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I Ron and Stony-Iron Meteorites 324 Achondrites impact into the hOl crust of asteroid 4 Vesta -4.5 Ga ago. acapu!coites: noble gas isotopic abundances, che mi c~! composition, cosmic-ray exposure ages, and solar cosmic Geochim. Cosmochim. Acta 65, 3577- 3599. Yamaguchi A., Clayton R. N., Mayeda T. K., Ebihara M., Qura ray effects. Geochim. Cosmochim. Acta 61 , 175-192. Y. , Miura Y. N., Haramura H. , Misawa K., Kojima H., and Wheelock M. M., Keil K. , Floss c., Taylor G. J., and Crozaz G. Nagao K. (2002) A new source of basaltic meteorites inferred (1994) REB geochemistry of oldhamite-dominatcd cl~sts from Northwest Africa OIl. Science 296, 334- 336. from the Norton County aubrite: igneous origin of oldhamtte. Yanai K. (1994) Angrite Asuka-88 1371: preliminary examin­ Ceochim. Cosmochim. Acta 58, 449-458. ation of a unique meteorite in the Japanese collection of Wolf R. Ebihara M" Richter G. R., and Anders E. (1983) Antarctic meteorites. Proc. NIPR Symp.: Amarct. Meteorit. Aub ri~es and diogenites: trace element clues to their origin. Ceochim. Cosmochim. Acta 47, 2257 - 2270. 7,30-41. Yanai K. and Kojima H. (1991) Yamato-74063: chondntlc Wlotzka F. and Jarosewich E. (977) Mineralogical and meteorite classified between E and H chandrite groups. Proc. chemical compositions of silicate inclusions in the ~1 1 12 NIPR Symp.: Antarct. Meteorit. 4, 118-130. Taco , Camp del Cieio, iron meteorite. Smithson. COfZtnh. Yanai K. and Kojima H. (1995) Catalog ofAntarctic Meteorites. Earth Sci. 19, 104- 125. National Institute of Polar Research Tokyo, Japan. Yamaguchi A., Taylor G. J., Keil K., Floss C., Crozaz G., Zipfel J., Palme H. , Kennedy A. K., and Hutcheon I. D. (l9?5) Iron and Stony-iron Meteorites Nyquist L. E. , Bogard O. D ., Garrison D., Ree~e .y., Chemical composition and origin of the Acapulco meteonte. Wiesman H. , and Shih c.-Y. (200 1) Post-cryslalhzatlon Geochim. Cosmochim. Acta 59, 3607- 3627. reheating and partial melting of eucrite EET 90020 by H, Haack University of Copenhagen, Denmark and 1. J, McCoy Smithsonian Institution, Washington, DC, USA 1.12.1 INTRODUCTION 325 1.12.2 CLASSIFICATION AND CHEMICAL COMPOSITION OF mON METEORITES 327 1.12.2.1 Group lIAB Iron Meteorites 327 /./2.2.2 Group W AE Iron Meteorites 327 1.12.2.3 Group IVA Iron Meteorites 328 1.12.2.4 Group IVB Iron Meteorite.~ 328 1.12.2.5 Silicate-bearing [ron Meteorites 329 J.12.2.6 Mesosiderites 329 1.12.2.7 Ungrouped Iron Meteorites 329 1.12.3 ACCRETION AND DIFFERENCES IN BULK CHEMISTRY BETWEEN GROUPS OF IRON METEORITES 330 1.12.4 HEATING AND DIFFERENTIATION 332 1. \2.5 FRACTIONAL CRYSTALLIZATION OF METAL CORES 333 /.12.5.1 Imperfect Mixing during Crystalliz.ation 335 1.12.5.2 Lale-stage Crystallization and Immiscible Liquid 335 1.12.5.3 The Missing Sulfur-rich Meteorites 336 1.12.6 COOLING RATES AN D SIZES OF PARENT BODIES 336 1.1 2.7 PALLASITES 338 1.I2.8 PARENT BODIES OF IRON AND STONY-IRON METEORITES 339 1.1 2.9 FUTURE RESEARCH DIRECTIONS 340 REFERENCES 341 1.12,1 INTRODUCTION solar system. Parent bodies of iron and so me stony-iron meteorites completed a geological Without iron and stony-iron meteorites, our evolution similar to that continuing on Earth­ chances of ever sampling the deep interior of a although on much smaller length- and time­ differentiated planetary object would be next to scales-with melting of the metal and silicates, nil. Although we li ve on a planet with a very differentiation into core, mantle, and crust, and substantial core, we will never be able to sample probably extensive vo lcan.i sm. Iron and stony­ it. Fortunately, asteroid collisions provide us with iron meteorites are our only available analogues a rich sampling of the deep interiors of differ­ to materials fo und in the deep interiors of Earth entiated asteroids. and other terrestrial planets. This fact has been Treatise on GeochemiSI1 Iron and stony-iron meteorites are fragments of recognized since the work of Chladni (1794), Published by Elsevier Ltd. ISBN (set): 0_08_0437 51 - a large number of asteroids that underwent who argued that stony-iron meteorites must have 324 Volume 1: (ISBN: 0_08_044336-2): pp. 291- significant geological processing in the early originated in outer space and fallen during 325 Classification and Chemical Composition of Iron Meteorites 327 326 Iron and Stony-iron Meteorites Although the original classification was entirely fireballs and that they provide our closest trace-element compositional trends in most consequences for the geochemical and thermal based on chemistry, additional data on structure, analogue to the material that comprises our own groups of iron meteorites are consistent with evolution of the surviving material. Later impacts cooling rates, and mineralogy suggest that it is a planet's core. This chapter deals with our current fractional crystallization of a metallic melt (Scott, on the meteorite parent bodies dispersed fragments genetic classification where each group represents knowledge of these meteorites. How did they 1972), thus constraining peak temperatures. The in the solar system, some of which have fallen on one parent core (Scott and Wasson, 1975). The form? What can they tell us about