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

From: Steve Schoner <schoner_at_meteoritecentral.com>
Date: Mon, 13 Apr 2009 15:22:49 GMT
Message-ID: <20090413.092249.10320.1_at_webmail04.dca.untd.com>

Paul,

Right. And the fact that they occur only in two places on earth and are found no where else, further complicates the issue.

I think that the Earth and moon were asteroid bombarded very intensely in two prolonged events. 2.9 and 3.7 billion years ago. And this leads to me really wonder about these carbonado diamonds that are so distinct from the others which were clearly formed in the Earth's upper mantle. The fact that these have an odd amorphous polycrystalline structure and are tougher than the normal terrestrial diamonds makes me wonder more about their origins, too.

Steve Schoner
IMCA 4470

P.S. I noticed that these are suddenly selling on ebay from the best and till now only supplier of carbonados (aaaroughy). One must watch out though, as the term "carbonado" is a term used loosely in the diamond trade. To most legit diamond dealers all dark, brown to black diamonds are "carbonado." The distinction for a true carbonado is its porous structure which is clearly evident to the eye either directly or under magnification.


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Message: 10
Date: Sun, 12 Apr 2009 19:23:49 -0700 (PDT)
From: Paul <bristolia at yahoo.com>
Subject: [meteorite-list] Black Diamonds: A interesting PBS NOVA
        article
To: meteorite-list at meteoritecentral.com
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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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Received on Mon 13 Apr 2009 11:22:49 AM PDT