HR: 11:05h
AN: MR22A-04 INVITED     [Abstracts]
TI: OBSERVATIONS AND MODELING OF EARTH'S DEEP MANTLE BOUNDARY LAYERS
AU: * Garnero, E J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: McNamara, A
EM: allen.mcnamara@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Lay, T
EM: thorne@es.ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064 United States
AB: Traditional ideas regarding Earth's core-mantle interface as a simple division between solid silicate rock lower mantle and liquid iron alloy outer core material are significantly challenged by a broad range of recent discoveries. These include the recent evidence for a lower mantle phase change from perovskite (Pv) to a post-perovskite phase (PPv), lowermost mantle thermo-chemical layering at multiple scales, partial melting, seismic anisotropy from mineralogical texture or fabric development, and small-scale convection with whole mantle plume genesis. Recent seismological analyses have imaged strong topographical variability in the D" discontinuity: up to 100 km over short lateral scales. Published values for the PPv Clapeyron slope imply lateral temperature variations of 600-700 K to induce such topography, if due to a phase change alone. Thus a chemical component to D" and/or strong short scale circulation above D" may be required. Recent analyses of seismic wavespeed anisotropy in D" have similarly provided a richness of results. These include abundant evidence for SV waves slower and faster than SH, as well as coupling between the SH and SV components that implies azimuthal anisotropy. These findings can be fit with tilting of the anisotropy symmetry systems (e.g., the vertical axis of symmetry in vertical transverse isotropy) to varying degrees, as a result of deep mantle dynamics. In addition, more careful analysis of the vertical extent of anisotropy may be used to search for the proposed Pv-to-PPv phase change. As material passes through the phase change, preexisting anisotropic fabric is likely reset, and only after a finite degree of strain will a new anisotropic fabric be developed. One prediction is that if the D'' layer is indeed due to the PPV phase, the site at which material passes into should have a region of very little seismic anisotropy adjacent to (i.e., just below) the transition. Ultra-low velocity zones (ULVZ) may be isolated partially molten pockets at the CMB with significant iron enrichment to account for anomalously high density; while ULVZ layering is an order of magnitude thinner than the proposed PPv layer, better understanding the role of Fe in the PPv phase is paramount. It is also possible that ULVZ may be composed of another material all together such as CMB sediments or subducted crustal material. In this presentation we will summarize several seismological and geodynamical findings, especially as they relate to the PPv discovery.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting