HR: 11:50h
AN: T42A-07    [Abstracts]
TI: Record of Earth's Secular Cooling in Mantle Xenoliths from the Continental Lithosphere
AU: Michaut, C
EM: michaut@ipgp.jussieu.fr
AF: Institut de Physique du Globe, 4 place Jussieu, Paris, 75005 France
AU: * Jaupart, C
EM: cj@ccr.jussieu.fr
AF: Institut de Physique du Globe, 4 place Jussieu, Paris, 75005 France
AB: The time-scale for diffusive heat transport in a 250 km thick rock column is 2 Gy, showing that cratonic lithosphere is potentially able to record secular changes of mantle temperature. In the lithospheric mantle, in-situ radiogenic heat production also decays with a characteristic time of about 3 Gy. Thus, the thermal structure of Archean lithosphere is not in equilibrium with the instantaneous rate of radiogenic heat generation and with heat supply at the base of the lithosphere. One consequence is that temperatures in the lithospheric mantle are not steady and continuously decrease with time. Comparison of xenolith (P,T) arrays with time-dependent thermal models allows constraints on time-changes of basal heat flux (or temperature) and on heat production in the lithospheric mantle. Model results depend strongly on lithosphere thickness. Using values of cooling rates determined from isotopic desequilibrium profiles in mantle minerals (F. Albarede, Geophys. Res. Lett. 30, 1015, 2003; R.M. Bedini et al., Earth Planet Sci. Lett. 223, 99-111, 2004) considerably tightens the solution range. All models that are consistent with xenolith (P,T) arrays from the Kaapval craton, South Africa, and with available heat flow and heat production measurements have been determined using a Monte-Carlo procedure. Taking no account of cooling rate estimates but requiring that lithosphere thickness is larger than 200 km, the characteristic decay time for the basal heat flux must be larger than 1.1 Gy. With larger values of lithosphere thickness, such as those obtained from seismological studies, this characteristic decay time increases to very large values exceeding 6 Gy, which would imply no significant change of basal heat flux. Taking into account available cooling rate estimates, there are no solutions with significant variations of basal heat flux for all values of lithosphere thickness. These results suggest that thermal conditions beneath cratons have changed very little since Archean times, and show that much can be learnt about Earth's thermal evolution by studying samples from thick continental lithosphere.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting