Bernd and List,
Aside from the origin of ureilites, it appears that traces of chondrules are found in polymict ureilites and are probably from impactors - like the chondrules sometimes found in howardites. See Mr. Weir's page on DaG 319. I might be lucky enough to have a thin section of DaG 319 that contains a chondrule. Tell me what you think.
http://www.johnkashuba.com/Ach_DaG_319_Ureilite_-_Polymict.html
Regards,
John Kashuba Ontario, California
----- Original Message ----- From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, September 06, 2004 11:31 AM
Subject: [meteorite-list] Ureilite Origins
They had to have been carbonaceous meteorites of some sort to begin with, but the articles I've seen don't seem to offer a clear picture of what they were like before they were shocked. CM, perhaps?
Hello Marc, Fr�d�ric, and List,
Here is what I've harvested during the last few minutes:
Cyrena Anne Goodrich, Lunar and Planetary Laboratory, University of Arizona
Invited Review - Ureilites: A critical review (Meteoritics 27-4, 1992, pp. 327-352):
1) Nilpena contains clasts of carbonaceous chondrite matrix material. Detailed petrographic and mineralogic studies have shown that this material has close affinities to CI - and differs substantially from CM-matrix (Brearley and Prinz, 1989; 1992).
Fr�d�ric, "close affinities to CI" would also explain why we do not find any chondrules or relict chondrules in ureilites - there have never been any.
But, ... now look at this - it is from the same review by C.A. Goodrich:
2) CI-matrix clasts in Nilpena have an oxygen-isotope composition plotting on the extension of the Allende mixing line on the 17^O-rich side of the terrestrial fractionation line, rather than within the field of CI matrix compositions (Brearley and Prinz, 1992).
So the starting material may have been CI- o r CV-like. If it was CV-like, we
might really expect to find traces of chondrules or at least chondrule precursor
material.
Best wishes,
Bernd
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