[meteorite-list] Re: H vs. L survival

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:48:12 2004
Message-ID: <20011017214730.7339.qmail_at_web10402.mail.yahoo.com>

Hello Rob, Rodrigo, Don, and List:

I think it only fair, since Rob used data from my
study area at Lucerne Valley, that I use his data from
another study area to make a stronger case for
metallic iron content having MINIMAL bearing on the
survivability of a chondritic stone from weathering.

Rob's data is still unpublished, but the main point is
that, within one "area of high concentration", he has
found a number of VERY weathered chondrites that are
nearly devoid of "metallic Ni-iron", yet the stones
are still quite competent - with a solid interior and
still retaining a relatively well preserved exterior.
Granted, the hematite from the oxidized metal may
actually be holding the stone together, but the point
is that chemical weathering alone will not have a
bearing on a chondritic stones survivability. During
the very complex process of meteorite weathering, it
is the degree of energy in an environment (mechanical
weathering) that will be the determining factor in a
meteorites survivability. In this case, the amount of
"metallic Ni-iron" has little bearing on the matter.

To be specific, here is Rob's data:

H6, S1, W4, Fa 19.4 +/- 0.1
H6, S2, W6, Fa 19.0 +/- 0.3
L6, S4, W3, Fa 25.1 +/- 0.3
L6, S3, W5, Fa 25.0 +/- 0.5
L6, S3, W6, Fa 25.4 +/- 0.2
L4, S2, W2, Fa 24.3 +/- 0.2
H6, S3, W2, Fa 18.5 +/- 0.7
Within one "area of high concentration" where Rob has
found 9 chondritic stones representing ~7 meteorite
falls (3ea. H6 & 4ea. L4-L6), there are an equal
number of H and L of W4-W6.

In this particular case, the metal grains in the H6's
appeared to have been much finer-grained than those in
the L's. When those grains were oxided, the resultant
finer pores facilitated "hematite cementing" of the
matrix, making for a more weather resistant stone.
Granted, the smaller metal grains oxided faster than
the larger metal grains in the L-chondrites, but this
was more than compensated by the greater degree of
"hematite cementation".

As I said earlier, terrestrial weathering is a very
complex process. But my hunch is, if you went to all
the bother to rank all the contributing factors to
this process, that the POROSITY of a meteorites
interior will have a greater role in the rate of
weathering, probably by a factor of 10, more so than
the proportion of its metal grains.

I could go on and on about weathering, but the bottom
line is that what I'm observing in the field and in
the interiors of these meteorites is that there is no
over-riding factor for preferential weathering based
upon H vs. L chemistry. If, for a given area, all
that has been found are L-chondrites, then all this
means is that you haven't found the H-chondrites yet.
;-)

Bob V.


------------------- Original Message
------------------
[meteorite-list] H vs. L survival

From: Matson, Robert ROBERT.D.MATSON_at_saic.com
Date: Tue, 16 Oct 2001 19:06:19 -0700

Hi Rodrigo,

> [Do]The Meteorites found in the areas of high
density
> that you cite have low proportion of Metal?

Well, let's see:

Lucerne: L6, LL4, H6, H4, L6, H6, L5, LL6, L6 and one
ordinary chondrite (type not determined). 3 H's, 4
L's
and 2 LL's. A pretty good spread.

Roach: H5, LL6, L5, L6

Other undisclosed locations also seem to have a good
mix
of L's and H's. Time may indeed favor the L's
survival
over the H's in some locations, but so far the finds
in
the desert southwest don't show an obvious trend of
L's
over H's.

Cheers,
Rob





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Received on Wed 17 Oct 2001 05:47:30 PM PDT