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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
JSS011 Oral Presentation 2116
Water flow into the mantle transition zone
Dr. Takashi Tonegawa
Earthquake Research Institute University of Tokyo IASPEI
Kazuro Hirahara, Takuo Shibutani, Katsuhiko Shiomi, Hiroo Kanamori
It has recently been proposed that the mantle transition zone is a water reservoir in the Earths interior.
Transition zone minerals, such as wasleyite and ringwoodite, have unusual property in comparison with
the upper and lower mantle minerals. The solubility of water in the upper and lower mantle minerals is
less than 0.2 wt%, whereas several experimental researches have shown that the wasleyite and
ringwoodite can contain significant amounts (~several wt%) of water in the crystal structure, and some
seismic researches have also estimated the amount of water in the transition zone. One of the problems
for water in the transition zone is why water exists in the transition zone that is shut out by anhydrous
minerals in the upper and lower mantle. We applied receiver function (RF) analysis with Hi-net tiltmeter
recordings, provided by NIED, that are observed at 700 stations in Japan and contain relatively lower
frequency component (~1000 sec). For deconvolution to calculate RF, since tiltmeter recordings have
just horizontal component, we obtained source-time function by stacking all of vertical components
observed at F-net broadband stations. We applied a bandpass filter of 0.02-0.16 Hz. Usually, radial
receiver function is stacked to image seismic discontinuities, but in this study we used transverse
receiver function that is usable to detect P-to-S phase converted at a dipping layer, such as subducting
slab. In RF transects, the top surface of the Pacific slab descending underneath the Japanese Islands
could be traced down to a depth of 400 km or greater. In addition, another seismic discontinuity just
above the slab surface would be traced by positive RF amplitude, which is likely to be related to water
transportation from the Earths surface to the mantle transition zone. Although, in subducting process,
most of hydrous minerals in the oceanic crust lose water by dehydration reaction at depths of 100 km,
lawsonite can preserve it up to 9~10 GPa (~300 km) under low temperature condition. The water
expelled from lawsonite would rise and react with the mantle wedge peridotite, and hence phase A will
be formed in the mantle wedge (Komabayashi et al., 2005). The seismic discontinuity just above the
Pacific slab detected in this study seems to correspond to the upper limitation of the existence of phase
A in the mantle wedge. This discontinuity detected down to 410 km or greater implies that the water
within phase A is transported into the mantle transition zone.
Keywords: receiver function, transition zone, water