Based on the mineralogy, petrography, bulk chemical, oxygen, and nintrogen isotopic compositions and 40Ar-39Ar age, Isheyevo is genetically related to CH and CB carbonaceous chondrites and provides a link between these group of pristine meteorites

Brandstätter, F.

Franchi, I.A.

Ivanova, M.A.

Kononkova, N.N.

Korochantseva, E.V.

Krot, A.N.

Trieloff, M.

Verchovsky, A.B.

English

Open Research Online (Open)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIsheyevo Meteorite: Genetic link between CH and CBchondrites?Conference or Workshop ItemHow to cite:Ivanova, M.A.; Kononkova, N.N.; Franchi, I.A.; Verchovsky, A.B.; Korochantseva, E.V.; Trieloff, M.; Krot,A.N. and Brandsta¨tter, F. (2006). Isheyevo Meteorite: Genetic link between CH and CB chondrites? In: 37th Lunarand 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/1100.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.ukISHEYEVO METEORITE: GENETIC LINK BETWEEN CH AND CB CHONDRITES?  M.A. Ivanova1, N.N. Kononkova1, I.A. Franchi2, A.B. Verchovsky2, E. V. Korochantseva1,3, M. Trieloff3, A.N. Krot4, and F. Brandstaetter5. 1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russia (venus2@online.ru), 2Open University, UK, 3Mineralogisches Institut, Ruprecht-Karls-Universitat Heidelberg, Germany, 4Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, USA, 5Natural History Museum, Austria.  Introduction: The metal-rich CH and CB chondrites consist largely of the mineralogically pristine compo-nents (chondrules, Fe,Ni-metal, and rare CAIs) which escaped thermal metamorphism and aqueous alteration. The origin of these meteorites is highly controversial; both nebular and asteroidal models have been proposed to explain their unique mineralogical, isotopic and chemical properties [e.g., 1-4]. The recently discovered metal-rich meteorite Isheyevo [5] contains several lithologies, which have mineralogical characteristics intermediate between CH and CBb chondrites. Here we report the petrography, mineralogy, bulk chemical, oxy-gen and nitrogen isotopic compositions and 40Ar-39Ar ages of Isheyevo and discuss its possible origin. Results: Lithologies. Isheyevo consists of Fe,Ni-metal, chondrules, heavily-hydrated matrix lumps, and rare CAIs.  No fine-grained matrix-like material is ob-served between these components. The meteorite con-tains several lithologies with different contents of Fe,Ni-metal (7-70 vol%) and chondrules (90-30 vol%); the latter have different sizes and dominant textural types between the lithologies. The metal-rich lithology con-tains smaller chondrules (0.1-0.4 mm) than the metal-poor lithologies (0.2-1 mm); the former is dominated by non-porphyritic (cryptocrystalline and skeletal olivine) chondrules; the latter contain abundant olivine, olivine-pyroxene, and pyroxene chondrules of porphyritic tex-tures. Metal grains in all lithologies have similar size range (0.1-0.3 mm). The boundaries between the litholo-gies are generally gradual (Fig. 1), sometimes sharp. Olivine grains in porphyritic chondrules show mosaic extinction, planar fractures and planar deformation features, indicative of S4 shock stage [6].  Fe,Ni-metal. About 20-30% of metal grains in Ish-eyevo are chemically-zoned: Ni and Co content decrease and Cr content increases towards the edge of the grains. The compositionally uniform metal grains contain 4.2-8.4 wt% Ni, 0.2-0.5 wt% Co, and 0.03-0.6 wt% Cr. Some compositionally uniform metal grains are Ni (26 wt%) and Co-rich (0.6 wt%) and contain rounded inclu-sions of Cr-rich (2.5-13 wt%) troilite. Ni and Co in metal grains are positively correlated and Cr and Ni negatively correlated; the Co/Ni ratio is solar. Chondrules. Based on the bulk chemical composi-tions, chondrules can be divided into ferromagnesian and Al-rich (>10 wt% Al2O3). The Al-rich chondrules are divided into Al-diopside-rich and plagioclase-rich; the