SHEAR-WAVE DISPERSION AND ATTENUATION IN FINE-GRAINED SYNTHETIC OLIVINE AGGREGATES - PRELIMINARY-RESULTS

Low-frequency torsional forced oscillation tests have been performed o n a fine-grained olivine polycrystal to determine shear wave dispersio n and attenuation. The specimen was a dense hot-pressed San Carlos oli vine aggregate with uniform grainsize of about 50 mu m, < 0.1 vol.% of melt and minor amount of hydroxyl (similar to 100 ppm). Mechanical te sts were conducted under 200 MPa hydrostatic pressure, low oscillation frequencies (0.01-1 Hz) and within the linear regime of strain (ampli tude < 5x10(-5). The specimen was first annealed at 1300 degrees C, an d subsequently measured at a series of progressively lower temperature s. At 1300 degrees C evidence was obtained of marked viscoelastic rela xation: the shear modulus G is relatively low and strongly frequency-d ependent and the attenuation Q(-1) is high (G similar to 33 GPa, Q(-1) similar to 0.14 for 1 Hz). In the temperature range 1100-1300 degrees C, G depends strongly on temperature with l partial derivative G/part ial derivative Tl several times greater than the value measured in ult rasonic experiments at MHz frequencies. Q(-1) varies with temperature and frequency as Q(-1) = A(o)[omega e(E/RT)](-n) with activation energ y for the relaxation rate E = 420+/-30 kJ/mol and exponent n = 0.31+/- 0.02. The presence of thermal cracks complicates the interpretation of data obtained at temperatures below 900 degrees C. Grainsize-sensitiv e processes may be responsible for much of the observed viscoelastic r elaxation. The strong temperature sensitivity of the shear modulus sug gests much smaller thermal anomalies than are commonly inferred to be responsible for the variability of seismic wave speeds in the upper ma ntle.