HR: 16:35h
AN: V44B-03    [Abstracts]
TI: Dynamics of Thermochemical Plumes
AU: * Farnetani, C G
EM: cinzia@ipgp.jussieu.fr
AF: Institut de Physique du Globe, LDSG, boite 89 4, pl. Jussieu, Paris, FRA 75252 France
AU: Samuel, H
EM: henri.samuel@yale.edu
AF: Yale University, Dept. of Geology and Geophysics PO Box 208109 , New Haven, CT 06511 United States
AB: We investigate the dynamics of thermo-chemical plumes to enlighten the fundamental differences with purely thermal plumes. The key features of our 3D numerical model include: (1) a compressible mantle with an endothermic phase transition at 670km depth, (2) a mantle 'wind' induced by the imposed surface plate motion, (3) twenty million active tracers simulate denser material initially in the lowermost mantle, (4) plumes form naturally i.e., without imposing any temperature perturbation. First, we investigate the widely accepted head-tail structure of plumes. Our results show that thermo-chemical plumes reaching the surface may or may not have a head since, in some cases, only a narrow 'tail' of hot material is able to ascend in the upper mantle. Therefore, we suggest that the existence of a large igneous province at the onset of hotspot volcanism is not a valid prerequisite for a deep plume origin. Second, we investigate the entrainment of deep heterogeneities. Our results show the generation of narrow, long lasting, distinct filaments in the plume's tail. Therefore, the plume conduit is laterally heterogeneous, rather than concentrically zoned. Third, we calculate the shear wave velocity anomalies in the lower mantle, using the temperature field and the distribution of chemical heterogeneities provided by the convection model. The great variety of plume's shapes and sizes differs strikingly from the expected 'mushroom' shape of purely thermal plumes, bearing important implications for the interpretation of seismologically detected plumes. Finally, our model predictions will be compared with a variety of observations in the Central Pacific.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7200 SEISMOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting