[meteorite-list] Terrestrial ages

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:24 2004
Message-ID: <20020829043558.32336.qmail_at_web10403.mail.yahoo.com>

----------- Original Message ---------------
Date: Mon, 26 Aug 2002 16:20:35 +0200
From: Bernd Pauli
To: David Freeman

Subject: [meteorite-list] Terresterial ages

David Freeman asked:

> Is there any time frame trend in the terrestrial
ages
> of meteorites found in the western deserts of the
US?


Hello Dave and List,

I think Bob Verish is your man with regard to this
question.
Bob even (co-) authored two papers about
"Extraterrestrial Materials From Cold And Hot Deserts
in which he discusses such problems:

VERISH R.S., RUBIN A.E. et al. (1999) Deflation and
Meteorite Exposure on Playa Lakes in the Southwestern
United States:
Unpaired Meteorites at Lucerne Dry Lake, California
(Workshop on Extraterrestrial Materials From Cold And
Hot Deserts, July 6-8, 1999, Kwa-Maritane,
Pilanesberg, South Africa, LPI Contr.#997).

RUBIN A.E., VERISH R.S. et al. (2000) Numerous
unpaired meteorites exposed on a deflating playa lake
at Lucerne Valley, California
(MAPS 35-5, 2000, Suppl., A181-A183).

Best wishes,
Bernd

P.S.: There should also be something in our archives
because we discussed it about a year or two ago ... if
I am not mistaken here.
------------ End of ORIGINAL Message ------------

Yes, Bernd,

You're right. There is much in the Archives about
this subject, just over the past two years, alone.

And you're right, Bernd, I haven't been contributing
that many posts, lately. But that is mostly due to
what I hope is a temporary problem, where a pinched
nerve in my neck gives me carpal tunnel syndrome-like
symptoms in my arm when I type on a keyboard. (Those
of you who have been getting my replies so late, now
you know why.) I guess I should get one of those
voice to text converters.

But back to Dave's question about weathering rates of
meteorites from the different Hot Deserts. That's a
good question.
 
In research papers that study terrestrial weathering
it is typical to compare the Saharan meteorites with
meteorites from Roosevelt County, New Mexico. The
conventional wisdom, or consensus, is that there are
more similarities than there are differences between
these two groups of Hot Desert meteorites.

In the Lucerne Dry Lake papers that Bernd referenced,
we compared the Lucerne Valley meteorites to the
Roosevelt County finds. But I've changed my thinking,
since then. I now see more differences than
similarities between Mojave Desert and Roosevelt
County meteorites, and even more distinction from
Saharan finds.

But before we entertain a discussion comparing
terrestrial age-dates, I would like to suggest a
reading of some of the more recent studies. I've
listed two URLs at the end of this message for that
purpose.

The only other thing that I can add are some
unpublished terrestrial age-dates, as determined by
Tim Jull, for specimens taken from six Mojave Desert
chondrites (W3) that I submitted to the Univ. of Az.
Without going into detail, they generally ranged from
2,000 to 9,000 years, with the six ages having an
average at 4,000 years.

PLEASE, don't make the inference that all Mojave
Desert chondrites with a weathering grade of W3 have a
terrestrial age range from 2k-4k years. All six of
these stones probably penetrated deeply into lakebed
sediment when they fell, and would have had a long
residence time in an impervious clay layer until they
were exhumed. Should a fallen stone not penetrate
into a clay layer or a sand dune, the condition of the
local environment could produce a W3 grade in less
than 2 years.
 
But what I want to leave you with is this - the oldest
terrestrial age among the six, was the stone that was
the LEAST "weathered-looking"! The stone that "looked
the freshest", was the oldest of the six stones. The
most likely explanation is that, among these six, this
stone was the most recently exhumed. Which is to be
expected, if you agree with the basic geologic tenet
that stratigraphically lower layers are older, and
that those layers take longer to be exposed. So, this
is just another example of why the degree of
weathering of a meteorite is not a good indication of
its terrestrial age.

One last pitch. We need a lot more Mojave Desert
meteorites age-dated. (I'm doing my best, to get more
of those into the hands of researchers.;-)

Bob V.

References:

Searched the web for terrestrial age meteorites found
in the U.S. deserts:
Results 1 - 2 of about 333.

[PDF] Bland, P. A
File Format: PDF/Adobe Acrobat -
... Although surprising, a possible explanation is
found when we ... Climate and rock weathering:
a study of terrestrial age dated ordinary chondritic
meteorites ...
http://psri.open.ac.uk/staff/blandpubs.pdf


[PDF] 5168.pdf
File Format: PDF/Adobe Acrobat -
... in contrast to hot deserts ... terrestrial age
meteorites are found ... An expanded 14 C terrestrial
age dataset for meteorites ... The 14 C age dataset
... data, may allow us ...
http://www.lpi.usra.edu/meetings/metsoc97/pdf/5168.pdf









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Received on Thu 29 Aug 2002 12:35:58 AM PDT