AW: [meteorite-list] Tafassasset classification
Hi Frank, John, and List, While discussing Tafassasset, Dr. Otto explained to me in much detail the fundamental difference between chondrites and achondrites - something that is quite obvious in viewing various thin sections. First, achondrites - including primitive achondrites - exhibit much larger crystals than any chondrite due to igneous melting and recrystalization processes. The matrix of a type 6 or type 7 chondrite is very fine grained compaired to primitive achondrites. True chondritic matter has never been melted by igneous processes, and it looks vastly different from achondritic matter through the microscope. Tafassasset is a very good example - it shows a coarse-grained crytalline matrix that exhibitis all features of a total igneous recrystalization of it's chondritic precursor - one of these features are the triple-junctions of the olivine, usually found in brachinites. Dr. Otto showed me various thin sections of chondrites, including CR's, and there is nothing even remotely resembling this coarse grained, recrystalized texture. However, it is very similar to Divnoe, Zag (b), and the brachinites. To put it short: the crystal size and texture of a primitive achondrite are very different from type 7 chondrites - even I could see this without much guideance. However, the elemental compositions and REE patterns of primitive achondritic matter remain more or less chondritic - and hence these meteorites have been defined as primitive achondrites. But since they have been completely recrystalized it's impossible to say that they have been an CR, CO, or CV etc. before recrystallization. You can't take some similarities, such as the O-isotopic composition and the approximate amount of metal, to justify such a wild guess. With the same logic you could guess that the aubrites might be lunar or terrestrial meteorites. Now, what to make of the relict chondrules in Tafassasset - and what about relict chondrules in general? Dr. Otto explained to me that relict chondrules have escaped the process of recrystalization and remained more or less intact in a few primitive achondrites, such as NWA 725 (Tissemoumine), and Monument Draw, both acapulcoites. The structures in Tafassasset, however, look much more like something Dr. Otto calls Sammelkristalle - I don't find the proper translation, sorry - something like agglomerates of crystals that usually form during melting and recrystalization processes; Dr. Otto even showed me similar agglomerates in thin sections of terrestrial igneous rocks. Well, does that mean that these structures can't be interpreted as relict chondrules? Yes and no. Dr. Otto said that he has seen these structures, too, but there's one fundamental fact that makes him doubt that they could be relict chondrules. Most of these structures are primarily composed of plagioclase poikilitically enclosing minor olivines and pyroxenes, often accompanied by nickel-iron metal. The predominance of plagioclase in these agglomerates is strange since chondrules usually aren't composed of plagioclase. There would be just one possibility - glas-rich or glas chondrules that have been transformed into plagioclase during the melt process. But, according to Dr. Otto, this is rather improbable and nothing more than a wild guess. It's much more probable that these structures are indeed Sammel- kristalle, and no relict chondrules. Anyway, even if it should be relict chondrules in the end, Tafassasset should be regarded per definition as a quite typical primitive achondrite because of its completely recrystalized, coarse-grained texture. Now, I hope this helps to explain Dr. Otto's point of view, and the difference between type 7 chondrites and primitive achondrites. To me, all of this sounded more than convincing. I just hope that I've been able to present his view coherently in my poor English. And sorry for this lengthy email... Best regards, Norbert -Original Message- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]Im Auftrag von fcressy Gesendet: Donnerstag, 3. Oktober 2002 06:08 An: [EMAIL PROTECTED]; John Divelbiss Betreff: Re: [meteorite-list] Tafassasset classification Hi John and all, I believe that, in general, primative achondrites are no longer chondrites in that the rock has rxperienced melting and chondrules are no longer present. The rock has recrystallized and it has a crystalline texture. However, compositionally they remain the same as the parent chondrite material. So texturally they are achondrites, compositionally they are chondrites. If I remember correctly from my acapulcoite samples, a lot of metal is present between the crystals. In a regular differented achondrite, the metal has separated out as has other material and the resultant composition of the rock is significanrtly different from the parent body. As for the difference between a petrographic grade 7 and a primative achondrite, I agree that this line is fuzzy. Grade 7 chondrites should still
Re: AW: [meteorite-list] Tafassasset classification
Hello all Another paired is probably Grein 004 Regards Matteo --- Norbert Classen [EMAIL PROTECTED] wrote: Hello Bernd, Svend, Anne, and List, I discussed this subject several times with my old friend Dr. Otto, the German researcher who did the original analysis on Tafassasset (Freiburg), a stone that was originally submitted to the NomCom. as Tenere 001, a new ungrouped primitive achondrite similar to Divnoe, and the brachinites. He was more than suprised that the paired Tafassasset (Paris) has been submitted as an equilibrated CR chondrite. He gave me several reasons for his original classification of Taf- Frei as a primitive achondrite, but I can't remember them all right now (you know, I don't have a degree in mineralogy, petrology, and/ or cosmochemistry). But I remember his central argument: primitive achondrites show a more or less completely recrystallized matrix that looks entirely different from even highly equilibrated chondrites. He showed me several thin