Experimentally determined attenuation and modulus in Earth analogue materials over a wide range of frequency

Seismic observations, particularly in areas of active tectonics, show large variations and can be affected by many variables, including temperature, composition, grain size, volatile content and the presence of small quantities of melt or other fluids. The understanding and interpretation of seismic wave dispersion and attenuation data can be significantly enhanced through improved knowledge of material dynamic response at the grain scale (and smaller). The organic binary system borneol-diphenylamine is a suitable analogue of melting in the Earth’s mantle. Not only does the solid phase deform through the same deformation mechanisms (diffusion creep, dislocation creep) observed in geologic materials, but the moderate and controllable dihedral angle (40° at 316 K) exhibited by this system provides an equilibrium melt-geometry very similar to that of the olivine + basalt system [Takei, 2000]. With a eutectic melting temperature (TE = 316 K at ambient pressure) just above room temperature, borneol-diphenylamine provides an attractive chemical analogue to study the properties of solid-melt interaction, obviating the difficulties associated with high-temperature, high-pressure experimentation. We have initiated compression-compression cyclic loading experiments on single phase (borneol; TM = 477 K) and multiphase (borneol-diphenylamine) aggregates. Our newly developed apparatus is able to apply and measure very small stress and strain amplitudes with high accuracy using a piezoelectric actuator and optical displacement meters, respectively. The planned experimental conditions are P = 1 atm, T = 298 - 333 K, strain amplitude <10-5, f = 10-4 - 102 Hz, and melt fraction φ = 0 - 0.1. The combination of low (sub seismic) frequency data obtained from these forced oscillation experiments and high-frequency data (f = 102 - 106) from ultrasonic wave transmission measurements allows us to explore the dissipation and Young’s modulus over a frequency range of some ten decades. Also, our ability to make very fine-grained samples (~1 μm) by using a cold ball-milling and hot-pressing technique allows us to explore the effect of grain size. In a separate study, compression creep, microcreep and load relaxation tests will be conducted on the same aggregates. The independently obtained values for activation energies, modulus and dissipation determined from such static tests will provide a good check of consistency and greatly aid in both the interpretation of results and the extrapolation to mantle conditions. Takei, Yasuko (2000) Acoustic properties of partially molten media studied on a simple binary system with a controllable dihedral angle, J. Geophys. Res., 105:B7, 16,665-16,682.