former consist of Al-diopside, forsterite and ±spinel; the latter consist of forsterite, low-Ca and high-Ca py-roxenes, Fe,Ni-metal, anorthitic plagioclase, and ±spinel. Ferromagnesian chondrules can be divided into MgO-rich (Type I) and FeO-rich (Type II); both types have porphyritic (PO, POP, PP) and non-porphyritic (BO, SO, CC) textures. Type I chondrules are much more abundant than Type IIs. Olivine and low-Ca py-roxene in porphyritic Type I and Type II chondrules range in composition from Fa2.5 to Fa10-38 and from Fs2.1Wo1.7 to Fs8-12Wo0.8-1.8, respectively. Some POP chondrules contain numerous micron-size blebs of Fe,Ni-metal. Ferrous cryptocrystalline chondrules often show inverse compositional zoning, suggesting reduc-tion. One ferrous CC chondrule contains euhedral Fe,Ni-metal grains, like in other CHs [7]. Silica-rich chon-drules commonly observed in CH chondrites [8] are very rare in Isheyevo. Several CC chondrules consist of magnesian pyroxene cores and fayalitic olivine (Fa72) rims, suggesting formation under different redox condi-tions [8,9]. One CC pyroxene-normative (Fs7) chondrule is surrounded by a phyllosilicate rim.  Matrix lumps are rare and composed of phyllosili-cates, Fe,Ni-sulfides, Ca- and Mg-rich carbonates, and magnetite. Ca,Al-rich inclusions are found in all lithologies of Isheyevo; most CAIs occur in the metal-rich lithology. Based on the major mineralogy, the CAIs can be divided grossite-rich, hibonite-pyroxene-rich, melilite-rich, and spinel-rich. CAIs are typically surrounded by a single- or double-layered rim composed of ±melilite and Al-diopside; occasionally, there is the innermost layer of spinel±hibonite. Most CAIs appear to be igneous; they are texturally and mineralogically similar to those in other CHs [10-12]. Amoeboid olivine aggregates are virtually absent. One AOA-like object consists  of forsterite, Al-diopside, and anorthite. Bulk chemical compositions. The refractory litho-phile abundances determined by INAA show enrich-ments of about 1.4×CI, between those in Hammadah al Hamra (1.3×CI) [13], and QUE94411 (1.5×CI) [14]. Volatiles are highly depleted with Na abundance 0.18×CI. Isheyevo is highly enriched in refractory and normal siderophile elements, but less than in CBs; the abundances decrease with increasing volatility. Bulk oxygen isotopic composition of Isheyevo (δ17O, 1.17, 1.07‰; δ18Ο 4.21, 4,29‰) is in the range of other CH chondrites, lies along the CR-CH-CB trend [15]. It is distinct from that of other CBs but does fall on a mix-ing line defined by components from the matrix in Ben-cubbin [16]. Bulk nitrogen isotopic composition. A large release of nitrogen was observed around 1000°C, with peak δ15N of +1523‰. Total content of N is 106 ppm with  δ15N of +1124‰. Above 600°C, the meteorite contains 73 ppm of N with δ15N of +1360‰. This is the highest Lunar and Planetary Science XXXVII (2006) 1100.pdfbulk δ15N value of any whole rock sample ever reported for a meteorite [17]. 40Ar/39Ar and cosmic-ray exposure ages. Concentra-tion of total trapped 36Ar is 19.6*10-8cm3STP/g, less than that in other CHs [18]. The presence of the trapped Ar and possible recoil effects complicate the 40Ar-39Ar -spectrum, so we derive a rough estimate of <3.5Ga for the last thermal event that caused partial argon loss. The probable cosmic-ray exposure (CRE) age of Isheyevo is ~36 Ma and similar to that of the CB carbonaceous chondrites [19]. Discussion: Based on the mineralogy, petrography, bulk chemical, oxygen, and nitrogen isotopic composi-tions, and 40Ar-39Ar ages, Isheyevo is genetically related to CH and CB carbonaceous chondrites and potentially provides a link between these groups of meteorites. The presence of the metal-rich and metal-poor lithologies, mineralogically similar to the CBb and CH chondrites, respectively, supports this conclusion. The very refrac-tory