HR: 0800h
AN: T41C-1322    [Abstracts]
TI: Evidence for Little Shallow Entrainment in Starting Mantle Plumes
AU: Lohmann, F C
EM: flohmann@ifm-geomar.de
AF: Leibniz-Institut fr Meereswissenschaften IFM - GEOMAR, Dienstgeb„ude Ostufer Wischhofstr. 1-3, Kiel, SH 24148 Germany
AU: * Phipps Morgan, J
EM: jp369@cornell.edu
AF: Department of Earth & Atmospheric Sciences, Cornell University, Snee Hall, Cornell University, Ithaca, NY 14853-1504 United States
AU: Hort, M
EM: hort@dkrz.de
AF: Institut fr Geophysik, Universit„t Hamburg, Bundesstr. 55, Hamburg, HH 20146 Germany
AB: Basalts from intraplate or hotspot ocean islands show distinct geochemical signatures. Their diversity in composition is generally believed to result from the upwelling plume entraining shallow mantle material during ascent, while potentially also entraining other deep regions of the mantle. Here we present results from analogue laboratory experiments and numerical modelling that there is evidence for little shallow entrainment into ascending mantle plumes, i.e. most of the plume signature is inherited from the source. We conducted laboratory experiments using glucose syrup contaminated with glass beads to visualize fluid flow and origin. The plume is initiated by heating from below or by injecting hot, uncontaminated syrup. Particle movement is captured by a CCD camera. In our numerical experiments we solve the Stokes equations for a viscous fluid at infinite Prandtl number with passive tracer particles being used to track fluid flow and entrainment rates, simulating laboratory as well as mantle conditions. In both analogue experiments and numerical models we observe the classical plume structure being embedded in a `sheath' of material from the plume source region that retains little of the original temperature anomaly of the plume source. Yet, this sheath ascends in the `slipstream' of the plume at speeds close to the ascent speed of the plume head, and effectively prevents the entrainment of surrounding material into the plume head or plume tail. We find that the source region is most effectively sampled by an ascending plume and that compositional variations in the source region are preserved during plume ascent. The plume center and plume sheath combined are composed of up to 85% source material. However, there is also evidence of significant entrainment of up to 30% of surrounding material into the outer layers of the plume sheath. Entrainment rates are found to be influenced by mantle composition and structure, with the radial viscosity profile of the mantle being an important factor in determining plume flow and entrainment rates. Prominent mantle discontinuities (e.g. 410km, 660 km) can slow down or speed up plume ascent significantly, thus affecting which regions of the mantle are sampled efficiently.
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Tectonophysics [T]
MN: Fall Meeting 2005