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Anisotropy explained

Variations in the speed of seismic waves in the Tonga slab may be explained by the presence of the mineral akimotoite, writes Sarah Day.

Geoscientist Online 8 October 2008

Research carried out at the Institute of Mineralogy, Petrology and Economic Geology at Tohoku University in Japan is the first to test experimentally the properties of akimotoite - a phase that exists at the bottom of the mantle transition zone and within the uppermost lower mantle - under conditions similar to those it would experience in its natural habitat. Performing such experiments requires some fairly impressive laboratory kit – in this case a Kawai-type multi-anvil apparatus.

The Tonga slab is a piece of oceanic lithosphere falling into the mantle along the Tonga trench subduction zone in the Western Pacific. The slab demonstrates seismic anisotropy, in that waves originating in deep earthquakes occurring along the zone travel through it at varying speeds, depending on their direction. It has previously been suggested that these variations in speed, which differ according to depth and temperature, could be due to the presence of akimotoite, thought to be a major constituent of the harzburgite layer of subducting slabs.

S waves produced by deep earthquakes in the Tonga subduction zone, where a slab of lithosphere (crust and uppermost mantle, shown in blue) is diving into the deeper mantle. From Wookey et al., Nature 415

The researchers, led by Rei Shiraishi (Institute of Mineralogy, Petrology and Economic Geology,Tohoku University, Japan) carried out plastic deformation experiments on polycrystalline akimotoite. The mineral was synthesised at high pressures and temperatures, and shown to have no anisotropic properties. It was then deformed using the multi-anvil apparatus, under uniaxial compression and simple shear.

After deformation, the akimotoite displayed high levels of seismic anisotropy, with P waves travelling at different speeds depending on the sample’s orientation. The amount of anisotropy also varied according to temperature. At 1000°C, P waves were found to travel slowest when moving perpendicular to compression; whereas at 1300°C they were slowest in the compression direction.

The experimental results suggest seismically anisotropic behaviour similar to that observed in the Tonga slab. In the southern segment of the slab, P waves travel more slowly in the compression direction - a situation similar to akimotoite at 1300°C, while in the northern segment they are slower in the direction perpendicular to compression, similar to akimotoite at 1000°C.

Ref:  Rei Shiraishi et al., ‘Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab’, Nature, Vol. 455 pp. 657-660 (2008)