Transient geotherms in Archean continental lithosphere: New constraints on thickness and heat production of the subcontinental lithospheric mantle

Because of its large thickness and thermal relaxation time, Archean lithosphere cannot be in thermal equilibrium with the instantaneous rate of heat production in the lithospheric mantle and heat supplied to its base. Comparison of xenolith ( P , T ) data with time‐dependent thermal models allows constraints on lithosphere thickness, in situ heat production in the lithospheric mantle and time changes of basal heat flux. In this paper, the lithosphere is defined as the rigid part of the thermal boundary layer where heat transport occurs by conduction only. A Monte Carlo procedure is used to determine the full range of thermal models consistent with xenolith ( P , T ) arrays from Newlands and Finsch orangeites, Kaapvaal craton, South Africa. These xenolith suites from an early phase of kimberlite magmatism in the Kaapvaal record thermal conditions in unperturbed lithosphere. Together with constraints on surface heat flow and crustal heat production, these data require the lithosphere thickness to be between 200 and 270 km, with most values between 210 and 250 km. Additional constraints, including the condition that lithospheric temperatures have remained below the solidus, estimates of the cooling rate in mantle xenoliths, and the depth extent of seismic anomalies beneath cratons tighten the solution range. Present‐day values of basal heat flow and heat production in the Kaapvaal lithospheric mantle lie between 12–16 mW m −2 and 0–0.02 μ W m −3 respectively. At 240‐km depth, lithospheric material undergoes secular cooling at a rate of 40 to 110 K/Ga due solely to in situ radioactive decay. Changes of basal heat flow must be less than 5% per Ga, suggesting that thermal conditions in the underlying mantle have not changed much since the Archean.