Re: [meteorite-list] xenolith vs. polymict breccia?
Walter Branch [EMAIL PROTECTED] wrote: Hi Yes the term xenolith would apply to the individual clast. It is a term used to describe something foreign or exotic that really shoulnd't be there. If the clast was the same as the matrix I would hesitate to use it but if it were different then it seems like a good term. Igneous rocks can certainly have xenolithic clasts of other igneous rocks, sedimentary rocks and metemorphic rocks. As an intrusion forces its way through country rock it will often pick up such clasts. If individual crystals are picked up which do not belong to that melt, but to another, these are termed xenocrysts. I would struggle to envisage chondrites being caught up in an igneous melt on an asteroid though. This would surely imply only partial melting of the body. If the two were found together it would surely be in a breccia and the chondritic component wouldn't be encased in the igneous component. A rubble pile could certainly have igneous clasts such as eucritic fragments in an LL body for instance. Aubrey 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 axenolithic clast of non-igneous material or could a sedimentary (e.g., rubble pile) type host have an igneous clast, thus making bothpolymict 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 Mezö-Madaras and Ghubara (GCA 32, 299-317). Hope this helps. Thanks again for your comments. -Walter --www.branchmeteorites.com - Original Message - From: Aubrey Whymark To: [EMAIL PROTECTED] 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 brecciacan 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 brecciaor 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
[meteorite-list] xenolith vs. polymict breccia?
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
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 brecciacan 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 brecciaor 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 --www.branchmeteorites.com Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now
Re: [meteorite-list] xenolith vs. polymict breccia?
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 axenolithic clast of non-igneous material or could a sedimentary (e.g., rubble pile) type host have an igneous clast, thus making bothpolymict 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 Mezö-Madaras and Ghubara (GCA 32, 299-317). Hope this helps. Thanks again for your comments. -Walter --www.branchmeteorites.com - Original Message - From: Aubrey Whymark To: [EMAIL PROTECTED] 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 brecciacan 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 brecciaor 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 --www.branchmeteorites.com Yahoo
