[meteorite-list] Ureilite Origins

[bernd.pauli at paulinet.de]

> 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, Frédéric, 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).

Frédéric, "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