Effective CO 2 -storage in the shallow solid Earth mainly occurs by the formation of carbonates. Although the possibility of transport and storage of carbonates to great depth is demonstrated experimentally, ultra-deep mantle carbonates have not been found before. Applying several in situ analytical techniques on inclusions in diamonds from Juina (Brazil) originating from the lower part of the transition zone (N 580 km) or even the lower mantle (N 670 km), reveal the existence of deep Earth carbonates. These finding unquestionably show that at least locally carbonates exist within the deep Earth and may indicate that the Earth's global CO 2 -cycle has an ultra-deep extension.

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Nyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wiistite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase- wustite series belong to two separate groups: wustite and Mg-wustite with Mg# = 1.9—15.3, and Fe- periclase and periclase with Mg# = 84.9—92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5—90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wustite inclusions with Mg# = 1.9—3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

More than 5,000 diamond crystals (or fragments) from kimberlite sills and placer deposits in the Guaniamo area of Venezuela have been characterized in terms of morphology, internal structure, carbon isotopic composition, syngenetic mineral inclusions, and the abundance and aggregation state of nitrogen. Ours is the first comprehensive mineralogical study of diamond from the Guaniamo area. About 50% of the crystals are resorbed dodecahedral forms; octahedra are the next most common form. In most cases, the diamond is colorless; 55-90% show radiation-induced pigmentation. About 20% of the stones have very low N contents (Type II); the remainder belong to the transitional IaAB type, with B > A. Ninety-three mineral inclusions were extracted from 77 crystals or fragments of diamond and analyzed by electron microprobe and LAM-ICP-MS to establish their traceelement compositions and the pressures and temperatures of diamond crystallization. In all, 86% of the diamond samples contain inclusions of the eclogitic paragenesis, represented by garnet, omphacite, rutile, ilmenite, pyrrhotite, and probable coesite. Inclusions indicative of the peridotite paragenesis are pyrope, chromian spinel and olivine. One inclusion of ferroan periclase may indicate a lower-mantle origin. The ␦ 13 C of 108 diamond samples ranges from -3.2‰ to -28.7‰, but most stones have ␦ 13 C ≤10‰. We contend that in large part, the diamond in placers in the Guaniamo area was derived from the Guaniamo kimberlite sills. P-T estimates on mineral inclusions suggest that most originated near the base of the lithosphere (T 1200-1300°C); this zone may contain a substantial proportion of eclogite formed by subduction of crustal material. The very high proportion of diamond derived from an eclogitic association in the Guaniamo deposits, and several features of the mineral inclusions trapped in diamond, show striking parallels to the Argyle deposit of Australia. Both deposits occur within cratons that have experienced extensive Proterozoic tectonothermal activity.Keywords: diamond, carbon isotope, nitrogen, mineral inclusions, eclogitic association, Guaniamo, Venezuela. SOMMAIRENous avons caractérisé plus de 5,000 cristaux (ou fragments) de diamant provenant de filons-couches de kimberlite et de dépôts alluvionnaires dans la région de Guaniamo, au Vénézuela, selon leur morphologie, leur structure interne, la composition isotopique du carbone, leurs inclusions minérales syngénétiques, et l'abondance et l'état d'agrégation de l'azote. Ce travail * Publication No. 213 of the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC). § E-mail address: felixvkaminsky@cs.com 1348 THE CANADIAN MINERALOGIST constitue la première étude minéralogique compréhensive du diamant de la région de Guaniamo. Environ 50% des cristaux ont une forme dodécaédrique résorbée; l'octaèdre est la deuxième forme la plus courante. Dans la plupart des cas, le diamant est incolore; entre 55 et 90% des échantillons font preuve d'une pigmentation due à une i...

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