CONSIDERABLE uncertainty surrounds the nature and evolution of the Earth’s mantle in Archaean times (>2.5 Gyr ago). Mantle-derived eclogite xenoliths erupted by kimberlites provide important clues in this regard, because their basaltic composition suggests that they may be remnants of an early (>4 Gyr) magma ocean(1,2), subducted Archaean oceanic crust(3-5), or crystallized high-pressure mantle melts(6-9). Better constraints on the age and origin of mantle eclogites are thus important for our understanding of early Earth processes. Here we present rhenium-osmium isotope data for diamond-bearing eclogites from the Udachnaya kimberlite pipe in Siberia, which indicate formation in the Archaean (2.9 +/- 0.4 Gyr). This age is too young for the eclogites to be remnants of early Earth differentiation, but overlaps the age range for crust generation and craton stabilization on both the Aldan and Anabar shields of the Siberian craton (2.85-3.2 Gyr)(10,11). These data, together with 3.1 Gyr Re-Os model ages for diamond-bearing Udachnaya peridotites(12), show that the Siberian craton lithosphere was at least 150 km thick (the minimum required for diamond stability) by the Mid-Archaean.