[meteorite-list] Brachinites - primitive or not?

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:29 2004
Message-ID: <5.1.0.14.2.20020423080357.04577e30_at_gsvaresm02.er.usgs.gov>

Brachinites are indeed primitive achondrites. This class of meteorites is
defined as those "whose bulk compositions are approximately chondritic, but
whose textures are igneous or metamorphic. [They] are generally thought to
be ultrametamorphosed chondrites or residues of very low degrees of partial
melting on small parent bodies. Winonaites, silicates from IAB and IIICD
irons, acapulcoites, lodranites, and recently even ureilites ... have been
considered to be primitive achondrites." (Mittlefehldt et al., 1998, Rev.
Mineral. 36, Ch. 4). The controversy over brachinites is whether they are
residues of partial melting, or cumulates derived from primitive
melts. Either way, because they are ultramafic rocks that are relatively
close to chondritic in composition, and they have equigranular textures,
they are "primitive achondrites" by definition. The most recent work,
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1008.pdf, by Mittlefehldt and
Berkley, suggests that some brachinits are cumulates.

jeff


At 04:38 PM 4/22/2002, Bernd Pauli HD wrote:
>herbert.raab_at_ris.at wrote:
>
> > I noted that McSween has included the Brachinites
> > with the primitive Achondrites in his second edition
> > of "Meteorites and their parent planets", whilst
> > Norton's Encyclopedia exludes the Brachinites from
> > the chapter on primitve achondrites, but rather
> > lists them with asteroidal (differentiate) achondrites.
> >
> > So, should we consider the Brachinites as
> > primitive meteorites, or not...? Any ideas?
>
>
>Hello Herbert and List,
>
>O.R. Norton does not really "e x c l u d e" them because he
>says (and also explains the different terms) on page 165:
>
>"Three a d d i t i o n a l small achondrite groups are currently
>recognized and should be briefly mentioned to complete our inventory
>of the achondrites. They (= acapulcoites, lodranites, and winonaites)
>are called primitive achondrites, and as a group are distinguished
>from the differentiated achondrites by having been only partially
>melted and therefore not completely diffeentiated."
>
>Reference:
>
>NORTON O.R. (2002) The Cambridge Encyclopedia of Meteorites.
>
>
>Best wishes,
>
>Bernd
>
>______________________________________________
>Meteorite-list mailing list
>Meteorite-list_at_meteoritecentral.com
>http://www.pairlist.net/mailman/listinfo/meteorite-list

Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Tue 23 Apr 2002 08:19:07 AM PDT