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Iron Meteorites Crystallization, Thermal History, Parent Bodies, and Origin

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Iron Meteorites Crystallization, Thermal History, Parent Bodies, and Origin ARTICLE IN PRESS Chemie der Erde 69 (2009) 293–325 www.elsevier.de/chemer INVITED REVIEW Iron meteorites: Crystallization, thermal history, parent bodies, and origin J.I. Goldsteina,Ã, E.R.D. Scottb, N.L. Chabotc aDepartment of Mechanical and Industrial Engineering, 313 Engineering Lab, University of Massachusetts, 160 Governors Drive, Amherst, MA 01003, USA bHawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA cThe Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA Received 9 August 2008; accepted 6 January 2009 Abstract We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmansta¨tten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 1C are about 100–10,000 1C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 1C/Myr for the IIIAB group and 100–6600 1C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150750 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, o1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets. r 2009 Elsevier GmbH. All rights reserved. Keywords: Widmansta¨tten pattern; Crystallization; Cooling rates; Chemical groups; Parent bodies; Iron meteorites; Fractional crystallization; Magmatic; Parent bodies ÃCorresponding author. Tel.: +1 413 545 2165; fax: +1 413 545 1027. E-mail address: [email protected] (J.I. Goldstein). 0009-2819/$ - see front matter r 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2009.01.002 ARTICLE IN PRESS 294 J.I. Goldstein et al. / Chemie der Erde 69 (2009) 293–325 1. Introduction In this paper we discuss how this new view of the formation of the iron meteorites was developed. We Recent studies have argued that the iron meteorites, review the general structure and classification of iron like stony-irons and achondrites, come from solar meteorites and outline the crystallization and formation system bodies that melted allowing mm-sized and of the iron meteorite chemical groups and their thermal smaller metal and silicate grains in chondrite material history as revealed by metallographic cooling rates. We to segregate into much larger domains (e.g., Krot et al., discuss other evidence, chiefly isotopic, that elucidates 2008; Weisberg et al., 2006). According to the textbooks, the thermal and igneous histories of irons, and finish iron meteorites are derived from over 50 bodies that with conclusions about the possible origins and forma- were 5–200 km in size, most of which melted to form tion of their parent bodies. metallic cores and silicate mantles. These bodies are thought to have accreted in the asteroid belt after the chondrites and to have been broken open by impacts long after the bodies had cooled slowly. 2. Composition, structure, and chemical groups Other studies suggest that almost all of these of the iron meteorites statements may be incorrect. Iron meteorites may have been derived originally from bodies as large as 1000 km 2.1. Composition and structure or more in size that melted. There is a growing consensus that most iron meteorites come from bodies that Iron meteorites are Fe–Ni alloys containing minor accreted early – even before the parent bodies of the amounts of Co, P, S, and C. The Ni content varies from chondrites – and that 26Al, which has a half-life of a minimum of 5.1 up to 60 wt% although the vast 0.7 Myr, is the major heat source that melted them. majority of irons have between 5 and 12 wt%. The 10 Evidence from Hf–W radiometric dating of irons largest irons, which each weigh more than 10 tons (e.g., and chondrites shows that most irons come from Buchwald, 1975), are meter sized and most were large bodies in which metallic cores formed o1 Myr after single crystals of taenite (fcc Fe–Ni) after solidification the growth of the oldest objects, Ca–Al-rich inclusions and at high temperatures in their parent bodies. The in chondrites (Kleine et al., 2005; Markowski et al., Widmansta¨tten pattern of the irons was revealed 2006a, b; Qin et al., 2008; Burkhardt et al., 2008). independently by Thomson and by von Widmansta¨tten The early accretion of igneously differentiated in 1804 and 1808, respectively (see review by Clarke and asteroids is also inferred from consideration of Goldstein, 1978) when polished sample surfaces were the heating effects of 26Al in planetesimals. Homo- etched by various chemicals (Fig. 1). This structure can geneity of Mg isotopic compositions of diverse be used to determine the cooling rate of each meteorite, meteorite parent bodies suggests that 26Al was homo- as discussed in Section 4. geneously distributed in the solar system (Thrane The Widmansta¨tten pattern develops as a two-phase et al., 2006). Therefore there would have been sufficient intergrowth of kamacite (a-bcc, ferrite) and taenite thermal energy from 26Al to melt cold planetesimals (g-fcc, austenite), and forms by nucleation and growth that accreted within 1.5 Myr of CAI formation and of kamacite from taenite during slow cooling of the were large enough (420 km radius) so that little heat parent body (Owen and Burns, 1939). The conventional was lost for several half-lives of 26Al (Hevey and explanation of Widmansta¨tten pattern formation, which Sanders, 2006). In addition, chondrule ages determined is only partly correct but will suffice for this introduc- by Al–Mg and Pb–Pb dating are 1.5–5 Myr after tion, is based on the binary Fe–Ni equilibrium phase CAI formation indicating that chondrites accreted diagram (Yang et al., 1996). A meteorite of a given after differentiated asteroids when 26Al concentrations Fe–Ni content cools from the one-phase taenite (g) were no longer adequate to melt asteroids (see Scott, region into the two-phase a+g region, where kamacite 2007). (a) nucleates and grows as the meteorite continues to The requirement that iron meteorite parent bodies cool. Kamacite nucleates on the close packed octahedral accreted before those of the chondrites is one of {1 1 1} planes of taenite, forming a Widmansta¨tten several arguments advanced by Bottke et al. (2006) for pattern (Fig. 1). In three dimensions, kamacite grows their claim that these bodies accreted not in the as two-dimensional plates into the surrounding taenite. asteroid belt but closer to the Sun at 1–2 AU As cooling continues, kamacite grows at the expense of where planetesimals accreted faster (see Section 7.7). taenite and the Ni content of both kamacite and taenite Thus, the parent bodies of the iron meteorites could increases. have been much more diverse than those of the The nature and scale of kamacite has led to a chondrites and the irons may tell us more about the structural classification of iron meteorites. Hexahedrites bodies that accreted to form the terrestrial planets than (H) are one-phase kamacite with Ni of 5–6.5 wt% Ni. the chondrites. Octahedrites (O) have visible (to the eye) Widmansta¨tten ARTICLE IN PRESS J.I. Goldstein et al. / Chemie der Erde 69 (2009) 293–325 295 Fig. 1. Polished and etched slices of iron meteorites showing Widmansta¨tten patterns. (a) Carlton – group IIICD, Of. Kamacite plates (blue) formed on the close-packed planes of the parent taenite phase. Plessite, a fine mixture of kamacite and tetrataenite, formed in the prior taenite regions between the kamacite plates. Schreibersite precipitates are observed in the centers of some of the kamacite plates. Scale – 1 cm along the bottom. (b) Canyon Diablo – group IAB, Ogg. Note cohenite precipitates in the centers of several kamacite plates. A large rounded sulfide occurs in the right-hand bottom corner. Scale – 10.5 cm along the bottom. (c) Mt. Edith – group IIIAB, Om. Narrow bands are kamacite, gray angular areas enclosed by these are plessite.
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