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. iron meteorite parent bodies formed under diverse melted asteroids. Samples from these latter meteorite history. conditions. These differences reflect both the asteroids provide us with a rare opportunity to In a series of 12 papers beginning in 1967, John observe core formation- frozen in place. Wasson and co-workers analyzed the chemistry of nebular source material and geological processing 1.12,2,1 Group lIAR Iron Meteorites in the parent bodies. Most iron and stony-iron meteorites came from the vast majority iron meteorites using INAA Can we be sure that the iron meteorites are asteroids that were sufficiently heated to com­ (Wasson, 1967, 1969, 1970, 1971; Wasson and Group IIAB iron meteorites (103 members) indeed fragments of cores? Since no differentiated pletely differentiate. The better-sampled iron Kimberlin, 1967; Wasson ef al., 1989, 1998; have the lowest nickel concentrations of any asteroid has yet been visited by a spacecraft, we meteorite groups provide us with an unparalleled Schaudy et al., 1972; Scott et aI., 1973; Scott and group of iron meteorites (5.3-6.6 wt.% Ni) (see rely on circumstantial evidence. Some M-type view of a crystallizing metallic magma. Except Wasson, 1976; Kracher et al., 1980; Malvin et al., figure 26, Chapter 1.05). The IIAB irons exhibit a asteroids have spectral characteristics expected for an enigmatic paucity of samples representing 1984), producing the definitive database on the distinct structural appearance, owing to their low from exposed metallic cores (Tholen, 1989), while the late stage Fe-S eutectic melt, the trends chemistry and classification of iron meteorites. In bulk nickel concentration and, thus, predomi­ others exhibit basaltic surfaces, a hallmark of recorded by the larger groups of iron meteorites the early papers, the meteorites were analyzed for nance of kamacite. Structural1y, they range from global differentiation. Although olivine-rich man­ cover the entire crystallization sequence of their nickel, iridium, gallium, and germanium, whereas hexahedrites (former group IIA) to coarsest tles should dominate the volume of differentiated parent-body cores (Scott and Wasson, 1975). the later papers include analyses for chromium, octahedrites (former group lIB). In hexahedrites, asteroids, there is an enigmatic lack of olivine-rich Within individual groups, the abundance of cobalt, nickel, copper, gallium, germanium, kamacite crystals are generally larger than the asteroids (and meteorites) that could represent meteorites in each compositional range is also arsenic, antimony, tungsten, rhenium, iridium, specimen, whereas coarsest octahedrites have mantle material (Burbine et al., 1996). Until we in good agreement with the expectations based on platinum, and gold. Other papers by Wasson and kamacite lamellae wider than 3.3 mm (Buchwald, visit an asteroid with parts of a core- mantle numerical models (Scott and Wasson, 1975). co-workers that include analytical data have 1975). The low nickel concentration, absence of boundary exposed, our best evidence supporting a For some asteroids, near-catastrophic impacts focused on individual groups or improved tech­ phosphates (Buchwald, 1975, 1984), and occur­ core origin is detailed studies of iron meteorites. played an important role in their early geological niques (Scott, 1977a, 1978; Pemicka and Wasson, rence of graphite in lIAB iron meteorites suggest Iron- nickel alloys are expected in the cores of evolution. Impact events ranging from local 1987; Rasmussen et al. , 1984; Wasson and Wang, formation from highly reduced material. differentiated asteroids, but what other evidence melting of the target area to complete disruption 1986; Choi et al., 1995; Wasson, 1999; Wasson supports the notion that i.ron meteorites sample of the parent body are recorded in most groups of and Richardson, 2001). Additional resources on the metallic cores of differentiated asteroids? meteorites (Keil et al., 1994). Major impacts in the ~he chemical composition of iron meteorites 1.12,2,2 Group IIIAR Iron Meteorites What suggests that these asteroids were suffi­ early solar system caused remixing of metal and mclude Buchwald (1975), Moore et al. (1969) ciently heated to trigger core formation, and that silicate and large-scale redistribution of cold (nickel, cobalt, phosphorus, carbon, sulfur, and With 230 classified meteorites, IIIAB is by far iron meteorites sample cores rather than isolated and hot material in the interior of some of the COpper), and Hoashi et al. (1990, 1992, 1993a,b) the largest group of iron meteorites. llAB irons pods of once molten metal? First and foremost, parent bodies. These processes had profound (platinum group elements). are medium octahedrites, often heavily shocked Classification and Chemical Composition of Iron Meteorites 329 328 Iron and Stony-iron Meteorites Although the metal in the latter two crystaJlized systematics (Smoliar ef aI., 1996) suggest that Mayeda, 1996; Ruzicka et al., 1999), although the (Buchwald, 1975; Steffler ef al., 1988), and have rVB irons, unlike other groups, require a non­ match is not perfect (Bogard et al., 2000). uniform exposure ages and cooling rates. They within the solid silicate matrix, it has normal structure, chemistry, and cooling rate for group chondri tic osmium reservoir. High metallographic The most perplexing feature of this group is the have the by far highest abundance of phosphates, cooling rates of IYB irons (Rasmussen et at., presence of silicate inclusions which give chrono­ suggesting formation from relatively oxidized IVA (Schaudy et al., 1972; Haack et al., 1996a; Scott er al., 1996). Metallographic cooling rates of 1984) suggest a small parent body. metric ages of ~3 . 8 Ga, suggesting formation material (Kracher et al., 1977; Olsen and -0.8 Ga after the fonnation of the solar system. A Fredriksson, 1966; Olsen ef al., 1999). Silicate group IV A irons are the subject of dispute between those who believe that an apparent cooling rate range of models have been proposed in the past inclusions are documented in only two members I.l2.2.S Silicate-bearing Iron Meteorites decade, including both impact-induced (Olsen (Haack and Scott, 1993; Kracher et al., 1977; variation correlated with nickel concentration is an artifact (Willis and Wasson, I 978a,b; Wasson and Group lAB, JIICD, and lIE irons differ in a er al., 1994; Ruzicka ef aI., 1999) and incipient Olsen et al., 1996). Observed compositional parent-body melting (Bogard et aI., 2000). Young trends can be broadly matched by numerical Richardson, 2001) and those that favor a true number of properties from other iron groups. The variation (Moren and Goldstein, 1978, 1979; compositional trends in these groups are unlike chronometric ages would virtually require that models for fractional crystallization of a common impact played a role in the formation of some Rasmussen, 1982; Rasmussen ef al., 1995; Haack those in other irons (e.g., lIIAB) for which an core (Haack and Scott, 1993; U1ff-M0I1er, 1998; members of this group. Chabot and Drake, 1999; Wasson, 1999; Chabot et aI., 1996a). Haack et al. (I 996a) argued that the origin by fractional crystallization of a common and Drake, 2000). Metallographic cooling rates of di verse, but slow, metallographic cooling rates metallic core is inferred. Although sometimes and evidence of rapid cooling from 1,200 'C of the termed "nonmagmatic," lAB, IIICD, and lIE irons IIIAB iron meteorites are fairly uniform with an 1.12.2.6 Mesosiderites average of 50 K Myr- I (Rasmussen, 1989), cor­ tridymite-pyroxene assemblage in one of the clearly experienced melting during their history, responding to a parent-body diameter of -50 km stony-irons, Steinbach, implied that the IV A parent although perhaps of a different type than, for Mesosiderites are arguably the most enigmatic (Haack et al., 1990). An interesting feature of body went through a breakup and reassembly event example, group IIIAB. Unlike other groups, group of differentiated meteorites. Mesosiderites IIIAE's is that 19 of 20 measured cosmic-ray shortly after core