Strange Diamonds:C N 6 E

y framesite 8 Strange Diamonds: y c 6 The Mysterious Originsn e u of Carbonado and Framesiteq e r F 4 Peter J. Heaney1, Edward P. Vicenzi2, and Subarnarekha De3 2 olycrystalline aggregates of diamond called carbonado andB framesite microcrystalline diamonds found have excited the attention of scientists because their crystallization in association with kimberlites all over the world. The unusual phys-single crystal Phistories are thought to depart markedly from established modes of ical features observed in carbona- diamond genesis. In contrast to kimberlitic diamonds, the100 geochemical dos and framesites have provoked signatures of carbonados are systematically crustal. Since the apparent age a wide array of hypotheses regard- of carbonados is Archean (~3.2 Ga), a number of exotic formation theories ing their origin. This article will focus on the chemical and struc- have been invoked, including metamorphism of the earliest80 subducted y tural properties that unite and lithosphere, radioactive transformation of mantle hydrocarbon,c and mete- divide these two puzzling diamond n varieties. orite impact on concentrated biomass. Unlike carbonados,e framesites are u known to originate in the mantle. They appear to haveq crystallized60 very e CARBONADO rapidly, shortly before the eruption of the kimberlites thatr brought them to Earth’s surface, suggesting that old cratonic materialsF can be remobilized Mineralogical after long-term storage in the lithosphere. 40 Characteristics Carbonado nodules typically are KEYWORDS: carbonado, framesite, polycrystalline diamond pea-sized or greater. Indeed, the largest known diamond of any INTRODUCTION 20 type is the Carbonado of Sergio (Brazil), weighing in at Most people who like diamonds like them large. 3,167 carats. Named for the Portuguese word for “burned,” Nevertheless, a small cadre of geochemists and mineralo- carbonadosC are black polycrystalline masses. As with frame- gists has focused its efforts on diamondiferous grunge—the0 site, carbonado nodules often exhibit a “microporphyritic” black multigranular masses of uncertain pedigree that fall texture in which euhedral crystals measuring hundreds of below even the basest standards of the Gemological -35microns -30across are -25cemented -20by a matrix -15 of micron-sized -10 -5 0 5 Institute of America grading scales. Generically called bort, grains (Fettge and Sturges 1932). In13 contrast to framesites, these diamonds have earned some measure of respect not however, X-ray microtomographyδ revealsC pervasive(‰) macro- as gems but as the best abrasives for jobs that require super- porosity with tunnel sizes in excess of 1 mm (Vicenzi et al. F hardness and supertoughness (Feenstra 1985). Single-crys- 2003). tal diamonds cleave so easily that they wear rapidly during Haggerty (1999) has described carbonados as “the most drilling applications, and inclusions can explode a single- enigmatic of all diamonds,” and the number of scenarios diamond host when heated. In contrast, the high densities proposed for their formation would seem to support this of grain boundaries in polycrystalline diamonds (PCD) claim. Unlike framesites, carbonados have never been impede fracture propagation, and the porosity in natural found in direct association with kimberlite pipes, raising PCD allows for thermal expansion of included mineral the possibility that the Earth’s mantle was not their ulti- grains without catastrophic failure of the entire compact. mate source. Although carbonados have been reported The taxonomy for polycrystalline diamond tends to be from Venezuela (Kerr et al. 1948) and various Russian local- based on qualitative external characteristics, as the names ities (e.g., Gorshkov et al. 1996), carbonados sensu stricto are of a cultural rather than scientific origin. Consequently, are found in Mid-Proterozoic (1–1.5 Ga) metaconglomer- even the modern literature is inconsistent in its application ates overlying the São Francisco and Congo-Kasai cratons of the terminology. In this review, “carbonado” refers in Brazil and the Central African Republic, respectively. The specifically to multigranular diamond aggregates from the São Francisco craton contains the oldest rocks in South Central African Republic and Brazil with distinctive proper- America, at least ~3.2 Ga (Martin et al. 1997; Magee 2001), ties (summarized in TABLE 1 and FIG. 1). “Framesite” is used and the great age of carbonado is part of its fascination. more broadly to describe clusters of randomly oriented Both bulk and in situ analyses of radiogenic lead isotope compositions of carbonado diamond and its inclusions indicate that crystallization occurred between 2.6 and 3.8 1 Department of Geosciences Penn State University Ga ago (Ozima and Tatsumoto 1997; Sano et al. 2002). University