[meteorite-list] new isotopic results suggest new classifications for some meteorites

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Mon Jul 10 20:41:18 2006
Message-ID: <44B2F3AA.4090908_at_earthlink.net>

Hello Adam,

I would like to thank you and your brother for your generous
contribution to my website/collection of your "first true brachinite
from the Sahara", NWA 3151. You have implied that the previously
considered first Saharan brachinite, NWA 595, was now determined not to
be a member of this rare group. I have included a paragraph from my NWA
595 webpage below, followed by new information from a 2006 MetSoc
(non-peer-reviewed of course) abstract:

-------------------------------------------------------
The brachinite group consists of chemically and mineralogically diverse
members. According to preliminary information from research in progress
by Drs. Anthony Irving, Scott Kuehner, and Douglas Rumble, III, several
factors indicate that NWA 595 may not be a member of the brachinite
group. Despite its similarities to the brachinites in chemical
composition and Xe-isotopic ratios, the O-isotopic ratios plot outside
of the brachinite field towards the TFL. In addition, FeO/MnO ratios of
both olivine and clinopyroxene are lower than for typical brachinites
and plot outside of the brachinite field. Moreover, NWA 595 contains
more magnesian olivine, lacks plagioclase (along with ALH 84025 and
Eagles Nest), and contains more orthopyroxene than is typically found in
brachinites (10 &#215; the next highest abundance in Hughes 026). Still,
one viewpoint is that the range of the brachinite group is too narrowly
defined, and that perhaps some of the brachinite-like primitive
achondrites such as NWA 595, Zag (b), Divnoe, and Tafassasset may be
genetically related.
------------------------------------------------------------

With the questionable classification of NWA 595 now understood, I was a
little surprised to read the new abstract by A. Irving and D. Rumble in
which they state that the D17O-isotopes of NWA 3151 plot in the same
narrow range as NWA 595, both of which have more positive D17O values
than that of the type specimen Brachina. In addition, they suggest that
similar isotopic values measured for the petrographically similar and
ungrouped, NWA 4042, may mean that it too is genetically related to NWA
595 and NWA 3151. This is good news for type collectors since only two
more members and we could have a new group; that is, IF the brachinites
are not eventually found to have a considerable isotopic heterogeneity,
as proposed in the last line of the paragraph above.

Adam, can you add any inside comments about this revelation since you
are in such close communication with the authors? Contrary to the idea
that all of these meteorites are derived from a significantly
isotopically heterogeneous object, it would seem more likely to me that
neither NWA 595, NWA 3151, nor NWA 4042 are actually brachinites, but
that they likely represent a separate but similar parent body.

Thanks for any inside revelations on this one since all I have to go on
is the new abstract:

http://www.lpi.usra.edu/meetings/metsoc2006/pdf/5288.pdf

Interestingly, in the same abstract, results of new O-isotopic studies
have determined that NWA 1054 is not an acapulcoite as it is presently
classified, but is likely paired with the NWA winonaites
(metachondrites), 725, 1052, 1058, and 1463 (most of which are also
presently classified as acapulcoites). This isotopic data provides
further confirmation of the mis-classification of all these meteorites
as first brought to my attention by Stan Turecki. Good job Stan. Isn't
the scientific method remarkable!

David
Received on Mon 10 Jul 2006 08:41:14 PM PDT