Nature 343, 742 - 746 (22 February 1990); doi:10.1038/343742a0

Diamond inclusions in garnets from metamorphic rocks: a new environment for diamond formation

N. V. Sobolev & V. S. Shatsky

Institute of Geology and Geophysics, Siberian Division of the USSR Academy of Sciences, 630090 Novosibirsk 90, USSR

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 m 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.

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