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Re: Carbonates and CI Chondrites

	I came across an interesting paper and thought I would share it
with the List. It is a recent article dealing with how carbonate(i.e.
calcite, dolomite, ect.) minerals may tell us about the early history of
the parent body of CI chondrites.
	First off, what is a CI chondrite exactly? CI chondrites, in the
simplest terms, are meteorites from a small parent body that experienced
flowing water(hydrothermal viens). This left the parent body with a wide
range of hydrous minerals, and veining due to water movement. 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).
	It seems that there are some charcteristics(trends) recorded in CI
chondrites. The first is that they are all regolith breccias. Orgueil
shows the greatest amount of brecciation, whereas Tonk and Alais show the
least. Next is the degree of alteration due to water. It seems the more
brecciated the CI is, the more water has affected it. Differences have
been recognized in sulfides in these meteorites. Sulfides are minerals
that have sulfur in their atomic structure without silicon. These show an
aleration rim. It increases again with brecciation. There is a higher
amount of mafic mineral( minerals that have more Mg, and Fe, than say K,
Na, ect.) in extremely brecciated CI chondrites than slightly brecciated
chondrites. Finaly, the less brecciated a CI chondrite is the more
phyllosilicates( minerals that are similar to mica)are present.
	Let's look at the history of carbonates on the CI parent body.
First off, we need to establish a way of moving water through the body.
There are four possibilites:
1.Increased pressure of formation may have moved solid water to liquid
  without heating up.
2.Radioactive decay of short lived nuclei in the core of the asteroid may
  have led to melting.
3.During the T-Tauri stage of the Solar System, electromagnetic induction
  may have heated ice on the parent body. 
4.Impact bobmbardment may have played an important role in the heating of
  the parent body.
	It seems that nuclear decay and impacts are responsible for the
heating. This heating may have help to propogate fractures in the parent
body, and allowed for the movement of the liquid water through the new
plumbing system, so to speak. The water would have mixed with the
surrounding minerals as it worked it's way throught the parent body.
Eventually the water would have cooled, and carbonates would have been
precipitated(deposited) out of solution. 
	The above was just a brief look at CI chondrites and the origin of
carbonate material. These are complex meteorites, with a even more complex
past. They are an unique story in the ever evolving Solar System. 
	
Frank Stroik

Reference: EndreSS, M , and Bischoff, A: Carbonates in CI chondrites:
               Clues to parent body evolution. Geochimica et Cosmochimica
	       V. 60, 3, 1996 p. 489-507
References: