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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS012 Oral Presentation 2194
The influence of water on seismic wave speeds and attenuation in the
mantle wedge: an exploratory experimental study
Dr. Yoshitaka Aizawa
Seismology, Volcanology and Disaster Mitigation Nagoya University IASPEI
Auke Barnhoorn, Ulrich H. Faul, John D. Fitz Gerald
Water, either as a free fluid phase or dissolved in nominally anhydrous minerals, may strongly influence
seismic wave speeds and attenuation especially in subduction zones where water is released into the
mantle wedge by dehydration of subducting oceanic lithosphere. As the first step in a new experimental
campaign, we have performed torsional forced-oscillation experiments on a natural rock, Anita Bay (NZ)
dunite, consisting mainly of the dominant upper-mantle mineral olivine along with accessory hydrous
phases. These experiments were conducted at teleseismic frequencies (mHz-Hz) under conditions of
high temperature to 1300C and high confining pressure (200 MPa) within internally heated 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 1300C followed
by slow staged cooling. The results for three such specimens indicate broadly similar absorption-band
viscoelastic behaviour, but with systematic differences in the frequency dependence of strain-energy
dissipation, attributed to differences in the small volume fraction of silicate melt and its spatial
distribution. 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 seismic-frequency,
high-temperature measurements under high water pore pressure. A marked reduction in shear
modulus, without concomitant increase in dissipation, observed at temperatures >1000C is attributed to
the widespread wetting of grain boundaries resulting from hydrofracturing and the maintenance of
conditions of low differential pressure (confining pressure minus pore pressure). During staged cooling,
accompanied by decreasing pore pressure and progressive restoration of significant differential
pressure, a different microstructural regime is encountered in which the fluid is increasingly
accommodated in arrays of isolated grain-boundary pores. The more pronounced viscoelastic behaviour
observed within this regime for the Pt-encapsulated specimen than for the essentially dry specimens is
tentatively attributed to enhanced grain-boundary sliding facilitated by the presence of interconnected
pore fluid and lower grain-boundary viscosity. Such processes may help explain localized regions of low
seismic wave speeds and high attenuation in the mantle wedge above subducting, dehydrating
lithosphere.
Keywords: seismic wavespeed, seismic attenuation, water