[meteorite-list] Abstract: EL3 Chondrite (not Aubrite) NorthwestAfrica 2828

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 25 Mar 2007 03:06:17 -0500
Message-ID: <08b901c76eb4$779dafe0$ab7e4b44_at_ATARIENGINE>

Hi, All,

> an ancient fluvial and/or acidic lacustrine environment...

Most people think of the Sahara as an ancient,
primordial environment. It's a relatively new feature.

The Sahara was a well-watered mixed forest and
glassland temperate environment, with lakes and
many rivers (whose ancient courses are still visible
in many places) 14,000 years ago and more.

There was plentiful game and a large human population.
The NE Sahara seems to have desertified first, driving
humans into the Nile Valley. By 8 to 10 thousand
years ago, it was a dry grassland and the lakes and
rivers were vanishing rapidly. The Sahara "grows"
from its center, where the bulk of the sand is generated
that flows out to make the Great Sand Sea. The process
is on-going and the remains of vast Roman "plantations"
can be found 100 miles or more into the Sand that were
thriving and productive 1600 years ago! North Africa
was the Breadbasket of the Roman Empire, green
and growing.

Like so many deserts, it is unlikely to revert to a
paradise again when the present Ice Age resumes after
this interglacial, because of the smothering effect of
the Sand. The Amazon Rain Forest, another temporary
Interglacial abnormality, will likely recover from the
damage done by its runaway forestation and revert to
the vast rolling Sea of Grass it was 12 to 16 thousand
years ago, when things get back to normal.

Any meteorite in the Sahara need not be highly
ancient to be completely weathered out. One sees
statements that completely weathered NWA's "must"
have terrestrial ages of 40 to 50 thousand years.
They would IF the Sahara had always been as dry
as it is, but it hasn't been. They need only be old
enough to have been exposed during the "wet" times.

This one seems to have sat in the lake bottom for
a long time, though, for all those changes. Still, I
doubt it's more than 20,000 years old, tops, and
it could be much younger. Chondrites don't last
that long in water!

Sterling K. Webb
----- Original Message -----
From: "Jeff Kuyken" <info at meteorites.com.au>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Sunday, March 25, 2007 1:39 AM
Subject: [meteorite-list] Abstract: EL3 Chondrite (not Aubrite)
NorthwestAfrica 2828

Hi all,

Thought some may find this abstract that I just found interesting.





EL3 Chondrite (not Aubrite) Northwest Africa 2828: An Unusual
Paleo-meteorite Occurring as Cobbles in a Terrestrial Conglomerate

Kuehner, S. M.; Irving, A. J.; Bunch, T. E.; Wittke, J. H.

American Geophysical Union, Fall Meeting 2006, abstract #P51E-1247

Publication Date:

Although we recently classified NWA 2828 as an aubrite [1], our examination
of new material (now comprising over 120 stones totaling >27 kg) requires
revision of that classification. New information on the find site in Algeria
indicates that these stones were excavated from a subsurface deposit, and we
have found terrestrial rhyolite pebbles and sandy matrix attached to several
NWA 2828 stones (see images at http://www.ess.washington.edu/meteoritics).
Thus this is a rare example of a paleo-meteorite or 'fossil' meteorite. Some
stones contain sparse (<5 vol.%) but very distinct round, radial pyroxene
chondrules (up to 3 mm across), as well as rounded, fine-grained aggregates
(up to 6 mm across) rich in either enstatite or sodic plagioclase. Remnant
Na-Al-Si-rich glass is present within cavities in chondrules, both between
enstatite blades and in annular zones. The matrix contains pervasive 0.2-0.5
mm cavities with coatings of calcite and minor halite and gypsum. Iron
sulfate (after troilite), jarosite, an inhomogeneous (possibly amorphous)
phase rich in Fe, Cr, Si, Ca, Ti, P, S and Cl, minor native sulfur and
silica also are present, and brown Fe-rich rinds on one stone contain up to
6.5 wt.% Ni. These secondary minerals signify terrestrial alteration of
primary metal, sulfides, phosphides, nitrides and glass in an ancient
fluvial and/or acidic lacustrine environment. The dominant primary phase in
NWA 2828 is enstatite (En98.4Wo1.4), which forms stubby prismatic grains
(lacking polysynthetic twinning indicative of inverted clinoenstatite [cf.,
1]). Our original classification was based on a very small specimen of an
apparently igneous-textured rock, but the discovery of chondrules and the
absence of twinned enstatite now suggests that it is instead an
unequilibrated enstatite chondrite. Additional primary phases noted
previously [1] are sodic plagioclase (An14- 15Or3-4), troilite, graphite,
daubreelite, alabandite, oldhamite, schreibersite, glass and very rare
kamacite. The well-formed, round chondrules containing glass coupled with
the unrecrystallized matrix lead us to re-classify NWA 2828 as an EL3
chondrite. We also must revise our opinion [1] about the relationship
between NWA 2828 and material classified as EL chondrites NWA 2965 and NWA
2736, which evidently come from the more extensively weathered top of the
same ancient conglomerate layer as NWA 2828. [1] Irving A.J. et al. (2006)
69th Met. Soc. Mtg., #5264 (MAPS 41 Suppl., A84)

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Received on Sun 25 Mar 2007 04:06:17 AM PDT

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