Archean Geodynamics and the Thermal Evolution of Earth

Possible geodynamic regimes that may have prevailed in the Archean are investigated by back-tracking the thermal history of Earth from the present-day conditions. If the temporal evolution of plate-tectonic convection is modulated by strong depleted lithosphere created at mid-ocean ridges, more sluggish plate tectonics is predicted when the mantle was hotter, contrary to commonly believed, more rapid tectonics in the past. This notion of sluggish plate tectonics can simultaneously satisfy geochemical constraints on the abundance of heat-producing elements and petrological constraints on the degree of secular cooling, in the framework of simple whole-mantle convection. The geological record of supercontinents back to ~2.7 Ga is shown to be broadly consistent with the accelerating plate motion as predicted by the new model. Furthermore, the very fact of repeated continental aggregation indicates that thicker depleted lithosphere in the past needs to move more slowly to become negatively buoyant by thermal contraction and also needs to be strong enough to support resulting thermal boundary layer. The concept of many small plates covering Archean ocean basins is thus physically implausible. As a consequence of reduced heat flux in the past, mantle plumes are expected to have been weaker in the Archean. The chemical evolution of Earth’s mantle may have been encumbered by sluggish plate tectonics and weak mantle plumes, maintaining its compositional heterogeneity at various spatial scales to the present day. Internal heat production probably played an important role in controlling plate dynamics in the early Archean, for which a different mode of mantle convection is suggested.