Journal of Petrology Advance Access published online on March 4, 2008
Journal of Petrology, doi:10.1093/petrology/egn007
| ||||||||||||||||||||||||||||||||||||||||||||||||||
Seismic Properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water
1Research School of Earth Sciences, Australian National University, Canberra, Act 0200, Australia
2Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan
Received December 8, 2006; Revised typescript accepted February 11, 2008
 |
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, 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.
KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing
*Corresponding author. Telephone: +61 2 6125 2498. Fax: +61 2 6125 8253. E-mail: Ian.Jackson{at}anu.edu.au
Present address: Japan Manned Space Systems Corporation, MY Bldg., 1-1-26, Kawaguchi, Tsuchiura, Ibaraki 300-0033, Japan.
Present address: Department of Earth Sciences, Boston University, Boston, MA 02215, USA.
Present address: Faculty of Geosciences, Department of Earth Sciences, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands.