HR: 08:20h
AN: T41H-02    [Abstracts]
TI: Flux estimates from tomographic plume images yield evidence for chemical stratification in the mantle.
AU: * Nolet, G
EM: nolet@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Department of Geology and Geophysics, Yale University, New Haven, CT 06520 United States
AU: Montelli, R
EM: montelli@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AB: We performed a new resolution analysis of the plumes visible in P wave tomography (Montelli et al, {\it Science 303},338, 2004), using realistic plume sizes matched to the observed ones, and including estimates of both the thermal and the volume flux. This experiment revealed 15 segments of lower mantle plumes that are sufficiently well resolved to estimate the plume flux. To do so we solve the Stokes equation, assuming that there is an approximate local equilibrium between the thermochemical buoyancy and the viscous drag.\To handle the large uncertainties in physical model parameters, we generated an ensemble of 600,000 earth model parameter combinations, varying viscosity, excitation enthalpy, geotherm, thermal expansivity, $\partial V_p/\partial T$, melting temperature, iron content, attenuation and heat capacity. For commonly accepted values of the viscosity of the lower mantle around $5 \times 10^{22}$ Pa\ s, the heat flux through the plumes is unacceptably high unless the buoyancy of the hot material is reduced by a denser component, most likely iron (as in our modeling).\For five of the well resolved plume segments, located beneath the hotspots of Afar, Kerguelen, Tahiti, Cape Verde and La Reunion, independent estimates of the buoyancy flux are available, which allow us to find combinations of `acceptable' model parameters, in particular the iron content and the viscosity. For Afar, Cape Verde and Tahiti this results in a maximum likelihood estimate of the extra iron along a line given by $\Delta X_{Fe} = 12.3 - 0.5\log \eta$ % (viscosity $\eta$ in Pa s), typically requiring $\Delta X_{Fe} = 1%$, or an iron enrichment of about 10% in the plume (Kerguelen and Reunion plot above and below this line, respectively). We use this to estimate the flux in other plumes.\It is possible that the buoyancy flux as estimated by the topographic swell underestimates the flux at depth because some of the plume flux is used to maintain the asthenosphere, as originally proposed by Morgan in 1972, allowing for lower $\Delta X_{Fe}$ or $\eta$. But even then our findings seems to support the hypothesis that the region above the Earth's core has a chemical composition that is distinct from the rest of the mantle. Iron enrichment of plumes would also explain the absence of plume heads in the tomographic images and leads to an alternative explanation of flood basalts: a starting plume will tap into the top of the lower mantle reservoir and be only slightly enriched in iron, leading to very high values of heat and volume flux, as our calculations indicate.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
DE: 5754 Physical properties of materials
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting