From: Slava Solomatov Slava Dao

Measurements of Velocities and Elastic Properties at High Pressures and Temperatures

The best-resolved properties of Earth’s interior are the sound velocities Vp and Vs, and the density. Beyond average radial properties, seismological studies now reveal lateral heterogeneity and anisotropy of the mantle, yielding a fully 3-D view of mantle structure and properties. A major challenges at the interface of mineral physics and seismology is to understand this new view of the mantle, the causes of seismic heterogeneity, and their likely geodynamical implications. Indeed, it appears that both variations in chemical composition and thermal structure contribute to the observed velocity heterogeneity at all levels in the mantle.In order to fully exploit the information emerging from seismic investigations, it is essential to know shear properties of mantle constituents as well as their bulk moduli (K). While the bulk modulus (and it’s P and T derivatives) provides necessary information on the density at depth, in many cases the shear modulus and Vs are more sensitive to compositional variations. An example is the substitution of aluminum in Mg-silicate perovskite, where the addition of only 5% Al substantially reduces the shear modulus while having a much smaller (if any) effect on K. Shear properties can therefore provide much tighter constraints on the range of acceptable chemical and thermal models for the mantle. However, relatively few direct experimental measurements have been available on the relevant shear properties. This situation has changed dramatically in the past few years. Shear velocities and moduli at high P and/or T have been measured using several techniques, including light scattering (eg., Brillouin scattering, laser phonon spectroscopy or LIPS), ultrasonic interferometry and resonance, and x-ray diffraction with the diamond cell. We review a selection of the results, emphasing measurements at high pressures and/or temperatures. An additional technical challenge in high-pressure experimentation is accurate determination of pressure. Combining acoustic measurements of velocity with density measurements by diffraction provides a means for defining primary pressure scales. For that purpose, a Brillouin spectrometer has been interfaced with synchrotron radiation at sector 13 (GSECARS) of the Advanced Photon Source. Techniques that are likely to play an increasingly important role in determination of sound velocities under extreme conditions are those developed at third-generation synchrotron sources, such as inelastic scattering of phonons by x-rays, and nuclear resonant inelastic x-ray scattering.