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Sunday, 12 April 2009

Black Diamonds: A interesting PBS NOVA article

Black Diamonds: A interesting PBS NOVA article

Paul bristolia at yahoo.com
Sun Apr 12 22:23:49 EDT 2009

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.

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