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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy<br />

(S) - <strong>IASPEI</strong> - International Association of Seismology and Physics of the Earth's<br />

Interior<br />

JSS012 Poster presentation 2202<br />

Effect of superhydrous phase B and phase D on the density of hydrated<br />

slabs in the deep mantle<br />

Dr. Konstantin Litasov<br />

Dept. Mineralogy, Petrology and Economic Geology Faculty of Science, Tohoku University<br />

Eiji Ohtani<br />

Recent geophysical data indicate that al least locally upper mantle and transition zone may be<br />

substantially hydrated. High-pressure experiments reveal existence of several dense hydrous<br />

magnesium silicates (DHMS) stable along the subduction slab geotherms in a peridotite composition.<br />

After breakdown of serpentine and chlorite at 6 GPa the most important of them, with increasing<br />

pressure, are phase A, phase E, superhydrous phase B (SuB), and phase D. SuB and phase D are most<br />

important hydrous phases of post-spinel assembly at the top of the lower mantle. After transformation<br />

of hydrous ringwoodite to Mg-perovskite and ferropericlase at the 660-km depth, the density of<br />

hydrated slabs may be reduced by subsequent formation of SuB and/or phase D. Pressure-volumetemperature<br />

relations have been measured for SuB and Fe-bearing phase D using synchrotron X-ray<br />

diffraction with a multianvil apparatus of SPring-8 facility. A fit of P-V-T data for superhydrous phase B<br />

to high-temperature Birch-Murnaghan equation of state yields K0 = 135.8 2.6 GPa; K= 5.3 0.2; dKT/dT<br />

= -0.026 0.003 GPa/K and zero-pressure thermal expansion a = a0 + a1T with a0 = 3.2 (1) x 10-5 K-1<br />

and a1 = 1.2 (4) x 10-8 K-1. A fit of P-V-T data for phase D to high-temperature Birch-Murnaghan EOS<br />

yields K0 = 139.6 3.0 GPa; K= 6.6 0.4; dKT/dT = - 0.023 (8) GPa K-1 and zero-pressure thermal<br />

expansion a = a0 + a1T with a0 = 3.4 (2) x 10-5 K-1 and a1 = 0.4 (6) x 10-8 K-1. Present EOSs<br />

enables accurate estimation of the density of SuB and phase D in pyrolitic composition in the deep<br />

mantle conditions. The density reduction of hydrated subducting slab at the top of the lower mantle due<br />

to presence of about 10 vol.% of SuB/phase D (with extrapolation to Fe-bearing composition) would be<br />

1.5-2.0%. This indicate that hydrated slab (>1 wt.% H2O) may be buoyant in the surrounding mantle<br />

rocks and does not penetrate into the deep lower mantle.<br />

Keywords: subduction, mantle, hydration

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