[meteorite-list] Two Questions

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Tue May 24 11:50:22 2005
Message-ID: <42934D04.BC0D95B5_at_bhil.com>

Hi,

    I'm no good for the first question, but...
    Obviously, if a fall can contain many thousand stones that were once
one stone and if 90% or more of the original meteoroid was ablated away in
the descent and it was a clean ablation (no series of fragmentations),
then the many meteorites would all be from the core of the original
meteoroid, and their CRE dates would be very uniform.
    But, if there was a series of fragmentations, we could very well end
up fragments from differing "depths" in the original meteoroid and a
(possible) variation in CRE dates. In that case you would take the oldest
date as most representative of the actual exposure time.
    But this variation would only occur in stones with long exposure
dates.
    Here's why. "Cosmic" rays are the highest energy particles (protons
and nuclei) known, so you would think that they are energetic enough to
zap right through a 100 meter rock like a proton through hot butter. If
that happens, we'll never know because we can only detect the cosmic ray
particles that run into some poor atom and catastrophically breakdown into
pions, which breakdown into muons, which breakdown into electrons, and
leave the detectable tracks of their demise. They also leave behind an
altered atom, the poor sucker they hit; it's been turned into a different
isotope.
    It's just a matter of sheer luck from the viewpoint of the particles,
all traveling at 99.9999-something percent of lightspeed. For example, we
all assume cosmic rays are all coming down into the atmosphere, but there
is an occasional detection of a cosmic ray coming UP, out of the earth!
Why? Some really unlucky neutrino, which could normally expect to
traverse lightyears of lead without hitting anything, hits something on
its way through the earth and zap! When your time's up, your time's up.
    Like all radiometric dating, there is a coarseness factor. Put a rock
into space for 10,000 years and retrieve it -- no detectable tracks will
be found. Put enough rocks into space for 20 or 30 thousand years and
you'll start finding tracks in some. So, good for dating millions of
years, the more many millions the better, but no good under 100,000 years,
and flakey under a million or so.
    The question you ask is a hot one and the subject of experiment and
debate. In calculating CRE dates, you search the matter for tracks (hard)
but we also base them on the amounts of odd isotopes that are produced
when such a particle collides with an atom. In the equation, there is a
"shielding parameter" to account for the "depth" in the parent body, but
it has been demonstrated to be a slushy "fudge factor," whose actual value
can be off by a factor of 200% to 400% (some claim).
    Some physicists tackle this problem by writing even more complicated
ways of calculating it and claim improved accuracy. Some physicists
bombard samples with the highest energy particles they can cook up and
measure the results. The two methods differ in outcomes, so... We can't
generate anything like the energy of cosmic rays, so scaling is a problem.

    I'm guessing you don't want to do this messy stuff yourself, and
that's good. And why I won't give a list of such references, unless you
really like the word "nucleotide" a lot. You turn the rock over to the
experts and they yell (quietly) at each other until they arrive at
something they can all agree on within limits.
    Well, OK, here's a sample of references. If you like these, there's
lots more...

Meteoritics & Planetary Science 35 (2000)
    Cosmogenic neon in mineral separates from Kapoeta: No evidence for an
irradiation of its parent body regolith by an early active Sun
    Rainer Wieler, Anselmo Pedroni and Ingo Leya


Meteoritics & Planetary Science 35 (2000)
    The production of cosmogenic nuclides in stony meteoroids by galactic
cosmic ray particles
    Ingo Leya, Hans-J?rgen Lange, Sonja Neumann, Rainer Wieler and Rolf
Michel


Meteoritics & Planetary Science 35 (2000)
    Simulation of the interaction of GCR protons with meteoroids: On the
production of radionuclides in thick gabbro and iron targets irradiated
isotropically with 1.6 GeV protons
    I. Leya*, H-J.Lange, M. L?pke, U. Neupert, R. Daunke, O. Fanenbruck,
R. Michel*, R. R?sel, B. Meltzow, T. Schiekel, F. Sudbrock, U. Herpers, D.
Filges, G. Bonani, B. Dittrich-Hannen, M. Suter, P.W. Kubik and H-A. Synal



Meteoritics & Planetary Science 37 (2002)
    Campo del Cielo iron meteorite: Sample shielding and meteoroid's
preatmospheric size
    Liberman R. G.*, Fern?ndez Niello J. O., di Tada M. L., Fifield L. K.,
Masarik J., and Reedy R. C.
    "Long-lived cosmogenic radioisotopes, 10Be, 26Al, 36Cl, 41Ca and 59Ni,
have been measured in five samples from the Campo del Cielo iron meteorite
by accelerator mass spectrometry (AMS)... The measured 36Cl activity
allowed an estimate of the meteoroid's preatmospheric size: a radius
larger than 300 m and a mass of at least 840,000 kg. We conclude that this
meteorite might be one of the largest meteorites to have been recovered."
    And there's so much of it, too...

    Can you tell I had nothing to do this morning but read my email?


Sterling K. Webb
-------------------------------------------------------------------------
Walter Branch wrote:

> Second try at posting this email:
>
> Hello Everyone,
>
> I been researching but I can't find the answers to two questions.
>
> First, what is the mechanism by which atmospheric gasses
> are trapped in the formation of basalts? I have been doing some
> lit reviews on martian meteorites and I find it interesting that some
> were formed in magma chambers deep undergound. indeed, some are
> thought to have formed several kilometers down. How does the
> Martian atmosphere get trapped in cooling rock so far underground.
>
> Second, when determining Cosmic Ray Exposure ages, can atmospheric
> fragmentation and ablation of a meteoroid affect the results from such
> testing and if so, how are fragmentation and ablation taken into
> consideration when determiniing CREs?
>
> Thanks to anyone who can help me understand these processes.
>
> -Walter Branch
>
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Received on Tue 24 May 2005 11:49:24 AM PDT