The Carbonado Conundrum

Carbonados are polycrystalline diamond aggregates of generally irregular shapes that have been observed in placer deposits and low-grade metamorphic rocks of mainly Brazil, Russia, South Africa, Ukraine, Venezuela, and the Central African Republic. Some major occurrences have been exploited commercially for the production of industrial diamond (e.g., Trueb and de Wys 1971; Kaminskiy et al. 1978). Carbonados occur in crustal lithologies and have crustal signatures in terms of stable isotope characteristics, and can not be related to an origin from kimberlites (e.g., Ozima et al. 1991; Haggerty 1999). They do have variable S13C values that overlap those of diamond-lonsdaleite aggregates in Ukrainian placers (15.8 to -20.5 %o, Kaminskiy et al. 1977; -29.7 to 24.2 %o, Kamioka et al. 1996; -23 to -30 %o, De et al. 2001) and the diamond-lonsdaleite bearing aggregates in suevitic breccias from Popigai (-12.3 to -17.6 %o, Galimov et al. 1980). Kaminskiy et al. (1978) reported the presence of lonsdaleite in some carbonados (an observation that has not been confirmed since) and that carbonados were generally of Precambrian age. Further evidence for a crustal source for the origin of carbonados includes their isotopically light character, noble gas contents interpreted as representing trapped atmospheric composition (Ozima et al. 1991), and rare earth element abundance patterns (Shibata et al. 1993; Kamioka et al. 1996) consistent with this interpretation. Also, the individual occurrences have widely different mineral parageneses and, thus, suggest a variety of different sources. Reviewing such evidence, Smith and Dawson (1985), consequently, suggested that carbonados could have been formed as a consequence of Precambrian impact events into carbon-bearing crustal rocks. All other traces of these impacts and the related impact structures apparently had been eroded, and only the carbonados had survived erosion and were then incorporated into sedimentary rocks.

No direct evidence for a shock origin of carbonados has ever been reported from any of these occurrences, and the geochemical evidence does not agree with this hypothesis either (Koeberl et al. 1997c). Shelkov et al. (1998) compared the 4He signatures in diamond crystals from kimberlites with those in carbonado samples and concluded that both types of samples had very similar signatures. In general, currently favored

corresponding interpretation of the various stratigraphie intervals in terms of their impact generation. After Grieve et al. (1991).

hypotheses for the formation of carbonados include mutually exclusive preferences involving extraterrestrial, crustal, impact and mantle provenances (Kaminskiy 1987; Kagi et al. 1994; Kletetschka et al. 2000; De et al. 1998, 2001; Haggerty 1999).

The carbonado occurrence of the Central African Republic (and of Brazil) has repeatedly been related to the possible existence of a very large impact structure, termed the Bangui structure (Girdler et al. 1992; De et al. 1998) in the region of the Bangui basin in the northern parts of the Democratic Republic of Congo and southern Central African Republic. However, all evidence quoted in support of the existence of such a structure has been derived from regional geophysical data and does not constitute reliable first-order evidence for the existence of such a Bangui impact structure.

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