HR: 1340h
AN: DI53A-1099    [Abstracts]
TI: Lehmann discontinuity due to dehydration of phengite
AU: * Ono, S
EM: sono@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AB: It is known that the Lehmann discontinuity has several unique features. The depth of this seismic discontinuity is around 220 km depth. However, this discontinuity has not been detected everywhere at around 220 km depth. It is observed under continents more than as often as under oceans. An increase in the compressional or shear wave velocity and in seismic reflections has been reported. A regionally varying negative seismological Clapeyron slope has been estimated. A seismic transition from anisotropic to a more isotropic state occurs at depths corresponding to the Lehmann discontinuity. Although several models have been proposed to explain these features of this discontinuity, previous models failed to clear these unique features. Here, we propose a new model to explain the origin of the Lehmann discontinuity. We conducted experiments with hydrous sediment. The temperature was varied between 1073 and 1673 K, while a pressure of 6-15 GPa was applied using the multi-anvil press. The typical pelite composition was used as the sediment [1]. Garnet, clinopyroxene, and silica phases were present in all of the experiments. Three hydrous phases were observed at temperatures below 1573 K. The stable crystalline hydrous minerals consisted of phengite below 8 GPa, topaz-OH from 9-12 GPa, and phase egg above 12 GPa. The breakdown boundaries of topaz-OH and phase egg show a positive Clapeyron slope. In contrast, the breakdown reaction of phengite gave a negative slope at about 7 GPa corresponding to 220 km depth. The upper temperature limit for phengite is greater than 1473 K. This phase is thus likely to be stable within average adiabatic mantle conditions [1]. Above ~220 km depth the deformation mechanism of olivine is dislocation creep, which gives a preferred orientation to the crystals. The seismic anisotropies are likely to result from the preferred orientation of olivine. As pressure increases, the diffusion creep becomes dominant. If sediments migrate by the convective flow in the upper mantle and intersects the dehydration boundary of phengite, the released water migrates into the surrounding mantle rock and the sharp boundary between the anisotropic and the isotropic structure is formed at around 220 km depth, because water enhances the change in the deformation mechanism of olivine. The Lehmann discontinuity is likely to be related with this boundary corresponding to the change in the deformation mechanism. As this exists only in regions of stagnated sediment, the local detection of the discontinuity is consistent with the hypothesis of sediment dehydration. The most characteristic features of the Lehmann discontinuity, such as 220 km depth, negative seismological Clapeyron slope, local detection, seismic velocity jump, and the seismological anisotropic to isotropic transition can be reasonably explained by our new model [2]. [1] Ono (1998) J. Geophys. Res., 103, 18253-18267. [2] Ono (2007) The Open Mineralogy Journal, 1, 1-4.
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 7208 Mantle (1212, 1213, 8124)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting