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

[David Weir]

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 × 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