ISOTOPIC CONSTRAINTS ON THE COOLING OF THE CONTINENTAL LITHOSPHERE

Abstract

A new model of continuous diffusion of radiogenic isotopes was applied to mineral 147Sm–143Nd and 176Lu–176Hf data on low-temperature garnet-peridotite xenoliths from Cretaceous South African kimberlites. The radiometric ages are younger than the Archean whole-rock Re–Os and U–Pb ages and reflect that both the Sm–Nd and Lu–Hf chronometric systems remained open under the thermal conditions of the lithospheric mantle. The radiogenic character of Hf in garnets, however, indicates that even if essentially no pyroxene remained immune to the effects of metasomatic events, the core of many garnets may preserve memory of the long history of this mineral in the subcontinental lithosphere. The cooling rates deduced from the garnet Sm–Nd ages in the South African lithosphere are fairly low (40–105 °C Gy−1), but compare well with values obtained on similar samples from different regions. These unexpectedly low values imply that the heat flow at the base of the subcontinental lithospheric mantle has changed only very slowly through time. They further support the recent suggestion that, as a result of viscous dissipation by plate bending, convection vigor and heat flow are to some extent decoupled, which argues against a thermal feedback on geodynamics. Modern convection may still be mining fossil heat stored in the lower mantle.