[meteorite-list] impact melts
From: Jeff Kuyken <info_at_meteoritecentral.com>
Date: Tue Apr 5 22:15:20 2005 Message-ID: <007201c53a4e$7a27dca0$2b558b90_at_mandin4f89ypwu> Very interesting Frank. Does this mean that we technically should define chondrites and achondrites something like the below? Rather than just the old "Chondrites - contain Chondrules" & "Achondrites - do not contain chondrules"? Chondrites - (kon-drites) meteorites which are primitive aggregates of early Solar System materials and have a composition similar to that found in the Sun. They also usually contain chondrules. Achondrites - (a-kon-drites) meteorites whose composition has been significantly altered from the early/primative chondritic material. Chondrules are not present. Thanks, Jeff Kuyken I.M.C.A. #3085 www.meteorites.com.au ----- Original Message ----- From: Frank Prochaska To: 'stan .' ; meteorite-list_at_meteoritecentral.com Sent: Monday, April 04, 2005 3:18 AM Subject: RE: [meteorite-list] impact melts I haven't seen a reply to your post yet, so I'll take a shot. The terms chondrite and achondrite are not used to denote texture any more. They were originally coined for that purpose, but that was long before the very accurate chemical analysis techniques we have today, and the greater understanding of the formation of meteorites. A chondrite is a meteorite that has a 'chondritic' composition, that is a composition similar to that found in the Sun. (This is assumed to be the composition of protoplanetary disk, except for some changes due to a few billion years of nuclear fusion.) So a CI chondrite is still a chondrite even though there are no chondrules to be seen. An achondrite is a meteorite whose composition has been altered significantly from the chondritic precursor material by, I guess I have to call it "geological" processes. In other words, they are differentiated rocks. Just completely melting a meteoroid doesn't make it an achondrite. Something like a cumulate eucrite is essentially a basalt or lava rock that can be found on Earth. It formed from a precursor rock that was completely melted and grains of particular minerals precipitated and settled out of the melt, probably settling to the bottom of the magma chamber. Meanwhile other material in the melt, with a different composition since the remaining minerals would crystallize at a different temperature (different time), would perhaps float on top and become a different rock above or perhaps be drawn off or 'move' somewhere else while still fluid to crystallize somewhere else etc. A cumulate eucrite is a highly differentiated rock. You can envision scenarios where a melt would differentiate slightly, but not nearly so dramatically. These would be a more primitive differentiated rock or achondrite. An impact melt is just that -- a rock that melted from an impact. A chondrite that melted from an impact is still a chondrite, because the bulk composition has not changed, just the texture of the rock since it melted and solidified again. There are certainly impact melts that appear to be entirely melted precursor material. Happy Canyon was originally considered an E6, then an E7, and now an E - impact melt (but a chondrite nonetheless). A breccia is a rock that is made up of broken pieces of rock that re-lithified some way or another. A breccia could be made up entirely of pieces of the same rock (pieces of the impact melt), or be a mixture of pieces of different rocks (pieces of the melt, unmelted precursor material, etc.). Sorry for the long answer, but some background into "why" seemed more useful than a "yes" or "no." Frank Prochaska -----Original Message----- From: meteorite-list-bounces_at_meteoritecentral.com [mailto:meteorite-list-bounces_at_meteoritecentral.com] On Behalf Of stan . Sent: Saturday, April 02, 2005 6:42 PM To: meteorite-list_at_meteoritecentral.com Subject: [meteorite-list] impact melts are all impact melts breccias of melt and original matrix? if the matrix is totally melted is is still considered an impact melt, or does the stone become an achonderite (a non primative achonderite, young achonderite, etc) thanks for any clarification ______________________________________________ Meteorite-list mailing list Meteorite-list_at_meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ Meteorite-list mailing list Meteorite-list_at_meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list Received on Tue 05 Apr 2005 10:15:13 PM PDT |
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