ARCHEAN RE-OS AGE FOR SIBERIAN ECLOGITES AND CONSTRAINTS ON ARCHEAN TECTONICS

CONSIDERABLE uncertainty surrounds the nature and evolution of the Ear th's mantle in Archaean times (>2.5 Gyr ago). Mantle-derived eclogite xenoliths erupted by kimberlites provide important clues in this regar d, because their basaltic composition suggests that they may be remnan ts of an early (>4 Gyr) magma ocean(1,2), subducted Archaean oceanic c rust(3-5), or crystallized high-pressure mantle melts(6-9). Better con straints on the age and origin of mantle eclogites are thus important for our understanding of early Earth processes. Here we present rheniu m-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 remna nts of early Earth differentiation, but overlaps the age range for cru st generation and craton stabilization on both the Aldan and Anabar sh ields of the Siberian craton (2.85-3.2 Gyr)(10,11). These data, togeth er with 3.1 Gyr Re-Os model ages for diamond-bearing Udachnaya peridot ites(12), show that the Siberian craton lithosphere was at least 150 k m thick (the minimum required for diamond stability) by the Mid-Archae an.