[meteorite-list] Mike's NWA 2092 LL6/7

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:55 2004
Message-ID: <4066F8C2.25BA39F_at_earthlink.net>

Hello list,

Ted Bunch has given his okay to post the following information on Mike's
NWA 2092, for all those interested, as I am. Also of interest, a nice
thin section image of an L7 showing those features characteristic of
type 7 chondrites can be seen on my NWA 1839 page of
meteoritestudies.com (courtesy of NAU). When I get a piece of the LL6/7,
I also have a nice TS photo of that one to show for comparison.

Regards,
David
__________________________________________________

Preface: the overall texture of 2092 is complicated by a shock
metamorphism overlay;
brecciation and thermal annealing along fractures, clast outlines and
matrix, i. e., instead
of an equigranular texture (grains of similar size), which we find in
true and uncomplicated
sevens, the grain size is highly variable from a shock recrystallization
event.

Dodd criteria:

1.the presence of poorly defined chondrules in type 6, but only relict
chondrules in type 7 – There are relict chondrule fragments of mostly
radial chondrules in
2092. I respect the Dodd criteria, although they were formulated ~ 24
years ago and nothing
else has come along, even though many more sevens have been recorded. If
we are to have a seven
category, then we feel that there should be no chondrule remnants in a
seven, otherwise why
have the additional compartment. Regardless of whether they are very
fuzzy or only partly
fuzzy.

2.low-Ca pyroxenes in type 6 contain no more than 1.0 wt% CaO (1.0 wt% =
~1.9 mol% Wo), but more than 1.0 wt% in type 7; conversely, the CaO
content of high-Ca pyroxenes decreases from type 6 to type 7. 2092 CaO
contents average 1.0 wt
% in orthopyroxene – the fence line on this issue.

3.feldspar grains gradually coarsen to reach a size of at least 0.1 mm
in type 7. The 0.1 mm size is a good point, but is not all of the story
if the size range of
all minerals are considered. We feel that the similar grain size of all
silicates is the
critical issue. In addition, in true sevens, some plagioclase shows
simple twinning. Not so in
lesser grades.

In addition to Dodd’s criteria, we have observed that the amount of
metal (modal/vol. %)
decreases significantly in sevens. NWA 3100 has <1 vol % metal, which is
taenite. Apparently,
during the last stages of metamorphism, low metal is consumed to make
olivine. In fact, our
modal analyses of sevens show that considerable amounts of olivine have
been made at the
expense of metal and pyroxenes, which leaves only small amounts of
Ni-rich metal and lower
amounts of orthopyroxene and clinopyroxene relative to those in lower
metamorphic grades. There
are other observations, but none of those given here (original to us)
have been tested in a
court of science review.

Later, perhaps.

Bottom line: We could go either way on the classification (6 or 7), but
the shock metamorphic
overlay and the Dodd margins suggested to us that the best
classification was and is a
transitional 6/7. We will stand by this until our future study or
studies by other
investigators suggest otherwise.


Best regards,

Ted
Received on Sun 28 Mar 2004 11:09:38 AM PST