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
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
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