Seismic properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water

Abstract

As a pilot study of the role of water in the attenuation of seismic waves in the Earth's upper mantle, we have performed a series of seismic-frequency torsional forced-oscillation experiments on a natural (Anita Bay) dunite containing accessory hydrous phases, at high temperatures to 1300°C and confining pressure (Pc) of 200 MPa, within a gas-medium high-pressure apparatus. Both oven-dried and pre-fired specimens] wrapped in Ni-Fe foil within the (poorly) vented assembly were recovered essentially dry after 50-100 h of annealing at 1300°C followed by slow staged cooling. The results for those specimens indicate broadly similar absorption-band viscoelastic behaviour, but with systematic differences in the frequency dependence of strain-energy dissipation Q-1, is attributed to differences in the small volume fraction of silicate melt and its spatial distribution. In contrast, it has been demonstrated that a new assembly involving a welded Pt capsule retains aqueous fluid during prolonged exposure to high temperatures - allowing the first high-temperature torsional forced-oscillation measurements under high aqueous fluid pore pressure Pf. At temperatures >1000°C, a marked reduction in shear modulus, without concomitant increase in Q-1, is attributed to the widespread wetting of grain boundaries resulting from grain-scale hydrofracturing and the maintenance of conditions of low differential pressure Pd = Pc = Pf. Staged cooling from 1000°C is accompanied by decreasing Pf and progressive restoration of significantly positive differential pressure resulting in a microstructural regime in which the fluid on grain boundaries is increasingly restricted to arrays of pores. The more pronounced viscoelastic behaviour observed within this regime for the Pt-encapsulated specimen compared with the essentially dry specimens may reflect both water-enhanced solid-state relaxation and the direct influence of the fluid phase. The scenario of overpressurized fluids and hydrofracturing in the Pt-encapsulated dunite specimen may have some relevance to the high Q-1 and low-velocity zones observed in subduction-zone environments. The outcomes of this exploratory study indicate that the presence of water can have a significant effect on the seismic wave attenuation in the upper mantle and provide the foundation for more detailed studies on the role of water.