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RE: Orgueil:Is it really from Mars?

	The hypothesis that CI chondrites originated on Mars is an 
intriguing one. Is it possible that Orgueil is sediment from an 
early Martian lake? I think before I talk about this, I should talk a 
little about sediments, and Isotopic data.
	Sedimentary rocks are rocks that originated in water, or by the 
evaporation of water. There are three main types of sedimentary 
rocks,Carbonates(limestone), sandstone, and shales(there are others, but 
these are a broad catagory). All sedimentary rocks form from particles 
suspended in water that settle to the bottom of the water body. 
Limestones however percipitate out of water, when calcium, carbon, and 
oxygen reach a point where the water can no longer support the elements 
in solution. 
	Now, when these sediments settle to the bottom of a water body, 
they layer. If you look at a shale, you will see fine layers, these 
indicate a period of deposition, and how much sediment was deposited at 
that time. Remember this, because it will be more relevent later.
	Oxygen isotopes are used to correlate different meteorites to 
Earth rocks. These help us to distinguish between meteorite groups, and 
relate them to the Earth. The Earth has a certain 18oxygen to 17oxygen 
ratio. This ratio never changes, and is considered a standard. It plots a 
straight line on a graph, and meteorites fall somewhere near that line, 
depending on their composition. This aids in identifying different 
groups, and relationships in those groups. 
	Okay, get on with Orgueil already! Well, according to this 
hypothesis, Orgueil(CI's in general) formed as deposited sediments in a 
Martian lake. The evidence is this, if you plot an 17 oxygen/18 oxygen 
isotope ratio line for Mars, the main isotope composition of minerals, 
and organic matter in CI's, fall almost directly on this line. In addition 
water extracted by heating of SNC's, and Orgueil contain the same 
Deuterium/Hydrogen ratio(deuterium is a H gas molecule wuth one extra H). 
ALH84001 is different, but that is due to it's great age(remember this 
also..). 
	How did the material come together? In the article it is said 
that after Mars formed, a period of intense meteoritic bombardment 
occured. The material that landed was then weathered and transported to a 
lake, and deposited on the bottom. The author uses the mineral serpentine, 
and smectite clays to justufy this assumption. Serpentine is a mineral 
that contains water in it's atomic structure, and smectitie clays are 
highly water altered igneous minerals. He uses this because the 
Lafayette SNC metorite contains these same minerals.
	Can Orgueil be from Mars? I don't think so. Lets look at the 
evidence. If Orgueil was sediment from a lake, we would expect to see 
layering in the rock. I know of no report that has shown layering, or 
evidence of a sedimentary deposition in Orgueil. Here on Earth, every 
sedimentary rock has layering. For instance, The Green River Formation in 
Wyoming contains thinly layered shales. If you look at a piece of it you 
may count 50 layers, while holding it in your hand, but make a thin 
section out of it, and you could count 500-1000 individual layers. So if 
Orgueil was truly sedimentary, a thin section anaylisis would have shown, 
at the very least, some fine layering.
	Next the Isotopic data are extremely close to that of what you 
find on Earth. If I plot the Earth istotope abundance line, Orgueil would 
plot almost directly on it. Does this mean that Orgueil came from Earth? 
No, what it means is that Orgueil encompasses a primitive abundance of 
elements of 
all planetary bodies in the Solar System. That is why it is used to 
compare igneous rocks on Earth, and see how much reprocessing has occured 
on the Earth, and other Solar system bodies.
	Orgueil has an age of 4.6 billon yrs. AlH84001 has an age of 4.3 
billon(?)yrs. When the Deterium/Hydrogen data were presented, he said 
Orgueil was the same as all other SNC's except for ALH84001, due to it's 
age. You see, This anaylisis shows what the atmoshpere was like when the 
water precipitated on the planet. Now If Orgueil, and ALH84001 are almost 
the same age, why would these values be different? That needs to be 
explained more. 
 	Serpentine, and Smectite are formed by water percolating through 
a rock of igneous origin. They form veins in the rock as the do Orgueil. 
I know of no occurance of these minerals in sedimentary rocks(as being 
formed by sedimentary processes). These minerals tell us that at some 
point in a rocks existence, wether here on Earth, or a meteorite, a small 
amount of water traveled through small cracks, altering the minerals 
already found on the matrix.
	Finally, How did it get ejected from Mars? There is no explanation 
for this. CI's are crumbly, and easily broken, how could this material 
survive an impact? I don't know, and The author does'nt know either.
	So, There is the evidence for and against the hypothesis that 
Orgueil is from Mars. If Mr. Brandenberg can resolve these problems in 
the theory,it would be quite a profound implication for life on Mars. It 
would show us that Mars was wetter, and warmer than previuosly thought, 
allowing the evolution of organic material into life. Until then, I will 
let the scientific method decide on how I view this hypothesis.

Frank Stroik


Reference: Mars as the Parent Body of CI carbonaceous chondrite
           Brandenberg John E.
           Geophysical Research Letters Vol23, No.9
           May 1, 1996
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