Albarede, F

Bland, PA

Bourot-Denise, M

Hewins, RH

Zanda, B

Meteoritics and Planetary Science

Spiral - Imperial College Digital Repository

AN ACCRETIONARY ORIGIN FOR ORDINARY CHONDRITE GROUPS.  B. Zanda1,2, R. H. Hewins2, M. Bourot-Denise1, P. A. Bland3,4, and F. Albarède5.  1MNHN-CNRS, UMS2679, 75005 - Paris. E-mail: zanda@mnhn.fr. 2Geological Sci., Rutgers Univ., Piscataway NJ08855. 3IARC, Dept. Earth Sci. & Eng., Imperial College London. 4BMNH, London. 5ENS Lyon, 69364 – Lyon.  Introduction: OC groups are based on bulk Fe and metal abundance and might derive from separate nebular reservoirs cre-ated in part by metal gain/loss. As metal formed in type I chon-drules [1], we explore the relationship between their abundances. Method: We measured type I and type II chondrule [2] abundances in 7 primitive OC falls to compare with their bulk Fe, Ni and metal derived from wet chemistry [3]. OCs are compared with CCs [2-4], an EH3 [3,5] and CH ALH85085 [6]. Results: OC data are displayed in Fig. 1. Chondrule abun-dances vary little, but their nature changes significantly: Type Is (small circles) dominate in Sharps (H), but are only a third of type IIs (big circles) in Krymka (LL). Metal abundance (triangles) correlates with type Is (vol% metal = 0.035 vol % type I; R2 = 0.9) and this correlation extends to the EH and the CH. This trend is also present but less distinct in the other CC groups. CI- and Mg-normalized Fe and Ni also correlate with type Is and metal in OCs, CH and EH, but not in the other CC groups.   Type II      Type I   Metal x 3 Discussion: The groups for which type I, metal and bulk Fe and Ni correlate are those in which metal physically separated from the chondrules where it formed [1]. Bulk Fe and Ni correlation with metal shows that metal loss/gain caused siderophile fractiona-tion and that the metal-type I correlation resulted from differential accretion: small dense metal grains have stopping times closer to those of the smaller silicate chondrules [7] and would have ac-creted preferentially with type Is in a turbulent regime. Rather than being derived from more or less reduced reservoirs, OC groups thus simply result from a differential accretion of larger objects (mostly type IIs) in LLs and smaller or denser objects (type Is and metal grains) in Hs. References: [1] Campbell A. et al. 2005. Proceedings of the Astronomical Society of the Pacific Conference Series, in press. [2] McSween H. Y., Jr 1977. Geochimica Cosmochimica. Acta 41, 477-491. [3] Köblitz, J. 2005. Metbase 7.1, meteorite retrieval software, CD-ROM. [4] McSween H. Y., Jr 1977. Geochimica Cosmochimica Acta 41, 1777-1790. [5] Weisberg M. K. And Prinz M. 1998. Abstract #1741. 29th Lunar & Planetary Science Confer-ence. [6] Scott. E. R. D. 1988. Earth and Planetary Science Letters 91: 1-18. [7] Kuebler K. E. and McSween H. Y. Jr. 1999, Icarus 141 96-106.  68th Annual Meteoritical Society Meeting (2005) 5216.pdf

An accretionary origin for ordinary chondrite groups

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