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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 2188
Variation of Seismic Discontinuity Depths at ~660 km: Role of Garnet
Associated with Stagnant Slab
Prof. Fumiko Tajima
Earth and Planetary Systems Science Hiroshima University IASPEI
Ikuo Katayama, Tsuyoshi Nakagawa
The role of mid-oceanic ridge basalt (MORB) with its primary component of garnet is important in slab
dynamics. Early 1990s, seismic tomography models visualized the bulk of stagnant slab in long
wavelength images. However, structural variation is apparent within the zones of high velocity anomaly
(HVA), i.e., stagnant slab, in the transition zone. Waveform modeling of triplicated regional broadband
waves that have ample information to resolve the finer structure in the transition zone derived P wave
velocity models M3.11, M2.0 or others associated with stagnant slab (Tajima and Grand, 1995, 1998).
Model M3.11 is characterized by HVA of up to +3% in the deeper part of the transition zone and
depression of the discontinuity depth to 690 km. This model is consistent with the structure predicted
for a cold slab in the stable field of ringwoodite (Ringwood and Irifune, 1988). Model M2.0 is
characterized by HVA which is similar to that in M3.11 indicating the existence of a flattened cold slab in
the transition zone, but not accompanied by broad depression of the 660 km discontinuity. Taken the
profile for the assemblage of ringwoodite, the lack of depression of the 660 km discontinuity may be
interpreted with the temperature which is normal beneath the flattened slab. However, the observed
change of structure from M3.11 to M2.0 in the northwestern Pacific is more rapid than what can be
expected from the temperature gradient associated with cold slab. Instead, we postulate the change of
geochemical properties with stagnant slab at the bottom of the upper mantle, in which the contrast of
MORB and peridotite involves. Results of two recent experiments under high pressure and temperature
conditions are supportive for this hypothesis. Sano et al. (2006) has shown that the Clapeyron slope of
hydrous garnet at the phase transformation is positive, and there may be no depression of the phase
transformation depth if T