , geothermal gradient , seismology , physics , lithosphere , thermodynamics , turbulence , quantum mechanics , optics , tectonics
We use a new method to construct an upper mantle model based on self‐consistent computation of phase equilibria and physical properties. Computation of the isotropic elastic wave velocities of a pyrolytic bulk composition in thermodynamic equilibrium shows a distinct low‐velocity zone with a minimum velocity V S = 4.47 km s −1 along the 100 Ma geotherm. In the vicinity of the low‐velocity zone the velocity of this null hypothesis is approximated along oceanic geotherms by V S = 4.77 + 0.0380( P , z /29.80) − 0.000378( T − 300), with pressure P in GPa, depth z in km, temperature T in K, and velocity V S in km s −1 . The null hypothesis predicts a minimum V S 0.1–0.2 km s −1 higher than that in seismological models of 100 Ma Pacific. We find that dispersion, estimated solely on the basis of seismological attenuation models, can account for this residual velocity deficit. Except in the immediate vicinity of the ridge ( t < 5 Ma), a solid‐state low‐velocity zone provides a satisfactory quantitative explanation of seismic observations. We do not find a satisfactory explanation for the magnitude of the Gutenberg discontinuity or for the high shear wave velocity gradient zone.