Many lines of evidence suggest that heat loss from the earth should have been significantly greater in the Archean. The presence of high temperature komatiites, greater radiogenic heat production and heat from the secular cooling of the earth all imply higher mantle temperatures in the Archean. However, these lines of evidence are difficult to reconcile with Archean metamorphic PT data, diamond thermobarometry, mantle xenoliths in kimberlites, the ominous lack of minimum melting granites and estimates for crustal thickness which all suggest that geothermal gradients in the Archean, at least on the continents, were not very different from today. This paradox presents problems for reconstructing Archean tectonic processes and environments.
It has been suggested (1) that if the rate of plate creation and subduction in the Archean occurred much faster than today, then the ratio of oceanic to continental heat loss would increase, preserving continental integrity. However, a clear physical basis for for this has, historically, been lacking. Recent modeling work (2) suggests that for a simple fluid-dynamical system, with continents represented as a chemically distinct boundary layer, this partitioning of heat loss occurs as a natural consequence of increasing convective vigour. One of the main criticisms of this model is the lack of realism in the oceanic plates. That faster convection necessitates faster plate creation and subduction is by no means certain. It has been proposed that to test the robustness of this result it is necessary to include an isothermally defined rigid boundary layer to the convective cell. In this way the effectiveness of the partitioning of heat transfer between a thermal chemical boundary layer (ie continents) and a thermal mechanical boundary layer (ie oceanic plates) can be systematically simulated. In this project, I propose to construct such a computer model based upon a particle in a cell code in the manner of Lenardic and Moresi. Its validity will be tested against reconstructed geotherms from present day heat flow data and existing Archean PT data. In this way it is hoped to assess whether the ratio of heat loss between the oceans and the continents increases with convective vigour under a plate-tectonic like mode of convection.
Dr R. D. Müller, The University of Sydney
Dr Louis Moresi, CSIRO, Nedlands and The University of Sydney, Research Associate
Dr Roger Buick, The University of Washington
Craig O’Neill, The University of Sydney, PhD student