Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.s11g..07l&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #S11G-07
Mathematics
Logic
7203 Body Wave Propagation, 7207 Core And Mantle, 8120 Dynamics Of Lithosphere And Mantle: General, 8124 Earth'S Interior: Composition And State (Old 8105), 8147 Planetary Interiors (5430, 5724)
Scientific paper
The boundary layer at the base of the mantle has been shown to have strong seismic heterogeneity, dominated by large scale provinces of higher or lower than average P and S velocities. Most attempts to account for the observations involve a hybrid boundary layer similar to that at the Earth's surface, involving large, enduring chemical heterogeneities embedded in a dynamically mixed boundary layer that participates in overall mantle flow. Thus, high velocity regions are associated with ponds of subducted slabs and low velocity regions are continent-size chemical aggregations that accumulate beneath upwellings. An alternate perspective of the boundary layer that merits consideration is that the lowermost mantle is chemically stratified, a natural consequence of the chemical differentiation of the planet and the proximity to the massive density and compositional change at the core-mantle boundary. It is difficult to account for the observed seismic attributes of D" with such a model unless one allows for the possibility of variable partial melting of the boundary layer. If the eutectic solidus of the chemically distinctive D" region is very close to the CMB temperature, lateral variations in the thermal structure of D" may cause variable degrees of partial melting. Proximity of D" temperatures to the solidus has gained viability by the observation of ultra-low velocity zones with 10 to 30 percent velocity reductions right above the CMB, and the profound affect of even very small amounts of melting on seismic shear velocity provides a means by which to account for large scale variations of +/- 4 percent in shear velocity as observed. Chemical contrasts and melting effects in a stratified thermo-chemical boundary layer can plausibly account for all of the salient seismological observations about the boundary layer. Testing of this hypothesis, versus the notion of the hybrid boundary layer presents a basic challenge in deep Earth research.
Garnero Edward J.
Lay Thome
Williams Quentin
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