Hi Zelimir,
Vesicles in achondrites have always interested me too. I believe around half
of the Dho 700 stones showed some form of vesicles. I have one of the 12
stones which does show a few small vesicles in the surface. You never know
what they are like in the interior though.
http://www.meteorites.com.au/features/dho700.html
D'Orbigny... one of my all time favourite meteorites and as you pointed out
is an excellent example of an achondrite with vesicles.
http://www.meteorites.com.au/features/d'orbigny.html
Anne Black has a stunning slice of D'Orbigny on her sale page with vesicles
and open druses:
http://www.impactika.com/Meteorities/Dorbig.htm
And then there are the melts such as NWA 3159 which have vesicles:
http://www.meteorites.com.au/features/nwa3159.html
And of course there is the wonderful anomalous basaltic achondrite; Ibitira.
I have pics and a brief outline on the theory of vesicle formation for
Ibitira here:
http://www.meteorites.com.au/features/ibitira.html
You might find this one interesting Zelimir as it talks about how the vapour
phase that deposited the vesicle minerals was in complete equilibrium with
the matrix materials.
Cheers,
Jeff
----- Original Message -----
From: "Zelimir Gabelica" <[email protected]>
To: "Ron Baalke" <[email protected]>; "Meteorite Mailing List"
<[email protected]>
Sent: Friday, March 27, 2009 5:18 AM
Subject: [meteorite-list] origin of vesicles in achondrites ?
Dear Ron, List,
Interesting story.
It is said that Almahata Sitta is a fragile, porous olivine and
pyroxene-bearing ureilite. Although said to be generally coarse-grained,
ureilites (major olivine, minor pyroxene-pigeonite embedded in a dark
carbon-rich matrix) are rarely porous.
I believe the propsity of Almahata Sitta is surprising, intriguing?
Vesicles (bubbles) were found in angrites and, recently, in one diogenite
(see below), perhaps on some other achondrites (I did not check the whole
group of achondrites for such oddities).
Looking at recent literature reveals that the origin of such bubbles is
diverse and can be very controversal.
My question concerns the origin of vesicles recently discovered in a
selected piece of Dhofar 700.
To better provoke some answer, here I am attempting to summarize what is so
far known as theories that could explain the presence of such bubbles in
achondrites.
1) Angrites.
D'orbigny (ANG) is a well known case.
Angrites are composed of variable amounts of fassaitic pyroxene and other
minor phases. They can be considered as basaltic rock involving cumulate
textures.
Their pores and vesicles (not so rare in D'Orbigny), often filled with
augite or anorthite xenocrysts, often well seen as single, sometimes well
isolated needles, are thought to have been formed before the rock had
crystallized on the earliest differentiated asteroids (289 Nenetta ?...3819
Robinson ?...3628 Boznemcova ? or other with a smaller diameter, as
reported by Weir ?) during the early days of the solar system genesis.
The origin of such vesicles is consistent with the formation and
coalescence of smaller CO2 bubbles in magma, before it rapidly cooled and
solidified near the surface (McCoy, cited in Weir's web site).
There are other theories as well, such as the exsolved solid sphere
hypothesis (Kurat, cited by Weir).
2) Diogenites.
So far, diogenites, composed of Mg-rich pyroxene and minor olivine and
plagioclase are currently coarse-grained (as the new ureilite Almahata
Sitta) but very rarely involve bubbles or vesicles. They are supposed to
come from magma chambers in deep regions of Vesta.
In a recent post (ad), Adam Hupé reported that several slices of Dhofar 700
contained vesicles that in some cases attained almost 1 cm in diameter.
This was a surprise since no other diogenite was reported to involve such
quite regular bubbles.
Adam's post reminded me that I was also the lucky owner of such a slice of
DHO 700, full of bubbles (almost 30 bubbles - some over 1 cm diameter - can
be discerned onto the 38x44 mm surface, some passing completely through the
slice (4 mm thick) !
See pic here (click on "Dhofar 700"):
http://www.agab.be/meteorites/historique/Connoisseur.html
The text below the pic describes in a somewhat popularized way (in French)
possible (perhaps still speculative) origins of such bubbles.
One theory argues for a mechanism involving the genesis of gas bubbles due
to some complex process on Vesta. Upon rapid cooling, such bubbles could
have remained occluded in the cooled magma in a similar way as for Angrites
(D'Orbigny).
The weak point of that theory is that such a process is quite unlikely to
occur in a cumulate rock.
Another source (Blaine Reed, personal communication) reported that no extra
minerals were found growing on the interior surfaces of the gas bubbles.
It appears as if the surrounding minerals have been crushed by some
increased pressure rather generated by a liquid, leading to think that it
was possibly hot water (!) that formed these bubbles.
But how to imagine that hot water, if any, ever could subsist on Vesta ?
A very recent article published in New Scientist reported that, unlike the
large Ceres, smaller Vesta (530 km diameter) must once have melted almost
completely so that oceans of molten rock or magma covered its surface,
resulting in a vesicle-forming scenario
similar to those describing the vesicular texture of angrites.
See details here:
http://www.newscientist.com/article/dn7522-magma-oceans-sloshed-across-early-asteroids.html
As curious (but not competent) person, I have that simple question:
Does someone know more about this weird and completely unexpected presence
of such a large amount of bubbles in a diogenite, in particular in (one out
of the 12 pieces found) of DHO 700 ?
Neither pic from Met. Bull. database shows such vesicles.
Thanks if someone has thoughts to share!
My best,
Zelimir
http://www.sciam.com/article.cfm?id=asteroid-meteorite-sudan-fireball
Rock Science: First Meteorites Recovered on Earth from an Asteroid
Tracked in Space
Fragments in the Sudanese desert make up an "asteroid trifecta":
discovery, prediction and recovery
By John Matson
Scientific American
March 25, 2009
Last October, asteroid monitors at the Catalina Sky Survey at the
University of Arizona in Tucson picked up a small object on an immediate
collision course with Earth. The asteroid was too small to present a
real threat - just a few meters across, it stood little chance of
penetrating the atmosphere intact. Indeed, it exploded in a
stratospheric fireball over northern Sudan less than 24 hours later - an
event witnessed by people on the ground as well as the pilots of a KLM
airliner- conforming well to astronomer's predictions for its trajectory.
But the asteroid, dubbed 2008 TC3, was
nonetheless a momentous discovery: Among the countless small objects
that strike Earth's atmosphere every year, none had ever been detected
and tracked before it impacted. Now the Sudan bolide has yielded yet
another first: Researchers report in Nature today
<http://www.nature.com/nature/journal/v458/n7237/full/nature07920.html>
that they have recovered 47 meteorites from the object in the Nubian
Desert. And lead author Peter Jenniskens, a meteor astronomer at the
SETI Institute in Mountain View, Calif., says that another search completed
earlier this month, after the paper was submitted, has upped the
meteorite count to about 280.
Astronomer Donald Yeomans, manager of NASA's Near-Earth Object Program
office <http://neo.jpl.nasa.gov/> at the Jet Propulsion Laboratory in
Pasadena, Calif., calls 2008 TC3 "a perfect asteroid trifecta,"
referring to "pre-impact discovery, successful impact prediction, and
successful sample return." (Yeomans did not contribute to the recovery
research, but his office played a leading role in tracking the
asteroid's entry <http://www.cfa.harvard.edu/mpec/K08/K08T50.html>.)
The find allows astronomers to connect the chemical composition of the
meteorite to its orbit and reflectance in the sky during tracking. "The
holy grail of asteroid science is to uniquely link a specific meteorite
and its detailed composition to a specific asteroid type," Yeomans says.
"And that has now been done without an expensive sample-return mission."
This object, which the study's authors call Almahata Sitta (Arabic for
Station Six, a train station in the desert where eyewitnesses saw the
fireball and that served as the researchers' base camp), appears to
belong to a rare class of bodies called F-class asteroids, which
constitute just 1.3 percent of all asteroids.
Chemically speaking, Almahata Sitta is a meteorite whose specific
composition is unique among meteorite collections. It is a fragile,
porous ureilite (a relatively rare kind of olivine- and
pyroxene-bearing meteorite)
containing graphite and nanodiamonds, among other materials. Its
fragility, Jenniskens says, helps explain why it broke apart so high in
the atmosphere.
With the benefit of the object's rarity as an F-class body and its
orbit, tracked backward through time, the researchers established a
possible link to a larger F-class asteroid, the 1.6-mile-
(2.6-kilometer-) diameter 1998 KU2, which may have originated from the
same parent body as Almahata Sitta.
"The orbit of the asteroid, by just tracking it for 20 hours, is 10,000
times better than anything you can get from just observing a fireball
Jenniskens says. "What's neat about this is that the big asteroid allows
you to extend back in time the evolutionary history." He notes that
scientists might be able to pinpoint the specific region of the asteroid
belt that 2008 TC3 came from with more F-class asteroids from the same
parent body.
Even the brief amount of time 2008 TC3 was tracked provided an excellent
lead on where to look- and the desert surface provided an ideal surface
for turning up the dark fragments. "The entry trajectory was very
precisely known," Jenniskens says.
The first samples were found in early December by a 45-person search
team from the University of Khartoum. (Three scientists from that
university and one from the University of Juba in Sudan are among the
co-authors of the study.) "We had many eyes and hands," Jenniskens says,
trying "to find these."
______________________________________________
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Meteorite-list mailing list
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Prof. Zelimir Gabelica
Université de Haute Alsace
ENSCMu, Lab. GSEC,
3, Rue A. Werner,
F-68093 Mulhouse Cedex, France
Tel: +33 (0)3 89 33 68 94
Fax: +33 (0)3 89 33 68 15
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