This study looks at the sulphide abundance and composition of Fe-Ni sulphide grains in 12 CM chondrites to determine an alteration sequence for these chondrites

Bullock, E. S.

Gounelle, M.

Grady, M.

Russell, S. S.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsFe-Ni Sulphides as Indicators of Alteration in CMChondritesConference or Workshop ItemHow to cite:Bullock, E. S.; Grady, M.; Gounelle, M. and Russell, S. S. (2007). Fe-Ni Sulphides as Indicators of Alterationin CM Chondrites. In: 38th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXVIII), 12-16Mar 2007, 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/lpsc2007/pdf/2057.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.ukFE-NI SULPHIDES AS INDICATORS OF ALTERATION IN CM CHONDRITES Bullock E. S.1, Grady M. M.2, Gounelle, M.3, Russell S. S.4 1Department of Mineral Sciences, National Museum of Natural History, Smith-sonian Institution, Washington, DC 20560 USA (BullockE@si.edu). 2Planetary Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, Bucks MK7 6AA, UK. 3 Laboratoire d'Étude de la Matière Ex-traterrestre, Muséum National d'Histoire Naturelle, 57, rue Buffon, 75005 Paris, France. 4Department of Mineral-ogy, The Natural History Museum, Cromwell Road, London SW7 5BD UK   Introduction: There are more CM chondrites known than any other type of carbonaceous chondrite [1], and CM material often occurs as clasts within other types of meteorite [2]. The CM chondrites show varying degrees of aqueous alteration, from relatively pristine CM2 chondrites that contain abundant anhy-drous silicates, through to highly altered CM1 chon-drites in which all olivine and pyroxene grains have been altered to form phyllosilicates [3]. Various meth-ods have been proposed to define a sequence to deter-mine a sequence from least altered to most altered. Previous work has focused on phyllosilicate abun-dance and composition [4], oxygen isotopic composi-tion of carbonates, [5] and chondrule olivine and py-roxene abundance and composition [6] to determine alteration sequences, but these have not been consis-tent as to the sequence of alteration.  In addition the mineral alteration index (MAI) devised by [4] has been to shown to correspond well with the year of fall of the chondrite, suggesting that it may not be a reliable measure of pre-terrestrial alteration [7]. In this study, we look at the composition of Fe-Ni sulphides in a range of CM chondrites, to see if sulphide composition and/or abundance correlates with other alteration fea-tures, and so whether they can provide information on the alteration undergone by individual CM chondrites. Fe-Ni sulphides are found in all CM chondrites, unlike carbonates and anhydrous silicates, which are absent from CM1 chondrites. Previous work on Fe-Ni sul-phides in CI1 chondrites has demonstrated that in-creasing alteration increases the proportion of Ni pre-sent in pyrrhotite grains [8], and it appears that the same may be true for CM2 chondrites [9]. Method: Sulphide grains in thick and thin sec-tions of ten CM2 chondrites (ALH81002, Bells, Cold Bokkeveld, Essebi, Mighei, Murchison, Murray, Nogoya, Pollen, and QUE99355) ranging from rela-tively pristine to extensively aqueously altered, and two CM1 chondrites (ALH88045 & MET01070) were analysed. The samples were primarily studied using a Jeol 5900LV SEM operating at 20kV and 1nA, and a working distance of 10mm. This allowed investigation of the morphology and size range of the grains, and also element mapping of the sections. Accurate quanti-tative analyses were obtained using a Cameca SX50 Electron Microprobe operating at 20kV and 20nA. Results and Discussion: The CM1 chondrites contain abundant sulphide grains (~4.5 vol% in MET01070 and ~6.5 vol% in ALH88045). The sul-phide grains are finely intergrown mixtures of pyr-rhotite and pentlandite (fig. 1), so overlap of the phases during analyses was inevitable. The sulphides are rich in Ni, and no troilite was identified (fig. 2) The average composition of pyrrhotite and pentlandite in each chondrite is given in Table 1.  