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Re: Total No. of Meteorites

Groupers,

The problem of establishing pairing among Antarctic meteorite finds
is a very difficult one.  Basically, when the Antarctic Meteorite
Newsletter comes out, pairings have been determined several simple ways.

(1) Two fragments actually fit together.  In some cases, I think
that these might even get a single number.  In other cases it may
be discovered later.  In any case, this is very rare.

(2) The meteorite is a rare type, the various specimens look
alike, and they are found on the same (or a very nearby) ice field.
Here, pairing can be fairly confidently assumed, and the meteorites
are [often] listed as paired.

(3) In case (2), and in other cases, like the thousands of ordinary 
chondrites, we rely on Brian Mason's [Smithsonian, Emeritus] well-trained
eye and vast storehouse of knowledge and memories.  I doubt that anybody
could do this better than Brian, but it is an absolutely impossible
task.  If he thinks that meteorites "may be paired" he says so,
and this gets listed in the AMN.  Many small ordinary
chondrites are not even analyzed by microprobe... their olivine 
compositions are determined by refractive index, and a type (L/LL/H) 
is declared.  In some cases, where a lot of identical fragments are
found in one place, and all seem to be the same type, Brian might
announce a pairing group (like EET 90053, an L6 with 678 paired 
specimens, most of which have never been looked at).

So, the pairings of US Antarctic meteorites listed in most catalogs 
derive from Brian Mason, armed with his wits, his microscope, and some 
probe data.

After these initial pairings are published, other analyses may be done
that bear on the question of pairing.  Natural and induced 
thermoluminescence may be measured, as may the activity of 
radionuclides like Al-26.  These are published in AMN as well, and
they often suggest that published pairings are either wrong, or that 
new pairings exist.  However, there is no recognized "czar" of 
meteorite pairings who makes it his or her business to sanction such 
findings, so they rarely make it into catalogs.  Of course, if a 
pairing changes is found for a rare type of meteorite, this DOES 
sometimes get into catalogs; in the case of US Antarctic meteorites, 
Brian Mason is again consulted, and the change would be published in AMN.
This only happens if the scientist who discovers it calls it to the
attention of the Curator.

You should also bear in mind that the same problems of pairing plague
meteorites from the Sahara Desert and the Nullarbor region.

You can find a recent abstract by Benoit & Sears on this subject at:
  http://cass.jsc.nasa.gov/meetings/LPSC98/pdf/1454.pdf
They have published a lot of other stuff as well.

You can find another on-line paper on this subject, by the Curator of 
the US collection, Marilyn Lindstrom, at:
  http://www-curator.jsc.nasa.gov/curator/antmet/ppr/ppr.htm

jeff