crystallization. The model was metals in lAB and IIICD irons show considerable are breccias composed of roughly equal pro­ exposure ages point to a breakup ofthe parent core disputed by Wasson and Richardson (2001), who ranges in nickel, gallium, and germanium concen­ portions of Fe-Ni metal and silicates. Unlike 650 :+: 100 Myr ago (Voshage, 1967; Voshage argued that the apparent correlation between trations but very restricted ranges for all other pallasites, where the silicates are consistent with a and Feldmann, 1979; see Chapter 1.13), metallographic cooling rates and chemistry for elements (see figure 26, Chapter 1.05). Iridium in deep mantle origin, the silicates in the mesosider­ suggesting catastrophic dispersal then (Keil er aI., IV A iron meteorites implies that the cooling rates these groups varies within a factor of 10, compared ites are basaltic, gabbroic, and pyroxenitic 1994). The large number of IIIAE iron meteorites are in error. Like IIIAE, they are often heavily to more than three orders of magnitude in llIAB. (Powell, 1971; see Chapter 1.11). Metal compo­ and the almost complete absence of unusual or shocked and have similar cosmic-ray exposure These trends cannot be explained by simple sitions of mesosiderites are almost uniform poorly understood features make group IIlAB iron ages of -400 :+: 100 Ma (Voshage, 1967; Voshage fractional crystallization (Scott, 1972; Scott and suggesting that the metal was molten whe~ meteorites the best available samples of a crystal­ and Feldmann, 1979; see Chapter 1.13) suggesting Wasson, 1975). lAB irons also have metal mixed with the silicates (Hassanzadeh et al., lized metal core from a differentiated body. parent-body disruption at that time (Keil ef aI., compositions close to cosmic abundances (Scott, 1989). Many innovative models have attempted to 1994). A recent compilation of the chemical 1972), suggesting limited parent-body processing. explain the enigmatic mixture of crustal materials compositions of IV A iron meteorites may be Silicates provide further evidence for the with core metal (see review in Hewins, 1983). found in Wasson and Richardson (2001). unusual origin of IAB - IIICD. While differen­ Some relatively new models include impacts of 1.12.2.3 Group IVA Iron Meteorites tiated silicates might be expected in association molten planetesimals onto the surface of a large IV A irons are fine octahedrites (Buchwald, with iron meteorites, silicates in IAB-IllCD i.rons differentiated asteroid (Rubin and Mittlefehldt, are broadly chondri tic (Mittlefehldt ef al., 1998; 1975). With 48 members, IVA is the third-largest 1.12.2.4 Group IVB Iron Meteorites 1992; Wasson and Rubin, 1985) and breakup and group. They have very low volatile concentrations Benedix et al., 2000; see Chapter 1.11). Models for reassembly of a large differentiated asteroid with a and an unusually low Jrl Au ratio. The chemical Despite containing only 13 members, group the origins ofIAB- lIICD iron meteorites include still molten core (Scott et aI., 2001). trends defined by group IV A are broadly IVB exhibits unique properties and deserves crystallization of a sulfur- and carbon-rich core in Another unusual characteristic of mesosiderites consistent with numerical models of fractional special mention. Group IVB irons have the lowest a partially differentiated object (Kracher, 1985; is their very slow metallographic cooling rates of I crystaJlization (Scott et al., 1996; Wasson and volatile concentrations of any group (Table 1; see McCoy et al., 1993), breakup and reassembly of a less than 1 K Myr- (Powell, 1969; Haack er al., Richardson, 2001). Several features of group IV A figure 26, Chapter 1.05). IVB is also enriched in partially differentiated object at its peak tempera­ 1996b; Hopfe and Goldstein, 2001). These cool­ suggest a complex parent body evolution after core refractory elements, with iridium, rhenium, and ture (Benedix er al., 2000), or crystal segregation ing rates are the slowest for any natural geological crystallization. Two IV A irons contain rare osmium concentrations an order of magnitude in isolated impact melt pools on the surface of a material, suggesting that the mesosiderite parent tridymite crystals and two others contain higher than in any other group of irons (Scott, porous chondri tic body (Wasson and Kallemeyn, body must have been large. Mesosiderites have 2002). A recent compilation of the chemical ~50 vol.