Dear Listees,
This is a very timely picture, thanks to most kindly to both Bernd Pauli and
Michael Johnson's Rocks from Space Picture of the Day :-)
Just a click away and an interested person without the materials and
microscope can vicariously participate with Bernd's clear and colorful image
of his nice thin section of Lodranite/Acapulcoite material. Nice!
Bernd, could you kindly mention which of the grains were measured for those
of us who want to confirm our little fun with a tape measure on our computer
screens?
Another click away is David Weir's free Internet Meteorite Encyclopedia:
http://www.meteoritestudies.com/protected_LODRAN.HTM
http://www.meteoritestudies.com/protected_MONUMENT.HTM
there are insightful summaries on the current state of research into the
evolution of these materials. One could conclude from the information on
David's site that there is a consensus among leading researchers that
Acapulcoites and Lodranites were "identical" in their early history to
believe them from a common precursor parent body, though the fragmentation
history is believed to be varied. Apparently, the grain size was originally
thought to be indicative of the depth of the sample excavated in a simple
model of the deeper it is the slower the solidification and interrupting of
grain growth in size (thus, Acapulcoites propsed nearer to the surface,
cooled quicker in space and had smaller grains. This simplified concept has
been put under the loupe most recently by Rubin, pointing to a likely more
violent history of near anniliation impacts, re-assembling, and a final
special delivery to earth, the last which liberated meteoroids from the
mother lode less than 7 million years ago. This is most exciting to me as
4-7 million years is in the same blink of the Universe's eye we are living,
and it gives hope that this can be sorted out and paired to something
floating around space.
I hope this hasn't been already covered on the list (Martin,et. al., feel
free to remind me), but would like to comment that if the current
classification system is simply so lacking considering current knowledge,
that a more modern approach like that of A. Rubin, T. McCoy, or C. Floss,
authors Patzer _et. al._, that these grain sizes in the range that separates
the classifications. Perhaps, based on these comments a similar system to
aqueous/thermal alteration H-L's nomenclature is motivated, only in this
case instead of looking at aqueous alteration, it is primarily grain growth.
Rubin (Geochimica et Cosmochimica Acta, Volume 71, Issue 9, 1 May 2007,
Pages 2383-2401) states:
"Acapulcoites and lodranites experienced moderate post-shock annealing,
presumably resulting from burial beneath material of low thermal
diffusivity. The annealing process repaired damaged olivine crystal
lattices, lending acapulcoites and lodranites the appearance of unshocked
(i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed
during initial shock reverted to their low-pressure polymorphs during
annealing. Some samples were subsequently shocked again; several
acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the
lodranite MAC 88177 reached S5."
To trace the evolution of thought on this issue, which touches on the
formation and migration of the metal cores in asteroids, in the words of Dr.
Christine Floss (2000) at Washington University:
"The simple bimodal classification of these meteorites based primarily on
petrographic criteria, which has been used to date, appears to be inadequate
to describe this diverse group of samples, as they represent a range of
degrees of partial melting, both with and without accompanying melt
migration. In some instances secondary processes on the parent body, such as
cryptic metasomatism*, have further modified sample compositions."
quote abstracted from:
Floss C. (2000) Complexities on the acapulcoite-lodranite parent body:
evidence from trace element distributions in silicate minerals. Meteorit.
Planet. Sci. 35, 1073-1085.
*cryptic metasomatism: a fancy way to describe alteration (metasomatism)
where the relative abundances of components of minerals changes but not into
new minerals (cryptic). Puzzling, for sure to work backwards through this.
Comments by Patzer, Hill and Boynton (2004) regarding Floss' statement:
"In view of the growing number of samples, however, this scheme has recently
been suggested to prove too limited. We share this assessment as far as we
can judge from our preliminary INAA results and we support an extended
classification modus similar to that developed by [Floss].
Not to be forgotten was TimMcCoy's PhD thesis (1994) suggesting the Rosetta
Stone Acapulcoite-Lodranite LEW 86220 and foreshadowing the entire subject:
"In one meteorite (LEW 86220), these basaltic, Fe, Ni, FeS-rich partial
melts from a lodranite source region were injected into a cooler,
acapulcoite region. The acapulcoite-lodranite parent body experienced a
range of partial melting and melt migration. Cooling of this body may have
been complex, with slow cooling at high temperatures, rapid cooling at
intermediate temperatures, and slow cooling at low temperatures, indicating
that the body may have broken up and gravitationally reassembled. One to
three impact events liberated the acapulcoites and lodranites from their
parent body. These meteorites represent our best opportunity to understand
partial melting and incomplete differentiation of asteroids."
The other somewhat parallel example might be the H-E-D system, which also
suffers by the one-dimensional classification that probably fails in some of
the more interesting cases by using an arbitrary diogenitic content to
separate polymict Eucrites from Howardites. And not to forget the parallel
situation with Winonaites, thankfully, being just one major class and sub
classifications and a bunch of unofficial pairings.
Maybe the scientists who work out this classification system better are
limited by access to material as 20 grams might not always be enough to go
chondrule hunting in such complex cases. After all, the 20 gram research
retainer is arbitrary in itself is probably partly responsible for these
classification issues in the first place, not to mention the practical
problems relating to locality less pairings and the financial realities that
affect our less than perfect world . No solutions at this point, from me,
except it would seem that scientists and dealers in these materials
hopefully have a special friendship when classifying material that is
destined to be cut up and distributed anyway, it seems that a few more thin
sections from diverse portions of each locality, like Bernd's, could really
be just what the doctor ordered ... and if all images were in a central
library ...
Best health,
Doug
Just for fun, how would a classification system look for the Peruvian fall
if we considered the sorting, mud and meteoritic powder content,
atmospheric, aqueous and oxidative alteration of the yet to be announced
main mass, powder, or fragment size and exterior shape, considering all of
the terrestrial mixing, not to mention what happens to the pieces after they
get into ten different Peruoid primary family members (primary dealers), and
then a thousand different hands and mailboxes. Not going to say more, but
if I were meteorite collector from another planet, I think I know who's
anomalous sample I'd want most. All that from an impact hole...not to
mention those containing llama hair spindles...
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <meteorite-list@meteoritecentral.com>
Sent: Monday, November 05, 2007 6:15 AM
Subject: [meteorite-list] Rocks From Space Picture of the Day - November
5,2007
http://www.spacerocksinc.com/November_5_2007.html
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