Clues to Carbonado's Enigmatic Origin

Carbonado is a natural polycrystalline diamond variety with unusual characteristics such as low 13C/12C isotope ratios and high porosity. It is absent from kimberlites and lacks mantle-derived mineral inclusions. Many non-mantle origins have been proposed although other polycrystalline diamonds are thought to be mantle-derived.

Carbonado is a natural polycrystalline diamond variety with unusual characteristics such as low 13C/12C isotope ratios and high porosity. It is absent from kimberlites and lacks mantle-derived mineral inclusions. Many non-mantle origins have been proposed although other polycrystalline diamonds are thought to be mantle-derived. No conclusive evidence supports or refutes the hypotheses for its formation.

In the current study [1], Japanese researchers examined carbonado from the Central African Republic using infrared spectroscopic and microscopic analyses. Transmission electron microscope observation revealed a negative crystal that is interpreted as a primary fluid inclusion in a single crystal of carbonado diamond. Observations by field-emission scanning electron microscope and electron backscatter diffraction analysis show characteristics similar to those of crystals formed in liquid.

The study builds upon these points and considers possible redox conditions of carbonado formation to hypothesize that the cratonic upper mantle is a possible formation location for carbonado. The research team proposes the following carbonado formation scenario: a C-O-H fluid was highly oversaturated in carbon under diamond-stable conditions, and then diamond crystals crystallized quickly to form a porous polycrystalline body of carbonado.

However, the 13C-depleted nature of carbonado means that it was derived from a 13C-depleted carbon reservoir. Some evidence suggests that 13C-depleted carbon reservoirs may exist in the mantle, but the origin of carbonado remains somewhat enigmatic until more conclusive evidence emerges.

Figure:  (a) Transmission electron microscope bright-field image of a film of a carbonado-forming single diamond crystal. (b) Magnified image of a hexagonal void in the film. The inset shows the [110] incident electron diffraction pattern around the void. The void is surrounded by a smooth surface of {111} planes, indicating it represents an octahedral negative crystal.

Reference:

1. Ishibashi H, Kagi H, Sakuai H, Ohfuji H, Sumino H (2012) Hydrous fluid as the growth media of natural polycrystalline diamond, carbonado: Implication from IR spectra and microtextural observations, American Mineralogist 97:1366-1372

 

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