% silicates mixed with metal on a 1972; see also Table 1). No chondrite groups have young Ar-At ages of -3.95 Ga (Bogard and centimeter scale (Haack et al., 1996a; Reid er aI., siderophile compositions similar to group lYE compositions of lAB and IIICD iron meteorites Garrison, 1998). The young At- At ages have 1974; Scott et aI., 1996; Ulff-M0J1er er al., 1995). iron meteorites (Rasmussen et al., 1984). Re- Os may be found in Wasson and Kallemeyn (2002). been attributed to extended cooling within a large . Group IIE is a much smaller group with very asteroid (Haack et al., 1996b; Bogard and dIverse characteristics in terms of metal textures Garrison, 1998) or impact resetting (Bogard Table 1 Average compositions of meteorites from the major iron and stony-iron meteorite groups. a~~ silicate mineralogy. The chemical compo­ er al., 1990; Rubin and Mittlefehldt, 1992). SItIon of the metal is very restricted and incon­ Ge S Re Ir Ni Co Cu Au Go Group I (flgg-I) (flg g- I) (wt.%) sistent with fractional crystallization (Scott and (ngg- I) (flgg- I) (wt.%) (mgg- ) (flg g- I) (flgg- I) Wasson, 1975; Wasson and Wang, 1986). The 1.12.2.7 Ungrouped Iron Meteorites 63.6 247 silicate inclusions range from metamorphosed lAB 260 2.0 9.50 4.9 234 1.75 0.71 (1.0) 58.63 174 (17) chondrites (e.g., Netschaevo) to highly differen­ Iron meteorites require a large number of parent IIAB 1,780 (250) 12.5 (1.3) 5.65 4.6 133 19.79 39.1 (12) IIlAB 439 (200) 3.2 (5.0) 8.33 5.1 156 1.12 (0.7) tiated silicates (e.g., Kodaikanal, Weekeroo bodies to account for their diverse properties. 2.14 0.12 (3) IVA 230 (150) 1.8 (1.8) 8.51 4.0 137 1.55 (1.6) Station), with intermediate members present Ungrouped irons alone require 40-50 different 0.055 (0) IVB 2,150 (3.500) 18.0 (22) 17.13 7.6 <9 0.14 (0.15) 0.23 (e.g., Watson, Techado) (Bogard ef al., 2000). parent bodies (Scott, 1979; Wasson, 1990), a Data are from the references listed in Section \ .12.2. Numbers in parentheses arc the calculated initial liquid compositions of the core from Chabot The most primitive members of the group number unmatched by the types of meteorites that and Drake (2000) (Re, S) and Jones and Drake (1983) (lr, Au). Note the significant differences between the estimates of the initial liquid and the resemble H chondrites in both mineral chemistry represent mantle and crust materials (Burbine average meteorite compositions. For elements such as Re and If with distribution coefficients between solid and liquid metal that are far from and oxygen isotopic composition (Clayton and er al., 1996). In addition, gallium and germanium unity, the average meteorite composition may be different from the initial composition of the liquid core (see also table 3, Mittlefehldt et al., 1998). 331 330 Iron and Stony-iron Meteorites Accretion and Differences in Bulk Chemistry concentrations of ungrouped irons are not ran­ in volatile concentrations observed among iron Temperature (K) domly distributed as expected if they represent a meteorite groups has no counterpart in chondrites. large and continuous population of poorly The main processes that control the compo­ sampled parent bodies (Scott, 1979). Ungrouped sition of nebular metal--condensation and frac­ irons tend to have gallium and germanium tionation from the gas at high temperatures Condensation of metal concentrations in the same ranges as those defined and oxidation-reduction processes (Kelly and at lo-s atm by the original Ga-Ge groups I-IV. The origin of Larimer, 1977)- are illustrated in Figure I. Oxi­ ungrouped irons is poorly understood. While dation of iron from the metal will enrich the many of them sample poorly known asteroidal remaining metal in elements less easily