The CM2 chon-drites, as would be expected for such a diverse range of meteorites, show varying abundances of sulphides, from ~0.05% to 0.6%. Most contain pyrrhotite and pentlandite, although only pentlandite was observed in Pollen and ALH81002, and several also contain a sul-phide of an intermediate composition. Troilite was observed only in Murchison (fig. 2) The original sulphide present in the CM chon-drites was probably troilite: troilite is the sulphide pre-dicted to have formed first from a cooling nebular gas [10]. With increasing alteration, the troilite grains in the CM chondrites lose iron and nickel, forming pyr-rhotite. The closer the stoichiometry of the pyrrhotite grains to troilite, the less altered the CM chondrite. According to this, the alteration sequence would be, in order of increasing aqueous alteration: Murchison ≤ Murray < Mighei < QUE99355 < Essebi < Bells < Cold Bokkeveld< Nogoya < MET01070 ≤ ALH88045 This broadly agrees with previous proposed se-quences [4, 5, 6]; however there are some complexities in the overall picture of the composition and abun-dance of sulphide grains, so that sulphides alone can-not be used to judge the degree of aqueous alteration undergone by a particular CM chondrite. However, the Ni content of pyrrhotite grains does increase with in-creasing alteration, and so, used in conjunction with other features, can provide clues as to the degree of aqueous alteration. References: [1] Grady M. M. (2000) Catalogue of Meteorites; [2] Gounelle M. (2003) GCA 67 p507-527; [3] Zolensky M. E. et al (1997) GCA 61 p5099-5115; [4] Browning L. B. et al (1996) GCA 60 p2621-2633; [5] Benedix G. K. et al (2003) GCA 67 p1577-1588; [6] Hanowski N. P. & Brearley A, J. (2001) GCA 61 p495-518; [7] Benedix G. K. & Bland P. A. (2004) 67th MetSoc, #5184; [8] Bullock E. S. et al (2005) GCA 69 p2687-2700; [9] Zolensky M. E. & Loan L. LPSC XXXIV #1235. [10] Grossman L (1972) GCA 36 p597-619. Lunar and Planetary Science XXXVIII (2007) 2057.pdf Figure 1. Intergrown pyrrhotite (dark grey) and pent-landite (light grey) in the CM1 chondrite ALH88045.     Figure 2. Composition of Fe-Ni sulphide grains in CM1 and CM2 chondrites, shown alphabetically.   Table 1. Average pyrrhotite and pentlandite composi-tions within CM chondrites. Meteorite Sulphide content (%) Average pyr-rhotite com-position Average pent-landite compo-sition ALH88045 6.5 (Fe,Ni)0.86S Fe4.87Ni4.25S8 ALH81002 0.1 N/A Fe4.67Ni4.31S8 Bells 0.5 (Fe,Ni)0.9S N/A Cold Bok-keveld 0.05 (Fe,Ni)0.9S Fe4.57Ni4.40S8 Essebi 0.3 (Fe,Ni)0.91S Fe4.87Ni4.14S8 MET01070 4.5 (Fe,Ni)0.86S Fe4.76Ni4.20S8 Mighei 0.4 (Fe,Ni)0.93S Fe4.74Ni4.22S8 Murchison 0.6  (Fe,Ni)0.99S Fe4.92Ni3.96S8 Murray 0.3 (Fe,Ni)0.99S Fe4.89Ni4.12S8 Nogoya 0.4 (Fe,Ni)0.88S Fe4.50Ni4.50S8 Pollen 0.5 N/A Fe4.90Ni3.98S8 QUE99355 0.1 (Fe,Ni)0.93S Fe4.78Ni3.84S8   Lunar and Planetary Science XXXVIII (2007) 2057.pdf

Fe-Ni Sulphides as Indicators of Alteration in CM Chondrites

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Fe-Ni Sulphides as Indicators of Alteration in CM Chondrites

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