[meteorite-list] NWA 2828 EL3 Fossil Meteorite and Pairings

From: Greg Hupe <gmhupe_at_meteoritecentral.com>
Date: Thu, 4 Jan 2007 09:38:20 -0500
Message-ID: <01e301c7300d$fb248090$cc135c18_at_Gregor>

Dear List Members,

Since some members do not remember the very lengthy thread regarding NWA
2828, the first in this series, and recent pairings, here is the most up to
date description and an abstract I have for NWA 2828, an EL3 Fossil
meteorite originally classified as an aubrite, but not publicly sold as such
because of ongoing scientific work being performed at the time. I held off
for almost a year before offering this material publicly so the most
accurate classification could be conducted. I am very glad I did as I would
not want the financial headache (or moral headache) attached to such
irresponsible acts being performed by self-classifiers and other
"too-quick-to-make-a-buck" "dealers":

>>
NWA 2828 Very Rare EL3 Fossil Meteorite



NWA 2828, an unusual EL3 Chondrite "Fossil Meteorite" occurring as cobbles
in a terrestrial conglomerate. We first called this as a "Paleo-Meteorite"
but a consensus in the scientific community prefers the term "Fossil
Meteorite". This enstatite-rich meteorite was first discovered in Algeria
and/or Western Sahara in 2005. It was originally thought to be an aubrite,
but scientific examination of new material has revealed the presence of
features that require revision of that classification. Several stones
contain sparse (<1 vol.%) but very distinct round, radial pyroxene
chondrules up to 3 mm across (image 2), as well as rounded, fine-grained
aggregates up to 6 mm across composed mainly of either enstatite or sodic
plagioclase (image 3). The dominant primary phase in NWA 2828 is essentially
pure enstatite, which forms stubby prismatic grains.



New information on the find site in Algeria indicates that the NWA 2828
material was excavated from a subsurface deposit, and terrestrial rhyolite
pebbles and sandy matrix are attached to several NWA 2828 stones (image 1).
The pebbles cling to them via iron oxide/carbonate cement. The oxides are
finely layered and alternate between magnetite and hematite, with inter
layers of saline deposits. This suggests times of past wet to dry periods in
a river or salt lake. However, the iron oxides contain nickel in a range of
up to 6.5 percent, which strongly suggests that the pervasive 0.5-2 mm
cavities in the NWA 2828 stones are the result of solution etching. The
scientists who conducted the studies have concluded that incoming solutions
replaced the primary nickel-iron grains with salts, carbonates, etc., while
outgoing solutions formed nickel-rich iron oxides on the exterior of the
stones, and cemented all of the neighboring terrestrial debris to the
meteorite stones.



All this evidence for acidic alteration and precipitation in cavities
combined with the terrestrial rhyolite pebbles cemented by sand seems to
indicate that the NWA 2828 stones are partially altered EL3 chondrite
fossil-meteorite cobbles in an ancient fluvial conglomerate. In such a
river environment with intermittent high water volumes it is quite possible
over time for original metal grains to be completely dissolved out of
cobble-sized EL3 chondrite fragments. EL3 chondrites are extremely rare
meteorites - other than this new specimen, only three Antarctic and one, (or
possibly two), African examples are known, making EL3 chondrites rarer than
aubrites!



Photograph of a 24.9g NWA 2828 slice with rhyolite pebble (image 1):

http://www.lunarrock.com/nwa2828/nwa2828pebble.jpg



Photograph of magnified radial pyroxene chondrule (image 2):

http://www.lunarrock.com/nwa2828/nwa2828chondrule.jpg



Photograph of magnified whitish enstatite-rich clast (image 3):

http://www.lunarrock.com/nwa2828/nwa2828clast.jpg



Photograph of a 14.3g complete slice of NWA 2828 (image 4):

http://www.lunarrock.com/nwa2828/nwa2828slice.jpg



Oxygen isotope results were obtained at the Carnegie Institution in
Washington D.C. Further studies are underway on NWA 2828 including bulk rock
chemical analysis at the University of Cologne, and attempted measurement of
terrestrial age using carbon isotopes at the University of Arizona. The
Total Known Weight (TKW) for NWA 2828 is officially recorded as 34,186 grams
consisting of many stones. With pairings, the combined weight is in access
of 100kg.



Copyright Hup? Collection 2006

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>>

NWA 2828 ABSTRACT:

Eos, Trans. Amer. Geophys. Union, 87, Fall Meet. Suppl., Abstract P51E-1247
(2006)



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



Scott M. Kuehner1, Anthony J. Irving1, Ted E. Bunch2, James H. Wittke2 and
Gregory M. Hup? 1Earth & Space Sciences, University of Washington, Seattle,
WA 98195, USA, irving at ess.washington.edu, 2Dept. of Geology, Northern
Arizona University, Flagstaff, AZ 86011, USA.





            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 Western Sahara 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 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
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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I hope this helps everyone with this most unusual meteorite. It was a fun
and frustrating one to classify. When in doubt, buy only from reputable
dealers!!



Best regards,
Greg

====================
Greg Hupe
The Hupe Collection
NaturesVault (eBay)
gmhupe at tampabay.rr.com
www.LunarRock.com
IMCA 3163
====================
Received on Thu 04 Jan 2007 09:38:20 AM PST