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Hi Aubry,
Thanks very much for your comments.
Yes, I was considering rocks with a matrix. I must admit, I had not
thought of rocks without a matrix - clast supported vs. matrix supported.
That is a very interesting distinction. Now I have yet another term
with which I can try to impress my wife (not an easy thing to do after 17 years
of marriage:-)
Would the term xenolith apply to the individual clasts in a polymict
breccia? Could an igneous rock have a
xenolithic clast of non-igneous
material or could a sedimentary (e.g., rubble pile) type host have an igneous
clast, thus making both polymict breccias?
I suppose given enough collisions among asteroids,
any combination is possible (e.g., chondritic monomict breccia's with exoctic
igneous clasts).
>Perhaps in descriptions of meteorites the terms polymict
>xenolithic breccia and monomict xenolithic breccia
>should be used as oppose to just xenolithic breccia
>which to me does not distinguish whether the clasts are
>the same or not. It merely says that at least some of the
>clasts are different to the matrix.
Yes, as you have described it, this makes
sense.
Aubry (and everyone)
I have received an email from a recently former
list member who reports that there is a paper which describes formal guidelines
on the description xenolithic inclusions in meteorites:
Binns R.A. (1968) Cognate xenoliths in chondritic meteorites: Examples in
Mez�-Madaras and Ghubara (GCA 32, 299-317).
Hope this helps.
Thanks again for your comments.
-Walter
----- Original Message -----
Sent: Saturday, January 24, 2004 9:41
AM
Subject: Re: [meteorite-list] xenolith
vs. polymict breccia?
Hi
Sorry for the length of this reply. Got carried away! Go to
bottom 3 paragraphs for summary.
You are right. A polymict breccia descibes a sedimentary rock made
up of angluar clasts of different rock whilst a monomict breccia refers
to a sedimentary rock made up of angular clasts of the same rock. A
breccia can be clast supported (the clasts are touching) or matrix
supported (clasts are not touching). If clast supported, a matrix may or
may not be present.
The term xenolith refers to a foreign rock or clast incorporated into
another rock. Traditionally I would tend to only use this term when
refering to igneous rocks but it could be used for exotic clasts in
sedimentary rocks (and chondrites). Ghubara is an example, being
classified as a xenolithic chondrite. If using the term xenolith you are
probably implying a matrix is present. The matrix is local, in the case
of chondrites, and the xenolith not local - it came from elsewhere. If
you were to apply the term xenolith to a matrix free polymict breccia the
xenolith would simply have to be the less abundant type of clast. This
is not satisfactory as just because it is less abundant does not neccessarily
make it the 'foreign' bit.
This is my understanding of rock names:
Angular clast of same composition, no matrix = monomict clast
supported breccia
Angular clasts of different composition, no matrix = polymict clast
supported breccia
Angular clasts of same composition with matrix also of same composition =
monomict clast or matrix supported breccia
Angular clasts of same composition in a matrix of different composition =
monomict clast or matrix supported breccia or xenolithic
chondrite.
Angular clasts of different compositions in a matrix = polymict clast or
matrix supported breccia or xenolithic chondrite ( but if the matrix is
the same as some of the clasts the term xenolith would only apply to the
different composition clasts)
So, to summarise, in my opinion a monomict or polymict breccia can be the
same thing as a xenolithic chondrite. Xenolithic simply implies some or
all the clasts are different to the matrix of the rock. The terms
polymict and monomict do not consider the matrix.
Perhaps in descriptions of meteorites the terms polymict xenolithic
breccia and monomict xenolithic breccia should be used as oppose to just
xenolithic breccia which to me does not distinguish whether the clasts are the
same or not. It merely says that at least some of the clasts are
different to the matrix.
Does anyone know of some formal guidelines on the description of
meteorites? There must be some somewhere which give the formal
definition of a xenolithic chondrite.
Regards,
Aubrey
Walter Branch <[EMAIL PROTECTED]>
wrote:
Hello Everyone,
What is the difference between a xenolith and a
polymict breccia?
Is it that the xenolithic host is igneous,
whereas the polymict breccia is sedimentary?
-Walter
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