HR: 0800h
AN: U11A-0010    [Abstracts]
TI: Cratons Formation and the Earth Episodic Thermal History
AU: * Davaille, A
EM: davaille@ipgp.jussieu.fr
AF: IPGP/ CNRS UMR 7154, 4 place Jussieu, PARIS cedex 05, 75 252, France
AU: Arndt, N
EM: Nicholas.Arndt@ujf-grenoble.fr
AF: LGCA / CNRS UMR 5025, Maison des Geosciences, BP 53, Grenoble cedex 09, 38401, France
AB: U-Pb ages of zircons in granites and large rivers record a three-stage evolution of the continental crust. Plate tectonics operated in the first stage, from ~4.4 to 2.7 Ga. Huge peaks of crustal growth separated by long troughs dominated the second stage, from 2.7 to 1.8 Ga. Semi-continuous growth punctuated by large peaks characterized the last stage, from 1.8 to 0 Ga. Individual peaks in the second stage open with massive mafic- ultramafic volcanism and climax 30 Ma later with intrusion of voluminous granitoids: each peak opened with enhanced mantle plume activity, climaxed with accelerated plate tectonic activity, and was followed by a long quiet period when little crust formed. We develop a fluid-mechanics model to explain the three-stage evolution and the pronounced peak-and-trough pattern of the second stage. Low temperatures in the upper mantle during the first and last stages allow the formation of thin, subductable oceanic crust, and lead to a plate-tectonic regime. The crustal-growth peaks of the second regime results from destabilization of a hot denser layer at the bottom of the lower mantle. Domes rising from this layer partially melt to form voluminous mafic magmas. Moreover, when the domes hit and spread under the top surface, they peel off the cold thermal boundary layer there, which triggers a ring of enhanced cold instabilities around each thermochemical dome. This period of enhanced downwellings (i.e. subduction) is however followed by a lull, during which the cold thermal boundary layer is growing again. The onset of the first thermochemical instabilities is predicted to be synchroneous on the whole mantle , and followed by several more disorganized events, in agreement with observations. These experiments closely link continental growth and the thermal history of our planet, which appear strongly episodic. The importance of the 2.7 Ga peak, related to the sudden destabilization of a lower denser layer, suggests that extraction of heat from the core could have changed drastically at this time. This could have triggered the inner core crystallization and a new dynamo regim.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8100 TECTONOPHYSICS
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8125 Evolution of the Earth (0325)
SC: Union [U]
MN: 2007 Fall Meeting