Geophysical Research Abstracts, Vol. 9, 00822, 2007 SRef-ID: 1607-7962/gra/EGU2007-A-00822 © European Geosciences Union 2007 SEISMIC P-WAVE VELOCITIES - DENSITY RELATION IN THE UPPER MANTLE OF THE WESTERN USA T. Romanyk (1), W. Mooney (1) Institute of Physics of the Earth named after O.Yu.Shmidt, RAS, B. Gruzinskaya 10, Moscow 123995, Russia, (romanyuk@ifz.ru / +7-495-2556040), (2) US Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025, United States (mooney@usgs.gov ) An investigation of the relation between seismic P-wave velocity and density is es- sential to adding new constraints on the physical state of the mantle. In particular, it can reveal whether the density contrasts driving the system are due to a purely thermal process or to both thermal and petrological/chemical effects. For a rough estimation, a linear velocity-density relation, ρ = a + b · V p (V –seismic P-wave velocity, ρ–density) can be used. Generally, variations of the upper mantle P-wave velocities and densities are primarily due to changes in pressure, temperature, composition, partial melts, and fluid content, etc. Variations in upper mantle composition are primarily due to varying degrees of basaltic melt removal. However, the influence of compositional changes is only mi- nor, giving a b-coefficient of ∼0.09 (g/cm3)/(km/s) (Jordan, 1978; 1981). Labora- tory experiments on upper mantle rock samples reveal that near and far from the solidus temperature the b-coefficient is ∼0.04 (g/cm3)/(km/s) (Anderson and Bass, 1984; Duffy and Anderson, 1989), and ∼0.18 (g/cm3)/(km/s) (Sato et al., 1988, 1989; Sato and Sacks, 1989), respectively. Thus the combined effect of composition and temperature on the b-coefficient can not exceed 0.27 (g/cm3)/(km/s). Birch’s (1961) velocity-density bulk ratio coefficient of ∼0.3 (g/cm3)/(km/s), reflecting the influence of mineral composition without temperature-pressure effects, is much higher than for a purely thermal effect. The basalt-eclogite transformation (gabbro: V p =7.0 km/s, ρ =3.0 g/cm3; eclogite: V p =8.5 km/s, ρ =3.5 g/cm3) would be the most impor- tant compositional change in down-welling lower crustal flow. A formal estimation