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Re: Carbonates and CI Chondrites



On Thu, 5 Feb 1998, Martin Horejsi wrote:

> Hello Frank,
> 
> Thank you for the interesting observations about CI condrites. I'm a little
> curious as to where the line is drawn between igneous and sedimentary
> occurrences in meteorites. From what I understand, there are no sedimentary
> meteorites, but carbonate deposits and alteration due to water movement
> seem to indicate sedimentary-like processes.
> 
> When looking closely at my fragment of Orgueil, it does remind me of the
> carbonates formed due to chemical weathering I see on many of the basaltic
> rocks in my backyard. I've read that carbonate formations are considered
> sedimentary when occurring in special circumstances such as in a cave.
> 
> Could CIs such as Orgueil have formed in a specific environment such as a
> lake bottom-like environ or hydrothermal cave formations such as those
> found in Yellowstone park?
> 
> One last thing, were does science currently stand regarding Orgueil as
> containing bio-indicators?
> 
> Sincerely,
> 
> Martin
> 
> 
>
Martin,
	Carbonates are considered sadimentary rocks on Earth. However when
applied to meteorites, it seems best to consider them chemical
precipitates, rather than actual settling of partices from suspention.
	I am not a carbonate petrologist, but carbonates form on Earth in
warm enviroments such as the Bahamas, or any shallow ocean water in an
equatorial location. It seems that Ca is fed into the ocean by rock
weathering and transport. As a rock weathers it is broken down until it
is no longer a rock, but only individual elements. These elements are
transported by rivers to the ocean, where they mix with salt water. If the
temperature is just right Ca will bond to O and C to form a
carbonate(limestone) layer on the ocean floor. There is more, but this is
the process in a nutshel.
	Carbonates in CI chondrites are precipitates in a CLOSED(?)
SYSTEM. In the oceans on Earth, waves capture O from the atmosphere which
helps to combine it with Ca and C already in the water(OPEN SYSTEM). The
elements that
make up the carbonates in CI chondrites are found in the matrix of the
meteorite. 
	It seems that ice had to be incorporated into the parent body of
CI chondrites. This ice was heated by a single mechanisim, or a couple of
mechanisims. This would have formed cracks in the parent body allowing for
the water to travel in the cracks. These were a result of the heating, and
impacts. This in turn results in dissolving the matrix, and mixing of
elements. When temperatures cooled the carbonates would have precipitated
out.
	As far as having origins in a lake bed, or a hydrothermal caves,
it seems these to be unlikely parents of CI chondrites. Water is vaporized
in the absence of pressure. This vaporization would not allow for 
standing water to form on the parent body, due to it's lack of atmosphere.
As for a formation in a cave, this is unlikely as well. It does not seem
that the parent body had a any differentiation of materials. This
indicates that there was no crust, mantle, or core. As we continue in this
logic, there was not any crustal thinning(as seen in Yellowstone), and
therefor no internal heat rising to the surface to warm the water.
However, nuclear decay is probobly a main contributor to the heating. It
was pervasive throughout the body, but had not heated it enough to melt
it. 
	As far as the Bio-indicators in Orgueil, I cannot answer that
question. I have not seen an articles published recently on the subject,
however this does not indicate that nothing has been done. I am more apt
to physical rather than organic geochemistry. 


	I hope this answers your questions.

Frank Stroik
References: