Compressional and shear wave velocities in granulite facies rocks and eclogites to 10 kbar

Abstract

Compressional and shear wave velocities (Vp and Vs) have been measured to 10 kbar in 17 granulite facies rocks and 15 eclogites. The former included quartzo-feldspathic, gabbroic, and garnet granulites as well as mangerites from the Adirondack region. The eclogites included the three types described by Coleman et al. from California, Norway, South Africa, and Tasmania. The mean ranges of the values of (∂Vp/∂P)T and (∂Vs/∂P)T in the 8- to 10-kbar range are slightly higher (0.015–0.022 and 0.008–0.012 km/s kbar) for eclogites than for granulites. Velocity-density systematics, based on the 10-kbar data, is evaluated in the light of Birch's law, D. L. Anderson's seismic equation of state, Wang's C-ρ relation, and the K-V relationship of O. L. Anderson and Nafe and D. L. Anderson and O. L. Anderson. On close analysis, there is a distinction in Wang's relations for ∼ 21 and ∼ 22. Most of the granulites and eclogites have values of ∼22; Birch's relationship for these and previously studied basalts ( ∼ 22) is Vp = −1.85 + 2.87ρ, where r2 = 96%. For Vs the best fit of the data for granulites and eclogites ( ∼ 22) is Vs = −0.33 + 1.40ρ, where r2 = 88%. The best-fit equation for Vs that includes the calcium oxide effect in all the granulites and eclogites is Vs = −0.63 + 0.21(21 − ) + 1.56ρ + 0.016 [CaO]. On the basis of the laboratory results, it is shown that elastic properties of garnet granulite with ∼ 22 are compatible with those of the 7.1- to 7.8-km/s crustal layer. Analysis of crust-mantle seismic data in various regions, and the present laboratory results, shows that the assumption of ∼ 22 for the lower crust as well as for the upper mantle fits better than ∼ 21. On the other hand, the ∼ 21 model fits better for some geophysically anomalous regions in the western United States.