Park, PA 16802, USA Carbonados are distinguished from other polycrystalline E-mail: [email protected] diamonds by the inclusions that line pore spaces (Trueb 2 Department of Mineral Sciences and Butterman 1969; Trueb and deWys 1969, 1971; Smithsonian Institution Washington, D.C. 20013-7012, USA Dismukes et al. 1988). The hydrated rare-earth phosphate florencite is the most abundant of these, but over 30 other 3 6338 Nepo Drive San Jose, CA 95119, USA E LEMENTS, VOL. 1, PP. 85–89 85 MARCH 2005 TABLE 1 COMPARISON OF CARBONADO, FRAMESITE, AND ECLOGITIC SINGLE-CRYSTAL DIAMOND. Carbonado Framesite Single-Crystal Eclogitic Diamond Color and Surface Properties Black, dark gray, or brown Light gray or brown Clear to yellow single with smooth exterior with irregular surface crystals are most common Grain Sizes Euhedral grains Shares bimodal texture Variable, but mm-sized (Typically up to 200 µm) with carbonado but and larger crystals are set in microcrystalline euhedral grains are larger common matrix (<0.5–20 µm) Porosity High (10% void space) Low (1% void space) Zero Common Inclusions Florencite-goyazite- Pyrope and almandine- Pyrrhotite, omphacite, gorceixite, xenotime, pyrope garnet, almandine-pyrope garnet kaolinite, quartz, Cr-rich clinopyroxene, orthoclase, zircon, chromite Fe, Fe-Ni, SiC, Si, Sn δ13C Composition –23 to –30‰ with –1 to –24‰ with +3 to –34‰ with modes a mode at –27‰ a mode at –19‰ at –5‰ and –12‰ δ15N Composition –17 to +8‰ –3 to +15‰ –12 to +5‰ minerals have been identified, including orthoclase, quartz, the case of framesites, as described below. It is possible that and kaolinite. These phases are distinctly crustal, and, con- carbonados are merely composite diamond clusters that versely, the mantle-derived inclusions commonly found in formed in the same way as kimberlitic diamond nodules kimberlitic diamond (e.g., pyrope-rich garnet, chromian but with enrichment in light isotopes of C and He. If so, clinopyroxene, pyrrhotite) have never been reported from carbonados may represent the end member of a chemical carbonado. The TEM results of Gorshkov et al. (1996) and continuum in which framesites serve as a bridge to eclogitic De et al. (1998) also have revealed inclusions of native met- diamonds. In this scenario, the strong crustal signature in als and metal alloys, such as Fe, Fe-Ni, Ni-Pt, Si, Ti, Sn, Ag, carbonados is related to the subduction of cold slabs Cu, and SiC. These metal phases are nanometers to hun- beneath continental margins, such that organic matter is dreds of microns in size, and they can be either enveloped imported to the mantle with virtually no mixing of carbon within the diamond itself or present as a coating of the reservoirs (Robinson 1978). This idea could perhaps explain pores. Gorshkov et al. (1999) describe similar metal assem- light rare-earth element (LREE) patterns suggestive of a blages in kimberlitic polycrystalline diamonds from the crustal origin (Shibata et al. 1993; Kamioka et al. 1996), the Udachnaya pipe in Yakutia. high concentrations of polycyclic aromatic hydrocarbons in carbonado pores (Kaminsky et al. 1991), and the pres- Geochemical Indicators for Formation ence of aggregated N defects (Nadolinny et al. 2003). The similarities in the inclusion assemblages of carbonados If carbonados represent mineralized organic carbon, their from the Central African Republic and Brazil provide strong measured age of ~3.2 Ga, when added to the time required evidence that these now geographically distant carbonados for plate subduction, suggests that these diamonds are are genetically related. Equally compelling support for a vestiges of some of the oldest biological material known. common origin comes from the tight clustering of carbon Nevertheless, it should be noted that mantle xenoliths are isotope compositions of nodules from the two localities, often depleted in 13C, and efforts to model the range of 13 with δ C values ranging from –21 to –32‰ (Vinogradov et δ13C values in these samples by simple mixing models of al. 1966; Galimov et al. 1985; Ozima et al. 1991; Kamioka carbon from organic and mantle reservoirs have not suc- et al. 1996; Shelkov et al. 1997) (Fig. 1). In situ isotope ceeded (Deines 2002). Deines’ study suggests that poorly analyses (De et al. 2001) have shown that within individual understood kinetic fractionation effects may play a role in 13 samples, δ C values are very uniform, with a slight the generation of isotopically light diamonds. bimodality between the larger euhedral crystals (–26‰) and the finer-grained matrix (–24‰). Further confirmation The implications of the radiogenic gases in carbonados are for a connection between Central African and Brazilian car- especially controversial. Some authors have proposed

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