nature of the Isheyevo CAIs, nearly complete ab-sence AOAs and rare occurrences of anorthite replacing melilite may indicate a short residence time of CAIs in the high-temperature region(s) of their formation. Most CAIs appear to have been melted; some might have been melted during chondrule formation. Zoned Fe,Ni-metal grains most likely formed by gas-solid condensation [3]. The magnesian CC and SO chondrules could be geneti-cally related to zoned Fe,Ni-metal condensates [19]. The magnesian porphyritic chondrules most likely formed in the same nebular region as zoned metal grains and mag-nesian non-porphyritic chondrules by incomplete melt-ing of solid precursor materials; since they lack sulfides, they may have been physically or kinetically isolated from low-temperature reactions with S-bearing vapor. Silica-rich chondrules or their precursors could have formed by condensation from a chemically fractionated gaseous reservoir [8]. The mineralogically-zoned chon-drules require multi-stage formation under variable re-dox conditions, possibly resulting from evaporation of dust. The lack of fine-grained matrix material in Ish-eyevo suggests that it was either absent or destroyed in the region where chondrules and metal grains formed or it did not accrete into the Isheyevo parent body. Heav-ily-hydrated matrix lumps show no genetic relationships to  the coarse silicate objects of Isheyevo, which escaped aqueous alteration, indicating the lumps were either added during regolith gardening or accreted together with other components of Isheyevo. The Isheyevo parent body experienced an impact event at <3.5 Ga which led to 40Ar loss. Acknowledgements: This work was supported by grant RFBR-BSTS (project N14/04 and 03-05-20008), Austrian Academy of Sciences (FWF, Austria), and PPARC, UK. References: [1] Scott E.R.D. (1988) EPSL 91, 1-18. [2] Wasson J.T. and Kallemeyn G.W. (1990) EPSL 101, 148-161. [3] Meibom A. (2000) Science 288, 839-841. [4] Krot A.N. et al. (2005) Nature 436, 989-992. [5] Ivanova M.A. et al. (2005) MAPS 40, A74. [6] Stöffler D. et al. (1991) GCA 55, 3845-3867. [7] Krot A.N. et al. (2000) MAPS 35, 1249-1259. [8] Hezel D.C. et al. (2003) MAPS 38, 1199-1215. [9] Ivanova M.A. et al. (2003) MAPS 38, A28. [10] Krot A.N. et al. (2005) Chem. Erde, in press. [11] Kimura M. et al. (1993) GCA 57, 2329-2359. [12] Weber D. et al. (1995) GCA 59, 803-823. [13] Zipfel J. et al. (1998) LPS 29, #1417. [14] Weisberg M.K. et al. (2001) MAPS 36, 401-418. [15] Krot A.N. et al. (2002) MAPS 37, 1451-1490. [16] Franchi I. A. et al (1998) Mete-oritics & Planet. Sci, 33, A53. [17] Krot A.N. et al. (2003) in Treatise on Geochemistry, 83-129. [18] Weber H. W. et al. (2001) MAPS 36, A220. [19] Kelly S. and Turner G. (1987) Meteoritics 22, 427.  [20] Krot A.N. et al. (2001) Science 291, 1776-1779.                                                   Fig. 1. Elemental maps in Ni Kα and Mg Kα X-rays of Isheyevo showing a gradual contact between metal-rich (top) and metal-poor litholo-gies. The metal-rich lithology is texturally and mineralogi-cally similar to the CBb chon-drites Hammadah al Hammra 237 and QUE94411. It is dominated by zoned Fe,Ni-metal grains and non-porphyritic (cryptocrystalline and skeletal olivine) chon-drules. The metal-poor lithol-ogy is mineralogically similar to CH chondrites; it contains much less metal grains and higher abundance of chon-drules compared to the metal-rich lithology; most metal grains are chemically-zoned; large proportion of chondrules has porphyritic textures, al-though non-porphyritic chon-drules are also common. Lunar and Planetary Science XXXVII (2006) 1100.pdf

Isheyevo Meteorite: Genetic link between CH and CB chondrites?

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Isheyevo Meteorite: Genetic link between CH and CB chondrites?

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