ELASTICITY OF MINERALS AT HIGH-TEMPERATURE: NOVEL PHASE TRANSITIONS AND APPLICATIONS TO GEOPHYSICS

Seismic tomographic studies of Earth’s mantle indicate the presence of low-velocity zones as well as lateral heterogeneity and anisotropy, thus stressing the need for fully anisotropic elasticity data at appropriate temperatures and pressures.  Brillouin scattering is an optical spectroscopic technique that allows the determination of sound velocities and adiabatic elastic moduli in material, thus providing information that is more directly comparable with observed seismic wave velocities.  Brillouin scattering also provides the opportunity to investigate elastic mode-softening of materials as they undergo structural phase transitions.  To date, few investigations have determined the temperature-dependence of the single-crystal acoustic properties of mantle materials. 

The focus of this presentation will be on the elasticity of upper mantle minerals at high-temperature, determined using Brillouin spectroscopy and resistive heating methods.  Two examples will be discussed:  synthetic ringwoodite (γ-Mg₂SiO₄), a high-pressure polymorph of olivine that is stable in Earth’s transition zone, and natural orthoenstatite (simplified to Mg₂Si₂O₆), an abundant mineral in mafic rocks of Earth’s crust and upper mantle.  In both materials, the full single-crystal elastic tensors (C_ij’s) were determined to 800°C, which allow better constraints to be drawn for upper mantle mineral assemblages.  Finally, single-crystal Brillouin measurements on OEN to 1350°C show significant softening prior to a reconstructive phase transition, which occurs between 1090(±10)°C £ T_tr £ 1175(±10)°C.  Our results on OEN are interpreted in terms of elastic softening ahead of a displacive phase transition. Before the displacive transition can occur, however, the elastic softening appears to trigger the observed reconstructive transition to the more stable protoenstatite (or high clinoenstatite) structure.  We suggest that the displacive phase transition would be to a previously unreported pyroxene structure with Cmca symmetry. The Landau free energy expansion for this transition will be discussed, as well as, the analogies to the well-known P2₁/c ↔ C2/c displacive transition in pigeonite and clinoenstatite.  Implications for low-velocity zones within Earth’s upper mantle will also be discussed.