ELASTIC PROPERTIES OF MANTLE MINERALS AT HIGH PRESSURES: NEW INSIGHTS INTO COMPOSITION AND STRUCTURE OF THE UPPER MANTLE

The single-crystal elastic properties of minerals are essential for interpretation of the seismic velocity structure of the Earth's mantle. Elastic properties also provide fundamental insights into structure, bonding, and the nature of phase transitions in minerals. Despite this importance, the elastic properties of many mantle minerals are poorly constrained, particularly for low-symmetry species at elevated pressures. Brillouin scattering is a method whereby one can measure the complete elastic tensor by recording the frequency shift induced in scattered laser light by thermally generated sound waves. We have used this technique in the diamond anvil cell to investigate the elastic properties of a variety of upper mantle minerals to pressures corresponding to ~300-400 km depth. The high-pressure elastic tensors of fayalite, grossular, and almandine have been measured. This has enabled us to construct mineralogical models that place improved constraints on the abundance of olivine at 410 km depth. In conjunction with recent seismic data, we can also constrain possible variations in Fe content near the transition zone. The various mantle rock types can differ greatly in their response to local variations in iron. The elastic properties of subduction zone minerals are necessary for understanding basic features of the subduction factory including the existence and distribution of hydrous phases, and the effect on seismic wavespeeds of thermal and metamorphic processes. Modeling of seismic velocities in subduction zones is greatly limited by lack of constraints on the elastic properties of the metamorphic phases expected to exist there. We have begun to address this problem by measuring the elastic tensors (at ambient and high pressures) of a suite of hydrous minerals including brucite, diaspore, alunite, and zoisite. As one example, brucite shows highly anomalous elastic properties including the largest shear modulus pressure derivative (G′=3.6) yet measured. The elastic anisotropy of brucite changes strongly with pressure. The ratio of the longitudinal moduli, C₁₁/C₃₃, decreases from 3.3 at ambient pressure to 1.2 at 15 GPa. Our results provide new insights into the behavior of hydrous minerals under subduction zone conditions.