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Re: Carbonates and CI chondrites: Orgueil Age
Hello All,
Since I have been primarily responsible for continuing the questions about
C1 chondrites, I felt the obligation to read through a year's worth of
postings to find the one Frank mentioned he had posted. At the risk of
avoiding the reinvention of the wheel (science ideas do evolve as they are
discussed over and over) I have compiled earlier postings about C1
chondrites and Mars from this month, but exactly one year ago. One other
note, I did run this posting through my spell-checker so it is possible a
word could have been inadvertently changed. My apologies to the authors is
such a case exists.
Enjoy,
Martin
In February 1997:
Frank S. wrote...
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 category). All sedimentary rocks form from particles
suspended in water that settle to the bottom of the water body.
Limestones however precipitate 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 relevant 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 18 oxygen to 17 oxygen
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 with 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
occurred. 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 justify 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 meteorite 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 analysis 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 isotope 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 occurred
on the Earth, and other Solar system bodies.
Orgueil has an age of 4.6 billion yrs. ALH84001 has an age of 4.3
billion(?)yrs. When the Deuterium/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 analysis shows what the atmosphere 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 occurrence of these minerals in sedimentary rocks (as being
formed by sedimentary processes). These minerals tell us that at some
point in a rocks existence, whether 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 doesn't 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 previously 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 Vol. 23, No. 9
May 1, 1996
Matt M. replies...
I would like to address some of the statements and observations Frank
made about Orgueil and sedimentary rocks. First of all, sedimentary
rocks DO NOT have to be deposited via precipitation from water. There are
large deposits of loess (wind blown dust) and thick sand dune sediments
and all over the world. Obviously the primary force for these sediments
is wind. Volcanic fields are huge sedimentary deposits, i.e., lava
flows, ash falls and the like. The Mars-Orgueil (all CI's) connection
is an intriguing one. Orgueil, does not necessarily have to come from a
lake bed, just because it contains water. Many clays and volcanic
deposits contain a high percentage of water and hydrous mineral phases.
I have not yet seen the chemical analysis of Orgueil, but it resembles
(in appearance) weathered basalts found here on earth. Also, who is to
say it is not an ancient earth or moon rock??
Regards,
--
Matt Morgan
Colorado Geological Survey
Frank replies...
" Of course I was not there at the beginning of the Solar System."
Harold Urey, Letter to NASA, 1961
I would like to thank Matt for pointing out that sedimentary rock
does not necessarily originate from water. I am aware of this, and
decided not to include those types not formed through hydrologic
processes. I did this, because I wanted to explain what the author of the
article was using as evidence for a sedimentary origin. This, in turn,
left an oversimplification in it's wake. I apologize for this oversight.
For completeness, there are also paleosoils(ancient dirt), Caliche
deposits(a dirt rich in calcium), evaporites(deposits formed by drying up
water), glacial related bulldozing of sediments in to piles forming what
is called a moraine. I could keep going, but I think everybody gets the idea.
John Brandenberg explicitly states in his article:" The material
(from meteorite bombardment) would form sediments of sorted fines
in large lakes or seas on the early Mars that then dried and lay relatively
undisturbed until a large impact ejected fragments into outer space". He
used the term " sorted fines" which, to me any way, indicate some sort of
layering, or preferential grain size orientation(larger grains on bottom,
because they are heavier, and smaller ones in succession until the
smallest are on top). This cannot be seen in Orgueil at all(if you own
Orgueil, look at it, do you see any layering? if anyone does I would like
to know...).
It is his choice of terms which leads to the questions of
his hypothesis. Also coupled with no experimental data of his own, leads
to doubt(he got all his information from other researchers).
If Mr. Brandenberg would explain in more detail how he concluded
that, among other things, Orgueil was a "sorted fine", then I would have
more confidence in it. Until then, I will stand on my feeling about the
paper,
and wait for more clarity in the matter.
end.
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