HR: 09:00h
AN: T21A-05    [PDF]
TI: Phase relations and phase transformation kinetics of the subducted oceanic crust and the seismic reflectors in the lower mantle
AU: * Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Inst. Mineral. Petrol. and Econ. Geology, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AU: Kubo, T
EM: tkubo@mail.tains.tohoku.ac.jp
AF: Inst. Mineral. Petrol. and Econ. Geology, Tohoku University, Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AB: We determined phase relations in dry and hydrous mid-ocean ridge basalt (MORB) at high pressure and temperature by in situ X-ray diffraction study together with conventional quenching experiments. In the lower temperature range around 1000-1100C at 20-30 GPa, we observed CaMg-cubic perovskite phase both under the dry and wet conditions. However, quenching experiments with a long duration revealed that the cubic perovskite is metastable, and it eventually decomposes into two perovskites. The metastable cubic perovskite might exist in the cold slabs. The phase boundary of the perovskite appearance in the wet MORB has a positive slope, and is about 1 GPa lower than that in the dry MORB (Hirose and Fei, 1998). The results presented above suggest that water can play an important role in slab dynamics near the 660 km discontinuity. The density relations between the basalt and peridotite implies that there is no density crossover between the basalt and the underlying peridotite layers in the wet slabs along the cold slab geotherm, resulting in higher density of the basalt layer throughout the transition zone and the lower mantle compared to the peridotite portion of the slabs. Thus, the hydrated slabs can penetrate effectively into the lower mantle. We also conducted a kinetics study of garnet-postgarnet transformation (Kubo et al., Nature, 2002), and found that the transformation rate is sluggish resulting in metasltable garnet in the dry and cold oceanic crust. On the other hand, our experiments on the in situ X-ray diffraction in MORB indicated that water enhances the reaction kinetics significantly. Therefore, the garnet-postgarnet transformation completes in the wet and cold subducted slabs. Recently, the seismic reflectors have been reported in the upper part of the lower mantle (e.g., Kaneshima and Helfflich, 1998; Niu et al., 2002). The seismic reflectors might be explained by the subducted oceanic crust in the lower mantle. Niu et al. (2002) reported that the seismic reflectors have characteristic properties, i.e., an increase in the density, nearly constant Vp, and a decrease of Vs in the reflectors. Recent mineral physics data on high pressure minerals in the oceanic crust including NAL phase and CF phase (Ono, 2002; Vanpeteghem et al, 2003) may provide information on the state of the oceanic crust in the lower mantle. Our preliminary analysis suggests that the change in physical properties of the seismic reflectors can not be accounted for by a simple chemical change from the lower mantle peridotite to the oceanic crust of the perovskite lithology with or without metastable garnet. Additional mechanisms are needed to explain a decrease of Vs in reflectors suggesting anelastic effects such as a possible existence of fluid or melt.
DE: 1025 Composition of the mantle
DE: 1212 Earth's interior--composition and state (8105)
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2003 Fall Meeting