[meteorite-list] Weathering Grades

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:29 2004
Message-ID: <20020602091751.20138.qmail_at_web10405.mail.yahoo.com>

--- Walter Branch <branchw_at_bellsouth.net> wrote:
> Hello Bob and List,
>
++++snip!+++++
> There may not be enough data to make meaningful
> conclusions but I would REALLY LIKE to see a
> breakdown by world geographical regions.
> For example, climates more conducive to the
> preservation (e.g., southwest US) of meteorite vs.
> those less favorable to meteorite preservation
(e.g.,
> southeast US).
>
> Bernd?
>
> -Walter
> -----------------------------------------------

Hello Bernd, Walter, and the few other people on this
planet that are interested in "Weathering Grades",
;-)

While were on this subject about trends, and averages,
and the mean being "skewed", I need to point out the
MAJOR "skew" that is currently developing for the set
of data for the Southwest USA Meteorites.

Check this out, Walter! The more "dry lake"
meteorites we find, the greater the trend toward a
higher average weathering grade for Southwest USA
Meteorites.

In a nutshell: Dry lakes in the Southwest USA have
seasonal wet and dry periods. There will be periods
of intermittent standing water on "dry lakes", while
other areas in the desert have long since dried-out.

What this means is that, for those meteorites which
are periodically lying in water on the lakebeds, their
interiors will be more weathered compared to those
meteorites lying up on well-drained alluvial fans
(such as at Gold Basin). The more "dry lake"
meteorites we find, the more higher will rise the
average weathering grade for Southwest USA Meteorites.


All you need to do is take a quick look at TABLE 3 of
the current Meteoritical Bulletin to see what I mean.
It is rare for a find to be W1. It would require a
find location having an environment with excellent
states of preservation. Only Borrego, Golden Mile,
and Payson are all W1, but none of them came from a
dry lake. Granted, Old Dominion Mine is a W3, but
I've seen how poor the condition of its exterior, so a
W3 grade for its interior means it is comparatively
"well-preserved".

Now, take a look at the "dry lake" finds, and see how
high the average weathering grade is. A "reverse
situation" is occurring here.

The following observation doesn't appear anywhere in
the classification data, but these dry lake meteorites
have surprisingly well-preserved exteriors compared to
their very weathered interiors. Can you imagine a
stone with a "W6" interior having an exterior
exhibiting well-preserved, relict fusion crust?
You will never see that in a NWA meteorite!

But to explain how all of this can happen will require
a separate thread [or article]. Later, - Bob V.






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Received on Sun 02 Jun 2002 05:17:51 AM PDT