oxidized cores, they include a variety of anomalous types, than iron. Differences in mineralogy and chem­ including highly reduced silicon-bearing irons istry between iron meteorite groups show that (e.g., Horse Creek) and at least one that essentially oxidation-reduction processes were important. I Oxidation I quenched from a molten state (e.g., Nedagolla). The oxygen fugacity of the molten core is reflected in the abundance of the oxygen-bearing phosphates found in some groups of iron meteor­ ites (Olsen and Fredriksson, 1966; Olsen et al., 1.12.3 ACCRETION AND DIFFERENCES IN 1999). Phosphate minerals are typical in group BULK CHEMISTRY BETWEEN lIIAB but have never been observed in lIAB iron GROUPS OF IRON METEORITES meteorites (Buchwald, 1975, 1984; Scott and The most striking differences in chemical Wasson, 1975). The higher nickel concentration composition between groups of iron meteorites in group IllAB compared to IIAB (Table I) is are the differences in concentration of volatile consistent with the latter forming from more reduced material. Whether this difference is a elements (several orders of magnitude) and the 5 6 7 8 9 20 25 result of a nebula process or a parent-body process smaller but important threefold variation in nickel %Ni concentration (Table I). These bulk chemical remains an open question. 5 The other important process in the nebula is Figure 1 Compositional evolution of metal condensing at a pressure of 10- atm. The first metal to condense differences cannot be attributed to fractional contains 19 ± 3 wt.% Ni and decreases to the cosmic value of 5.7 wt.% Ni as cooling commences. Elements crystallization of asteroidal cores (Scott, 1972), condensation and fractionation from the gas. more refractory than Ni are enriched in the early condensates, whereas elements more volatile than Ni are but suggest that processes during asteroid accre­ Condensation at high-temperatures results in depleted. Oxidation of Fe shifts the composition of the metal in the direction of the heavy arrows (Kelly and Larimer, tion and core-formation produced metallic melt refractory element enrichment and volatile 1977) (reproduced by pertTUssion of Elsevier from Geochim. Cosmochim. Acta, 1977,41,93-111). bodies with a range of compositions. These depletion. High-temperature condensates may be preserved if isolated from the gas before more processes occurred exclusively in the nebula 100 (condensation-evaporation), in both nebular and volatile elements condense (Meibom et aI., 2000; parent-body settings (oxidation-reduction) and Petaev el al., 2001). Processes operating within exclusively on the parent body (metal-silicate the parent bodies may mask this process. Volatile­ .... element depletions could result from degassing of segregation, degassing of volatiles and impacts). 0 ~ In some cases, it is possible to relate specific the parent-body during heating, partial melting, 10 • chemical characteristics of an iron meteorite and impact (Rasmussen ef al., 1984; Keil and Wilson, 1993). Parent-body degassing will not, s0- group to a specific process, but in most cases .e ~'t~~~~d\, • some ambiguity remains. In this section, we however, result in significant refractory element discuss the diverse chemical compositions of enrichment. Thus, it may be possible to dis­ '" • o CH metal grains iron meteorites and its possible origins. tinguish volatile depletions caused by high­ • IVA Chondrites are generally considered represen­ temperature condensation and parent-body , .IVB • tative of the material from which asteroids, degassing. • • ungrouped, low Ge including iron meteorite parent bodies, and The best evidence for the importance of 0.1 • planets formed. The heterogeneity of primitive condensation and fractionation is the composition 5 10 15 20 chondrites shows that solids in the early solar of group rYB irons and ungrouped irons of similar Ni (wt.