Seventh Annual V. M. Goldschmidt Conference 2330.pdf

CARBONADOS FROM CENTRAL AFRICA AND BRAZIL - ARE THEY RELATED TO IMPACT EVENTS? A GEOCHEMICAL, SPECTROSCOPICAL, AND X-RAY STUDY. Christian Koeberl1, Marcus Schrauder1, Marco A. G. Andreoli2, Franz Brandstätter3, Christian Lengauer4, and Iain Gilmour5. 1Institute of Geochemistry, University of Vienna, Althanstrasse 14, A- 1090 Vienna, Austria ([email protected]); 2AEC, P.O. Box 582, Pretoria 0001, South Africa; 3Naturhistorisches Museum, P.O. Box 417, A-1014 Vienna, Austria; 4Institute of Mineralogy and Crystallography, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; 5Planetary Sciences Research Institute, Open University, Milton Keynes, MK7 6AA, U.K.

Carbonado is a grey to black polycrystalline diamond variety that has been found in placer deposits in Central Africa and Brazil. Various hypotheses have been proposed for the origin of this unusual diamond type, including derivation from the Earth’s mantle, formation in metamorphic processes related to subduction, meteorite impact, extraterrestrial sources, or irradiation of carbonaceous materials at or near the surface of the Earth. However, detailed studies of carbonados are scarce and their origin remains unknown. In discussing their origin, any scenario needs to be able to explain the formation of individual carbonados with sizes of up to about 600 g. We studied six carbonados from Diamantia (Brazil) and four from Ubangi (Central Africa) by infrared spectroscopy, X-ray diffractrometry, electron probe microanalysis, electron microscopy, mass spectrometry, and neutron activation analysis. The samples were prepared using a laser sawing technique. Slices of 0.1-0.3 mm thickness were cut from the central parts of the carbonado samples. Preliminary X-ray diffraction data (on several square millimeters of the carbonado sections) suggest that all Brazilian diamonds consist of relatively pure diamond without any detectable lonsdaleite component. The occurrence of lonsdaleite, which is often associated the varieties of impact-derived diamonds that have formed after graphite (e.g., at the Popigai impact structure), would be an indication for an impact origin. The XRD data show the presence of quartz (up to a few wt%) and a not yet identified sheet silicate. Infrared spectra of Brazilian carbonados show all the intrinsic diamond modes and absorption bands that are due to impurities in this porous diamond type. Quartz (~1100 cm-1), sheet silicates (~1100 cm-1), carbonate (1430 -1 -1 -1 cm ), water (1640, 3420, 3700 cm ), and CO2 (~2350 cm ) are present in all samples. Water is present probably within the sheet silicate and other inclusions. The water contents of the bulk carbonados reach a few thousand ppm. Carbonate is present in all samples and is identified by an absorption at 1430 cm-1. The ratio between water and carbonate is variable. IR spectra of African diamonds are similar but the water and carbonate absorption bands are split into two bands. Electron microscopic (SEM and BSE) studies with mineral analysis by EDX on several carbonados shows the presence of rather unusual inclusions. One sample (shown in the Figure below) contains irregular inclusions of a rare earth element alumino-phosphate mineral (with La contents varying between about 3.8 and 9 wt%, and Ce contents of 5-11 wt%), probably florencite. Also found were various small metal-rich inclusions (e.g., containing Ta, Sn, and a Fe-Cr-Ni spinel), rutile, and anhydrite. It is hoped that upon completion of our study some clues regarding the origin of the carbonados will have emerged.

Acknowledgements: Supported by the Austrian FWF, project Y-58-GEO (to CK).

Fig. 1: BSE image of REE, Al,P-rich inclusions in Central African carbonado CAR/C.