[meteorite-list] what is the most primitve meteorite?

[Jeff Grossman]

There are two meanings to the word "primitive" that are in common use 
among researchers:

1) It means that the meteorite was relatively unaffected by secondary 
processes that occurred on asteroids, including thermal metamorphism, 
melting, shock effects, and aqueous alteration.  These meteorites are 
the chondrites whose chondrules, CAIs, matrix, and presolar grains 
are in the most pristine condition.  In this sense, the most 
primitive ordinary chondrite is Semarkona. There are several very 
primitive carbonaceous chondrites, including Acfer 094,  Adelaide, 
ALHA 77307, and a few CR chondrites.  Most of the meteorites people 
have mentioned in this thread are not particularly primitive by this 
definition.

2) It also can mean that the chemical composition of the meteorite is 
little different from that of the bulk solar system, as represented 
by the solar photosphere, minus the H and He.  In this sense, the CI 
chondrites are the most primitive meteorites, with meteorites like 
Tagish Lake and CM chondrites in second place.

Generally, age is not used to determine how primitive a meteorite may 
be.  Indeed, recent data suggest that some "primitive" chondrites may 
actually be younger than some meteorites which formed by melting on asteroids.

Jeff

At 05:39 PM 4/15/2008, Dave Harris wrote:
>Yep - I reckon either Tagish or Orgueil.
>
>My 2mg worth....
>
>Dave
>IMCA #0092
>Sec.BIMS
>www.bimsociety.org
>
>______________________________________________
>http://www.meteoritecentral.com
>Meteorite-list mailing list
>Meteorite-list at meteoritecentral.com
>http://six.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