[meteorite-list] Black Diamonds: A interesting PBS NOVA article

[Paul]

Personally, I do not know what to think about the
origin of carbonados. I do think that "there is much 
contradictory evidence in the literature" and "This 
is conundrum which still calls for much more research."
to quote from the abstract of: 

McCall, G.J.H., 2009, The carbonado diamond conundrum.
Earth-Science Reviews. vol. 93, no. 3-4, pp. 85-91.
http://dx.doi.org/10.1016/j.earscirev.2009.01.002

Whatever happened to create carbonados, happened a
very long time ago as discussed by:

Yuji, S., R. Yokochia, K. Teradab, M.L. Chaves, and 
M. Ozimad, 2002, Ion microprobe Pb-Pb dating of 
carbonado, polycrystalline diamond. Precambrian 
Research. vol. 113, no. 1-2, pp. 155-168
http://dx.doi.org/10.1016/S0301-9268(01)00208-X

Some quotes from Yuji et al. (2002).

"Pb–Pb age of mineral inclusions in GM01 carbonado,
3.3+/-0.7 Ga is older than the formation age of the 
Sopa conglomerate in carbonado-bearing deposit 
related to the first extensional event in southeast 
border of the Sao Francisco craton at 1750–1700 Ma 
(Uhlein et al., 1998). Therefore, the carbonados 
were originated somewhere else and later incorporated 
in the Sopa conglomerate."

"Mineral inclusions (rutile, florencite, quartz, 
zircon, and clay minerals) of GM01 and DO3 
carbonados are typically crustal. The upper mantle 
origin of the carbonados may be, therefore, ruled out."

"In addition we can not find any evidence of high 
pressure shock-metamorphism such as occurrence 
of coesite based on the mineral assemblage of the 
carbonado inclusions. Therefore, the early impact 
origin of the carbonados is probably excluded."

"207Pb/206Pb isochron age of DO3 carbonado
matrix is consistent with that of Central African 
carbonado reported by other workers, suggesting 
a close genetic relationship between Brazilian and 
African carbonados within a united landmass 
during the Archean, as supported by 3.6 Ga zircons 
observed in the conglomerate.

With the contradictory evidence for their origin, 
the lack of any carbonados being found in place where
they formed, and their great age, it might be very 
difficult to understand exactly how they formed. 
Because of their great age and having been eroded
and redeposited from the parent strata in which they
either form in place or accumulated as debris from
some extraterrestrial event means a lot of the 
critical evidence concerning their origin has been 
lost.

Yours,

Paul H.