HR: 14:10h
AN: DI43A-03    [Abstracts]
TI: New seismic constraints on the 660 km discontinuity at the base of the transition zone
AU: * Deuss, A
EM: deuss@esc.cam.ac.uk
AF: University of Cambridge, Dept. Earth Sciences, Cambridge, CB3 0EZ United Kingdom
AU: Redfern, S
EM: satr@esc.cam.ac.uk
AF: University of Cambridge, Dept. Earth Sciences, Cambridge, CB3 0EZ United Kingdom
AU: Chambers, K
EM: kitc@earth.ox.ac.uk
AF: Oxford University, Dept. Earth Sciences, Oxford, OX1 3PR United Kingdom
AU: Woodhouse, J H
EM: john.woodhouse@earth.ox.ac.uk
AF: Oxford University, Dept. Earth Sciences, Oxford, OX1 3PR United Kingdom
AB: The 660-km discontinuity separates the upper mantle from the lower mantle. Its characteristics determine the style of mantle convection in the Earth and are usaully interpreted in terms of a phase change in ringwoodite (γ-spinel). Seismic reflections of this discontinuity have routinely been observed on a global scale in precursors to the SS phase (e.g. Flanagan & shearer, 1998). However, it has not been seen before in precursors to the PP phase (Estabrook & Kind, 1996, Shearer & Flanagan, 1999). This was interpreted by the discontinuity having a zero change in bulk modulus, posing major problems for models of mantle composition. In particular, these observations cannot be reconciled with a pyrolite mantle composition. Here, we study the detailed characteristics of the 660 km discontinuity on both a global and regional scale by comparing a newly processed high quality data set of PP and SS precursors. We report the first observations of this discontinuity in many different regions using precursors to the PP phase. We find a very complicated structure, showing single and double reflections ranging in depth from 640 to 720 km. These observations require the existence of multiple phase transitions on a global scale at the base of the transition zone. Commonly used mantle models (i.e. pyrolite and piclogite) contain a mixture of olivine, garnet and pyroxene which have different phase transitions at 660 km depth. First, in the olivine component there is the transition from ringwoodite to perovskite and magnesiowüstite. Secondly, in the residuum there is a transition from majorite garnet to perovskite. The combination of these phase changes leads to a delicate balance that can impede or enhance convection and also lead to multiple discontinuities around 660 km depth, depending on the local temperature and mantle model (Weidner & Wang, 1998, Hirose, 2002). Computations of reflection amplitudes for different models show that our observations are consistent with a pyrolite composition and imply that phase transformations from the garnet component are of major importance for understanding the structure of the Earth's mantle and its convective state.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3924 High-pressure behavior
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005