HR: 08:48h
AN: DI51B-05    [Abstracts]
TI: Seismic observations of mantle discontinuities, and their mineral physical interpretation
AU: Andrews, J
EM: jra36@cam.ac.uk
AF: University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB0 3EZ, United Kingdom
AU: * Deuss, A
EM: deuss@esc.cam.ac.uk
AF: University of Cambridge, Bullard Labs Madingley Road, Cambridge, CB0 3EZ, United Kingdom
AB: We study transition zone discontinuities using a new large collection of stacked receiver functions, in addition to our previous data sets of PP and SS precursors. Such stacks provide a means of investigating hypotheses about the mineral physical nature of discontinuities and constrain the thermal and compositional structure of the Earth's mantle. The 410-km discontinuity is observed in all data types and is characterised by a simple single peak. The 660-km discontinuity, however, shows a much more complex structure. PP precursors only show the 660-km discontinuity in certain regions, while it is always seen in SS precursors and receiver functions. Our receiver function observations reveal a very complicated global structure with single and double reflections ranging in depth from 640 to 720 km. A weaker and possibly non-global discontinuity at approximately 520 km depth is also present in all data types. Confirming previous work on SS precursors, our receiver functions show that this 520-km discontinuity is split in certain regions, while in other regions one discontinuity is observed. So, the different data types are agreeing on the type of complexity observed at each of the transition zone discontinuities. These observations are explained by the presence of multiple phase transitions on a global scale in the transition zone. The combined phase transitions of olivine and garnet in a pyrolite mantle explain the seismic observations of double peaks (or splitting) at the 520 and 660 km discontinuities, while the single olivine phase transition at 410 km explains the simple seismic observations in this depth range. Thus, we can reconcile the complex seismic observations of the transition zone with mineral physical phase transitions. Additional regional variations in the seismic observations imply the need for compositional heterogeneity in minor elements such as water, Fe, Al and Ca. Our three data types also support evidende for reflections from additional upper and lower mantle discontinuities, the most consistent ones being at approximately 220, 800 and 1150 km depth. Discontinuities at 1100-1220 km have been proposed before by some regional studies and would be consistent with tomographic models, particularly in subduction zone areas.
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 the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting