ON THE PARTITIONING OF MANTLE HEAT-LOSS BELOW OCEANS AND CONTINENTS OVER TIME AND ITS RELATIONSHIP TO THE ARCHEAN PARADOX

The Archaean paradox stems from the fact that while certain lines of e vidence suggest that Archaean continental geotherms were similar to th ose at present, other lines of evidence suggest that the average mantl e heat flux was considerably greater. The simplest qualitative solutio n, which holds that a greater proportion of heat loss was carried by t he creation and subduction of oceanic lithosphere in the Earth's past, has lacked a clear physical basis. At a fundamental level, it require s that the ratio of mantle heat loss below oceans to that below contin ents be an increasing function of overall convective vigour and, to da te, no self-consistent model has been used to suggest a means by which this could be so. A simple means is suggested by models that allow ac cumulations of chemically light near-surface material, analogues to co ntinents, to form within the upper thermal boundary layer of a thermal ly convecting and chemically dense fluid, an analogue to the mantle. T he physical properties of continental crust cause the thermal coupling condition between a model continent and the convectively unstable man tle below to be different from that which exists at the interface betw een mantle and oceans. For the latter, a spatially constant temperatur e condition holds due to (1) the participation of oceanic crust in con vective mantle overturn and (2) the large effective thermal conductivi ty of oceans relative to the mantle. For the former, a spatially near- constant equilibrium heat flux condition holds due to (1) the long-ter m stability of continental crust against remixing into the mantle and (2) the comparable thermal conductivities of distinct rock types. This leads to a laterally variable thermal condition at the surface of the convecting mantle, which, in turn, leads to different local mantle he at flux behaviour in continental versus oceanic regions. As a result, heat flux below continents can increase at a slower rate, with increas ing convective vigour, than it does elsewhere. Model heat-flux scaling s are used to assess the degree to which continental geotherms can be time-stabilized by this means. Results suggest that the thermal surfac e heterogeneity imposed on the mantle by the presence of continental c rust can prevent wide-scale crustal melting in the Archaean by forcing oceans to carry a greater proportion of the Earth's heat-loss load in the past.