[meteorite-list] Tagish Lake, CH, and Bencubbin-like meteorites

From: Jeffrey N. Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:01 2004
Message-ID: <4.3.2.7.2.20010119075901.0182fe30_at_127.0.0.1>

There is no formal procedure for meteorite classification schemes to
be accepted. Only passage into common use among researchers constitutes
acceptance. The "5-member" rule you may have heard about, stating that
5 meteorites with identical heritage are needed to define a group,
was proposed by John Wasson, and has evolved into a tradition. But it
is also not a formal rule of any kind, as there are no formal rules
nor any group with authority to make rules formal.

As for Tagish Lake, there has only been one formal publication on it so
far, the original Brown et al. article in Science, and the authors said:

"We tentatively conclude that Tagish Lake is a new type of carbonaceous
chondrite. We note, however, that there are no examples of CI2 chondrites,
and we do not rule out the possibility that Tagish Lake's unusual chemical
and isotopic characteristics are due to its being a less altered CI
chondrite."

In the Bulletin, I called it "C2, ungrouped." As this is fully
consistent with Brown et al., I'd recommend using this until some
other classification develops in the literature.

There has also been discussion in this list about "bencubbinites" and CH
chondrites. What you have with these meteorites is basically science
in flux. It's clear that meteorites called CH and bencubbin-like (or
B or CB chondrite) are related to the CR chondrite clan. However nobody
has written the definitive paper delineating the properties of these
groups and differences with other groups. Both CH and Bencubbin-like
meteorites are heterogeneous in their physical and chemical properties.
The fuzziness in these groups is illustrated by abstract titles like
Sasha Krot wrote last year: "Chondrules of the very first generation
in Bencubbin/CH-like meteorites..." In fact, some people (myself
partially included) are even skeptical that these are all chondrites,
as opposed to what Wasson and Kallemeyn (1990) called "subchondritic."

The lists posted by others to this group saying which meteorites were CH
and which were Bencubbin-like are accurate. However, take all these
classifications with a grain of salt until researchers start to converge
on a common nomenclature. For now, my recommendation is to call them
"CH chondrites" and "Bencubbin-like meteorites." But that's just my
opinion at this moment in time.

jeff

p.s. The "type" CH chondrite was Allan Hills 85085. The name was introduced
by Bischoff et al 1993 (GCA paper). I don't know why they chose "CH"
meaning high metal over "CA" for Allan Hills or Acfer, as would be the
traditional way of naming C chondrite groups, but it caught on
and has never been countered.

Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Fri 19 Jan 2001 08:41:09 AM PST