HR: 08:12h
AN: DI51B-02 INVITED    [Abstracts]
TI: Phase relations of cold subducted slab: constraints on the slab temperature and on the chemical heterogeneity in the lower mantle
AU: * Komabayashi, T
EM: komabayashi.t.aa@m.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Omori, S
EM: omori@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Hirose, K
EM: kei@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AB: We will talk about two topics of phase relations in cold subducted slabs. They are stability relations of high- pressure hydrous phases in a peridotite layer and of Ca-perovskite in a mid-oceanic ridge basaltic (MORB) crust on the basis of the thermodynamic calculations and high-pressure experiments. In the hydrous peridotite system MgO-SiO₂-H₂O, seven high-pressure hydrous phases appear after serpentine dehydration (~150-km depth). These hydrous phases carry water to the deep mantle condition. At the transition zone, a series of dehydration reactions will occur if the slab temperature is above 1300K. In the case of lower temperature, high-P hydrous phases will further carry water into the deep lower mantle. At about 1300-km depth, hydrous phase D will transfer water to high-pressure ice if the temperature is lower than 1300K. After the ice formation, no fluid-forming reaction may occur in the slab except at the core-mantle boundary where the temperature increase is expected. The depth distribution of dehydration reactions in the slab is well consistent with that of seismic event in the subduction zones if the appropriate temperature is assumed. This suggests that the deep-focus seismicity is induced by the dehydration reactions in the slab, and further suggests that the seismic event is an indicator of slab temperature and water transport into the deep mantle. CaSiO₃-perovskite undergoes a structural phase transition from tetragonal to cubic symmetry at about 540 K, almost independent of pressure. This transition temperature significantly increases with increasing Al₂O₃ contents in Ca-perovskite. Unlike in multicomponent peridotite systems, in a mid-oceanic ridge basalt system, Ca-perovskite contains significant amounts of Al₂O₃ up to about 3 wt% where the structural phase transition may occur at about 1200 K. Seismological studies reported numbers of seismic scatters, reflectors, and low-V layers at a wide depth range (1100-1850-km depth) beneath Mariana subduction zone (e.g., Kaneshima and Helffrich, 2003). A key feature of these observations is a large drop in S-wave velocity without significant anomaly in P-wave velocity. The structural phase transition in Ca-perovskite may be ferroelastic type and therefore easily explain these abnormal seismic features. If these observations are indeed due to the structural phase transition in Ca-perovskite, the temperature of MORB crust of the slab at those depths should be around 1200K. We will further discuss the slab temperature and the chemical heterogeneity in the lower mantle caused by the subduction of the oceanic plate. References, Kaneshima and Helffrich, 2003. JGR, 108, NO. B5, 2272, doi:10.1029/2001JB001596.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting