HR: 10:50h
AN: V32B-03    [Abstracts]
TI: The Coupled Effects of Pressure and Mineralogy on the Dynamics of Axisymmetric Thermochemical Plumes in the Earth's Lower Mantle
AU: * samuel, h
EM: henri.samuel@yale.edu
AF: Department of Geology & Geophysics, Yale University, 210 Whitney avenue, New Haven, CT 06511 United States
AU: Bercovici, d
EM: david.bercovici@yale.edu
AF: Department of Geology & Geophysics, Yale University, 210 Whitney avenue, New Haven, CT 06511 United States
AB: Mass balance considerations based on chondrites compositions as well as the observed anticorrelation between bulk sound and shear wave velocities suggest that a significant part of the deep mantle is enriched in Si and Fe, relative to the bulk mantle composition. Several numerical studies and laboratory experiments have shown that the presence of these compositional heterogeneities may have a first order impact on the dynamics of ascending mantle plumes, through their induced chemical density contrast Δ ρ_χ/ρ with respect to the surrounding mantle. However, using mineral physics data and thermodynamic considerations, we first show that contrary to what was assumed in previous geodynamic studies, Δ ρ_χ/ρ may significantly vary with pressure in the lower mantle (up to 200% variation), depending on the Si and Fe content of the chemically distinct material. We therefore investigate the coupled effects of mineralogy and pressure on the dynamics of axisymmetric thermochemical plumes in the Earth's lower mantle, using both high resolution numerical experiments and analytical theory. Our results show that the effect of mineralogy is considerable: (1) For relatively low Si enrichment, an oscillatory behavior of the plume head is observed, similar to previous laboratory experiments [Davaille, Nature, 402, 756-760, (1999)] but without the need of large volume of chemically distinct material. (2) For Si-enriched compositions with respect to a reference pyrolitic mantle, the chemical density excess increases with decreasing depth, leading to the stagnation of large plume heads at various depths in the lower mantle. As a consequence, these thermochemical plumes may display broad (~ 1200 km wide and more) negative seismic velocity anomalies at various lower mantle depths, which may not necessarily be associated with upwelling currents. In addition, the coupling between mineralogy and dynamics may provide an efficient mechanism for the long term survival of compositional heterogeneity in the lower mantle, as suggested by geophysical observations and geochemical record.
DE: 7270 Tomography (6982, 8180)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8125 Evolution of the Earth (0325)
DE: 8130 Heat generation and transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005