Article

Mantle-related carbonados? Geochemical insights from diamonds from the Dachine komatiite (French Guiana)

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Abstract

Carbonado is a unique type of polycrystalline diamond characterised, among others, by 13C-depleted isotope compositions (δ13C∼-25‰ vs. PDB), little advanced nitrogen aggregation (Ib-IaA) and sintered (ceramic-like) diamond grains. Its origin remains an enigma, with models proposing a formation either in the Earth's crust or even within an exploding super-nova. The possibility that carbonado formed in the Earth's mantle is often rejected because diamond with carbonado-like geochemical features has never been found in rocks, such as kimberlites, that carry diamonds from the mantle.In this study, it is shown that the C- and N- stable isotope compositions, nitrogen contents and nitrogen speciation of diamonds from the Dachine komatiite (French Guyana) exhibit unambiguous similarities with carbonados. These include C-isotopes (from -32.6 to +0.15‰, mode at ∼-27‰), N-aggregation (only Ib-IaA diamonds, from 2 to 76% of N-pairs) and N-isotopes (from -4.1 to +6.9‰, average ∼ +2.1±2.9‰), which all strikingly match the carbonado data. This evidence illustrates that the main geochemical arguments usually called to reject a mantle origin of carbonado are no longer valid. A model linking carbonado crystallisation from komatiite volatiles is developed. In this model, the sintering is produced by the high temperature of the komatiite magma thus accounting for their absence in colder kimberlites. The low δ13C compositions of carbonados would be inherited from the transition zone (> 300km depths), which is known to yield diamonds with distinct C-isotope distributions compared to most lithospheric diamonds (150–300km depths). This model can account for most available observations of carbonados, including their large size, sintering, photoluminescense/cathodoluminescence features and geochemical characteristics.

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... Type Ib diamonds are very rare in nature (<1% of cape series yellow diamonds i nature). So far, type Ib diamonds have only been discovered in Helam/Swartruggens i the eastern block of the Kaapvaal craton [46,47], Lac de Gras in the Slave Craton [48], Da chine in the Amazon Craton [49][50][51], Orapa rock tubes in the southwest of the Zimbabw Craton [52], Qilalugaq of the Rae Craton and the Kankan region of the West African Cra ton [53,54], Tibet of China and Pozanti-Karsanti of Turkey [19]. ...
... Type Ib diamonds are very rare in nature (<1% of cape series yellow diamonds in nature). So far, type Ib diamonds have only been discovered in Helam/Swartruggens in the eastern block of the Kaapvaal craton [46,47], Lac de Gras in the Slave Craton [48], Dachine in the Amazon Craton [49][50][51], Orapa rock tubes in the southwest of the Zimbabwe Craton [52], Qilalugaq of the Rae Craton and the Kankan region of the West African Craton [53,54], Tibet of China and Pozanti-Karsanti of Turkey [19]. ...
... Carbon isotopic compositions of diamonds from kimberlite, lamproite and metamorphic rocks have been extensively studied [23,49,[60][61][62]. According to Cartigny (2005), eclogitic diamonds (E-type) have δ 13 C ranging from −38.5 to +2.7‰ and peridotitic diamonds have δ 13 C from −26.4 to +0.2‰. ...
Article
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Most of the diamond deposits in China are in the North China Craton. In addition to gem diamonds in kimberlite, a large number of microdiamonds have also been discovered in alkaline dolerites. These microdiamonds show very different characteristics from those recovered in kimberlite. Here, we report the morphology, colour, nitrogen contents, and carbon isotopic compositions of the diamonds recovered from the alkalic dolerites in eastern China. The microdiamonds are mainly cube and rhombic dodecahedron with diameters of 0.2 to 0.6 mm. Infrared spectrum analysis shows that these microdiamonds are mostly type Ib with a small amount of type Ia. The Y centre is obvious in type Ib diamond. Modelling mantle residence times for the IaAB diamonds is about 550 Ma. Nitrogen contents of the diamonds range from 4.5–503 ppm, with a median value of 173 ppm. The total δ13C range of the microdiamonds varies between −18.6 and −21.1‰ and are similar to those of ophiolite diamond.
... Although the study of Sumino et al. (2011) highlighted the occurrence of some mantle-derived rare gases, there is no doubt that, for the vast majority of UHP diamonds, the carbon reservoir is crustal, apparently isolated from mantle carbon. The crustal source of carbon is evident from the association of the diamonds with metasedimentary protoliths, the unusual chemical composition of their fluids (e.g., Hwang et al. 2005Hwang et al. , 2006, and their high-N content (up to 1 wt%) or heavy d 15 N (Cartigny 2005(Cartigny , 2010 that are not found among mantle-derived diamonds. ...
... The distribution of d 13 C-values of diamonds formed in Earth's mantle (Fig. 7) are usually divided into distinct diamond populations on the basis of several factors: (1) their eclogitic versus peridotitic paragenesis as inferred from their inclusions, (2) their relative crystallization ages as inferred by younger, often kimberlite-age fibrous coats of coated diamonds grown upon older, typically Archean and Proterozoic monocrystalline diamond cores, (3) their depth of origin in the lithosphere, transition zone or lower mantle as inferred from their inclusions, and (4) their morphology as monocrystalline versus polycrystalline aggregates (including boart, framesites), the latter whose formation age might be closely related to the kimberlite eruption age (e.g., Heaney et al. 2005). In addition, d 13 C values have been measured on (5) polycrystalline, sintered diamonds known as carbonado (Jeynes 1978), whose extra-terrestrial, mantle or crustal origin is unclear (see Cartigny 2010, and references therein), and (6) microdiamonds (typically < 500 microns) formed in crustal rocks subducted to pressures greater than 3.5 GPa along a cold, crustal geotherms. These latter two groups are considered petrogenetically distinct and will be discussed separately. ...
... The "main mantle range" defined by fibrous diamonds (i.e., kimberliterelated, see text for details), carbonatites, and carbonates from kimberlites. For references see Cartigny (2005Cartigny ( , 2010. Figure 7 is a summary diagram of the worldwide data set for the C isotopic composition of diamond. Several key points are clear from the distributions seen: ...
... Although the study of Sumino et al. (2011) highlighted the occurrence of some mantle-derived rare gases, there is no doubt that, for the vast majority of UHP diamonds, the carbon reservoir is crustal, apparently isolated from mantle carbon. The crustal source of carbon is evident from the association of the diamonds with metasedimentary protoliths, the unusual chemical composition of their fluids (e.g., Hwang et al. 2005Hwang et al. , 2006, and their high-N content (up to 1 wt%) or heavy d 15 N (Cartigny 2005(Cartigny , 2010 that are not found among mantle-derived diamonds. ...
... The distribution of d 13 C-values of diamonds formed in Earth's mantle (Fig. 7) are usually divided into distinct diamond populations on the basis of several factors: (1) their eclogitic versus peridotitic paragenesis as inferred from their inclusions, (2) their relative crystallization ages as inferred by younger, often kimberlite-age fibrous coats of coated diamonds grown upon older, typically Archean and Proterozoic monocrystalline diamond cores, (3) their depth of origin in the lithosphere, transition zone or lower mantle as inferred from their inclusions, and (4) their morphology as monocrystalline versus polycrystalline aggregates (including boart, framesites), the latter whose formation age might be closely related to the kimberlite eruption age (e.g., Heaney et al. 2005). In addition, d 13 C values have been measured on (5) polycrystalline, sintered diamonds known as carbonado (Jeynes 1978), whose extra-terrestrial, mantle or crustal origin is unclear (see Cartigny 2010, and references therein), and (6) microdiamonds (typically < 500 microns) formed in crustal rocks subducted to pressures greater than 3.5 GPa along a cold, crustal geotherms. These latter two groups are considered petrogenetically distinct and will be discussed separately. ...
... The "main mantle range" defined by fibrous diamonds (i.e., kimberliterelated, see text for details), carbonatites, and carbonates from kimberlites. For references see Cartigny (2005Cartigny ( , 2010. Figure 7 is a summary diagram of the worldwide data set for the C isotopic composition of diamond. Several key points are clear from the distributions seen: ...
... Synthetic diamonds contain mainly single-substitutional nitrogen and N pairs, typically to a lesser extent (e.g., most <20%) in comparison to natural diamonds, i.e., Ib-IaA diamonds with concentrations typically ~200 atomic ppm N (Collins, 2000). The occurrence of unaggregated nitrogen (type Ib) state reflects their short residence frames (from several minutes to weeks) at high temperatures (Finnie et al., 1994;De Corte et al., 1998;Cartigny, 2010;Howell et al., 2015;Smith et al., 2016). Because single N defects are responsible for the absorption of light that gives high pressure-high temperature synthetic diamonds their characteristic yellow color, as are the ophiolitic diamonds, being smaller than the currently investigated synthetic diamonds, i.e., from 100 to 500 μm, their colors are only pale yellow to colorless and the general observation is that their N content and N aggregation state are overall similar. ...
... Natural diamonds commonly contain nitrogen in abundance varying from a few to several thousand atomic ppm N and show large variations in aggregation states (Boyd et al., 1994). Kimberlitic diamonds type Ib are very rare (<0.1%; reviewed in Cartigny et al., 2004;Nadolinny et al., 2006;Hainschwang et al., 2013;Cartigny, 2010;Smit et al., 2016;Smith et al., 2016) and exhibit high nitrogen aggregation states and would plot on the right side of Figure 7 (IaAB diamonds, e.g., Javoy et al., 1984;Deines et al., 1997;Cartigny et al., 1997). This is because, in kimberlites and lamproites, most diamonds (>95%) are xenocrysts, which must have formed earlier when their host rocks equilibrated in the Earth's mantle (Richardson et al., 1984;Shirey et al., 2013, for review). ...
... In this respect, they appear less aggregated (i.e., 0% IaA) than previously analyzed metamorphic diamonds (most between 20% and 80% IaA; Finnie et al., 1994;De Corte et al., 1998, 1999Cartigny et al., 2004;Dobrzhinetskaya et al., 2007); ophiolitic diamonds are the least aggregated samples reported so far. There is a large consensus that the preservation of the type Ib feature of diamonds can be preserved only though rapid (a few million years; Fig. 7) tectonic exhumation of the metamorphic diamonds after their formation at temperatures <1000 °C (for metamorphic diamonds, see Howell et al., 2015;Finnie et al., 1994;De Corte et al., 1998, 1999; for mantle diamonds see Cartigny, 2010;Smit et al., 2016;Smith et al., 2016). Their nitrogen concentrations range from 108% to 589% ± 20% atomic ppm; these values are clearly higher than both mantle Ib-IaA diamonds and impact-related diamonds, respectively (Fig. 7); they are also lower by about one order of magnitude than previously studied metamorphic diamonds (see references above). ...
Article
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We report new δ13 C data and N content and aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray-Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 to 589 atomic ppm ± 20%) with a consistently low aggregation state and show identical infrared spectra dominated by strong absorption between 1130 cm –1 and 1344 cm –1 , and therefore characterize type Ib diamond. Microdiamonds from the Luobusa peridotites have δ 13 C (PDB) values ranging from −28.7‰ to −16.9‰, and N contents from 151 to 589 atomic ppm. The δ 13 C and N content values for diamonds from the Luobusa chromitites are −29‰ to −15.5‰ and 152–428 atomic ppm, respectively. Microdiamonds from the Ray-Iz chromitites show δ 13 C values varying from −27.6‰ to −21.6‰ and N contents from 108 to 499 atomic ppm. The carbon isotopes values have features similar to previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds we analyzed, in that they show enhanced δ 13 C variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N aggregation state and N content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e., <950 °C), and in a short residence time (i.e., within several million years) at high temperatures in the deep mantle.
... Capdevila et al. suggest a depth of origin N250 km for Dachine diamonds on the basis of the inferred depth at which a putative komatiitic host magma would be generated. Cartigny (2010) related Dachine diamonds to carbonado on the basis of their similar isotopic compositions, and suggested they originate in the mantle at depths N 300 km, crystallizing from volatiles components associated with komatiite formation. Previous conclusions about the origins of Dachine diamonds were made in the absence of any silicate inclusions that could place limits on the depth of formation and lithologies involved. ...
... The mean N concentration is hence lower than implied by the choice of data shown in the figure. With the exception of the complexities mentioned above, the range of N concentration and aggregation is similar that reported by Cartigny (2010) and the results for N concentration by FTIR and mass spectrometry (Table 1) for diamonds in our sample suite are also consistent. The key finding from FTIR is that the Dachine diamonds have very low N aggregation compared with the worldwide diamond populations where N 99% do not show single N defects (Allen and Evans, 1981), placing strong constraints on the thermal history of the diamonds between their growth and exhumation. ...
... Carbon and nitrogen data for Dachine diamonds are provided in Table 1, and isotopic compositions are plotted in Fig. 4. The results show ranges for δ 15 N and δ 13 C of −5 to +21‰ and 0 to −36‰, respectively, with a mean N concentration of 20 ppm. Such 13 C-depletion relative to the mantle values and the low N concentrations with a high proportion of Type II diamonds are consistent with prior observations of Dachine diamonds (Cartigny, 2010;McCandless et al., 1999). The identified 15 N-enrichment for the Type II samples (this study) is similar to Type I samples (Cartigny, 2010), and we observed no statistically significant relationships among δ 13 C, δ 15 N, and N concentrations. ...
Article
Diamonds from Dachine, French Guiana, are unique among worldwide diamond populations. The diamonds were transported to the surface in an unusual ultramafic extrusive magma with an affinity to boninite or komatiite, which was emplaced within an arc geological setting at ~ 2.2 Ga. Dachine diamonds have internal and external morphologies indicative of relatively rapid growth from carbon oversaturated fluids or melts, and exhibit internal features consistent with residence in a high-strain environment. On the basis of nitrogen (N) defects the diamonds are categorized as Type Ib-IaA. The unusually low aggregation state of N places severe constraints on the thermal history of the diamonds, effectively ruling out derivation in convecting mantle. The carbon and N isotopic compositions of Dachine diamonds are consistent with a sedimentary source of carbon, with the majority of diamonds having δ¹³C values < − 25‰ and δ¹⁵N values > + 4‰. The primary carbon was presumably deposited on an early Proterozoic seafloor. Sulphide inclusions have low Ni and Cr and are comparable to lithospheric eclogitic-type sulphide inclusions. Three garnet and one clinopyroxene inclusion are also eclogitic in composition, and one garnet inclusion has a majorite component indicating an origin around 250 km depth. The silicate inclusions are highly depleted in many incompatible trace elements (e.g. LREE, Nb, Hf, Zr), and modelling indicates an eclogitic source lithology that contained a LREE-enriched trace phase such as epidote or allanite, and an HFSE-rich phase such as rutile. Four of the five inclusions are unusually enriched in Mn, as well as Ni and Co, and modelling indicates a protolith with the bulk composition of subducted normal MORB plus about 10% ferromanganese crust component. We suggest a model wherein Dachine diamonds precipitated from remobilized sedimentary carbon at the slab-mantle interface from liquids derived ultimately by deserpentinization of slab peridotite at depths of ~ 200 to 250 km. These fluids may also trigger melting in wedge peridotite, resulting in a volatile-rich ultramafic melt that transports the diamonds rapidly to the surface. The process of diamond formation and exhumation from the slab mantle interface likely occurred in a Paleoproterozoic subduction zone and over a very limited timespan, likely less than a million years.
... Hydrogen ranges from 100 to 500 ppm and δD varies between −50 and −200‰ (Demeny et al. 2011). Framesite (unnecessarily termed diamondites by Maruoka et al., 2004) is isotopically heavier than carbonado in δ 13 C and is more akin to some eclogitic diamonds in kimberlites (see compilations in Cartigny, 2005Cartigny, , 2010. ...
... This was later supported by the inference that the largest magnetic anomaly on Earth, Bangui in the Central African Republic (Fig. 1), is a buried iron meteorite (Girdler et al., 1992). Other interpretations for the origin of carbonado include: ion-implantation by U and Th in coal deposits (Ozima et al., 1991), and unspecified carbon-bearing matter (Kaminsky, 1987(Kaminsky, , 1991; irradiation in carbon-uranium mineraloids (Daulton and Ozima, 1996); a complex series of reactions in the upper mantle and subsequent radiation sintering of microdiamonds in the crust (Kagi et al., 1994); small particle physics, that might modify the C phase diagram (Kaminsky, 1991); ocean floor subduction (De Carli, 1997), followed by mantle plume interaction on metastable graphite ; growth from a thermodynamically stable C-saturated H 2 O-bearing fluid in the mantle, based on the putative presence of hydrous fluid inclusions (Kagi and Fukura, 2008;Ishibashi et al., 2012); the similarities in C-and N-isotope compositions in carbonado (from Brazil and CAR), with diamonds from French Guiana suggest formation in extremely hot komatiitic lava flows (Cartigny, 2010); by a combined growth and sintering process at high P-T (Chen and Van Tenderloo (1999); by acoustic cavitation (Suslick, 1990) of frozen ocean floor clathrates (Haggerty, 1995); and by a comparison between robust carbonado and loosely bonded framesite (Heaney et al., 2005). ...
... Hren et al., 2009) and leads to the highly improbable conclusion for the origin of carbonado by "…mantle/rock melts, or subducted crustal materials and reduced C-H fluids…"; (8) putative fluid inclusions are almost certainly due to contamination by atmospheric water induced by changes in the surface properties of diamond from hydrophobic to hydrophilic as a result of strong acid treatment, heating (T = 700-1100°C), or immersion in a sample containment system that absorbs water, e.g. KBr (Mutschke et al., 1995;Andersen et al., 1998;Braatz et al., 2000;Chakrapani et al., 2007) moreover, the inferred fO 2 (FMQ -3) is clearly incompatible with the metal-mineral assemblage in carbonado, and given the repeated heating cycles one would expect decrepitation of the fluid-inclusions which is not reported; (9) the PAH content in carbonado is ten times higher than in diamonds from impacts, and 100 times higher than diamonds in kimberlites (McCall, 2009); (10) the hot komatiitic proposal is self-invalidated (Cartigny, 2010) in the statement on p. 332: "….the investigated samples were not polycrystalline (i.e. so they cannot be carbonado)…"; in fact the diamonds in these rocks are very similar to those from kimberlites (Fig. 16); there is some question as to whether the host rock is in fact komatiite as it lacks the typical spinefex olivine texture and is unusual in bulk chemistry; diamond, let alone carbonado has never been found in conventional komatiite; and there no evidence that these magmas originated in the diamond stability field at depths in excess of 180 km; the overall conclusion reached here is supported by Kaminsky et al. (2013) that the French Guinea diamonds are not carbonado; (11) there is a complete absence of mantle mineral inclusions (e.g. ...
... This means that the carbon and nitrogen isotopic systems are potentially useful indicators for the interactions between crustal and mantle volatile during subduction zone plate tectonics, and associated magmatism/volcanism (Wallace, 2005). Based on this premise, the carbon and nitrogen isotopic compositions of diamonds formed in the mantle have been widely used as a tool for investigating the origins of diamond-forming carbon in the terrestrial mantle (Javoy et al., 1984;Boyd et al., 1987Boyd et al., , 1992Boyd and Pillinger, 1994;Cartigny et al., 1997Cartigny et al., , 1998aCartigny et al., , 1998bCartigny et al., , 1999Cartigny et al., , 2003Cartigny et al., , 2004Cartigny et al., , 2009Shelkov 1997;Bulanova et al., 2002;Hauri et al., 2002;Gautheron et al., 2005;Harte et al., 1999;Shiryaev et al., 2005;Thomassot et al., 2007Thomassot et al., , 2009Palot et al., 2009Palot et al., , 2013Cartigny, 2010;Klein-BenDavid et al., 2010;Mikhail et al., 2013). Monocrystalline diamonds (MCDs) from the upper and lower mantle, containing both peridotitic and eclogitic silicate and sulphide inclusions, show a strong mean δ 13 C of −5 ± 3‰ (Cartigny, 2005 and references therein) (Fig. 1a). ...
... The use of nitrogen isotopes alongside carbon can theoretically provide a more accurate insight into the origin(s) of diamond-forming carbon, because the mean nitrogen isotope value for peridotitic diamonds is −5 ± 4‰ (Fig. 1c) (Cartigny, 2005) and the mean δ 15 N value for pre-Cambrian crustal organic nitrogen is +6 ± 4‰ (Fig. 1d) (Thomazo et al., 2009). This has lead several studies to argue that negative δ 15 N values coupled with low δ 13 C values are evidence against the subduction of crustal organic carbon as a source of diamond-forming fluids (Cartigny et al., 1998a(Cartigny et al., , 1998bCartigny, 2010). However, the data presented in Fig. 1 are derived from bulk stable isotope determinations achieved by oxidation of single diamonds followed by isotopic analyses of the resulting gas (i.e. ...
... Sample analysis was bracketed every two runs with blank determinations, and the blank subtraction and calculations for the error propagation for a given sample account for two blank runs during the same analytical session. Our blank corrected total uncertainties are directly comparable with the data from other laboratories working on the nitrogen isotope values from diamond Cartigny, 2010;Palot et al., 2012). The error bars shown for Δ(δ 15 N) are the combined blank-corrected total uncertainties for the end-members used to determine the range. ...
... Additionally, carbon-saturated fluid: It is also suggested that carbonados formed from a carbon-saturated fluid (Cartigny, 2010). It is important to note that: The mineral inclusions found in carbonado are secondary, formed after the creation of the diamond (Afanasiev et al., 2024). ...
... It is important to note that: The mineral inclusions found in carbonado are secondary, formed after the creation of the diamond (Afanasiev et al., 2024). Carbonados exhibit low nitrogen aggregation (Cartigny, 2010). The growth of carbonados could be related to a decrease in carbon saturation (Petrovsky et al., 2010). ...
Preprint
This study presents a detailed analysis of the lonsdaleite diamond Capii-6, an exceptional specimen originating from the Earth's lower mantle. The presence of lonsdaleite in this diamond was confirmed through Raman spectroscopy and X-ray diffraction (XRD), techniques that also allowed the identification of characteristic lower mantle mineral inclusions, such as breyite (formerly Ca-perovskite), bridgmanite (or its retrogressed product, enstatite/corundum), and ferropericlase. The diamond's formation pressure was estimated between 25 and 29 GPa, based on both the composition of ferropericlase and the relationship between the full width at half maximum (FWHM) of the D-peak in Raman spectra and pressure. This pressure range supports its origin in the lower mantle. Raman spectroscopic analysis revealed three main spectral types within the diamond, suggesting that it is not a defect-free natural cubic diamond. Additionally, an inverse relationship was observed between the intensity of the D-peak and the FWHM values, a behavior that contrasts with typical natural diamonds. X-ray diffraction patterns showed a prominent peak near 75.6° 2θ (a 220 reflection from diamond, overlapping with that of lonsdaleite), a less intense peak around 43.72° 2θ, and a characteristic triplet peak for lonsdaleite. These findings significantly contribute to the understanding of ultra-deep diamond formation and the dynamic processes within the Earth's mantle, potentially linked to the subduction of the Nazca plate.
... French Guiana is also home to primary diamond-bearing igneous sources, but these are metamorphosed ultramafic and pyroclastic shoshonites or lamprophyres found in the Dachine region. These diamonds are very small (~1 mm), grayish yellow to brown, irregular to cuboid (figure 18, middle), and mostly contain sulfide inclusions (Cartigny, 2010;Smith et al., 2016). ...
... Top: Diamonds (0.10-2.12 ct) from Suriname; from Naipal et al. (2020). Middle: French Guiana diamonds; from Cartigny (2010). Bottom: Alluvial diamonds from Guaniamo, Venezuela. ...
Article
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Diamonds have been mined in Guyana for more than 130 years and are traded in major diamond centers in Belgium, Israel, and the United Arab Emirates. Notwithstanding this long history, the primary source rocks of Guyana’s diamonds remain a mystery. The diamonds are likely detrital material derived from sedimentary rocks of the Roraima Supergroup, but a primary igneous, kimberlitic source has not been eliminated. Diamond exploration and mining in Guyana remain a mostly artisanal endeavor. In a similar fashion, scientific studies have rarely addressed these diamonds’ provenance and formation, and very few were aimed at a gemological audience. Here we present a detailed gemological description of Guyana’s diamonds to serve as a comparison with other diamond populations in the Guiana Shield and globally. We use our direct observations of diamonds from various alluvial deposits in Guyana. We combine government reports and datasets as well as historical accounts to provide an overview of diamond production and mining practices in Guyana. Details concerning color, morphology, nitrogen content, and luminescence are also included.
... In contrast, the Kokchetav diamonds reported in other works always have significant amounts of Acenters corresponding to type Ib-IaA and, in general, contain very high amounts of total nitrogen (Fig. 7) (e.g., De Corte et al. 1998;Cartigny et al. 2004). The lowest nitrogen aggregation reported for natural diamonds was observed in microdiamonds from Dachine komatiites (French Guiana) (Cartigny 2010) and Zimmi alluvial deposits (West Africa) (Smit et al. 2016(Smit et al. , 2018. They may be comparable with Tolbachik and ophiolite diamonds by FTIR spectra, but clearly have some other distinctive features, such as sulfide inclusions (Smit et al. 2016(Smit et al. , 2018C. ...
... Note that most typical metamorphic diamonds correspond to clear Ib-IaA type with significant degree of A-centers aggregation. (Color online.) Figure 7. Nitrogen aggregation vs. nitrogen content in Tolbachik (this work) and ophiolite-hosted microdiamonds (Xu et al. 2018) in comparison with eclogite diamonds from kimberlite, metamorphic diamonds (mainly from Kokchetav massif), microdiamonds from Akluilak minettes (Canada) (Cartigny et al. 2004), diamonds from Dachine komatiite (French Guiana) and carbonado (Cartigny 2010), and Zimmi (West Africa) alluvial diamond (Smit et al. 2016). Black line from 100 to 500 ppm N indicate a range observed in ophiolite diamonds, which coincide in general with that for Tolbachik diamonds. ...
Article
Taking into account recent publications, we provide additional comprehensive evidence that type Ib cuboctahedral diamonds and some other microcrystalline diamonds from Kamchatka volcanic rocks and alluvial placers cannot be natural and undoubtedly represent synthetic materials, which appear in the natural rocks by anthropogenic contamination. The major arguments provided in favor of the natural origin of those diamonds can be easily disproved. They include the coexistence of diamond and deltalumite from Koryaksky volcano; coexistence with super-reduced corundum and moissanite, Mn-Ni silicide inclusions, F-Cl enrichment and F/Cl ratios, and carbon and nitrogen isotopes in Tolbachik diamonds, as well as microtwinning, Mn-Ni silicides, and other inclusions in microcrystalline diamond aggregates from other Kamchatka placers. We emphasize the importance of careful comparison of unusual minerals found in nature, which include type Ib cuboctahedral diamonds and super-reduced phase assemblages resembling industrial slags, with synthetic analogs. The cavitation model proposed for the origin of Tolbachik diamonds is also unreliable since cavitation has only been shown to cause the formation of nanosized diamonds only.
... Submillimeter to centimeter polycrystalline aggregates of cubic diamond found in alluvial placers, known as carbonados (from Mesoproterozoic deposits in Brazil and Central Africa) and yakutites/carbonados (from Yakutiya, Russia), have been attributed to formation by shock metamorphism (see Smith and Dawson, 1985;Kaminsky, 1994). However, Cartigny (2010) argued that a mantle origin for carbonados cannot be excluded. Formation mechanisms other than shock such as crystallization from a carbonsupersaturated fluid have been suggested (Ketcham and Koeberl, 2013), and the origins of carbonados remain poorly understood (Haggerty, 1999(Haggerty, , 2014Heaney et al., 2005;McCall, 2009;Cartigny, 2010). ...
... However, Cartigny (2010) argued that a mantle origin for carbonados cannot be excluded. Formation mechanisms other than shock such as crystallization from a carbonsupersaturated fluid have been suggested (Ketcham and Koeberl, 2013), and the origins of carbonados remain poorly understood (Haggerty, 1999(Haggerty, , 2014Heaney et al., 2005;McCall, 2009;Cartigny, 2010). Yakutites have been described as differing from carbonados in several aspects (but . ...
Article
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During the end of the last glacial period in the Northern Hemisphere near 12.9k cal a BP, deglacial warming of the Bølling–Ållerod interstadial ceased abruptly and the climate returned to glacial conditions for an interval of about 1300 years known as the Younger Dryas stadial. The Younger Dryas Impact Hypothesis proposes that the onset of the Younger Dryas climate reversal, Pleistocene megafaunal extinctions and disappearance of the Clovis paleoindian lithic technology were coeval and caused by continent-wide catastrophic effects of impact/bolide events in North America. While there are no known impact structures dated to the Younger Dryas onset, physical evidence of the impact/bolide events is argued to be present in sediments spanning several continents at stratigraphic levels inferred to date to the Bølling-Ållerod/Younger Dryas boundary (YDB). Reports of nanometer to submicron-sized diamonds in YDB sediments, in particular the rare 2H hexagonal polytype of diamond, lonsdaleite, have been presented as strong evidence for shock processing of crustal materials. We review the available data on diamonds in sediments and provide new data. We find no evidence for lonsdaleite in YDB sediments and find no evidence of a spike in nanodiamond concentration at the YDB layer to support the impact hypothesis.
... It occurs mostly in Brazil and the Central African Republic metaconglomerates ; its crustal inclusions may be primary or, given their high porosity, secondary. The origin of carbonado remains an enigma; models of formation range from impact related (Smith & Dawson 1985) to extraterrestrial (Garai et al. 2006, Haggerty 2014) to crustal (Kaminsky 1991) to mantle derived (Kagi & Fukura 2008, Cartigny 2010). For a detailed description, the reader is referred to Haggerty's most recent review (Haggerty 2014). ...
... Diamonds from Jagersfontein (Kaapvaal craton) in South Africa and from alluvial deposits in New South Wales in Australia are further potential examples of diamond formation related to the occurrence of subduction-related carbon, but at present, nitrogen and oxygen isotope information is lacking at both sources. Eclogitic diamonds from the Jericho kimberlite in the northern Slave craton in Canada and diamonds of unknown paragenesis from the Dachine komatiite in French Guiana have δ 13 C modes of approximately −38 and −28, respectively (Figure 5), and these values are difficult to interpret in light of the idea that isotope heterogeneity is subduction or mantle related (De Stefano et al. 2009, Cartigny 2010, Smart et al. 2011). Preservation of δ 13 C values less than −25 indeed requires the virtual absence of subducted carbonates, and δ 13 C values below −40 are exceedingly rare within the sedimentary geological record (De Stefano et al. 2009, Smart et al. 2011). ...
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Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earth’s mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the 15N/14N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are best seen in diamonds from the transition zone.
... Carbonado is a smooth-surfaced fine-crystalline (1-10 μm) and highly porous diamond aggregate, mostly black in color, which lacks most of the typical macroinclusions found in monocrystalline diamonds (Heaney et al., 2005;McCall, 2009). Their origin is still under debate with models ranging between crustal derivation, genesis in the upper Earth's mantle and an extraterrestrial origin (see Heaney et al., 2005;McCall, 2009;Cartigny, 2010;Haggerty, 2014 for detailed summaries). ...
... The nanoinclusion suites of carbonados thus show much more similarity to findings in mantle-derived diamonds (Klein-BenDavid et al., 2007;Cartigny, 2010;Jacob et al., 2011) and metamorphic diamonds from ultrahigh pressure terranes (e.g. Dobrzhinetskaya, 2012;Dobrzhinetskaya et al., 2013) than the larger inclusions. ...
... Thus, carbon further acts as an agent of mass transfer in the form of mobile carbonate-rich melts. has broadened to more than 40h (Galimov, 1991;Kirkley et al., 1991;Cartigny 2005Cartigny , 2010De Stefano et al., 2009;Bulanova et al., 2010), which has raised concerns about the homogeneous distribution of carbon isotopes in the upper mantle (Deines et al., 2009). Additionally, the low δ 13 C compositions of diamonds with inclusion derived from potentially deep mantle sources has been interpreted as evidence for the presence of recycled light carbon derived from subducted organic material (Walter et al., 2011;Smart et al., 2011). ...
... Despite several strong evidences that points to the low-temperature surface carbon fractionation for the origin of light carbon in the mantle, a recent experimental investigation of carbon isotope fractionation in the Fe-C system at high-temperature and high-pressure conditions has hypothesized a possibility of Earth's core as a light carbon reservoir, thereby proposing a new possible source for light carbon flux from core-mantle boundary (Satish-Kumar et al., 2011). To what extent the wide range of carbon isotope variation of diamond is a result of mantle fractionation processes, whether they preserve primordial heterogeneities (Cartigny, 2010;Deines et al., 2009), or an interplay of recycling of surface carbon and those released from the core still remain unanswered (Wood et al., 2013). ...
Article
Carbon isotope fractionation between graphite and carbonated silicate melt was determined at 5 GPa and in the temperature range between 1400 and 1900 °C. High pressure experiments were carried out in the carbon-saturated model harzbergite system (Enstatite–Magnesite–Olivine–Graphite), where carbonated silicate melt and graphite were the two stable carbon-bearing phases in the run products. Carbonated silicate melting resulted in an isotopic fractionation between graphite and carbon in the silicate melt, where the carbon in the melt is 13C enriched than co-existing graphite. 13C enrichment in carbonate melt when compared to graphite was further confirmed in experiments where redox melting between olivine and graphite produced carbonate melt as well as carbonate reduction experiments to form graphite. Although a quantitative estimate of fractionation between carbonate melt and graphite could not be obtained, our results indicate that mantle melting in the presence of graphite can result in progressive 13C carbon isotope enrichment in carbonate melt and depletion in graphite, which can be an alternate explanation for the carbon isotopic heterogeneity observed in the mantle derived carbon.
... If isotopically heavy diamonds crystallize together with carbides during redox interaction, then such diamonds keep their isotopic composition unchanged. This might provide the formation of carbon reservoirs of different isotopic compositions in the Earth's mantle, whose existence is widely discussed in the scientific literature (Deines, 2004;Stachel and Harris, 2009;Cartigny, 2010). At the same time, the presence of impurities of other transition metals, such as nickel or cobalt, in iron hinders the formation of iron carbides. ...
Article
—Subduction of marine carbonates is accompanied by numerous transformations and interactions, including reactions with reduced mantle rocks. At depths of 250–300 km, carbonates enter mantle zones where metallic iron can be stable. The interaction of carbonates with metals is one of the mechanisms of the release of elemental carbon and the formation of diamond. These processes are also accompanied by carbon isotope fractionation and can result in a significant isotopic heterogeneity of mantle carbon. In this work we study the partitioning of carbon isotopes between carbon and carbon-bearing phases obtained in experiments on the interaction of FeNi alloy with (Mg,Ca)CO3, which simulates mantle–crust redox reactions in the temperature range 800–1550 °C and at a pressure of 6.3 GPa. It has been established that at 800–1000 °C, the carbon of carbonate is reduced at the metal/carbonate interface and dissolves in the FeNi alloy. This process leads to a 17–20‰ depletion of the metal in the heavy carbon isotope. At temperatures above 1330 °C, the fractionation of carbon isotopes between carbonate and metal–carbon melts is reduced to 8.5‰, approaching the thermodynamic calcite–cohenite isotope equilibrium. At temperatures above 1400 °C, diamond crystallizes from metal–carbon and carbonate melts, which leads to isotopic depletion of the metal–carbon melt. As a result, the measured carbon isotope fractionation between the carbonate and metal–carbon melts increases and moves away from the thermodynamic CaCO3–Fe3C equilibrium line. The carbonate–metal redox interaction is supposed to be one of the probable mechanisms of the formation of isotopically light carbon in the mantle at the expense of the marine carbonate sediments subducted into the mantle. This mechanism also provides the formation of anomalous isotopically heavy carbonates found in kimberlites of the Siberian Platform.
... Among the many other forms of natural diamond, carbonado, a sintered type of polycrystalline diamond, remains the most enigmatic. Carbonados may form during meteorite impact, in Earth's mantle or in extraterrestrial environments (see Garai et al. 2006;Kagi and Fukura 2008;Cartigny 2010;Haggerty 2014 and reference therein). (110-250 km). ...
... Table 2. Averages for samples where combined N and C data are available (no data considered when only one of the isotope systems is reported). Note the large standard deviations for the mean nitrogen concentrations for PDAs reflecting the large ranges of concentrations and the absence of a significant mean nitrogen abundance for PDAs.Data sources: monocrystalline diamond (Cartigny et al. 1997(Cartigny et al. , 1998a(Cartigny et al. , b, 1999(Cartigny et al. , 2004Palot et al. 2009;Thomassot et al. 2009;Cartigny 2010;Palot et al. 2012;Mikhail et al. 2014b;Smith et al. 2016), fibrous diamond (Javoy et al. 1986;Boyd et al. 1987Boyd et al. , 1992Klein-BenDavid et al. 2010), and PDAs (Javoy et al. 1986;Boyd et al. 1987Boyd et al. , 1992Shelkov 1997;Klein-BenDavid et al. 2010 When crushed under vacuum, PDAs release gas (Gautheron et al. 2005;Mikhail et al. 2019a), and this requires the presence of fluid micro-inclusions to explain the release of detectable 3 He after crushing (Gautheron et al. 2005;Mikhail et al. 2019a). Therefore, to shed more light on the origin of diamond-forming fluids one can apply noble gas isotope data to complement and test hypotheses developed using C-N stable isotope data (e.g., 3 He/ 4 He). ...
... VOLCANIC ROCKS Diamond finds in lamprophyres, picrites, and analogous volcanic rocks were hotly debatable several tens of years ago, but at present are a common phenomenon. Within the last 15 years, several dozen known occurrences (Kaminsky, 2007) have been supplemented by diamondiferous lamprophyres in the Nunavut (MacRae et al., 1996;Sablukov, 2002), Northern Ontario (DeStephano et al., 2006;Wyman et al., 2015), and Northern Quebec (Birkett et al., 2004) provinces of Canada; komatiites of French Guiana (Cartigny, 2010;Smith et al., 2016) and Suriname (Naipal et al., 2019), shoshonite-absarokite-picrites of Uzbekistan (Golovko and Kaminsky, 2010), and others occurrences (Fig. 2). Many of them were found within ancient fold belts instead of ancient cratons. ...
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The best-known, most well-studied diamondiferous rocks are kimberlites and lamproites. Diamonds are also found in impactites, metamorphic rocks, ophiolites, and modern volcanic rocks. Diamonds from these rocks differ from kimberlitic diamonds in size, morphology, trace-element and isotope composition , and physical properties. Differences in these characteristics are related to their different mechanisms of origin. In some cases, diamonds can be formed in "metastable" conditions under disequilibrium thermody-namic parameters, supporting the conclusion that diamond is a polygenetic mineral, formed in nature under different physicochemical and geodynamic conditions. According to thermodynamic considerations and calculations , "metastable" crystallization of diamond is mainly controlled by the size of the forming crystallites. The main effectors in decreasing the energetic barrier for nanosized diamonds are surface tension and related surface energy.
... VOLCANIC ROCKS Diamond finds in lamprophyres, picrites, and analogous volcanic rocks were hotly debatable several tens of years ago, but at present are a common phenomenon. Within the last 15 years, several dozen known occurrences (Kaminsky, 2007) have been supplemented by diamondiferous lamprophyres in the Nunavut (MacRae et al., 1996;Sablukov, 2002), Northern Ontario (DeStephano et al., 2006;Wyman et al., 2015), and Northern Quebec (Birkett et al., 2004) provinces of Canada; komatiites of French Guiana (Cartigny, 2010;Smith et al., 2016) and Suriname (Naipal et al., 2019), shoshonite-absarokite-picrites of Uzbekistan (Golovko and Kaminsky, 2010), and others occurrences (Fig. 2). Many of them were found within ancient fold belts instead of ancient cratons. ...
... VOLCANIC ROCKS Diamond finds in lamprophyres, picrites, and analogous volcanic rocks were hotly debatable several tens of years ago, but at present are a common phenomenon. Within the last 15 years, several dozen known occurrences (Kaminsky, 2007) have been supplemented by diamondiferous lamprophyres in the Nunavut (MacRae et al., 1996;Sablukov, 2002), Northern Ontario (DeStephano et al., 2006;Wyman et al., 2015), and Northern Quebec (Birkett et al., 2004) provinces of Canada; komatiites of French Guiana (Cartigny, 2010;Smith et al., 2016) and Suriname (Naipal et al., 2019), shoshonite-absarokite-picrites of Uzbekistan (Golovko and Kaminsky, 2010), and others occurrences (Fig. 2). Many of them were found within ancient fold belts instead of ancient cratons. ...
Article
Full-text available
The best-known, most well-studied diamondiferous rocks are kimberlites and lamproites. Diamonds are also found in impactites, metamorphic rocks, ophiolites, and modern volcanic rocks. Diamonds from these rocks differ from kimberlitic diamonds in size, morphology, trace-element and isotope composition , and physical properties. Differences in these characteristics are related to their different mechanisms of origin. In some cases, diamonds can be formed in "metastable" conditions under disequilibrium thermody-namic parameters, supporting the conclusion that diamond is a polygenetic mineral, formed in nature under different physicochemical and geodynamic conditions. According to thermodynamic considerations and calculations , "metastable" crystallization of diamond is mainly controlled by the size of the forming crystallites. The main effectors in decreasing the energetic barrier for nanosized diamonds are surface tension and related surface energy.
... Combined with Mn-bearing inclusions within diamonds, these light carbon isotopic compositions are considered as evidence of recycled crustal materials in the mantle portions of ophiolites [5,37,138,139]. Nevertheless, it is worth noting that these low δ 13 C values also match those of some super-deep diamonds from kimberlites (−24‰ to −17‰) and carbonados that have been linked to komatiites (−27‰ to −21‰) formed in the transition zone and lower mantle [143]. Particularly, ophiolite-hosted diamonds are comparable with those diamonds from primitive chondrites (−32‰ to −38‰) [128]. ...
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Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.
... Diamonds formed beneath and within the continental lithospheric mantle are brought to the Earth's surface by various types of magma, including kimberlites, lamproites, lamprophyres and komatiites 118,134,[177][178][179] (Fig. 6). Mineral inclusions and light carbon isotopes from diamonds discovered in Brazil, South Africa and Canada confirm that they are formed at depths between 300 km and 800 km by recycled, subducted oceanic crust 151,[180][181][182][183] . ...
Article
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Ophiolites, which represent segments of oceanic lithosphere obducted onto the continental crust, provide an important window into processes such as continental rifting, mantle melting, asthenospheric upwelling and cooling of oceanic lithosphere. Traditionally, research has focused on crustal sections of ophiolites. However, there is a growing recognition that mantle sections contain important information on ophiolite formation and crustal recycling. In this Review, we outline the importance of chromium spinel and associated mineral inclusions for recording the tectonic setting and past histories of ophiolitic mantle sections, with a focus on the insights they provide into deep crustal recycling. In particular, the presence of ultra-high-pressure mineral inclusions, such as microdiamonds, metal alloys, Mn silicates and coesites, in podiform chromitites and other ophiolitic mantle rocks offer evidence of deep and reduced formation conditions. The composition of ultra-high-pressure minerals, and especially the light carbon isotope composition of ophiolite-hosted diamonds, indicates a contribution of recycled crustal material to the mantle portions of ophiolites. The details of crustal material transport through the mantle remain a subject of debate and ongoing research. A global investigation of high-pressure minerals in ophiolitic peridotites and chromitites should be a target of future research to help clarify understanding of deep crustal recycling.
... According to the identification of nitrogen species in diamond, I type diamond can be divided into two group: 1) Ia diamond in which nitrogen atom exists as nitrogen pairs (N 2 ) and tetra-nitrogen (N 4 ), and 2) Ib diamond in which nitrogen atom exists as single nitrogen (N) (Kaiser and Bond, 1959). Single nitrogen (N) could transform to nitrogen pairs (N 2 ) and tetra-nitrogen (N 4 ) if Ib type diamond stays for a long residence time at mantle depths, or at high temperatures (Cartigny, 2010;Navon et al., 1988;Shirey et al., 2013). Infrared spectroscopy analysis indicates that the diamonds from Lan'gan is Ib type (Zhu et al., 2018). ...
Article
The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatismcould provide a unique windowinto deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646–0.70925, εNd(t) = −2.1 to −4.9, 206Pb/204Pb(t) = 17.14–18.12, 207Pb/204Pb(t) = 15.28–15.61, 208Pb/204Pb(t) = 37.82–38.67, and zircon εHf(t) = −17 to −21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts fromthe Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithosphericmantle of the eastern NCCwere reduced in viscosity and intensity, and finally promoted partialmelting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.
... There are several diamond-bearing types of rocks or alluvial deposits, which are not related to kimberlites and lamproites, but have proven the deepseated origin and are associated with minerals of high-pressure origin. They include potassium lamprophyre of Gibson lake area (Northern Territories, Canada) (Macrae et al., 1995), carbonate-rich aillikites of Torngat (Labrador, Canada) (Tappe et al., 2008), alluvial diamonds from Kalimantan (Kueter et al., 2016) and Witwatersrand (Smart et al., 2016), komatiite of French Guiana (Capdevila et al., 1999;Cartigny, 2010; metamorphic diamonds from ultrahigh-pressure complexes (Ogasawara et al., 2002;Sobolev and Shatsky, 1990) and some others. In these rocks, we can observe single crystal diamonds with predominant octahedral habits and infrared characteristics with A (pair of N atoms) and B (four N atoms substituting for carbon) nitrogen centers (e.g., Zaitsev, 2001) suggesting long-time residence under static HPHT conditions. ...
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The enigmatic appearance of cuboctahedral diamonds in ophiolitic and arc volcanic rocks with morphology and infrared characteristics similar to synthetic diamonds that were grown from metal solvent requires a critical reappraisal. We have studied 15 diamond crystals and fragments from Tolbachik volcano lava flows, using Fourier transform infrared spectrometry (FTIR), transmission electron microscopy (TEM), synchrotron X-ray fluorescence (SRXRF) and laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS). FTIR spectra of Tolbachik diamonds correspond to typical type Ib patterns of synthetic diamonds. In TEM films prepared using focused ion beam technique, we find Mn-Ni and Mn-Si inclusions in Tolbachik diamonds. SRXRF spectra indicate the presence of Fe-Ni and Fe-Ni-Mn inclusions with Cr, Ti, Cu, and Zn impurities. LA-ICP-MS data show variable but significantly elevated concentrations of Mn, Fe, Ni, and Cu reaching up to 70 ppm. These transition metal concentration levels are comparable with those determined by LA-ICP-MS for similar diamonds from Tibetan ophiolites. Mn-Ni (+Fe) solvent was widely used to produce industrial synthetic diamonds in the former USSR and Russia with very similar proportions of these metals. Hence, it appears highly probable that the cuboctahedral diamonds recovered from Kamchatka arc volcanic rocks represent contamination and are likely derived from drilling tools or other hard instruments. Kinetic data on diamond dissolution in basaltic magma or in fluid phase demonstrate that diamond does not form under the pressures and temperature conditions prevalent within the magmatic system beneath the modern-day Klyuchevskoy group of arc volcanoes. We also considered reference data for inclusions in ophiolitic diamonds and compared them with the composition of solvent used in industrial diamond synthesis in China. The similar inclusion chemistry close to Ni70Mn25Co5 for ophiolitic and synthetic Chinese diamonds scrutinized here suggests that most diamonds recovered from Tibetan and other ophiolites are not natural but instead have a synthetic origin. In order to mitigate further dubious reports of diamonds from unconventional tectonic settings and source rocks, we propose a set of discrimination criteria to better distinguish natural cuboctahedral diamonds from those produced synthetically in industrial environments and found as contaminants in mantle- and crust-derived rocks.
... There are several diamond-bearing types of rocks or alluvial deposits, which are not related to kimberlites and lamproites, but have proven the deepseated origin and are associated with minerals of high-pressure origin. They include potassium lamprophyre of Gibson lake area (Northern Territories, Canada) (Macrae et al., 1995), carbonate-rich aillikites of Torngat (Labrador, Canada) (Tappe et al., 2008), alluvial diamonds from Kalimantan (Kueter et al., 2016) and Witwatersrand (Smart et al., 2016), komatiite of French Guiana (Capdevila et al., 1999;Cartigny, 2010; metamorphic diamonds from ultrahigh-pressure complexes (Ogasawara et al., 2002;Sobolev and Shatsky, 1990) and some others. In these rocks, we can observe single crystal diamonds with predominant octahedral habits and infrared characteristics with A (pair of N atoms) and B (four N atoms substituting for carbon) nitrogen centers (e.g., Zaitsev, 2001) suggesting long-time residence under static HPHT conditions. ...
Article
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Here we present trace element compositions of synthetic diamonds, which show spectacular similarity with the compositions of metallic inclusions in type Ib cuboctahedral diamonds in ophiolitic chromitites and peridotites. The compositions of inclusions in synthetic and ophiolite diamonds closely correspond to Ni:Mn:Co = 70:25:5 in wt.%, which is the most widely used catalyst for HPHT growth of synthetic diamonds in China. Thus, we claim for thorough reconsideration of diamonds in ophiolitic rocks and emphasize that most of them appear by anthropogenic contamination.
... More importantly, ophiolite-hosted diamonds are all characterized by a quite unique carbon isotopic composition with low  13 C values (< −18‰). These diamonds have a light carbon isotopic composition that is consistent with carbon sources of subducted organic matter [151,152]. A small portion of eclogitic diamonds also have positive  13 C values consistent with carbon sources of subducted inorganic matter, such as carbonate (Fig. 6) [37]. ...
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As reported in our prior work, we have recovered microdiamonds and other unusual minerals, including pseudomorph stishovite, moissanite, qingsongite, native elements, metallic alloys, and some crustal minerals (i.e., zircon, quartz, amphibole, and rutile)from ophiolitic peridotites and chromitites. These ophiolite-hosted microdiamonds display different features than kimberlitic, metamorphic, and meteoritic diamonds in terms of isotopic values and mineral inclusions. The characteristic of their light carbon isotopic composition implies that the material source of ophiolite-hosted diamonds is surface-derived organic matter. Coesite inclusions coexisting with kyanite rimming an FeTi alloy from the Luobusa ophiolite show a polycrystalline nature and a prismatic habit, indicating their origin as a replacement of stishovite. The occurrence in kyanite and coesite with inclusions of qingsongite, a cubic boron nitride mineral, and a high-pressure polymorph of rutile (TiO2 II)point to formation pressures of 10–15 GPa at temperatures ∼1300 °C, consistent with depths greater than 380 km, near the mantle transition zone (MTZ). Minerals such as moissanite, native elements, and metallic alloys in chromite grains indicate a highly reduced environment for ophiolitic peridotites and chromitites. Widespread occurrence of diamonds in ophiolitic peridotites and chromitites suggests that the oceanic mantle may be a more significant carbon reservoir than previously thought. These ophiolite-hosted diamonds have proved that surface carbon can be subducted into the deep mantle, and have provided us with a new window for probing deep carbon cycling.
... The PKO diamonds mainly have δ 13 C − 23 to − 29‰, with three grains having δ 13 C of ~ − 20‰, which is well within the carbon isotopic range of subducted organic carbon. Subducting sediments are commonly suggested to be the carbon sources for eclogitic diamonds with quite light carbon isotopic compositions due to the consistency between eclogitic diamonds and recycled carbon in carbon isotopic composition (Heaney et al. 2005;Tappert et al. 2005;Cartigny 2010;Walter et al. 2011;Nestola et al. 2018). Results from oxygen isotopic and bulk composition studies on mineral inclusions in diamond also provide evidence supporting a crustal carbon origin for diamond with low δ 13 C values (Walter et al. 2011;Schulze et al. 2013;Burnham et al. 2015). ...
Article
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The Pozanti–Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro-diamonds were recovered from the podiform chromitites, and these diamonds were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed-habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ¹³C range of the PKO diamonds varies between − 18.8 and − 28.4‰, with a principle δ¹³C mode at − 25‰. Nitrogen contents of the diamonds range from 7 to 541 ppm with a mean value of 171 ppm, and the δ¹⁵N values range from − 19.1 to 16.6‰, with a δ¹⁵N mode of − 9‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of (Ca0.81Mn0.19)SiO3, NiMnCo-alloy and nano-sized, quenched fluid phases were observed as inclusions in the PKO diamonds. We believe that the ¹³C-depleted carbon signature of the PKO diamonds derived from previously subducted crustal matter. These diamonds may have crystallized from C-saturated fluids in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts.
... Type Ib diamonds still preserve isolated nitrogen (C centre), rather the more common Type IaAB diamonds that contain nitrogen pairs (A centres) and nitrogen aggregates (B centres). Type Ib diamonds are extremely rare in cratonic settings (<1% of natural yellow diamonds; King et al., 2005) and other than at Zimmi (Shigley and Breeding, 2013a), have only been documented in a few localities worldwide: Helam/Swartruggens on the eastern block of the Kaapvaal craton (Logan, 1999;McKenna et al., 2004), Lac de Gras on the Slave craton (Davies et al., 2004), Dachine on the Amazon craton (Cartigny, 2010;Smith et al., 2010;Walter et al., 2013), Orapa on the southwestern portion of the Zimbabwe craton (Timmerman et al., 2016), Qilalugaq on the Rae craton (www.northarrowminerals.com/projects/qilalugaq) and in the fibrous coat of a Kankan diamond on the West African craton (Weiss et al., 2009). Preservation of C centres in natural Type Ib diamonds is typically attributed to either very short mantle residence of only a few million years, or otherwise to mantle storage at low temperatures <850°C (Taylor et al., 1996), or both. ...
Article
Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe–Cu–Ni–Co contents and Re–Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850 °C for any extended period. Such diamonds make up less than 0.1% of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic ¹⁸⁷Os/¹⁸⁸Os, all features characteristic of eclogitic sulphides. Each diamond has 3–4 individual inclusions. ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os data fall along three individual ∼650 Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond – unambiguously dating the time of diamond formation. The ∼650 Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial ¹⁸⁷Os/¹⁸⁸Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650 Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.
... A complete view of the magnitude of carbon isotope fractionation in the Earth's mantle is necessary for understanding a number of fundamental problems, including the global carbon cycle and diamond formation in the mantle. The available data on the carbon isotope composition of diamonds from kimberlites and lamproites are the only reliable source of information on the fractionation of carbon isotopes during mantle crystallisation (Deines 1980(Deines , 2002Galimov 1991;Cartigny et al. 2001; Thomassot et al. 2007;Stachel et al. 2009;Cartigny 2010). However, natural samples do not allow the study of all phases in diamond-generating systems. ...
Article
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We report first results of a systematic study of carbon isotope fractionation in a carbonate fluid system under mantle PT conditions. The system models a diamond-forming alkaline carbonate fluid using pure sodium oxalate (Na2C2O4) as the starting material, which decomposes to carbonate, CO2 and elementary carbon (graphite and diamond) involving a single source of carbon following the reaction 2Na2C2O4 → 2Na2CO3 + CO2 + C. Near-liquidus behaviour of carbonate was observed at 1300 °C and 6.3 GPa. The experimentally determined isotope fractionation between the components of the system in the temperature range from 1300 to 1700 °C at 6.3 and 7.5 GPa fit the theoretical expectations well. Carbon isotope fractionation associated with diamond crystallisation from the carbonate fluid at 7.5 GPa decreases with an increase in temperature from 2.7 to 1.6 ‰. This trend corresponds to the function ΔCarbonate fluid–Diamond = 7.38 × 106 T−2.
... A pulse of methane triggered by CAMP volcanism (Beerling & Berner, 2002) would have a value of −50 ‰ to −100 ‰ depending on whether the gas was thermogenic or biogenic in origin. Additionally, it is possible (though highly uncertain) that the emitted magmatic carbon itself may have been 13 C-depleted through interactions with pools of isotopically light carbon in the mantle (Deines, 2002; Cartigny, 2010; Paris et al. 2012). ...
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A sharp negative δ13C excursion coincides with the end-Triassic mass extinction. This is followed by a protracted interval of 13C enrichment. These isotopic events occurred simultaneously with the emplacement of the Central Atlantic Magmatic Province (CAMP). Here we use a carbon cycle box model to explore the effects of episodic carbon release - constrained by recently developed high-resolution chronology - on atmospheric pCO2, ocean chemistry and the δ13C of the ocean-atmosphere carbon pool. Our results are consistent with previous modelling efforts in suggesting that the sharp negative δ13C excursion and acidification event associated with the extinction are best explained by the rapid release (<20 ka) of highly 13C-depleted carbon (−70‰). However, our model also indicates that the likely short duration of the excursion requires organic carbon burial to have closely followed carbon injection. The age within the Hettangian of the large positive δ13C excursion which follows is currently uncertain. If early Hettangian in age, then our modelling indicates that the interval of 13C enrichment was closely associated with the volcanic CO2 pulses and pCO2 peaks. If late Hettangian in age, then the 13C enrichment must have lagged the carbon input substantially (by hundreds of thousands of years) and was associated with CO2 drawdown and over-cooling. Our modelling highlights the need for improved age constraints on Hettangian stratigraphic sections in order to test between two distinct and contrasting possibilities: continuing carbon cycle instability due to recurrent perturbations from CAMP activity or a delayed recovery arising from internal biosphere dynamics.
... Hypotheses have included meteorite impact (Smith and Dawson, 1985); formation during hot Archean subduction of early organic material; volcanic hydrothermal systems with metal-catalyzed diamond growth (McCall, 2009); extraterrestrial origin (Garai et al., 2006); and irradiation of carbonaceous materials (Ozima and Tatsumoto, 1997). Recent work has favored formation at mantle conditions, likely in the presence of a C-O-H fl uid supersaturated in carbon (Ishibashi et al., 2012;Petrovsky et al., 2010), a C-H-rich fl uid in lower crust transiently metamorphosed at mantle conditions (Sautter et al., 2011), or associated with komatiite magma intrusion into the continental lithosphere (Cartigny, 2010). ...
Article
Three-dimensional textural observations of inclusion and porosity patterns in a 23-carat carbonado diamond using high-resolution X-ray computed tomography reveal new information bearing on the nature and origin of this enigmatic material. A prominent patinaed surface is texturally linked to a banding and grading of inclusions and pore space beneath, extending several millimeters into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures, corroborating previous work indicating that the pore network is fully three-dimensionally (3-D) connected, and that virtually all macro-inclusions are secondary. Large metal inclusions are only found immediately adjacent to the margin of the specimen, and are thus also likely to be secondary or even tertiary. However, we also report pseudomorphs after a phase forming pristinely euhedral rhombic dodecahedra, individually and in clusters from 0.3 to 1 mm in diameter; although we could find no evidence of this phase persisting, it nevertheless represents the first "true" macro-inclusion reported in carbonado, which almost certainly formed syngenetically with the diamond material. The pore system is essentially trimodal, consisting of single and clustered pseudomorphs, oblate pores 0.1-0.3 mm in length with a clear preferred orientation, and 20 mu m to <1 mu m pores that form the connected network. Our observations support recent work suggesting that carbonado crystallized from a carbon-supersaturated fluid and suggest that the second stage may correspond with the creation of the pore alignment fabric. We further postulate that, although the present-day macroinclusions are certainly secondary, the bulk material that comprises them may not be, and may instead be broken-down remains of the original included phase(s). While further verification is needed, a model built around this hypothesis may provide the simplest explanation to many of the unusual features of carbonado.
... The model is able to reproduce the intensity of the observed negative δ 13 C excursion in the late Rhaetian sediments if we assume a low isotopic composition (−20‰) of the degassed mantle carbon. Although such a low value may appear unrealistic, the existence of heterogeneities in the mantle displaying very low carbon isotopic composition has been proposed recently by Deines (2002) and Cartigny (2010). The existence of such isotopic anomalies in the mantle could be considered in the future as an alternative to biogenic methane release systematically used to explain negative carbon isotopic excursions in the surficial environment. ...
... Thus, carbon reservoirs differing in isotope composition could be generated by iron carbide and diamond crystallization during the primary differentiation of the Earth, even if it was initially well-homogenized. The existence of such reservoirs is widely discussed on the basis of carbon isotope compositions of mantle xenoliths and diamonds (Cartigny, 2010;Deines, 2002;Stachel et al., 2009). ...
... Stachel et al., 2002;Tappert et al., 2005, see Section 5.1). In a recent study of diamonds from a komatiite (Cartigny, 2010), it was argued that the very negative d 13 C-values (mode at À27%) would reflect mantle values in the asthenosphere-transition zone, but no direct constraints about the depth of volatiles could be deduced. The following paragraph little relies on the mechanism leading to anomalous C-isotope composition in basaltic diamonds from the asthenospheric-transition zone. ...
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Sublithospheric diamonds that sample the transition zone and uppermost lower mantle provide a unique view into the deep Earth. In order to investigate the origin of diamond-forming C-H-O-N fluids within the deep mantle, within the framework of the terrestrial deep volatile cycle, we conducted a delta C-13-delta N-15-[N] micro-analytical study, by secondary ion mass spectrometry, of five Kankan diamonds from the asthenosphere/transition zone and the lower mantle. Abrupt and large changes in delta C-13 within KK-99 (up to 10.2 parts per thousand) and KK-200A (up to 6.9 parts per thousand) illustrate distinct episodes of diamond growth, involving different fluids, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. Despite limited variability of delta C-13 within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic delta C-13-delta N-15-[N] co-variations which can be modelled as a single diamond growth episode in a Rayleigh process from fluids/melts. These data constrain the carbon isotopic fractionation factors to be both negative (Delta
... A more recent 15 study exhibits diamonds from the Dachine komatiite with characteristics very close to the carbonados (diamonds with δ 13 C of ca. −25 ‰) the latter being sometimes interpreted as extra-terrestrial in origin, suggesting a possible mantellic origin of such light isotopic composition (Cartigny, 2010). This study suggests the existence of pools with carbon depleted in 13 C at the transition zone. ...
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The Triassic-Jurassic boundary (TJB) is associated with one of the five largest mass extinctions of the Phanerozoic. A deep carbon cycle perturbation and a carbonate production crisis are observed during the late Triassic. The Central Atlantic Magmatic Province (CAMP), one of the most important large igneous provinces of the Phanerozoic, emplaced at the TJB. To understand the carbon cycle perturbations observed at the TJB, we investigate the consequences of CO2 degassing associated to the CAMP emplacement on atmospheric and oceanic carbon cycle. The CO2 input within the atmosphere due to volcanism has been modeled using a global biogeochemical cycle box model (COMBINE) coupled with a climate model (FOAM). Weathering fluxes and CO2 equilibrium are constrained by the Rhaetian paleogeography and different scenarios of the CAMP emplacement are modeled. The study focuses (1) on the geological record and the carbonate productions crisis and (2) on the sedimentary carbon isotope record. For point (1), comparison of different modeling scenarios shows that a Gaussian CO2 emission distribution over the duration of the main activity phase of the CAMP fails in reproducing any of the geological observations, mainly the carbonate production crisis observed in the late Rhaetian sediments. Contrastingly, intense degassing peaks lead to successive decrease in carbonate production as observed in the geological record. For point (2), the perturbations of carbon cycle due to the degassing of CO2 with a mantellic carbon isotopic composition of -5‰ do not reproduce the intensity of the observed carbon isotope excursions. This was achieved in our model by assuming a mantellic carbon isotopic composition of -20‰. Even if this hypothesis requires further investigations, such low values may be associated to degassing of carbon from pools of light isotopic carbon located at the transition zone (Cartigny, 2010), possibly linked to setting of large igneous provinces (LIP's). Breakdown of biological primary productivity can also partially account for the sedimentary carbon isotope excursions and for the observed increase of atmospheric pCO2.
... Stachel et al., 2002;Tappert et al., 2005, see Section 5.1). In a recent study of diamonds from a komatiite (Cartigny, 2010), it was argued that the very negative d 13 C-values (mode at À27%) would reflect mantle values in the asthenosphere-transition zone, but no direct constraints about the depth of volatiles could be deduced. The following paragraph little relies on the mechanism leading to anomalous C-isotope composition in basaltic diamonds from the asthenospheric-transition zone. ...
Article
Diamond, as the deepest sample available for study, provides a unique opportunity to sample and examine parts of the Earth’s mantle not directly accessible. In order to provide further constraints on mantle convection and deep volatile cycles, we analysed nitrogen and carbon isotopes and nitrogen abundances in 133 diamonds from Juina (Brazil) and Kankan (Guinea). Host syngenetic inclusions within these diamonds indicate origins from the lithosphere, the asthenosphere-transition zone and the lower mantle.Juina and Kankan diamonds both display overall carbon isotopic compositions within the current upper mantle range but the δ13C signatures of diamonds from the asthenosphere-transition zone extend toward very negative and positive values, respectively. Two Kankan diamonds with both lower mantle and asthenosphere-transition zone inclusions (KK-45 and KK-83) are zoned in δ13C, and have signatures consistent with multiple growth steps likely within both the lower mantle and the asthenosphere-transition zone illustrating the transfer of material through the 670 km seismic discontinuity.At a given locality, diamonds from the upper and the lower mantle show similar δ15N distributions with coinciding modes within the range defined by typical upper mantle samples, as one might expect for a well stirred reservoir resulting from whole mantle convection.Kankan diamonds KK-11 (lower mantle), KK-21 and KK-92 (both lithospheric) display the lowest δ15N values (-24.9%, -39.4% and -30.4%) ever measured in terrestrial samples, which we interpret as reflecting primordial heterogeneity preserved in an imperfectly mixed convective mantle.Our diamond data thus provide support for deeply rooted convection cells, together with the preservation of primordial volatiles in an imperfectly mixed convecting mantle, thereby reconciling the conflicting interpretations regarding mantle homogeneity derived from geochemical and geophysical studies.
... Although the study of Sumino et al. (2011) highlighted the occurrence of some mantle-derived rare gases, there is no doubt that, for the vast majority of UHP diamonds, the carbon reservoir is crustal, apparently isolated from mantle carbon. The crustal source of carbon is evident from the association of the diamonds with metasedimentary protoliths, the unusual chemical composition of their fluids (e.g., Hwang et al. , 2006), and their high-N content (up to 1 wt%) or heavy d 15 N (Cartigny 2005(Cartigny , 2010) that are not found among mantle-derived diamonds. ...
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### Introduction Earth’s carbon, derived from planetesimals in the 1 AU region during accretion of the Solar System, still retains similarities to carbon found in meteorites (Marty et al. 2013) even after 4.57 billion years of geological processing. The range in isotopic composition of carbon on Earth versus meteorites is nearly identical and, for both, diamond is a common, if volumetrically minor, carbon mineral (Haggerty 1999). Diamond is one of the three native carbon minerals on Earth (the other two being graphite and lonsdaleite). It can crystallize throughout the mantle below about 150 km and can occur metastably in the crust. Diamond is a rare mineral, occurring at the part-per-billion level even within the most diamondiferous volcanic host rock although some rare eclogites have been known to contain 10–15% diamond. As a trace mineral it is unevenly distributed and, except for occurrences in metamorphosed crustal rocks, it is a xenocrystic phase within the series of volcanic rocks (kimberlites, lamproites, ultramafic lamprohyres), which bring it to the surface and host it. The occurrence of diamond on Earth’s surface results from its unique resistance to alteration/dissolution and the sometimes accidental circumstances of its sampling by the volcanic host rock. Diamonds are usually the chief minerals left from their depth of formation, because intact diamondiferous mantle xenoliths are rare. Diamond has been intensively studied over the last 40 years to provide extraordinary information on our planet’s interior. For example, from the study of its inclusions, diamond is recognized as the only material sampling the “very deep” mantle to depths exceeding 800 km (Harte et al. 1999; McCammon 2001; Stachel and Harris 2009; Harte 2010) although most crystals (~95%) derive from shallower depths (150 to 250 km). Diamonds are less useful in determining carbon fluxes on Earth because they provide only a small, …
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A COMPARATIVE ANALYSIS OF DIAMONDS IN NON-KIMBERLITIC ROCKS OF THE WORLD AND NEOGENE SANDS OF UKRAINE The diamond mineralogy from a number of different non-kimberlitic occurrences of the world was analyzed and a comparison to diamonds from Neogene placers of Ukraine was made. Diamonds from lamprophyres of the Canadian Shield, metakomatiites of the Guiana and West African Shields, and ultra-metamorphic rocks in Asian, European and African continental occurrences were considered. In general, Ukrainian Neogene placer diamonds have many similar mineralogical features to diamonds from lamprophyres and metakomatiites, which differ little from diamonds from kimberlites and lamproites. Ukrainian placer diamonds are characterized by their micron sizes (≤0.5 mm), many of them are colored and have a cubic habit and nitrogen-free compositions, relatively frequent spectral Ib and Iаb types, and orange photoluminescence behavior. In addition, many diamonds are characterized by a relatively high content of hydrocarbons (i.e., CH2, CH3 groups and bonds >C=CH2), OH groups, also C=O, N-H, CO3, NO3-groups. This is evidence for a volatile-rich environment during the diamond crystallization. The formation of diamonds from lamprophyres and metakomatiites, as well as from kimberlites and lamproites, occurred under mantle temperatures and pressures. The relatively recently discovered diamond-bearing lamprophyres and metakomatiites have a number of similarities. They are: i) both formed on the edges of Archean cratonic structures, ii) their old ages (2.7 billion years ago and 1.83 billion years ago for the lamprophyres and 2.2 billion years ago for the metakomatiites), iii) both are strongly metamorphosed, iv) both contain many fragments of rocks of various origins, v) both contain rare xenograins of mantle minerals having diamond-bearing peridotite and eclogite associations, and vi) the diamonds themselves are often microcrystals and many are colored and have a cubic habit. The main difference between diamonds from the lamprophyres and metakomatiites is in their carbon isotopic signatures and their thermal history in the mantle. This is expressed by their different nitrogen contents and the different degree of its aggregation. Diamond-bearing ultrametamorphic rocks are mainly Paleozoic-Mesozoic in age (i.e., 531-92 million years) and occupy a tectonic setting at convergent plate boundaries unlike diamond-bearing lamprophyres and metakomatiites. Diamonds from ultrametamorphic rocks differ from diamonds from lamprophyres and metakomatiites as well as from Ukrainian placer diamonds from Neogene sands in a number of ways. Keywords: non-kimberlitic diamonds, Ukrainian placer diamonds, lamprophyres, metakomatiites, ultrametamorphic UHPM rocks, Neogene placers.
Chapter
Extraterrestrial materials consist of samples from the moon, Mars, Vesta and a variety of smaller bodies such as asteroidsAsteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar systemSolar system. These heterogeneities are not observed on Earth, because they have become obliterated during high-temperature processes over geologic time. Nevertheless, isotopes have been used as a genetic link between meteorites and the Earth (i.e. Clayton in Treatise on geochemistry, Elsevier, Amsterdam, 2004).
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Structure, defects and inclusions in natural polycrystalline ballas-type diamonds are studied by complementary techniques: Scanning Electron microscopy, cathodo- and photoluminescence, X-ray diffraction, radiography and fluorescence, infra-red spectroscopy and carbon isotope analyses. Peculiarities of internal structure of some of the samples are explained by growth of the diamonds as volume-filling dendrites. Similarities of structure and mineralogy of trapped inclusions between ballas-type diamonds with fibrous and cuboid diamonds are established. However, significant differences between growth mechanisms of all these varieties exist and are possibly explained by variations in carbon supersaturation and diamond wet-tability by growth medium.
Chapter
Extraterrestrial materials consist of samples from the moon, Mars and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on Earth, because they have become obliterated during high-temperature processes over geologic time.
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The origin of diamonds in the lava and ash of the recent Tolbachik eruption of 2012–2013 (Kamchatka) is enigmatic. The mineralogy of the host rocks provides no evidence for the existence of the high pressure that is necessary for diamond formation. The analysis of carbon isotope systematics showed a similarity between the diamonds and dispersed carbon from the Tolbachik lava, which could serve as a primary material for diamond synthesis. There are grounds to believe that the formation of Tolbachik diamonds was related to fluid dynamics. Based on the obtained results, it was suggested that Tolbachik microdiamonds were formed as a result of cavitation during the rapid movement of volcanic fluid. The possibility of cavitation-induced diamond formation was previously theoretically substantiated by us and confirmed experimentally. During cavitation, ultrahigh pressure is generated locally (in collapsing bubbles), while the external pressure is not critical for diamond synthesis. The conditions of the occurrence of cavitation are rather common in geologic processes. Therefore, microdiamonds of such an origin may be much more abundant in nature than was supposed previously.
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The fate of subducted oceanic lithosphere and its role in the planet-scale geochemical cycle is a key problem in solid Earth studies. Asthenospheric and transition zone minerals included in diamond have been interpreted as representing subducted oceanic crust based on inclusion REE patterns and strong 13C depletion of their host diamond (d13C as low as -23 ‰). This view/explanation, however, has been challenged by alternative interpretations that variable carbon isotopic compositions either result from high temperature fractionation involving carbides, or reflect primordial, unhomogenised mantle reservoirs. Here, we present the first oxygen isotope analyses of inclusions in such ultradeep diamonds – majoritic garnets in diamond from Jagersfontein (South Africa). The oxygen isotope compositions provide unambiguous evidence for derivation of the inclusions from subducted crustal materials. The d18OVSMOW values of the majorites range from +8.6 ‰ to +10.0 ‰, well outside that of ambient mantle (+5.5 ±0.4 ‰) and indicate that the protoliths were very heavily weathered at relatively low temperatures. When this information is combined with the broadly eclogitic composition of the majoritic garnets, a derivation from subducted sea-floor basalts is implied. Based on the association between the heavy oxygen and light carbon, the light carbon isotope composition cannot relate to deep mantle processes and is also ultimately derived from the crust.
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Despite nearly two centuries of investigation, a comprehensive understanding of dinosaur biology has proven intractable. The recent development of means to study tissue-level growth, age these animals, and make growth curves has revolutionized our knowledge of dinosaur lives. From such data it is now understood that dinosaurs grew both disruptively and determinately; that they rarely if ever exceeded a century in age; that they became giants through accelerated growth and dwarfed through truncated development; that they were likely endothermic, sexually matured like crocodiles, and showed survivorship like populations of large mammals; and that basal birds retained dinosaurian physiology.
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A unique Brazilian sample of a carbonado, displaying unusually large amount of diamond clasts merged within a finegrained diamond matrix, has been studied by Secondary Ion Mass Spectrometry (SIMS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) on Focused Ion Beam (FIB)-extracted foils. We found for the first time in the diamond clasts (stage-1) micrometer to nanometer-sized inclusions of augite, ilmenite and phlogopite (all Fe-rich). Inclusions of metallic phases (Fe, Ti, Cr, Al and alloys of Fe-Cr, Al-Cr, Al-Fe Cr) described in worldwide carbonado occur in the studied sample exclusively within the fine-grained matrix (stage-2). The carbon isotopic composition of the diamond clasts and the fine-grained matrix falls within the range 27 % to 32 % , like worldwide carbonado. The iron-rich silicate-oxide assemblage isolated inside clasts points to an initial growth of that diamond from mafic-rock minerals under oxidizing conditions (f O2> IW). On the other hand metallic phases within the fine-grained matrix indicate an oxygen fugacity drop of at least 15 log units. This change in redox conditions is coeval with a deformation event under shearing stress at upper-mantle depth. During this metamorphic event, stage-1 diamonds were broken giving rise to the stage-2 fine-grained matrix, and syngenetic oxide inclusions were reduced to their metallic elements. This unique sample sheds new light on early 1970s hypotheses that interpreted carbonado as a high-pressure product from prograde metamorphism of crustal mafic rocks.
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The Reverend Charles Grenfell Nicolay (1815–1897) made an important contribution to early geological work in Western Australia as a scientific adviser to the Colonial government and founder of the Colony’s first public collection of rocks, minerals and fossils. During his early career he taught geography at King’s and Queen’s Colleges in London, before leaving London in 1858 to serve as the Anglican Church Chaplain to the British residents in the city of Salvador, Bahia, Brazil. We describe here some of his geological activities in Brazil over the period 1858–1869. He assisted Charles Frederick Hartt (1840–1878) and Louis Agassiz (1807–1873) on the Thayer Expedition of 1865–1866 in their geological investigations of the province of Bahia, most notably providing geological descriptions of the diamond deposits of the Chapada Diamantina, then a diamond province of world importance. After returning to England, he presented his findings on the Chapada Diamantina to the British Association for the Advancement of Science meeting in Norwich in 1868. From May to August 1869, he made a brief return visit to Brazil acting as a geological advisor to the Brazilian Turba Company, who were hoping to exploit bituminous sedimentary deposits adjacent to the Bahia de Camamu, Bahia, in the production of oil and gas. Following his arrival in Western Australia, he corresponded with the Reverend William B. Clarke (1798–1878), in 1871–1872, on the subject of Brazilian diamonds, as Clarke sought to understand the diamond occurrences in eastern Australia. Through Clarke, Nicolay’s description of the geology of the Chapada Diamantina was circulated to the Australian scientific community.
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A collection of 35 diamondite samples (polycrystalline diamond aggregates, sometimes referred to as framesites), assumed to be from southern Africa, have been studied to investigate their infrared (IR) spectroscopic characteristics. Due to the abundance of sub-micron, interlocking diamonds (polycrystalline) with mineral and fluid inclusions within the diamond material affecting their transparency, only fragments from 10 of the samples provided high quality data. The IR spectra showed a wide range of generally high nitrogen concentrations (386 – 2677 ppm), with a full range of nitrogen aggregation states, from pure IaA to pure IaB. Platelet characteristics were interpreted as being regular (i.e. not having been affected by deformation and/or heating events), meaning the nitrogen aggregation data could be interpreted with confidence. Surprisingly, the platelet data showed a positive correlation between their intensity (integrated area) and peak position. The primary hydrogen band (at 3107 cm-1) and secondary band (at 1405 cm-1) are both often present in the samples’ spectra, but show no correlation with any other characteristic. There is also no correlation between the samples’ paragenesis (as defined by their garnet chemistry) and any of the IR characteristics. Whilst we have no independent determination of the samples mantle residence age, nor the temperature they resided at, we infer that diamondite formation has occurred episodically over a large time frame in single and distinct growth events (as opposed to over a short time frame but over a large depth / temperature range). This idea is more in keeping with the theory that C-O-H diamond- (and diamondite-) forming fluids are the result of localized small volume processes. Interestingly, one sample contained fluid inclusions that exhibited a water:carbonate molar ratio (~0.8), similar to the saline and silicic end members of the monocrystalline diamond-forming fluid chemical spectrum.
Article
AbSTRAcT Samples of carbonado from Brazil and the Central African Republic were studied with the use of electron backscatter diffraction (EBSD) for quantitative textural analysis (QTA) and transmission electron microscopy (TEM). The grain size distribution in carbonado is either random or may be approximated by a log-normal distribution model with a mode at 6–8 μm. No bimodal distribution, as suggested previously for some carbonado samples, was observed. The crystallographic orientations of diamond grains in carbonado are quasi-random. The following minerals were identified among syngenetic mineral inclusions, enclosed in diamond grains of carbonado: garnet, apatite (including fluorapatite), phlogopite (or high-silica mica), SiO 2 , Ca-Mg-Sr-and Ca-Ba-carbonates, halides (sylvite and bismocolite BiOCl), native Ni and metal alloys (Fe-Ni, Cr-Fe-Mn, and Pb-As-Mo), oxides (FeO, Fe-Sn-O, TiO 2 , SnO 2 , and PbO 2), and Fe-sulfides, as well as fluid inclusions. Most of these occur over a very wide range of stability conditions. Only bismocolite, which is characteristic of the weathered crust, can be considered an entirely crustal mineral phase, which implies a possible crustal origin of the entire mineral association. Among syngenetic liquid inclusions in diamond grains comprising carbonado, silicate-carbonate ones overwhelmingly predominate. In addition to the usual silicate components, such as Si, Ti, Al, and Fe, they have Ca, K, and Cl; the latter three comprise 11.9 at.% of the only analyzed fluid inclusion.
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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 δ13C of 108 diamond samples ranges from -3.2‰ to -28.7‰, but most stones have δ13C ≤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 Protarozoic tectonothermal activity.
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To better understand the yellow diamonds currently in the marketplace, as well as identify possible changes in their trends seen over a five-year period, researchers at the GIA Gem Laboratory analyzed gemological data collected on more than 24,000 natural-color yellow diamonds examined in the calendar years 1998 and 2003. These data included color grade, type of cut, clarity grade, weight, ultraviolet fluorescence, and UV-visible and infrared spectra. Among natural-color colored diamonds, those with a yellow hue are some of the most abundant; even so, they are much less common than the colorless to light yellow diamonds associated with GIA's D-to-Z color grading scale. Since the yellow color is a continuation of the gradation of color associated with the D-to-Z scale, there can be misconceptions about the color grading, which involves different procedures from those used for D-to-Z grading. The grading and appearance aspects, as well as other characteristics of yellow diamonds, are discussed to clarify these differences. The authors have also identified five subgroups of type I yellow diamonds, which (with some overlap) are characterized by representative spectra and color appearances.
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Specimens of carbonado from Brazil and North Yakutia, USSR, were examined to evaluate the organic C present and its possible effect on delta 13C. The extracts were analysed using chromatography and MS. Only low levels of organic C were found, those from the North Yakutian carbonado being insufficient for analysis. Readings varied more than previously assumed in the Brazilian carbonado and differed between the two carbonados. This indicates that they should be considered as distinct varieties of diamond.-J.W.C.
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Carbonado and yakutite are both porous aggregates of polycrystalline micrometre-size diamond, with very different characters from those of monocrystalline diamond. The genesis of carbonado is very controversial, whereas yakutite is thought to have been formed by meteorite impact. Neutron activation analyses of trace elements in carbonado and yakutite indicate that their rare earth element (REE) abundance patterns have common characteristics: heavy REEs are not much depleted and a negative Eu anomaly is observed. These patterns are quite different from those of kimberlite and monocrystalline diamond and are similar to those of crustal materials such as shale, supporting the hypothesis of a crustal origin for carbonado and yakutite. -Authors
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Polycrystalline diamond aggregates of variety VIII (Orlov’s classification) with inclusions of chromite and, occasionally, ilmenite and sulfides from the Mir pipe have d13C from -6.1 to -2.6‰ PDB (average 4.5 vs PDB). The composition of mineral inclusions indicates that the samples affiliate with the ultramafic assemblage. The C isotopic composition of polycrystalline diamond aggregates from the Mir pipe is statistically different from that of single crystals from the same pipe (f-test = 0.06). In the sequence from octahedral to rhombododecahedral single diamond crystals the relative 13C concentration decreases. Individual crystals of bort are similar in isotopic composition within the volume of a sample, a fact testifying to their crystallization in an isotopically homogeneous environment.
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Carbonado diamonds from the Central African Republic were investigated using spectroscopic observations and C-isotopic analysis. Based on photo luminescence (PL) spectra, carbonado samples were classified into two groups: Group-A, which exhibits an intense PL band at 504 nm; and Group-B, which exhibits PL bands at 504, 575, and 638 nm at room temperature. PL spectra measured at 120 K gave well-resolved side-band structures of 504 nm bands. Consequently, the 504 nm band of Group-A can be assigned to the 3H center attributable to self interstitials in diamond, whereas the 504 nm band from Group-B can be assigned to the H3 center attributable to a vacancy (E trapped at nearest-neighbor substitutional nitrogen (N) pairs. The PL band at 575 nm, which is attributable to neutral N-square pairs, is known to increase its width with increasing residual stress in diamond aggregates. The average FWHM of the 575 nm band and the standard deviations were 3.80 and 0.54 nm for Group-A carbonado, and 2.80 and 0.38 nm for Group-B carbonado. These values suggest that Group-A carbonado samples have higher residual stress than the Group-B samples. The presence of an H3 center and the lower residual stress in Group-B carbonado both suggest that they originated from higher temperatures compared to Group-A. Radiation halos were observed in cathodoluminescence (CL) images of both Group-A and Group-B samples. The CL halos are traces of radiation damage from radioactive nuclides. The texture of the haloes suggests that the radiation damage was a secondary event after formation of the carbonado diamonds. The average C-isotopic composition of Group-A is -23.6 +/- 0.52%o in delta C-13(PDB) and that of Group-B is -26.3 +/- 0.65%o. Group-A carbonado was enriched systematically in C-13 compared with Group-B carbonado. Isotopic fractionation might occur as a result of the different thermal history of carbonado.
Conference Paper
Very few nitride minerals are known, but some occur widely in certain classes of meteorites; theyare simple compounds. We have found delicate crystals of several new nitride minerals inside a single broken sample of carbonado diamond, not previously reported from Earth or meteorites, but comparable to synthetic compound mixtures between Ti-Cu-N. In addition to chemical stoichiometries from electron microprobe data, we have obtained in-situ non-destructive X-raydiffraction data, of the actual cluster of crystals imaged by the SEM, using a high brightness microbeam. Three new nitride phases have been identified; Ti3CuN (tetragonal), CuTiN2(orthorhombic) and a Cu-N compound. However, we are less confident of the exact stoichiometry of the Cu-N phase and although there are several candidates ranging from Cu3N to CuN2 (azide), the X-ray diffraction pattern is most consistent with Cu3N (tetragonal). The three new Ti-Cu nitride minerals coexist in a sub-mm scale cavity with minor copper nitrate, silver chloride and faceted polycrystalline host diamond. The delicate nitride crystals (<20 microns) contain between ~6 and ~20 wt % N and occur in three growth forms: (a) slender hollow needles (b) platycrystals with round holes and (c) euhedral spinel-shaped equant grains. In addition to major Ti and Cu, which vary sympathetically, the crystals also contain variable amounts of silver (up to ~4 wt %). The unusual metal association (Ti-Cu-Ag) of the nitride, and delicate growth forms within unconnected cavities suggest an unusual origin. Occasional probe detection of oxygen probablyreflects surface alteration to oxynitride, similar to synthetic behaviour. Our working hypothesis for origin of the nitrides, is growth from a high temperature vapour; nitrides such as osbornite (TiN) can form at temperatures of ~2500. Such a high temperature is found in the mid-lower mantle at a depth in excess of 2000 km. This is consistent with recent synthesis of polycrystalline diamond in a few minutes at 2,300-2,500oC at mantle pressures (Irifune et al, 2003). If the new nitride minerals are stable within the Earth’s mantle, they may represent a significant reservoir for nitrogen, and could explain the large isotopic imbalance between nitrogen in mantle diamond and atmospheric nitrogen, as predicted by Javoy (1998).
Conference Paper
Diamonds from Jagersfontein kimberlite with isotopically light carbon include majoritic garnets and associated minerals that may be superdeep, perhaps from the asthenosphere or transition zone (Tappert et al, 2005). Metals and carbides are very rare in natural diamonds but have been reported in pyroxenitic garnet inclusions within polycrystalline diamond from Venetia kimberlite (Jacob et al, 2003; Fe3C “cohenite”). We describe the occurrence of metallic and carbide inclusions together with their host diamonds from Jagersfontein.
Article
In addition to exhibiting the resistance-to-wear characteristic of diamond, the natural polycrystalline varities carbonado and ballas have an inherent toughness, and they have a long history of industrial usage. This article surveys all the available published information on natural polycrystalline diamond and discusses this data from the point of view of texture, since, in the author's opinion, it is upon the texture that the physical property of toughness critically depends. An attempt is also made to clear up some of the confusion, apparent from the literature, regarding definitions and terminology for the various naturally occurring polycrystalline forms of diamond.
Article
Carbonado is the unusual type of polycrystalline diamond, whose origin is still disputed. On the basis of previous studies several hypotheses for the formation of carbonado have been put forward [1-4]. However, no one explanation is completely satisfying in terms of previous observations. The current work reports new data aimed at identifying links between the two populations of carbonados (Brazil and Central Africa), we also report some results for other forms of polycrystalline diamond. Based on our data, carbonados from Ubangui are indistinguishable from those collected in Brazil in respect of δ13C, N abundance and δ15N. Whereas Brazilian and Ubangui carbonados are similar they can easily be discerned from framesites from Orapa and Jwaneng which have a higher N abundance and are enriched in δ13C and δ15N isotopes. The polycrystalline aggregates of shock origin from the Popigai crater specimens are even more δ13C enriched were extremely low in nitrogen. The results are discussed with relation to various hypothesis for carbonado origin. The C and N isotopic composition of carbonado suggests some connection with a crustal reservoir of carbon because both δ15N and δ13C are in the range for organic materials. Given that some eclogitic type diamonds may have been produced from crustal carbon sources, it may be difficult to distinguish different processes where the crust is involved. Within our work we encounted several facts supporting relation of carbonado to the diamonds of mantle origin: one sample from Ubangui indicated δ13C of -5.8‰ and δ15N of -5.0‰; in one sample from Brazil reliable IR spectra indicated IaA type of nitrogen aggregation; appearance of large crystallites (40-200 μm in diameter) in some carbonado. Some specimens of carbonado have a very distinctly unequilibrated rim which could be explained by fast changing of growth conditions or partial transformations in the process which could be caused by shock [5].
Article
Native metals [iron, chromium, nickel, taenite γ-(Fe, Ni), and separate particles of Ag] were first detected in cryptocrystalline diamonds (carbonado) from the Ubangi region. It has been noted that taenite particles at the edges were frequently substituted by pentlandite (Ni5Fe4S8). In addition, florencite-containing REEs, magnetite, anatase, rutile, ilmenite, hematite, goethite, and kaolinite were fixed. Based on the experimental results obtained and taking into account the reference data, it has been concluded that different inclusions in carbonado from the Ubangi region (Central Africa) reflect various stages of its geological history. Native metals a-Fe, Cr, Ni, γ-(Fe, Ni), and Ag are syngenetic inclusions and reflect the highly reducing conditions of carbonado crystallization. Inclusions of rutile, ilmenite, and magnetite are syngenetic also and indicate the high temperatures and pressures of this process. Rare earth florencite reveals the hydrothermal metasomatic alteration of carbonado at lower physicochemical parameters, while kaolinite, hematite, and goethite reveal the late post-magmatic stage of their formation in carbonado pores.
Article
Consideration of the characteristics of terrestrial diamond and of synthetic diamond and current notions of conditions prevailing in the earth's interior, indicates that most terrestrial diamond probably crystallized just within its thermodynamic stability field out of liquids parental to various peridotitic and eclogitic rocks. Such diamond has been brought to the surface as xenocrysts in kimberlite, which is the only igneous rock originating from sufficient depths (i. e. more than 150 km) to have frequently tapped diamond-bearing regions. However, carbonado is possibly exceptional in its origin. Meteoritic diamond exhibits characteristics indicating a shock-induced origin in all cases.
Article
Diamonds were found in impact melt rocks and breccias at the Popigai impact structure in Siberia. The diamonds preserve the crystallographic habit and twinning of graphites in the preimpact target rocks, from which they formed by shock transformation. Secondary and transmission electron microscopy indicate that the samples are polycrytslline and contain abundant very thin lamellae, which could present stacking faults, with local hexagonal symmetry, or microtwins. Microrystalline units are ≤1 μm. Infrared spectroscopy indicates the presence of solid CO2 and water in microinclusions in the diamonds, CO2 being under a pressure greater than 5 GPa (at room temperature). Trace element and isotopic compositions confirm the derivation from graphite precursors.
Article
Carbon isotopic ratios and rare-earth element (REE) contents were measured for six Central African carbonados and two Brazilian ones. The δ13C values of all the Central African samples and one of the Brazilian samples are from -29.7‰ to -24.4‰, which are within the range of organic matters such as petroleum and coal, and are much lower than the typical values for ordinary diamonds of around -5‰. Another Brazilian sample gives diamond-like δ13C value of -8.8‰, suggesting that this is actually not a carbonado but an aggregate of ordinary diamond such as bort and ballas. Samples with a larger amount of impurities show higher REE contents and higher light-REE/heavy-REE ratios, implying the existence of a light-REE-enriched mineral, such as florencite, along grain boundaries between diamond crystals. Chondritenormalized REE abundance patterns of the samples are similar to crustal materials such as shales rather than to kimberlites and ordinary diamonds, which are much more light-REE enriched than most of the studied samples. The Brazilian sample with a higher δ13C value, however, shows a kimberlite-like REE pattern which is clearly different from that of the other Brazilian sample. From our data, the crustal origin of carbonado is preferable to its genesis in the mantle.
Article
Carbonado diamonds from the Central African Republic (CAR) were investigated using spectroscopic observations. Raman spectra for a polished section of carbonado showed the average Raman frequency as 1333.0 cm−1; measurements 10 μm below the sample surface showed a bimodal distribution of the Raman frequency with the average at 1333.5 cm−1. These contrasting results indicate that the carbonado interior retains considerable residual pressure. The maximum residual pressure detected in this study from the 10 μm subsurface was 0.49 GPa. From infrared (IR) absorption spectra for crushed carbonado samples, no absorption attributable to diamond was observed because absorption bands of mineral and fluid inclusions were too strongly observed. Acid leaching treatment of the crushed grains elicited IR bands assignable to intrinsic diamond vibration and nitrogen impurity in the diamonds. Nitrogen atoms in the CAR carbonado were not much aggregated and the degree of aggregation was intermediate between type Ib and type IaA. In the chemically treated samples, IR absorption bands from liquid water and carbonate were ubiquitous suggesting that the CAR carbonado samples contain fluid inclusions and are similar to diamonds containing mantle-derived fluids. The experimental results of this study suggest that the CAR carbonado originated from rapid heating event with a presence of fluid in the mantle and subsequent rapid cooling before aggregation of nitrogen impurity in the diamond lattice.
Article
The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ (n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies.
Article
Abstract— Nineteen diamond aggregate specimens (1–2 mm in size) from impactites of Popigai crater and five diamond samples (5–7 mm in size) from Ebeliakh river placers were studied. Our investigations indicate that samples from Ebeliakh were formed in an impact event with the exception of one specimen (Y7). The carbon isotopic composition of diamonds from Popigai varies within the previously reported limits (δ13C, −8 to −22%); whereas, diamonds from Ebeliakh placers show heavier values of δ13C (−7 to −10%). All the specimens studied contain very low amounts of N, mostly <20 ppm, but a few contained up to 60 ppm. For specimens, where the quantity of N allowed reliable analysis, δ15N values were found to be in the range of −3.9 to +11.9%. On the basis of combined Ar and N study, it was concluded that impact diamonds studied here can be a mixture of at least two types of gas carriers (e.g., different diamond components). A possible explanation would be involvement of a carbon vapour deposition (CVD) process or diamond growth in the impact melt in addition to the direct graphite-diamond shock transformation. The δ13C distributions and different N/36Ar correlations have indicated a difference between impact diamonds from Ebeliakh and diamonds extracted from Popigai crater. This could be explained by the existence of different diamond populations formed during the Popigai impact event. On the other hand, Ebeliakh diamonds could have resulted from a separate impact event to Popigai and an alternative crater is yet to be found.
Conference Paper
Four eclogites with both diamond and graphite, and 20 graphite eclogites are described. All the diamond-bearing rocks and most of the graphite eclogites contain minor amounts of Na2O in their garnet and K2O in their clinopyroxenes, and are classed as Group I eclogites (after MacGregor & Carter, 1970). Four of the graphite-bearing rocks are classed as Group II eclogites. The diamond eclogites tend to contain relatively magnesian garnets and to give higher estimated T of equilibration than the Group I graphite eclogites. A relatively deep origin is postulated for the diamond eclogites. The Group II graphite eclogites are geochemically distinct from those of Group I. They give relatively low estimated T of equilibration and appear to be derived from shallower depths. There is no reason to link the Group II eclogites with diamond-forming processes. Geochemical evidence suggests that many of the diamond-bearing eclogites are not directly related to the eclogitic inclusions in diamond and the silicates intergrown with diamond aggregates which have been studied from the Orapa kimberlite. Some fundamental differences between the xenolith diamonds and those from the kimberlite indicate that the former are not dominant in the Orapa mine diamond population. This is notwithstanding that the eclogitic paragenesis predominates amongst the Orapa diamonds and that the xenoliths studied are many thousands of times richer in diamonds than the kimberlite.-J.M.H.
Article
The infrared absorption spectra of a suite of twenty-one type lb synthetic diamonds have been recorded, and the concentrations of nitrogen in the form of single substitutional N atoms in these specimens determined by electron spin resonance measurements. It is found that an absorption coefficient, at 1130 wavenumbers, of 1 cm-1 is produced by 25.0+/-2 at.p.p.m. of nitrogen in this form, a result identical to that of Chrenko, Strong and Tuft and consistent also with that of Woods, van Wyk and Collins. Seventeen of these specimens were heated at temperatures between 1800 and 2050-degrees-C, under a stabilizing pressure, to bring about an appreciable degree of aggregation of the single N defects into A centres, or nitrogen pairs. For each specimen the residual, unaggregated single N defect concentration was then determined by electron spin resonance. The difference between this and the original value gave the concentration of nitrogen in the form of A defects. The infrared absorption strength of the A centres was obtained in each case by decomposition of the infrared spect-rum into the residual single N, and A, components. In this way it is found that the absorption coefficient at 1282 wavenumbers due to A centres is 1 cm-1 per 16.2 +/- 1 at. p.p.m. This value is in good agreement with an earlier result obtained by inert-gas fusion and the result given in the paper that immediately follows.
Article
We present a model in which the generation of komatiites in Archaean subduction zones produced depleted mantle residues that eventually formed the highly depleted portions of the Kaapvaal lithospheric mantle. The envisioned melting process is similar to that which has formed boninites in Phanerozoic subduction zones such as the Izu-Bonin-Mariana arc. The primary differences between the Archaean and Phanerozoic melting regimes are higher mean melting temperatures (1450 versus 1350 oC) and higher mean melting pressures (2.5 versus 1.5 GPa) for the komatiites. The komatiites from the Komati Formation in the Barberton greenstone belt are mafic enough to have produced the depletion seen in most Kaapvaal granular peridotite xenoliths. However, the most highly depleted Kaapvaal xenoliths require an even more Mg-rich magma than the Komati komatiites (Kk). Samples of boninite mantle residues from the fore-arc of the Marianas subduction zone are nearly as depleted as the Kaapvaal cratonic mantle, indicating that buoyant, craton-like mantle is being produced today. We speculate that production rates of cratonic mantle were greater in the Archaean due to the greater depth of melting for komatiites (relative to boninites) and greater worldwide arc length. The high production rates and high buoyancy of the komatiite mantle residues gave rise to the rapid growth and stabilization of the Kaapvaal craton in the Archaean.
Article
Based upon their geochemical similarity, we propose that the 3.5 Ga Barberton basaltic komatiites (BK) are the Archean equivalents of modern boninites, and were produced by the same melting processes (i.e. hydrous melting in a subduction zone). The Barberton komatiites also share some geochemical characteristics with boninites, including petrologic evidence for high magmatic H2O contents. Experimental data indicates that the Archean sub-arc mantle need only be 1500-1600°C to produce hydrous komatiitic melts. This is considerably cooler than estimates of mantle temperatures assuming an anhydrous, plume origin for komatiites (up to 1900°C). The depleted mantle residue that generates the Barberton komatiites and BK will be cooled and metasomatised as it resides beneath the fore-arc, and may represent part of the material that formed the Kaapvaal cratonic keel.
Article
Carbon isotope fractionation factors for 13C exchange between graphite, diamond and carbon dioxide are calculated and tabulated for various temperatures between 0° and 1000°C. Lattice dynamics and the root sampling method were used to calculate the needed reduced partition function ratios for graphite and diamond. Approximate ways for doing the calculation and the geochemical interest of the results are discussed.
Article
We have investigated the textural diversity in four Brazilian samples of carbonado diamond by cathodoluminescence petrography and spectroscopy, coupled with carbon, nitrogen and argon isotope analyses. Three types of textures were observed: homogeneous, porphyritic and flow textures. In these last two textural varieties, carbon isotopic composition and nitrogen contents covary in different ways. The complex texture and delta(13)C-[N] variability in our two heterogeneous samples are analogous to those in igneous rocks. All four grains yielded similar cathodoluminescence spectra with respect to peak, positions and shapes, but relative intensities exhibit great differences. The main band at 575 nm represents the N + V center, which can involve (i) an irradiation followed by annealing, (ii) a plastic deformation during sintering of Ib-type microcrystals of diamond, or (iii) a growth of diamond crystals by a chemical vapor deposition (CVD) mechanism. Both cathodoluminescence spectra and N(2)/(40)Ar values of carbonado are distinct from those of impact-generated diamond. The relatively low concentration of Ar also does not support a CVD origin of carbonado diamond.
Article
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 meteorite 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.