the early temperatures required to fonn a metallic melt are the Earth in the form of meteorites. These chemical composltions within each group form evolution of the solar system and its solid bodies? sufficiently high to cause substantial melting of fragments provide us with an opportunity to study coherent trends generally consistent with fractional How closely do they resemble the materials from the associated silicates and trigger core formation the geological evolution of a complete suite of crystallization and the members have similar or planetary interiors? What do we know and don't (Taylor et al., 1993). In addition, meter' sized rocks from differentiated asteroids in the taenite crystals and slow metallographic cooling laboratory . uniformly varying structure and mineralogy. we know? Despite significant overlap in the compositional Iron and stony-iron meteorites constitute ~6% rates in some iron meteorites suggest crystal­ clusters, multiple elements, along with structure of meteorite falls (Grady, 2000). Despite their lization and cooling at considerable depth. Due to and mineralogy, distinguish the groups. Most scarcity among falis, iron meteorites are our only the high thermal conductivity of solid metal 1.12,2 CLASSIFICATION AND CHEMICAL groups have uniform cooling rates, supporting the samples of ~75 of the ~135 asteroids from which compared with mantle and crust materials, metal COMPOSITION OF IRON METEORITES idea that each group represents an individual meteorites originate (Keil et aI., 1994; Scott, 1979; cores are believed to be virtually isothermal asteroid core. Cosmic-ray exposure ages fu.rther Meibom and Clark, 1999; see also Chapter 1.05), during cooling (Wood, 1964). Iron meteorites To use meteorites as a guide to parent-body support a genetic classification in groups IliAB suggesting that both differentiated asteroids and from the same core should therefore have evolution, we need groupings that represent and IV A, where most members have cosmic-ray the geologic processes that produced them were identical cooling rates. Metallographic cooling individual parent bodies. In this section, we exposure ages of 650 Ma and 400 Ma, rates are different from group to group but are, briefly discuss the characteristics of iron and commOD. respectively. Despite the highly evolved nature of iron and with the exception of group IVA, relatively stony-iron meteorites used to discriminate A detailed description of the individual iron uniform within each group (Mittlefehldt et al., between the different parent bodies. Historically, stony-iron meteorites, their chemistry provides meteorite groups is found in Scott and Wasson 1998). This is consistent with each group cooling iron meteorites were classified on the basis of important constraints on the processes operating (1975). We include a brief synopsis of some of the in a separate core surrounded by a thermally macroscopic structure, being divided into hexa­ in the solar nebula. Although most of them important characteristics of the largest groups, insulating mantle. hedrites, octahedrites (of varying types based on probably formed through similar mechanisms, particularly those with implications for the If the iron and stony-iron meteorites came from kamacite bandwidth), and ataxites (see Buchwald, their characteristics are diverse in terms of evolution of the parent bodies. Differences fully differentiated asteroids, how did these 1975 and references therein). Beginning in the chemistry, mineralogy, and structure. Significant between these groups serve to illustrate some of asteroids heat to the point of partial melting and 1950s, it was recognized that the chemical differences in bulk chemistry between iron the diverse characteristic of iron meteorites. The how did the metal segregate from the silicates? composition of iron meteorites varied, often in meteorites from different cores as well as groups range from the volatile and silicate-rich Unlike large planets, where potential energy concert with the structure. Today, the chemical variations in chemistry between meteorites from IAB - IIICDs to the highly volatile depleted release triggers core formation, small asteroids classification- based on bulk chemical analysis of the same core provide evidence of the complex groups IV A and, in particular, IVB. Differences require an additional heat source. The heat the metal-is the standard for iron meteorite chemical evolution of these evolved meteorites. in the oxygen fugacities of the parent cores are source(s) for asteroidal melting produced a wide groupings (see Chapter 1.05), with structural and Intergroup variations for volatile siderophile reflected in the difference in mineralogy between range of products, from unmetamorphosed chon­ other data (e.g., cosmic-ray exposure ages) elements (e.g., gallium and germanium) extend the highly reduced IIABs and the more oxidized drites to fully molten asteroids, as well as partially providing supporting information about iron more than three orders of magnitude, hinting that group llIAB iron meteorites.
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  • Geophysical Abstracts, 180-183 January-December 1960
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