%) system were, to some extent, chemically and composition (Figure 2). The bulk compositions isotopically diverse. Metal abundances range from of groups IV A and IVB irons are consistent with Figure 2 Ni versus Ir for group IV A and IVB iron meteorites and a number of ungrouped iron meteorites with low zero in some carbonaceous chondrites (e.g., CI high-temperature condensation of the source Ge concentrations and compositions intemlediate between IVA and IVB. Also shown are compositions of zoned and CM) to more than 60 wt. % in some CHICB material (Kelly and Larimer, 1977) (Figure 1). metal grains from CH-like chondrites (sources Campbell el at., 2001; Wasson and Richardson, 2001; Rasmussen et al., 1984). chondrites (Campbell et al., 2001; see Chapter These groups are depleted in the volatile to 1.05). The chemical composition of chondritic moderately volatile elements sulfur, phosphorus, metal is also diverse, primarily reflecting oxi­ gallium, germanium, phosphorus, antimony, cop~ elements rhenium, osmium, and iridium in group et al., 1984), although a few CB chondrites dation - reduction processes, although in rare cases per, and gold, with the nonmetals sulfur and IVB cannot (Kelly and Larimer, 1977; Scott, contain metal grains thought to sample high­ the metal formed as high temperature condensates phosphorus of particular importance, since they 1972; Wasson and Wai, 1976). High-temperature temperature condensates (Meibom et al., 2000; (Kong and Ebihara, 1997; Kong et aI., 1997; have significant effects on core crystallization and condensation for group IV A might also explain Campbell et al., 2001; see Chapter 1.05). These Campbell et al., 2001). Although ordinary chon­ Widmanstatten pattern formation. While the the unusually high nickel concentration, although metal grains are compositionally intermediate drites are depleted in gallium and germanium by depletion of volatile elements in group IV A can OXidatIOn cannot be excluded (Figure I). between IV A and IVB irons (Figure 2) and may up to an order of magnitude relative to CI result from parent-body degassing (Keil and The Source material for lVB irons is not be the first direct evidence that such materials chondrites (Wasson and Wai, 1976), the variation Wilson, 1993), enrichments in the · refractory preserved among known chondrites (Rasmussen were produced in the nebula. A close relationship 332 fron. and Ston)' ~ iron Meteorites Fractional Crystallization of Metal Cores 333 suggested that oxygen-rich melts at higher sulfur-dominated. The small sulfur rich cores source? Melting occurred very early in solar between type IV A and IVB irons and CB 182 chondrites is also suggested by the elevated B1sN pressure might readily segregate. Rushmer et al. would coexist with a mantle that contains essen­ system history. 182Hf _ W and 187Re_1870s values in all three groups (Kerridge, 1985; (2000) reviewed recent experimental evidence for tially chondritic silicate mineralogy and metal systematics in iron meteorites (e.g., Horan et al., Prombo and Clayton, 1985, 1993), Metal from metal-sulfide melt migration at pressures from abundance. As near-total melting is achieved, 1998) suggest core formation within 5 Myr of each IVB irons would have been isolated from a 1 atm to 25 GPa (Herpfer and Larimer, 1993; metal drains efficiently to the center of the body, other and formation of the first solids in the solar nebular gas above 1,500 K (Petaev ef al., 2001), Ballhaus and Ellis, 1996; Minarik et al., 1996; forming metal-dominated cores, which may be system. Early differentiation is also supported by even higher than metal grains in CB chondrites. Shannon and Agee, 1996, 1998; Gaetani and depleted in sulfur relative to the chondri tic the presence of excess 26Mg from the decay of The postulated chondri tic metal corresponding to Grove, 1999). For our purposes, many of these precursor as a result of explosive removal of the extinct 26Al (half-life of 0.73 Ma) in the eucrites IYB irons was either entirely incorporated into experiments are limited in their utility, since Fe,Ni-FeS eutectic melt (Keil and Wilson, 1993). Piplia Kalan (Srinivasan el al., 1999) and Asuka- bodies that differentiated, or these chondrites pressures in even the largest asteroids