[meteorite-list] xenolith vs. polymict breccia?
From: Aubrey Whymark <tinbider_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:08 2004 Message-ID: <20040124144112.13149.qmail_at_web25201.mail.ukl.yahoo.com> --0-1001753303-1074955272=:12714 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit 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 <branchw_at_bellsouth.net> 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 ------------------------------------------ www.branchmeteorites.com --------------------------------- Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now --0-1001753303-1074955272=:12714 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: 8bit <DIV>Hi </DIV> <DIV> </DIV> <DIV>Sorry for the length of this reply. Got carried away! Go to bottom 3 paragraphs for summary.</DIV> <DIV> </DIV> <DIV>You are right. A polymict breccia descibes a sedimentary rock made up of angluar clasts of <U>different</U> rock whilst a monomict breccia refers to a sedimentary rock made up of angular clasts of the <U>same</U> 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.</DIV> <DIV> </DIV> <DIV>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.</DIV> <DIV> </DIV> <DIV>This is my understanding of rock names:</DIV> <DIV> </DIV> <DIV>Angular clast of same composition, no matrix = monomict clast </DIV> <DIV>supported breccia</DIV> <DIV> </DIV> <DIV>Angular clasts of different composition, no matrix = polymict clast supported breccia</DIV> <DIV> </DIV> <DIV>Angular clasts of same composition with matrix also of same composition = monomict clast or matrix supported breccia</DIV> <DIV> </DIV> <DIV>Angular clasts of same composition in a matrix of different composition = monomict clast or matrix supported breccia <U>or</U> xenolithic chondrite.</DIV> <DIV> </DIV> <DIV>Angular clasts of different compositions in a matrix = polymict clast or matrix supported breccia <U>or</U> 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)</DIV> <DIV> </DIV> <DIV>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.</DIV> <DIV> </DIV> <DIV>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.</DIV> <DIV> </DIV> <DIV>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.</DIV> <DIV> </DIV> <DIV>Regards,</DIV> <DIV> </DIV> <DIV>Aubrey</DIV> <DIV> </DIV> <DIV> </DIV> <DIV><BR><B><I>Walter Branch <branchw_at_bellsouth.net></I></B> wrote:</DIV> <BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid"> <META content="MSHTML 6.00.2800.1276" name=GENERATOR> <STYLE></STYLE> <DIV><FONT face=Arial size=2>Hello Everyone,</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>What is the difference between a xenolith and a polymict breccia?</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Is it that the xenolithic host is igneous, whereas the polymict breccia is sedimentary?</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>-Walter</FONT></DIV> <DIV><FONT face=Arial size=2>------------------------------------------<BR><A href="http://www.branchmeteorites.com/">www.branchmeteorites.com</A></FONT></DIV></BLOCKQUOTE><p><hr size=1><font face="Arial" size="2"> <a href="http://uk.rd.yahoo.com/mail/tagline_messenger/*http://uk.messenger.yahoo.com"><b> Yahoo! Messenger</b></a> - Communicate instantly..."Ping" your friends today! <a href="http://uk.rd.yahoo.com/mail/tagline_messenger/*http://uk.messenger.yahoo.com/download/index.html"><strong>Download Messenger Now</strong></a></font> --0-1001753303-1074955272=:12714-- Received on Sat 24 Jan 2004 09:41:12 AM PST |
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