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Re: Carbonates and CI Chondrites



On Sun, 8 Feb 1998, Martin Horejsi wrote:

> Hello Frank,
> 
> Thank you very much for the additional information about Orgueil. I'm
> starting to put together the picture of Orgueil's formation and alteration.
> 
> One thing that you mentioned about Orgueil earlier has me curious. You
> stated:  "They do not have chondrules, and are considered to be extremely
> primordial (they are used as a standard to compare earth's igneous rocks to
> the mean composition of the Solar System).
> 
> What is the relationship between primativeness and age. According to Dodd
> (A Petrologic-Chemical Synthesis, 1981), Orgueil's age is not particularly
> old, nor is it uncommon among meteorites. Also, CI's, according to Dodd
> again, "are more complex than their nearly solar chemical composition
> suggests."
> 
> Dodd also mentions that "these, the least evolved of meteorites, had active
> physical and chemical histories."
> 
> How does one sort through the past activities of the Orgueil in order to
> get a baseline measure for comparison? And if the meteorite is so
> primitive, how can it contain, as Dodd points out, "Much of the abundant
> carbon in CI meteorites occurs in extremely complex organic compounds."
> Were complex organics present at the earliest stages of our solar system?
> 
> Sincerely,
> 
> Martin
> 
> 
> 
Martin,
	You have asked some difficult questions. It is going to be a
challenge to answer them in a meaningful way. I can only hope I don't
confound you with complexities.
	Primativeness is not, by any means, implying time of
formation. Look, for example, at modern alligators(off the subject I know,
but...).Paleontologists know they are primitive in their morphology. They
are related to birds, but they are obviously different. Looking at the
modern alligator, we can look into the past, and see the early history of
birds. So a primitive object does not mean it is old, only that it has
characteristics of an earlier time.
	How does a scientist know then that something is "primordial"(
primordial is just a expression to imply a great age)? In the case of
meteorites, they can look at the abundance of elements, types of elements,
and associations of elements in the bulk rock chemistry of the meteorite.
The data from anaylisis is plotted against modern abundances, which shows
what elements were present in the early solar system. 
	In order to extrapolate a primordial position of a meteorite, it
requires a knowledge of how elements, and isotpes evolve through time.   
Scientists have a good idea of what isotopes and elements are stable,
and how they become unstable. By knowing how, why, and when and isotope
decays, we can tell what the original element was that was present before
the decay process began. For example uranium-lead decay has a certain
pattern that the uranium isotopes follow until a stable form is
established. Looking at the abundances in the meteorite at present day
values allows us to work back the history of the evolution of the
isotopes.  
	Now when looking at Orgueil for a primative abundance, it requires
looking at how elements evolve in stellar lifecycles. These are model
driven theories, but they can be supported by astronomical data from
certain stars. We have a great example of a star in our solar system, the
Sun. By measuring the elemental abundance in the Sun, and compare it with
CI chondrites, they plot very near one another. Although the formation of
CI chondrites are not extremely old, they are "primordial" by virtue of
the solar element abundance relationship. 
	The Sun is a standard so to speak. Because of it's mass it is used
to see how other bodies in the solar system compare to it's elemental
compostion. By extension then, CI chondrites are used as a standard as
well, only this standard can be physically srutinized in a laborotory. We
can compare igneous rocks from throughout the solar system,and see if they
are enriched, or depleated in elements relative to the Sun.
	As for the organic compounds, it is hard to answer. People are
working on finding out the answer to your question. It is not my
speciality, but I will try to give a meaningful answer.
	Organic simply means any chemical compund with carbon in it. I
will make an assumption here that Dodd was speaking of amino acids when he
refered to organic molecules, or at least in part. Since the CI chondrite
parent body has been shown to have had aqueous activity, then it is not a
far stretch to see why they have such a complex organic record. Amino
acids are formed through periodic wetting and drying of compunds with
carbon in them. So it seems logical then, that, the aqueous activity was
not a one time event, infact it may have been many events. This would
lead to the prodution of complex organic molecules.
	Recently astronomers have seen complex organic molecules in nebula
in interstellar space. It seems that organic molecules are an inevitable
production in nature. However the relationship between the primitive solar
nebula and the organics in CI chondrites is unclear to me. I have not
found any research published relating to this subject, but that does not
mean it does not exist.  
	


Frank Stroik
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