We conducted experimental heating of two CM2 chondrites, Murchison and Mighei, to study changes in their oxygen isotopic compositions and mineralogy and explore possible genetic relationships between MCCs and normal CMs

Baker, L.

Brandstätter, F.

Franchi, I.A.

Greenwood, R.C.

Ivanova, M.A.

Lorenz, C.A.

Morris, A.A.

Nazarov, M.A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsExperimental study of laboratory-heated CM2chondrites Mighei and MurchisonConference or Workshop ItemHow to cite:Ivanova, M.A.; Lorenz, C.A.; Greenwood, R.C.; Franchi, I.A.; Nazarov, M.A.; Morris, A.A.; Baker, L. andBrandsta¨tter, F. (2006). Experimental study of laboratory-heated CM2 chondrites Mighei and Murchison. In: 37thLunar and Planetary Science Conference, 13-17 Mar 2006, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1086.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukEXPERIMENTAL STUDY OF LABORATORY-HEATED CM2 CHONDRITES MIGHEI AND MURCHISON.  M. A. Ivanova1, C. A. Lorenz1, R. C. Greenwood2,  I. A. Franchi2, M. A. Nazarov1, A. A. Morris2, L. Baker2, and F. Brandstaetter3. 1Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow 119991, Russia (venus2@online.ru). 2Planetary and Space Sciences Research Institute, The Open Univer-sity, Walton Hall, Milton Keynes, MK7 6AA, UK. 3Naturhistorisches Museum, Burgring 7, A-1010, Vienna, Aus-tria.    Introduction:  Metamorphosed carbonaceous chondrites (MCCs) of the Belgica-7904 group from Antarctica [1] and two non-Antarctic MCCs from the desert of Oman, Dhofar 225 and Dhofar 735 [2] – are very different in oxygen isotopic compositions from normal CMs (Fig. 1). They are characterized by δ18Ο 21.07-22.29 ‰, δ17O 9.22-11.59 ‰, and ∆ 17O −0.06+0.38 [3].  MCCs are also different from CMs in mineralogy and chemistry [2]. Unlike normal CMs, all MCCs have low H2O content, their phyl-losilicates are dehydrated and they lack tochilinite and P-rich sulfides [2]. We conducted experimental heat-ing of two CM2 chondrites, Murchison and Mighei, to study changes in their oxygen isotopic compositions and mineralogy and explore possible genetic relation-ships between MCCs and normal CMs. Experimental: The heating experiment was con-ducted in the Laboratory of Meteoritics of the Ver-nadsky Institute in Russia. Chips of Murchison and Mighei were powdered, homogenized and split into several aliquots. The aliquots were placed into copper crucibles and heated under vacuum for one hour. The Murchison aliquots were heated to temperatures of 450o, 600o and 930oC.  The Mighei aliquots were heated to 400o, 600o and 800oC. The hottest tempera-tures (800o-930oC) correspond to the temperatures at which olivine and enstatite  form by complete dehy-dration of serpentite and saponite [4]. The samples were weighed before and after heating. Oxygen iso-topic compositions of heated and unheated samples of Murchison and Mighei were determined by laser fluo-rination [5] at the Open University (UK), with changes measured by Thermo-Gravimetric/Evolved Gas Ana-lyser (TGA-EGA). Two chips of each meteorite, Murchison and Mighei, were also heated under vacuum for two hours at temperatures of 500o and 700oC, respectively. Pol-ished sections were prepared from these chips and the sections were studied by optical microscopy, SEM, and microprobe analyses at the Vernadsky Institute and the Natural History Museum and the University of Vienna (Austria). Results and discussion: Most CM2s fall on a mix-ing line with slope ≈0.6 and display δ18O of over 8‰ [3]. The unheated samples of Mighei and Murchison plot at the low 18O end of this trend.  The unheated and 450oC samples of Murchison are not distinguished (Fig. 1, 2). The unheated Mighei sample  is poorer  in 18O than all the heated samples and is very similar to published data [3]. The unheated Murchison sample lies at the low end of the CM2 range in both ∆17O and δ18O – quite distinct from previous data [3,6], which also displayed some variation. Given the isotopic het-erogeneity within CM2s, considerable variation in whole-rock measurements is to be expected. For both Mighei and Murchison, the heated sam-ples become more 18O rich with increasing temperature (Fig. 1). δ18O values vary from 5.92 to 9.69 ‰, in the range of CM2 chondrites. ∆17O falls from ~ -2.99-3.20 to lower values with increasing temperature (Fig. 2), away from the less negative MCCs and towards the level of partially dehydrated CMs (A881334, Y793321, Y82098, Y86695) [3]. The heated Murchi-son samples display larger isotopic shifts than the Murchison samples studied in [6], because they were heated in closed tubes which could permit isotopic equilibration at temperature above 500oC (Fig. 3). Significant variations in oxygen isotopic composi-tion result from dehydration of phyllosilicates, to-chilinite and decomposition of carbonates. Mineralogi-cal study of polished sections of the Murchison and Mighei heated samples showed that carbonates survived heating to 500oC and 700oC in both meteor-ites, as well as in all MCCs [1], although they should decompose between  400-700oC [7]. The TGA analy-ses of all Murchison samples showed a peak of CO2 at 600-700oC, indicating the presence of carbonates; however the extent of their decomposition during heat-ing is poorly quantified. Tochilinite-bearing objects in the matrices of the heated samples have different com-positions than in typical CMs. Usually tochilinite oc-curs in assemblages with phyllosilicates in different abundances, displaying negatively-correlated variation in S and Si contents [8]. Tochilinite should decompose at 245oC [8], but we observed it in the 500oC heated samples of both meteorites and traces of it in the 700oC heated samples of Mighei. Probably the decom-position rates of tochilinite and carbonates were not enough to form final products during our short ex-periments.  The abundance of tochilinite and carbon-ates in Murchison is notably less than phyllosilicates. Lunar and Planetary Science XXXVII (2006) 1086.pdfLunar and Planetary Science XXXVII (2006) 1086.pdfTherefore the oxygen isotopic system in our experi-ment was more affected by dehydration of phyllosili-cates than by decomposition of carbonates and to-chilinite. We suggest that the observed effects of oxygen iso-topic fractionation could take place during thermal metamorphism of carbonaceous chondrites.  However, the shift in δ18Ο of the heated samples is not out of the CM2 field (Fig. 1). Therefore, a genetic relationship between CM2s and MCCs could not be confirmed. Probably MCCs were affected by multiple processes of hydration-dehydration due to aqueous alteration and metamorphism, enriching the silicates in 18O. Alterna-tively, MCCs could represent a separate group of chondrites whose primary material was different from typical CM2 chondrites and formed in a different oxy-gen reservoir. Acknolegements: This work was supported by grant RFBR-BSTS (projects N14/04 and 03-05-20008) and from PPARC, UK. References: [1] Bischoff A. and Metzler K. (1991) Proc. NIPR Symp. Antarct. Meteorites, 4, 226-246. [2] Ivanova M.A. et al., (2005) LPS XXXVI, Abstract # 1054. [3] Clayton R.N. and Mayeda T.K. (1999) Geo-chim. Cosmochim. Acta 63, 2089-2104 [4] Akai J. (1992) Proc. NIPR Symp. Antarct. Meteorites, 5, 120-135, [5] Miller M. F. et al (1999) Rapid Comm. Mass Spectrm. 13, 1211-1217, [6] Clayton R.N. et al. MAPS  32, A30. [7] Ikornikova A, Sheptunov D. (1973) Crys-tallization processes under hydrothermal condition. Ed. By Lobachev, New York, Consaltants Bureau, 113-123, [8] Tomeoka K, Buseck P.R  (1985) Geochim. Cosmochim Acta. 49, 2149-2163.                                                                                                    -101234 5 6 7 8 9 10δ18O (‰)δ17O (‰)Murchison heatedMurchison Mighei heatedMigheiMurchison heated (Claytonet al. 1997)Murchison (Clayton et al.1997)Fig. 3-50510150 5 10 15 20 25δ18O (‰)δ17O (‰) MCCsCMCM-metaMurchison heatedMurchison MigheiMighei heatedFig. 1-5-4-3-2-10-2 0 2 4 6 8 10 12 14δ18O (‰)∆17 O (‰)Murchison heatedMurchisonMighei heatedMigheiCM-meta, Clayton,Maeyda, 1999CM Fig. 2Lunar and Planetary Science XXXVII (2006) 1086.pdfLunar and Planetary Science XXXVII (2006) 1086.pdf

Experimental study of laboratory-heated CM2 chondrites Mighei and Murchison

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Experimental study of laboratory-heated CM2 chondrites Mighei and Murchison

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