were only While most meteorites sample either primitive 881394 (Nyquist e/ al., 2001). remained unsampled. fractions of a GPa (Rushmer ef al., 2000). These or fully differentiated asteroids, a few offer direct Several heat sources for core formation can be experiments demonstrate that only anion-domi­ evidence for processes occurring during core ruled out. Early melting was not caused by decay nated metallic melts exhibit dihedral angles less formation. Acapulcoites and lodranites (McCoy of the long-lived radionuclides (e.g., uranium, than 60° and form interconnected networks el al., 1996, 1997a, b; Mittlefehldt ef al., 1996) potassium, and thorium) that contribute to the 1.12.4 HEATING AND DIFFERENTIATION (Figure 3). This is consistent with the work of offer tantalizing clues to the nature of asteroidal Earth's heat budget. Similarly, accretional heat­ ing-which could have caused planet-wide melt­ How did asteroidal cores come to exist in the Taylor (1992) and suggests that even Fe- S core formation. In the acapulcoites, millimeter- to ing in the terrestrial planets- would have first place? What were the physical processes? eutectic melts at low pressure (anions/cations centimeter-long veins of Fe,Ni-FeS eutectic produced heating of no more than a few tens of What were the heat sources? In this section, we ...... 0.8) would not migrate to form a core under melts formed in the absence of silicate partial degrees in asteroids. Keil ef al. (1997) argued that address these fundamental issues by considering static conditions. melting (Figure 4). The bulk composition of these the maximum energy released during accretion is evidence from iron meteorites, primitive chon­ Perhaps dynamic processes played a role in the samples is unchanged from chondritic, suggesting equal to the gravitational binding energy of the drites, achondrites, experiments, and numerical formation of asteroidal cores. Rushmer et al. that only localized melt migration occurs. In asteroid after accretion. For a 100 km body, this calculations. The simplest case for core formation (2000) found considerable evidence for iron sulfide contrast, lodranites are depleted in both plagio­ equates to a temperature increase of only 6 °C. is metal sinking through a silicate matrix that has melt mobility in the absence of silicate partial clase- pyroxene (basaltic) melts and the Fe,Ni ­ Finally, impacts sufficiently energetic to melt experienced a high degree of partial melting. melting in experiments on an H6 ordinary FeS eutectic melts, suggesting that silicate partial asteroids will also catastrophically disrupt the Numerous experimental studies (e.g., Takahashi, chondrite. These authors suggest that metal-sulfide melting opened conduits for metal-sulfide melt asteroid. 1983; Walker and Agee, 1988; McCoy ef al., 1998) segregation may be possible, although the migration. While sulfur-rich cores may have The most likely heat sources for melting of demonstrate that metal and sulfide tend to form pressure- strain regime used may not be applicable formed at low degrees of partial melting (e.g., asteroidal parent bodies are eleculcal conduction rounded globules, rather than an interconnected to asteroidal-sized bodies. Keil and Wilson (1993) <20%), they required silicate partial melting. heating by the T-Tauri solar wind from the pre­ network, at moderate degrees of silicate partial suggested that overpressure during Fe,Ni-FeS Impact during core formation is a complicating main-sequence Sun (e.g., Sonett ef al., 1970) or melt. These globules then sink through silicate eutectic melting might create veins and volatiles factor. Mesosiderites and silicate-bearing lAB and short-lived radioactive isotopes (Keil, 2000). mush. Taylor (1992) calculated that at silicate might cause these veins to rise in small bodies. IIICD irons may have formed by disruption of a Melting of asteroids by 26 Al has gained broader fractions of 0.5, metal particles - 10- 1,000 cm These results suggest two very different types of partially to fully differentiated parent body prior to acceptance