HR: 17:15h
AN: T14B-06    [Abstracts]
TI: Dense layer entrainment and structure of mantle plumes
AU: * Lin, S
EM: skylin@umich.edu
AF: Department of Geological Sciences, Univeristy of Michigan, 2534 C.C. Little Building, 425 E. University Avenue, Ann Arbor, MI 48109-1063 United States
AU: van Keken, P
EM: keken@umich.edu
AF: Department of Geological Sciences, Univeristy of Michigan, 2534 C.C. Little Building, 425 E. University Avenue, Ann Arbor, MI 48109-1063 United States
AB: The entrainment of a dense layer near the bottom of the mantle by thermal plumes has important implications for the style of the mantle convection, the structure of the mantle and the conditions of melt formation below midoceanic islands. We have carried out numerical experiments in axisymmetrical, spherical shell to investigate the influence of the thickness and the excess density of the dense layer as well as the plume viscosity on the effectiveness of the dense layer entrainment, plume formation and chemical heterogeneities in the plumes. Our results show that the denser material can be entrained by the plumes and reaches the surface within a wide range of model parameters. Both the thickness and density of the dense layer and plume viscosity control the entrainment and plume structure. Temperature at the plume axis is a function of the thickness and density of the dense layer and is about 40% - 90% of maximum initial thermal perturbation when denser material is entrained. We also find that the entrainment of denser material slows the ascending plume and changes the shape of plume head. In addition, the internal structure of the plume head becomes complicated and the uneven distribution of the denser material causes strongly heterogeneities in the plume head for models with temperature-dependent rheology. Our model results imply: (1) Subducted slabs and recycling oceanic crust near the bottom of the Earth­Ýs mantle can be sampled by the plumes and preserves their distinctive nature when they reach the surface. (2) The temperature reduction of the mantle plumes due to the presence of the dense layer near the bottom of the mantle can be small which suggests that more than one mechanism is responsible for the mismatch between the plume excess temperatures inferred from petrological studies (ca. 200-300 K) and the temperature increase across D­" (ca. 1000-1300 K). (3) The geochemical heterogeneities in flood basalts (if associated with plume heads) can be greater than that in the subsequent hotspot volcanism. The large deformation near the plume axis makes accurate representation of the chemically distinct material a task. Traditional absolute tracer method may not model the chemical heterogeneities properly; it causes instabilities and generates unrealistic physical behaviors in current scenario. We will compare several methods and show that cautious usage of the numerical approaches on the thermochemical convection problems is suggested/required.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7207 Core and mantle
DE: 3902 Creep and deformation
DE: 2753 Numerical modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting