CARBONADO
A POSSIBLE RELIC FROM URANUS OR NEPTUNE?

standby for carbonado photo
Recognized 1840's
Bahia Province, Brazil

Terrestrial History

The name carbonado, meaning burnt or carbonized in Portuguese, was given to this material upon its discovery in Brazil in the 1840's. This dark gray to black variant of diamond has been mined in the Bahia Province of Brazil since that time. Due to its much greater hardness than typical diamond, it is used industrially for drills and for edges in cutting implements. Carbonado was subsequently found to occur in the same sedimentary geological horizon on a separate continent—in the Ubangui region of the Central African Republic, and has also been reported to occur in Venezuela and the Soviet Union. Carbonado diamond is unique to these limited regions, and it has never been found during any conventional diamond mining and processing operations around the world. The largest recorded carbonado, weighing 3,167 carats (633.4 grams) and named 'Sergio', was found in Brazil in 1905.

In the ongoing debate to explain the circumstances surrounding the terrestrial occurrence of this unusual diamond, the theory which posits an arrival during a Precambrian (late Archaean) impact event has remained one of the primary contenders. The arrival occurred at a time when the present-day continents of South America and Africa formed a unified supercontinent known as Rodinia. A magnetic anomaly discovered in Central Africa, encompassing an area over 700,000 km², has also been dated to the Precambrian time. This impact feature, the largest known on Earth, might be associated with the delivery of the carbonado diamonds.

In a contrary opinion, S. Master (1998) proposed that the Precambrian feature known as the Kogo Structure in Equatorial Guinea (1° 11' N., 10° 1' E.), measuring 4.67 km in diameter, would have been situated exactly between the Brazilian and Central African carbonado source locations at the time of its occurrence. He determined that a direct relationship exists between the size of the carbonados and their distance from the Kogo Structure, and suggested that this is the probable impact point from which these diamonds were disseminated. The diagram below presented by Stephen E. Haggerty (2014) shows the only known site of carbonado as it appeared ~1.1 b.y. ago, on the Congo–São Francisco craton (age 3.3–3.7 b.y.; Barbosa and Sabate, 2004). This craton was either part of the Rodinia supercontinent or the pre-Rodinia supercontinent Nuna, the latter of which likely existed between ~1.78 b.y. and ~1.40 b.y. ago (Zhang et al., 2012). The Congo–São Francisco craton, at the time part of Gondwonaland, broke up ~180 m.y. ago into what is now Brazil and the Central African Republic.

standby for carbonado locations photo
Image credit: Stephen E. Haggerty, Earth-Science Reviews, vol. 130, pp. 49–72 (March 2014, open access link)
'Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model'
(https://doi.org/10.1016/j.earscirev.2013.12.008)

Physical Characteristics

As described by Garai et al. (2006), carbonados are porous (5–15 vol%; Haggerty, 2017), polycrystalline aggregates of sub-µm- to µm-sized, mostly cuboid-shaped diamond crystallites in random orientation. The nodules have a polished surface rind reminiscent of a fusion crust, and it is considered likely that entry- and impact-related processes, including partial ablation at extremely high temperatures in Earth's primordial oxygen-poor atmosphere, melted an initially porous texture. Thereafter, secondary mineralization filled the remaining surface pores with silica-based minerals. The interior of carbonado nodules is highly vesiculated, with some pores measuring up to 1 mm in size, and polycyclic aromatic hydrocarbons (PAHs) fill the pores (Kletetschka et al., 2000).

In their magnetization studies of carbonado, Kletetschka et al. (2000) found that magnetic carriers are only present at the smooth surface of carbonado nodules and that the nodules are completely non-magnetic throughout the interior. This suggests that the magnetic carriers were not present at the initial site of carbonado formation, but instead were added during the ablation process in Earth's atmosphere and/or during secondary terrestrial weathering processes. This finding is contrary to what one would find given an origin within Earth's crust or mantle.

While the C- and N-isotopic compositions and N abundances of carbonados recovered from Central Africa and Brazil are indistinguishable from each other, they are unlike any known terrestrial-sourced diamond varieties and are inconsistent with either a crustal or mantle origin (Shelkov et al., 1995; A. Pradenas, 2015). These unusual values, such as the isotopically light C content (δ13C ~ –27‰ compared to typical upper mantle values of δ13C ~ –5‰) and the low N abundances, are consistent with an extraterrestrial origin exemplified by solar system material having high contents of organic matter (e.g., carbonaceous chondrites, comets).

standby for carbonado c and n isotopic diagram
Image credit: Stephen E. Haggerty, Earth-Science Reviews, vol. 130, pp. 49–72 (March 2014, open access link)
'Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model'
(https://doi.org/10.1016/j.earscirev.2013.12.008)

Notably, P. Cartigny (2010) has described diamonds from Dachine komatiite (French Guyana) that exhibit high isotopic variability including some with isotopic similarities to carbonado. Therefore, and he suggests a possible formation for carbonado in the high-temperature transition zone at depths >300 km, as compared to depths of 150–300 km for typical diamonds. However, these findings of P. Cartigny (2010) are contrary to other studies which have found different isotopic values as well as a lack of isotopic variability in the transition zone (A. Pradenas, 2015). Moreover, his conclusion is at odds with the fact that carbonado has never been found associated with komatiites.

Spectroscopic analyses of mineral inclusions in carbonado diamond matrices from Brazilian and Central African source locations reveal the presence of highly reduced metals and metal alloys, including Fe, Si, Ti, Ni, Ag, FeNi-metal, FeCr-metal, and NiCr-metal, as well as the carbide SiC (De et al., 1998; Garai et al., 2006 and references therein); such highly reducing conditions occur within the deep mantle and are associated with certain solar system objects (S. Haggerty et al., 2014 and references therein). The presence of these exotic metals and alloys provides further evidence for an extraterrestrial origin for carbonado. The mineral osbornite (TiN) has also been identified in carbonado, a nitride previously found only in certain meteorites and recently acquired through NASA's Stardust mission to comet Wild 2 (Jones et al., 2003).

Theories of Origin

Over the years, various non-impact theories have been put forth to explain the formation of these enigmatic diamonds, including chemical vapor deposition (CVD), irradiation of carbonaceous material (e.g., kerogen, coal) over geologic timescales by highly energetic particles emitted from radiogenic U and Th, subduction of crustal organic matter into the mantle, and impact metamorphism of Archaean rock containing concentrated organic biomass. In their studies of carbonado, Yokochi et al. (2008) found that the cathodoluminescence spectra and the 40Ar values are inconsistent with an impact-generated formation for this diamond, and also that the relatively low concentration of Ar is inconsistent with a CVD origin of carbonado diamond.

Ozima and Zashu (1991) conducted noble gas isotopic studies in order to study the mechanism of diamond formation through irradiation by energetic particles. Their results do support the theory of micro-scale diamond formation through high-energy irradiation. They found that carbonado contains a high abundance of radiogenic 4He, nucleogenic 21Ne, and parentless 136Xe and 86Kr. This is consistent with implantation through an external radioactive decay process, most likely by U and Th in close contact with carbonado (e.g., Ozima and Zashu, 1991; Ozima and Tatsumoto, 1997). However, the exact nature of the metamorphic process (e.g., shock compression) by which micro-scale diamonds could have been aggregated into multi-carat-sized carbonado is only speculative at this time. Kletetschka et al. (2000) contend that a high-pressure crustal or upper mantle origin is inconsistent with the highly porous nature of carbonado as well as the indicated temperature of formation of <400°C. Based on the model of radiation-induced crystallization for micro-diamonds in carbonado, the crystallization age was constrained by the timescale of radiogenic Pb implantation from decay of U or Th—2.6 b.y. (instantaneous formation of micro-diamonds) to 3.8 b.y. (formation of micro-diamonds continued to the present) (Kaminsky, 1994; Ozima and Tatsumoto, 1997). This age range is in accord with that ascertained through Pb–Pb dating of quartz, rutile and clay mineral inclusions and matrix in Brazilian carbonado by Sano et al. (2002).

Robinson (1998) and Vicenzi and Heaney (2001) studied a formation theory which is based on the subduction of a slab containing organic sediments. They determined that the C and N isotopes and the N abundances have values that do not support such an origin for carbonado. Moreover, it is not understood how such diamonds could eventually end up in placer deposits, or why their sizes should only reach to µm-size. In addition, it is considered that subducted organic carbon would be too shallow for diamond formation, which is stable at ~150 km (A. Pradenas, 2015).

Based on examinations of carbonado by Smith and Dawson (1985), and the discovery of lonsdaleite in yakutite (a carbonado-like diamond from Yakutia (Soviet Union) which is significantly different from carbonado; F. Kaminsky, 1994), they favor the theory of impact metamorphism of Archaean crustal rock containing organic carbon or graphite. However, no evidence of high-pressure phases of silica such as coesite and stishovite have been identified in association with quartz. In addition, A. Pradenas (2015) recognized that the large sizes of some carbonado nodules are inconsistent with an impact origin, and also that the spectroscopic absorption band typically associated with impact-generated diamond is not present in carbonado.

