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

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Thu, 29 Mar 2007 07:55:15 -0700 (PDT)
Message-ID: <888436.53230.qm_at_web36912.mail.mud.yahoo.com>

Hello Sterling -

The "present ice age" is not going to return. The
currents of the Pacific Ocean were altered by a
massive impact at the end of the last ice age, and
most likely that impact was what ended it.

The important point here is how long NWA meteorites
have been accumulating, and as you point out it has
been a relatively short period.

Ed
E.P. Grondine
Man and Impact in the Americas
$34.95 at amazon, or contact me off list

--- "Sterling K. Webb" <sterling_k_webb at sb7
cglobal.net> wrote:

> 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.
>
>
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AGUFM.P51E1247K
>
> Cheers,
>
> Jeff
>
>
------------------------------------------------------
>
> Title:
> EL3 Chondrite (not Aubrite) Northwest Africa 2828:
> An Unusual
> Paleo-meteorite Occurring as Cobbles in a
> Terrestrial Conglomerate
>
> Authors:
> Kuehner, S. M.; Irving, A. J.; Bunch, T. E.; Wittke,
> J. H.
>
> Publication:
> American Geophysical Union, Fall Meeting 2006,
> abstract #P51E-1247
>
> Publication Date:
> 12/2006
>
> Abstract:
> 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 Thu 29 Mar 2007 10:55:15 AM PDT