[meteorite-list] H7 - L7 - LL7 statistics

From: Jeffrey N. Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:09 2004
Message-ID: <4.3.2.7.2.20010213081749.0267e008_at_127.0.0.1>

This is a repost of my comment to the list dated 4 Feb 1999:

>...there are few consensus answers to be found among researchers
>[about the term "type 7"]. "Type 7" has
>been used in a variety of different ways. However, Alan Rubin and I
>agree on what we think is the best way to use this term:
>Types 3-6 form a metamorphic sequence of increasing intensity. We think
>that type 7 should only be used (if EVER) for meteorites that have
>experienced even more solid-state metamorphism than type 6. An impact
>melt-rock should be called just that, not type 7. You can have impact
>melt sheets and pockets in low metamorphic-grade chondrites that have
>experienced shock, so it doesn't make sense to confuse the two.
>There is lots of room for confusion about what is an impact melt rock
>and what is a type 6-7 meteorite...
>
>Primitive achondrites are all differentiated rocks. They do not have
>chondritic compositions for all elements, although they are still
>fairly close.

The data summarized by Bernd and listed in the Catalogue simply repeat what
can be found in the literature. Because the term "type 7" has no accepted
meaning, these statistics are also meaningless. Some of those meteorites
clearly have impact melt origins, and others are poorly described.

SO... given the lack of any precise meaning for the term "type 7,"
consider all meteorites listed as type 7 as most likely to be either type
6 or impact melt rocks until somebody writes a paper to sort them all out.

jeff

At 05:42 PM 2/12/2001, Bernd Pauli HD wrote:
>Hello Listees and Listoids,
>
>In the summary section of M.M. Grady's Fifth Edition of the
>Meteorite Catalogue on p. 7, you find the following entries:
>
> Falls Finds Total (up to December 1999)
>H7: 0 5 5 (3 are listed on p. 19)
>L7: 0 15 15
>LL7: 1 8 9 (7 are listed on p. 27)
>E7: 0 1 1
>
>My personal database (METCAT - up to Met.Bull. 85, July 2001):
>
>H7: 0 4 4
>L7: 0 18 18
>LL7: 1 10 11
>E7: 0 2 2
>
>
>As for the transition from chondrites to achondrites:
>
>LEVI F.A. et al. (1994) L7 Chondrites in FRO 90 Antarctic
>Meteorite Collection (Meteoritics 29-4, 1994, A490):
>
>A major problem in stony meteorite research appears to still be
>unsolved. A clear identification of a petrologic transition from
>chondritic to achondritic material is not yet obtained.
>
>And:
>
>TOSHIYUKI BABA et al. (1992) Mineralogy of Frontier Mountains
>L7 chondrites with reference to the chondrite-achondrite transition
>(Meteoritics 27-3, 1992, A199):
>
>Achondrites have been generally assumed to be derived from magma
>formed from some chondritic melts or mixtures, and the genetic
>model often proposes specific crystallization sequences to account
>for particular achondrite properties. However, some achondrites such
>as primitive achondrites (Acapulco-type chondrites and/or Lodranites)
>and ureilites still preserve primitive signatures of chondritic source
>materials (Palme et al. 1981).
>
>
>Best wishes
>and Good Night,
>
>Bernd
>
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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 13 Feb 2001 08:49:05 AM PST