Thematic Articles

Since the first report of microdiamonds of metamorphic origin in crustal rocks of the Kokchetav Massif, northern Kazakhstan, diamonds have been described from several other ultrahigh-pressure (UHP) metamor- phic terranes. In situ diamond is the best indicator of ultrahigh-pressure conditions (>4 GPa), and testifies to subduction of continental crust to depths within the diamond stability field followed by relatively rapid exhu- mation. In contrast to other UHP terranes, the Kokchetav Massif contains rocks with unusually abundant diamonds, particularly in the Kumdy-Kol region. Kumdy-Kol diamonds exhibit diverse morphologies, dependent upon the host rock. Raman and cathodoluminescence spectra and carbon isotope composi- tions differ between core and rim, indicating two distinct growth stages.

Polycrystalline aggregates of diamond called carbonado and framesite have excited the attention of scientists because their crystallization histories are thought to depart markedly from established modes of diamond genesis. In contrast to kimberlitic diamonds, the geochemical signatures of carbonados are systematically crustal. Since the apparent age of carbonados is Archean (~3.2 Ga), a number of exotic formation theories have been invoked, including metamorphism of the earliest subducted lithosphere, radioactive transformation of mantle hydrocarbon, and mete- orite impact on concentrated biomass. Unlike carbonados, framesites are known to originate in the mantle. They appear to have crystallized very rapidly, shortly before the eruption of the kimberlites that brought them to Earth’s surface, suggesting that old cratonic materials can be remobilized after long-term storage in the lithosphere.

Most diamonds form in a relatively narrow depth interval of Earth’s subcontinental mantle between 150 and 250 km. From carbon isotope analyses of diamond obtained in the 1970s, it was first proposed that eclogitic diamonds form from crustal carbon recycled into the mantle by subduction and that the more abundant peridotitic diamonds formed from mantle carbon. More recent stable isotope studies using nitro- gen, oxygen, and sulfur, as well as carbon, combined with studies of mineral inclusions within diamonds, have strengthened arguments supporting and opposing the early proposal. The conflicting evidence is reconciled if mantle carbon is introduced via fluid into mantle eclogites and peridotites, some of which represent subducted oceanic crust.

Diamonds originate in the deep roots of ancient continental blocks (cratons) that extend into the diamond stability field beneath about 140 km. Over the last two decades, rare diamonds derived from even greater depths—the deep upper mantle, the transition zone (410–660 km), and the lower mantle—have been recognized. Inclusions in diamonds from the deep upper mantle and the transition zone document sources of basaltic composition, possibly related to subduction of old oceanic crust back into Earth’s mantle. Diamonds from the lower mantle carry inclusions that largely confirm predictions of the composition and mineralogy of the deep mantle based on a “pyrolite” (primitive peridotitic) composition of silicate Earth. For some inclusions, however, the chemical evidence again points to a connection with subducting oceanic slabs, possibly ponding at the top of the lower mantle.

Active research on diamond, a carbon mineral with superlative proper- ties, extends into many realms of natural and material sciences. Extreme hardness and transparency make diamond a valuable gem and a high-pressure research tool, as well as a superabrasive. Natural forma- tion at high pressure and resistance to weathering make diamonds our most informative messengers from Earth’s mantle. A review of diamond’s charac- ter and forms leads into the topics of the articles in this issue of Elements.