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DIAMONDS commonly occur in kimberlites, lamproites and alluvial sediments derived from these rocks. More recently, diamonds (or their graphite pseudomorphs) have been discovered in ultramafic massifs1 and picrites2. Here we report the occurrence of diamonds in situ in crustal rocks: highly retrograded high-pressure metamorphic garnet–pyroxene and pyroxene–carbonate–garnet rocks, biotite gneisses and schists from the Kokchetav massif, northern Kazakhstan, USSR. The diamonds are cubo-octahedral, averaging 12 μm in size, and occur in zircons, and with euhedral graphite as inclusions in unzoned garnets. We believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite. Mica, rutile, titanite, clinopyroxene, kyanite and zircon also occur as inclusions in garnet, often intergrown with the diamonds. Equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of ⩾40 kbar and temperatures >900–1,000 °C.

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 ocean1,2, subducted Archaean oceanic crust3–5, or crystallized high-pressure mantle melts6–9. 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 peridotites12, show that the Siberian craton lithosphere was at least 150 km thick (the minimum required for diamond stability) by the Mid-Archaean.

Analysis of inclusions1 has shown that natural diamond forms at pressures of 5-6 GPa and temperatures in the range 900-1,400 °C. In non-metallic systems2,4, diamond has been synthesized only at pressures greater than 7 GPa and temperatures of more than 1,600 °C. We find that diamond can crystallize in alkaline carbonate-fluid melts at pressures and temperatures that correspond to those of natural diamond formation.