Inclusion of kokchetavite in a diamond crystal from Venezuela: an evidence of continental crust subduction

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We have examined in detail diamond crystals from the Guanyamo placer deposit (Venezuela) with numerous mineral inclusions. Inclusions in the studied diamonds are represented by a typical set of eclogite-paragenesis indicator minerals: omphacite, garnet, kyanite, coesite and rutile. In addition to single mineral inclusions, polyphaser inclusions have also been diagnosed. The mineral associations of polyphase inclusions adjacent to each other within a distance of less than 100 micrometres within the same growth zone can vary significantly. For example, in one of the diamond crystals studied, polyphase inclusions are represented by the following associations: sanidine – dolomite – anatase and magnetite – rutile – dolomite – apatite – kokchetavite – graphite. It should be noted that this is the first discovery of kokchetavite as an inclusion in diamond crystals of kimberlite origin. Previously, this hexagonal polymorphic modification of KAlSi3O8 had only been diagnosed in minerals from continental crustal rocks formed under conditions of high or ultra-high mafic metamorphism. Thus, the presence of kokchetavite as inclusions in diamond crystals carried by kimberlites allows us to infer the subduction of continental crustal material to mantle depths and its important role in metasomatic alteration of upper mantle rocks.

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Sobre autores

A. Korsakov

Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences

Email: pazilovdenis@igm.nsc.ru
Rússia, Novosibirsk

D. Mikhailenko

Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences; Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences

Autor responsável pela correspondência
Email: pazilovdenis@igm.nsc.ru
Rússia, Novosibirsk; Ekaterinburg

A. Serebryannikov

Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences

Email: pazilovdenis@igm.nsc.ru
Rússia, Novosibirsk

A. Logvinova

Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences

Email: pazilovdenis@igm.nsc.ru
Rússia, Novosibirsk

D. Gladkochub

Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences

Email: pazilovdenis@igm.nsc.ru

Corresponding Member of the RAS

Rússia, Irkutsk

Bibliografia

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2. Fig. 1. Photograph of the studied diamond crystal from Venezuela with mineral inclusions. Black frames highlight fragments of the diamond crystal with coesite inclusions (Fig. 2) and coexisting polyphase inclusions of potassium minerals (Figs. 3, 4).

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3. Fig. 2. Raman mapping images showing residual stresses in a diamond crystal (a–d), where the brightness shows the intensity of the selected peak at a point. The shift of the diamond peak (1332 cm–1) and the most intense coesite peak (521 cm–1) by 4 cm–1 and 12 cm–1, respectively (Fig. 2 b) indicates significant residual stresses in the diamond around the coesite inclusion and in the inclusion itself (Fig. 2 d, f). (g)–(h) – representative Raman spectra for a diamond crystal and a coesite inclusion in it (enlarged fragment from Fig. 1).

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4. Fig. 3. Three-phase inclusion #1 in a Vn-65 diamond crystal. (a) photograph in transmitted light; (b) Raman map showing the mineral composition of the inclusion (colors correspond to the spectra in Figure c); (c) Raman spectra of minerals identified in inclusion #1.

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5. Fig. 4. Polyphase inclusion #2 in diamond Vn-65: (a) Raman map of inclusion #1 (colors correspond to spectra in Fig. 4b); (b) Raman spectra of minerals diagnosed in inclusion #2. Mag = magnetite, Rut = rutile, Dol = dolomite, Ar = apatite, Kok = kokchetavite, Gr = graphite.

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6. Fig. 5. IR absorption spectra of diamond with mineral inclusions. The individual fragments of the IR spectrum removed demonstrate the presence of probably liquid water in the area with kocchetavite inclusions. Center – the central part of the diamond crystal. Rim – the edge part of the diamond crystal without visible inclusions. Inclusion – corresponds to the area of ​​the diamond with polyphase inclusions.

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