An extraterrestrial origin for carbonado is supported by recent experiments utilizing Fourier transform infrared (FTIR) spectroscopy (Haggerty et al., 2006; Garai et al., 2006; Garai, 2012). By exposing carbonado to intense infrared light, they observed peaks primarily corresponding to C–H stretching of diamond hydride, spectra almost identical to that of presolar diamonds found in some meteorites. They assert that the hydrogen serves as the bonding agent (protonation) that sinters the microdiamonds together to form the carbonado. They determined that carbonado diamond is consistent with an origin in a hydrogen-rich environment similar to that of the solar nebula. Moreover, the presence of nitrogen mono-hydride substitution for C is more similar to that found in presolar diamonds than to terrestrial diamonds, providing a clear argument for an extraterrestrial origin. This substitution is inconsistent with the conditions under which conventional diamonds are formed, i.e., slow cooling over millions of years at high pressures.

As with the nanodiamonds present in some meteorites, carbonado diamonds may have also been produced in a supernova explosion. Over time, they would be accreted into a planetesimal or perhaps an iron core, as attested by their presence in the Canyon Diablo iron. On the other hand, perhaps the abundant carbonado that was produced broke up into asteroid-sized masses. The Florida International University and Case Western Reserve University research team has proposed that a carbonado-rich asteroid measuring ~1 km in diameter impacted the Earth billions of years ago when Africa and South America were part of a single supercontinent. They argue that the vesiculation present in carbonado was caused by gases escaping under conditions of low-pressure during formation, conditions which are inconsistent with the very high pressures existing at diamond formation depths of >150 km, but which do exist in space.

Other exotic mechanisms of carbonado formation have been proposed over the years. Scientists from Princeton University have postulated the existence of diamond layers within extrasolar carbon-rich planets. Haggerty (1996), supported by observations of astronomers from the Harvard–Smithsonian Astrophysical Observatory and other studies (e.g., Stroud et al., 2011), presumes that carbon was transformed into diamond by the intense shock waves generated during the explosive collapse of a red giant star (supernova), resulting in a white dwarf and its accompanying planetary nebula. White dwarf stars constitute ~6% of the stars in the solar neighborhood. Either of these mechanisms of diamond formation may have resulted in the injection of massive diamond asteroids into the protosolar cloud which become gravitationally attracted to Earth.

Closer to home, the ice-giant planets Uranus and Neptune are considered by some planetary scientists to potentially produce diamond from methane (CH4), which constitutes 10–15% of their dense atmospheres. It was theorized and experimentally verified that dissociation of CH4 will occur at high pressures (≈100 GPa) and temperatures >4000 K to form H2 and hydrocarbons, and possibly diamond (Richters and Kühne, 2013 and references therein). Extreme temperatures and pressures (0.6–1.1 TPa) are hypothesized to exist at great depth on Uranus and Neptune, possibly forming a liquid C metallic outer core which transitions to a solid diamond-rich layer at intermediate depths (Eggert et al., 2010). It was experimentally verified (e.g., Benedetti et al., 1999; M. Ross, Lawrence Livermore National Laboratory) that the conversion of methane to diamond did occur at the high-temperature (2000–3000 K) and high-pressure (10–50 GPa) conditions that exist on Uranus and Neptune (see Diamonds in the Sky, PBS–NOVA, P. Tyson). Under the ultra-high temperature and pressure conditions that exist in mid-layers on Uranus and Neptune, oceans of liquid diamond with solid chunks of diamond floating atop are thought to be plausible, in a manner similar to the unusual behavior of water and its less-dense form of ice (I. Silvera, 2010).

Notably, the Pb–Pb age of carbonado coincides with the period of Solar System history known as the Late Heavy Bombardment, 4.1–3.8 b.y. ago, during which time it is thought that the gas giant planets Jupiter and Saturn, and the ice giant planets Uranus and Neptune, underwent orbital migrations under the influence of mutual resonances ('Nice model'). Among the effects of the resulting gravitational instabilities was the perturbation of the smaller planetesimals into eccentric orbits, eventually leading some to intersect with the terrestrial planets. Significant collisions with Uranus and Neptune would be likely to occur at this time, and it has been argued that the large obliquity of Uranus is the result of a severe tangential collision with an Earth-sized proto-planet early in its history (A. Brunini, 1995, 2006). It may be conjectured that this unique cataclysmic event might also be responsible for the delivery of carbonado into an Earth-crossing orbit at this time.

While the exact origin of carbonado remains a mystery, the accumulating evidence for an extraterrestrial origin, or even possibly an extrasolar origin, provokes much excitement; however, further research is necessary. The photo above shows a 1.07 carat (0.21 g) Brazilian carbonado nodule measuring 6.1 × 5.5 × 4.3 mm with a shiny, porous surface.


Information for this page was obtained through many published works, including the following (arranged by date):


This website and all of its content, unless otherwise noted, is Copyright protected, © 1997–2018 by David Weir.