sections of various PACs, and equilibrated chondrites, and in fact, Tafassasset closely resembles other primitive achondrites, such as Divnoe, Reid 027, and Zag (b). No equilibrated chondrite has such a coarse grained matrix, which is a typical sign for recrystallization (if I got Dr. Otto right). There were other points in Dr. Otto's argumentation that convinced me of his point of view, and his view is obviously backed up by the research that Dr. Zipfel et al performed on Tafassasset, more recently. Obviously, there is some resistance to accept the fact that this nice meteorite is no extraordinary CR - something that has to do with the self importance of certain scientists that made extraordinary claims, but failed to provide extraordinary proofs. Just my two Tafs, Norbert -Original Message- Svend wrote: J. Zipfel from the MPI in Mainz says that refractory lithophiles fractionated compared to a typical CR composition is leading to a possible classification as primitive achondrite. Does anybody know about a final classification yet? Hello Anne, Svend and List, In the Abstract issue of MAPS (vol. 37-7, July 2002, p. A155), Jutta Zipfel et al. published an abstract re: Tafassasset. In the discussion part, the authors state: The figure illustrates that both samples have compositions clearly fractionated from CI and other chondrite groups. Tafassasset has AI/Mg and Mn/Mg ratios similar to other primitve achondrites, indicating incipient partial melting involving mobilization of SiO2 (+/- A1203, CaO, etc.) -, P205- and S-rich melts. Although the bulk compositions are heterogeneous, characteristic element signatures (e.g., low Zn, Mn/Mg,depletion in refractory lithophile elements) e x c l u d e a relationship to CR chondrites and support pairing of Taf-Pa and Taf-Frei*. * Taf-Pa is the Paris material - Taf-Frei the Freiburg material Cheers, Bernd __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list = M come Meteorite - Matteo Chinellato Via Triestina 126/A - 30030 - TESSERA, VENEZIA, ITALY Email: [EMAIL PROTECTED] Sale Site: http://www.mcomemeteorite.com Collection Site: http://www.mcomemeteorite.info International Meteorite Collectors Association #2140 MSN Messanger: [EMAIL PROTECTED] EBAY.COM:http:[EMAIL PROTECTED]/ __ Do you Yahoo!? New DSL Internet Access from SBC Yahoo! http://sbc.yahoo.com __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
AW: AW: [meteorite-list] Tafassasset classification
Hi Matteo, hi List, Grein 004 as well as Tenere 001 are just synonyms for the Tafassasset Freiburg mass, a single stone weighing 3610 grams. It's no pairing. Just to avoid further confusion... Best, Norbert -Original Message- Hello all Another paired is probably Grein 004 Regards Matteo --- Norbert Classen [EMAIL PROTECTED] wrote: Hello Bernd, Svend, Anne, and List, I discussed this subject several times with my old friend Dr. Otto, the German researcher who did the original analysis on Tafassasset (Freiburg), a stone that was originally submitted to the NomCom. as Tenere 001, a new ungrouped primitive achondrite similar to Divnoe, and the brachinites. He was more than suprised that the paired Tafassasset (Paris) has been submitted as an equilibrated CR chondrite. He gave me several reasons for his original classification of Taf- Frei as a primitive achondrite, but I can't remember them all right now (you know, I don't have a degree in mineralogy, petrology, and/ or cosmochemistry). But I remember his central argument: primitive achondrites show a more or less completely recrystallized matrix that looks entirely different from even highly equilibrated chondrites. He showed me several thin sections of various PACs, and equilibrated chondrites, and in fact, Tafassasset closely resembles other primitive achondrites, such as Divnoe, Reid 027, and Zag (b). No equilibrated chondrite has such a coarse grained matrix, which is a typical sign for recrystallization (if I got Dr. Otto right). There were other points in Dr. Otto's argumentation that convinced me of his point of view, and his view is obviously backed up by the research that Dr. Zipfel et al performed on Tafassasset, more recently. Obviously, there is some resistance to accept the fact that this nice meteorite is no extraordinary CR - something that has to do with the self importance of certain scientists that made extraordinary claims, but failed to provide extraordinary proofs. Just my two Tafs, Norbert -Original Message- Svend wrote: J. Zipfel from the MPI in Mainz says that refractory lithophiles fractionated compared to a typical CR composition is leading to a possible classification as primitive achondrite. Does anybody know about a final classification yet? Hello Anne, Svend and List, In the Abstract issue of MAPS (vol. 37-7, July 2002, p. A155), Jutta Zipfel et al. published an abstract re: Tafassasset. In the discussion part, the authors state: The figure illustrates that both samples have compositions clearly fractionated from CI and other chondrite groups. Tafassasset has AI/Mg and Mn/Mg ratios similar to other primitve achondrites, indicating incipient partial melting involving mobilization of SiO2 (+/- A1203, CaO, etc.) -, P205- and S-rich melts. Although the bulk compositions are heterogeneous, characteristic element signatures (e.g., low Zn, Mn/Mg,depletion in refractory lithophile elements) e x c l u d e a relationship to CR chondrites and support pairing of Taf-Pa and Taf-Frei*. * Taf-Pa is the Paris material - Taf-Frei the Freiburg material Cheers, Bernd __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list = M come Meteorite - Matteo Chinellato Via Triestina 126/A - 30030 - TESSERA, VENEZIA, ITALY Email: [EMAIL PROTECTED] Sale Site: http://www.mcomemeteorite.com Collection Site: http://www.mcomemeteorite.info International Meteorite Collectors Association #2140 MSN Messanger: [EMAIL PROTECTED] EBAY.COM:http:[EMAIL PROTECTED]/ __ Do you Yahoo!? New DSL Internet Access from SBC Yahoo! http://sbc.yahoo.com __ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
