HR: 10:20h
AN: U41C-01 INVITED     [PDF]
TI: D": The Enigmatic Magmatic Boundary Layer
AU: * Williams, Q
EM: quentw@emerald.ucsc.edu
AF: Dept. of Earth Sciences, U. of California, Santa Cruz, CA 95064 United States
AB: The structures and dynamics of the basal region of Earth's mantle lie at the crux of a broad suite of geophysical questions. These include: how Earth's silicate mantle and iron-rich core interact; how volcanic hot spots are generated; how (and at what rate) heat is transported from Earth's deepest interior; and how the Earth's silicate mantle has differentiated through time. The seismically anomalous lowermost 200-300 km of Earth's mantle are characterized by both small or negative changes in velocity with depth, and seismic discontinuities about 250 km above the core-mantle boundary (CMB), and 5-40 km directly above the core. The origin of the former discontinuity remains unclear, but it is likely that it marks the top of the anomalous region at the base of the mantle. The latter discontinuity, characterized by decreases in compressional wave velocity of about -10 percent and changes in shear velocity of about -30 percent (the Ultra-Low Velocity Zone, or ULVZ), is most readily explained by the presence of 5-30 percent partial melt in this zone. The key issue associated with the presence of partial melt at the absolute base of Earth's mantle is not why there is melt present at this depth--this is simply a natural consequence of intercepting the solidus of the basal material--but rather why the melt has not segregated from the coexisting solids into a pure melt layer. The apparent maintenance of a mixture of solid and liquid at the base of Earth's mantle strongly implies that the density difference between melt and solid under these conditions is vanishingly small, and that the solid and liquid compositions have evolved to the point where they coexist at the same depth. Moreover, the partially molten slurry is unlikely to be completely confined to the base of the mantle. The negative shear velocity and small compressional velocity gradients throughout the bulk of D", when coupled with the probable small values of the temperature derivatives of the elastic properties of minerals at the pressures of the core-mantle boundary, are most readily explained by the ubiquitous presence of a small fraction of partial melt (less than 1 percent) distributed throughout the lowermost 200-300 km of the mantle. Seismic evidence indicates that some of this melt may be confined within lamellae distributed at variable depths within D".This ubiquity of melt within the bottom boundary layer of Earth's mantle implies that the geodynamics of the bottom of the mantle is unlikely to be dominated by the solid-state convective circulation present within the bulk of Earth's mantle, but instead by classical magmatic processes: magma ascent and re-equilibration, fractional crystallization, and ubiquitous dissolution and reprecipitation.
DE: 7207 Core and mantle
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8439 Physics and chemistry of magma bodies
SC: U
MN: 2003 Fall Meeting