[meteorite-list] Re: Rubble pile asteroids

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:32 2004
Message-ID: <20020926085731.7782.qmail_at_web80313.mail.yahoo.com>

------------ Original Message ------------
[meteorite-list] Re: Rubble pile asteroids
 
Herbert Raab <herbert.raab_at_utanet.at> writes:

On the other hand, the real border between a L5 and a
L4 or L6 might be more blurry than the descriptions in
the textbooks suggest. There is an interesting paper
on the Dar al Gani region in the August 2002 issue of
MAPS. The authors conclude that pairing based on
class, shock level and weathering grade is
problematic, because individual specimens from
ordinary chondrite strewfields were assigned to
different classes. Either this is another strong
indication for frequent rubble pile meteoroids, or
maybe we just see the fuzzy borders between
petrographic types of meteorites.
Who knows? I certainly don't... ;^)

Best greetings,
  Herbert
-------------- End of Original Message --------------

Herbert makes reference to the "Dar al Gani meteorite
field" article in the August 2002 issue of MAPS. Not
only is the paper "interesting" as he describes it,
but I think this will be a very important paper; one
that we will be referring to many times in the future.
 So, it is for that reason that I want to make clear
what the authors of that paper concluded:

"that shock and weathering grades were problematic for
pairing purposes"

but they had no problem with petrologic class (or
metamorphic grade). In fact, they used a noble gas
isotope study to prove that all the L6 specimens were
indeed, paired to the same fall!

Truth is, most researchers already allow for a range
of shock stages and weathering grades when pairing
meteorites. But nearly all of them insist that
[unbrecciated] chondritic stones can only be of one
class. Meaning, that if two stones are of two
different metamorphic grades (one is L6 and the other
is L4), then they can't be paired.

But there is a "corollary" - if two different stones
(L6 & L4) ARE the SAME meteorite, then the parent
meteoroid MUST have been a breccia!
Ergo, a Gold Basin (L6) and a Gold Basin (L4) are
paired even though there is no visual evidence showing
it to be brecciated. In the current scheme of
classifications, there is no way to describe such a
meteorite without invoking brecciation.
(But then the current classification scheme doesn't
take into consideration the possibility of a "rubble
pile meteoroid";-)

So, it was because of this problem that I attempted to
put forth an explanation of how Gold Basin can have
various metamorphic grades (L6 thru L4) in one of my
MeteoriteTimes.com articles.
"Gradational" metamorphic grade was the term I coined
to explain how the meteoroid that produced the Gold
Basin meteorites could have this range of classes.

But I've changed my mind about this and would now like
to retract that proposed explanation.

I've concluded that no one meteoroid could be large
enough to have a "formed in place" range of
metamorphic grades, such as L6 out to L4. This form
of metamorphism is more akin to terrestrial "regional"
metamorphism, which is gradational over a much greater
areal extent than can be accounted for across the
diameter of just one meteoroid.

So, with no visual evidence of brecciation, I'm back
to having no explanation for the Gold Basin problem.

But the point that I wanted to make was that the MAPS
article is an example, by its own test methods, that
the only way, now, to really PROVE pairing is through
radio-isotopic testing (noble gases, carbon14, and
other terrestrial age dating methods).

Bob V.




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Received on Thu 26 Sep 2002 04:57:31 AM PDT