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

From: Jeff Kuyken <info_at_meteoritecentral.com>
Date: Sun, 25 Mar 2007 16:39:29 +1000
Message-ID: <004001c76ea8$58b8b6d0$0201010a_at_mandin4f89ypwu>

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)
Received on Sun 25 Mar 2007 02:39:29 AM PDT

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