[meteorite-list] AD - 86 Auctions - A Case for Mercury? or not?

From: Michael Fowler <mqfowler_at_meteoritecentral.com>
Date: Tue Dec 20 15:49:51 2005
Message-ID: <F76FF7C2-EFC0-4AC6-8711-546485021614_at_mac.com>

Hi Adam & David,

Angrites are indeed very enigmatic and may not all be from the same
parent body.
See below from: http://www.psrd.hawaii.edu/April03/asteroidalLava.html

However reading the article cited by Adam above it would seem that
item #6 especially, would rule out the Angrites being from Mercury.

"Are Angrites From Mercury?: Angrites are mafic to ultramafic
"igneous" or metamorphic rocks that probably derive from a large,
differentiated planetary body; yet, in the absence of any "ground
truth" the possibility that they are samples from Mercury rests only
on circumstantial arguments: (1) the virtual lack of Na implies a
highly refractory planet (near the Sun?) (2) oxygen isotopic
compositions are close to and parallel to the TFL (like planetary
rocks from Earth, Moon, Mars and Vesta) (3) preserved corona textures
in NWA 2999 require a parent body capable of km-scale tectonic uplift
of lithospheric material (by thrust faulting?) (4) each angrite
specimen is texturally different with a unique CRE age (5) the wide
range in CRE ages (55 to <6.1 Ma) suggests that the parent body (APB)
is large enough to be struck repeatedly and may still be extant (6)
very ancient formation ages (>4.555 Ga) imply very rapid core
segregation and cooling following APB accretion (consistent with
contraction?) (7) dynamical calculations predict that ~1% of material
ejected from Mercury could reach Earth (8) the limited shock effects
may mean that some angrites, including NWA 2999, were ejected by
spallation; others may be impact melts (could vesicles in some
quenched specimens be trapped impact rock vapor?). "

Why? Simply put, the Moon remained geologically active for more than
1 billion years. Since Mercury is larger than the Moon, it would
have retained its internal heat even longer. Since the formation
ages of Angrites are all extremely ancient and tightly clustered as
well, they had to come from a body that cooled quickly, even quicker
than Vesta, since the ages of Eucrites are spread out over about a 10
million year period.

Sincerely,

Mike Fowler
Chicago














The Angrites and Their Odd Namesake

So now we are up to four asteroids that provide us with volcanic
meteorites: two types of eucrites, the baffling mesosiderites, and
the angrites. However, David (Duck) Mittlefehldt and his colleagues
show that Angra dos Reis does not seem to be related to the other
five angrites, in spite of lending its name to the entire group.
Perhaps we have a fifth asteroid represented here.

Mittlefehldt and his coworkers made a detailed study of a new angrite
named D'Orbigny, a 16.6-kilogram meteorite found in Argentina. They
also did chemical analyses of Sahara 99555, a 2.7-kilogram meteorite
found in northern Africa, and compare these new data to what we knew
previously about the angrites. The new angrites appear to be volcanic
rocks, as shown by the shapes and intergrowths of minerals (see
photograph below) and the way the mineral olivine is chemically zoned
and rich in calcium. Zoning usually indicates that a crystal has
grown fairly rapidly from a magma that cooled too fast to allow
formation of minerals uniform in composition. .........

The chemical composition in individual olivine crystals varies in a
complicated way. The interiors are uniform in composition, but about
half way to the rims the composition begins to change, with the ratio
of magnesium to iron and the concentration of chromium decreasing
while calcium increases. There are even some reversals in the
chemical zoning patterns. Mittlefehldt suggests that the complicated
zoning patterns are caused by the crystallization of pyroxene in the
lava because pyroxene incorporates different amounts of iron,
magnesium, calcium, and chromium compared to the amounts incorporated
in olivine. There may be an added effect due to the addition of fresh
(uncrystallized) magma to the crystallizing portion of the lava.

The chemical compositions of angrites are strikingly different from
those of the eucrites. Mittlefehldt and his coworkers illustrate this
by drawing attention to the ratio of calcium to aluminum. The
angrites have Ca/Al ratios larger than those found in carbonaceous
chondrites, which are thought to represent the relative abundances of
most elements in the solar system. The eucrites, in contrast, have Ca/
Al ratios the same as in carbonaceous chondrites. This shift in Ca/Al
cannot be attributed solely to a mineral with a low Ca/Al ratio
separating from the angrite magma. For example, the likely mineral is
plagioclase feldspar. If this mineral separates, it also affects the
ratio of samarium (Sm) to europium (Eu). In eucrites, which seem to
have been affected by plagioclase removal somehow, Sm/Eu varies
widely. This ratio is quite uniform in angrites, suggesting that
there is some other cause for the high Ca/Al. Mittlefehldt suggests
it might have been caused by the presence of a mineral rich in
aluminum (such as spinel) in the interior of the angrite asteroid,
where the angrite magmas were created by partial melting. Angra dos
Reis plots in a drastically different place from the other angrites,
suggesting it formed in a different asteroid or in a very different
region on the same asteroid.

(missing graph)

ABOVE: This plot shows the ratio of samarium to europium (Sm/Eu)
versus the ratio of calcium to aluminum (Ca/Al) in eucrites and
angrites. Both ratios have been divided by the ratio in carbonaceous
chondrites. Angrites are clearly enriched in Ca relative to Al. This
enrichment is not caused by addition or separation of plagioclase
feldspar, which would affect the Sm/Eu ratio even more, as shown by
the eucrites. Note that Angra dos Reis is much different in both Ca/
Al and Sm/Eu than the other angrites. This suggests that Angra dos
Reis may have formed on a different asteroid.

The angrites seem to consist of two different groups: Angra dos Reis
is all by itself in one group. The other five meteorites make up the
other group. Angra dos Reis has been metamorphosed, whereas the
others have not been. Angra dos Reis is also chemically distinctive.
These factors point to an origin on a different asteroid. If so,
perhaps it has a different mix of oxygen isotopes than do the other
angrites, allowing us to use the same argument for distinctive origin
as Akira Yamaguchi used to identify a new type of eucrite. No such
luck. All six angrites have the same mix of oxygen isotopes.
Mittlefehldt and coworkers suggest that the oxygen isotope test might
not be definitive in this case because quite a few different
meteorite groups have oxygen isotopic compositions in this region of
the oxygen isotope plot. (Angrites are labeled "Ang" in the oxygen
isotope plot above.)
Received on Tue 20 Dec 2005 03:49:44 PM PST