[meteorite-list] Ordovician Meteorites...was New or maybe oldQUESTION???

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 5 May 2008 02:14:23 -0500
Message-ID: <136801c8ae7f$a546f670$db45e146_at_ATARIENGINE>

Hi, E-Man, List,

    I posted this in snippet form the night subject came up:

> There are fossil meteorites from limestone in Sweden
> that date to the Ordovician -- 440-480 million years ago.
> http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html

    The real hot news in the PSRD paper is the estimate infall
rate for the Ordovician L chondrites that fossilized is estimated
to have been 120 +/-50 TIMES (not percent) greater than the
present infall rate.

    Got a steel umbrella? The fossil layers cover several million
years. The timing fits the biggest breakup in the last billion years,
the disruption of the parent body of the Flora family (with 800
asteroidal members). It was the parent body of the L chondrites
which even now, a half billion years later, comprise 50% of all
falls on Earth.

    85% of those L chondrites show shock features that all date
from the same time: 465 +/-15 million years ago. The limestone
quarry's oldest fossil meteorites have short CRE dates, meaning
they fell shortly after some breakup and didn't spend much time
wandering around in space. The younger layers are older. The
high fall rate persisted for 2 million years, so it wasn't one shower.

    That breakup may not have been the initial breakup. There
is evidence of a bigger breakup at around 570-540 million years ago
(higher cratering rates throughout the inner solar system).
http://www.lbl.gov/Science-Articles/Archive/lunar-spherule.html
    "The data published by this team show that the impact
cratering rate had dropped steadily until the unexpected rise
when the impact rate returned to the same levels as 3.5 billion
years ago. The sudden increase coincides with the "Cambrian
explosion," a period in which life on Earth took off with a
dramatic burst in the number and diversity of species."

    Lots of nice graphs of cratering rates in this:
http://muller.lbl.gov/papers/Lunar_impacts_Nemesis.pdf
(You can forget the Nemesis parts if you want; they're
a footnote to the cratering rates, his explanation for the
high cratering rate.)

    It was a busy time. A one-hundred-fold increase in
meteorites, five ice ages in 100 million years, one of them
the worst in Earth history ("Snowball Earth"), massive
breakups of major bodies, the complete re-surfacing of
Venus (surface age 480 million years). And all tektites
have an original Rb/Sr melt date of 440-480 million years
ago. All just a coincidence, of course...

    And BTW, Glover's Bluff and Elm Rock craters are in
Wisconsin, not Illinois. (I know you're just quoting the guy.)
Not to worry. The Glover's Bluff crater trace is a quarry
and it'll be gone soon... The Kentland crater is a quarry too,
and so is Crooked Creek. Maybe what we need to do to
find Ordovician crater sites is map quarries?


Sterling K. Webb
-------------------------------------------------------------------------
----- Original Message -----
From: "Mr EMan" <mstreman53 at yahoo.com>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Sunday, May 04, 2008 10:34 PM
Subject: [meteorite-list] Ordovician Meteorites...was New or maybe
oldQUESTION???


I believe the paleo meteorites we are thinking about were found in a
limestone quarry of Ordovician age in Kinnekulle, Sweden. If memory serves,
only after the tiles containing the meteorites had been polished and
installed in a building were they identified for what they were. Scientist
went back to the quarry and found more in a narrow layer. There is also a
citation for a 10cm meteorite found in Brunflo. "The first fossil meteorite
found in ancient sediments was Brunflo, a heavily altered 10 cm chondrite
found in Ordovician limestone (Thorslund et al., 1981)" Perhaps someone can
reconcile this.

The Ordovician period was 488.3?1.7 to 443.7?1.5 mya. As explained below, a
disruption of the L parent body around 500mya lead to a bombardment on Earth
during Ordovician's early to mid epochs(470mya?). The bombardment is
thought to be the cause of the great diversification of life forms during
that time. It was also an ice age on earth and impact influence on the
climate hasn't been ruled out.

The Cincinnati Arch and the Nashville Dome are Ordovician geological units,
composed of very fossiliferous limestones as they were shallow seas during
the time of deposition. The uniform shallow depth could be advantageous for
preservation.

When collecting fossils from Indiana, Ohio, Kentucky and, Tennessee, one
should keep an eye out for meteorite shaped rocks amongst all the
brachiopods, bryozoans and trilobites. Fallback breccia from a blasted reef
has been found from the Oneota Formation, Glover's Bluff, Wisconsin.

Who knows-- trilobites might have evolved eyes on stalks just to keep a
better lookout given all the rocks falling out of the sky.
Elton

Here are some blurbs from Uncle Google:

Discovery of a second Ordovician meteorite using chromite as a tracer.

The small number of known fossil meteorites owe their discovery to the
preservation of a characteristic composition or structure. These features
are easily changed beyond recognition by diagenesis and other processes at
low temperature after a meteorite becomes incorporated into sediments,
because the meteoritic minerals are generally susceptible to even weak
alteration. The mineral chromite, however, is an exception. Here we report
the finding of a strongly altered fossil stony meteorite identified by the
composition of its relic chromite, and suggest that chromite chemistry can
be used as a basis for a systematic search for fossil meteorites.
 AND:
Abundant fossil meteorites in marine, condensed Lower Ordovician limestones
from Kinnekulle, Sweden, indicate that accretion rates of meteorites were
one to two orders of magnitude higher during an interval of the Early
Ordovician than at present. Osmium isotope and iridium analyses of
whole-rock limestone indicate a coeval enhancement of one order of magnitude
in the influx rate of cosmic dust. Enhanced accretion of cosmic matter may
be related to the disruption of the L chondrite parent body around 500
million years ago.

AND: From
<http://www.fossilmuseum.net/Fossil_Galleries/Stromatolites/DS612/Stromatolites61.htm>
This is unique stromatolite called impact fallback breccia. Coming from the
Lower Ordovician, it is also very young, from a time when stromatolitic
reefs no longer dominated the planet's marginal marine environments. Note
the heterogeneous patterning with sharp and angular fragments of various
size embedded in the reddish matrix. This beautiful pattern was formed
during the so-called Glover's Bluff meteorite strike during the Ordovician;
a fragment from the same meteor also struck in near Elm Rock Illinois,
forming a mile-wide crater. The meteor that struck what is now the Oneota
formation plowed through the stromatolite reef deep into the earth, spewing
molten rock upward, with everything eventually falling back to earth, with
one result being the "fallback impact breccia" seen here. It seems certain
that the living stromatolite would have been decimated.


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Received on Mon 05 May 2008 03:14:23 AM PDT