Seismic wave dispersion and attenuation in Åheim dunite: an experimental study

Abstract

A machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency (10-lo00 mHz) and strain amplitude under conditions of high pressure (to 300MPa) and temperature (to >lOOO"C). The eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the Earth's upper mantle. Measurements have been made on cylindrical specimens of an olivine-rich rock from Aheim, Norway which contains, in addition to olivine, about 10 per cent pyroxene and 5-10 per cent of hydrous silicate phases dominantly clinochlore, serpentine and talc. In order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and/or their dehydration products, the specimens were either previously fired under controlled oxygen fugacity at 1200 "C for 24 hours in order to effect complete dehydration within the olivine stability field, or simply oven-dried at 110 "C. Linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10-8-10-6, and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the Krarners-Kronig relations of linear theory. Both the pressure dependence at room temperature, and the temperature dependence at 300MPa, of the shear modulus and internal friction have been investigated. Much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration. Temporal evolution of Q-' towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and/or grain boundaries. At the highest pressures and temperatures achieved in this study, 300MPa and lOOO"C, marked dispersion of the shear modulus, amounting to 5 per cent between periods of 3 and 100 s, and concomitant strong internal friction, varying with oscillation period T, approximately as 0.011T,''6, are observed. The energy dissipation appears to be concentrated within, rather than at the boundaries between, the olivine grains. These results provide the clearest indication yet that solid-state, probably intragranular, anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the Earth's upper mantle, although the mechanistic basis for the observed anelasticity remains to be established.