since the discovery of excess 26Mg in sink readily through a crystal mush, although it cores result from partial melting. At low degrees of core crystallization and solidification (Benedix Piplia Kalan (Srinivasan et al., 1999). Taylor remains unclear how millimeter-sized metal partial melting, cores may be sulfur-rich or even el al., 2000; Scott el ai., 2001). The consequences (1992) points out an interesting conundrum. In particles in primitive chondritic meteorites attain of an impact during core formation/crystallization chondrites, 26Al is concentrated in plagioclase­ these sizes. Settling would be rapid, with core have not been fully explored. an early melting silicate. Loss of basaltic melts formation requiring tens to thousands of years, Iron meteorite parent bodies experienced heat­ leaves pJagioc1ase- and troilite-depleted residues depending on parent-body size and degree of ing and melting at temperatures in excess of akin to the lodranites (e.g., McCoy el al., 1997a), silicate melting. Many large iron meteorites (e.g., 1,500°C (Taylor, 1992). What was the heat which are sufficiently depleted in 26Al that no Canyon Diablo, Hoba) and shower-producing further heating occurs. Several authors (e.g., irons (e.g., Gibeon) exceeded 1- 10 m, supporting Shukolyukov and Lugmair, 1992) suggest 60Fe the idea that irons originated in central cores, Crystal Crystal as a heat source. Recent evidence for very high rather than dispersed metallic masses. concentrations of 60Pe in chondritic troilite A more interesting case is porous flow through (Mostefaoui ef ai., 2003; Tachibana and Huss, interconnected networks in a largely solid silicate 2003) raises the possibility that 60Fe could cause matrix. This scenario is of interest both because melting even in the absence of other heat sources. any fully melted body would have experienced an Occurring in both oxidized and reduced forms, earlier stage of partial melting and because early 60Fe would be retained throughout all layers of a partial melts are sulfur enriched. The Fe, Ni - FeS body during differentiation and could provide the eutectic occurs at - 950 °C and contains heat necessary for global melting and, ultimately, -85 wt.% FeS (Kullerud, 1963). Using Darcy's core formation in asteroids. law, Taylor (1992) calculated flow velocities of 270 m yr- I at 10% melting, suggesting core 2 4 formation _10 _10 yr. Taylor (1992) did note Fig~re 4 The Monument Draw acapulcoite contains considerable uncertainty about interfacial angles, Figure 3 Illustration of the dihedral angle () and a cen~meter scale veins of Fe,Ni metal and troilite formed 1.12,5 FRACTIONAL CRYSTALLIZATION which control melt migration, in Pe,Ni - FeS melts depiction of two end-member microstructures for static d.u~lllg the first melting of asteroids. In the absence of OF METAL CORES and that considerable experimental evidence systems. If () < 60°, an interconnected network will slhcate melting, these veins were unable to migrate argued against metal-sulfide melt migration at form and melt migration can occur. If () > 60°, melt substantial distances and, thus, would not have Although iron meteorites provide a guide to low degrees of partial melting under static forms isolated pockets. In experimental systems, inter­ contributed to core formation. Length of specimen is understanding the ongoing crystallization of the conditions. In contrast, Urakawa et ai. (1987) connectedness only occurs in anion-rich static systems. --7.5 cm (Smithsonian specimen USNM 7050). Earth's core, important differences in the physical 334 Iron and Stony-iron Meteorites Fractional Crystallization of Metal Cores 335 settings exist. First and foremost, the central fractional crystallization from a perfectly mixed of assimilation (Malvin, 1988), imperfect mixing pressures within the iron meteorite parent bodies liquid with constant distribution coefficients will of the liquid (Chabot and Drake, 1999), liquid did not exceed 0.1 GPa, compared to central result in straight chemical trends where the slope immiscibility (Ulff-M~ller, 1998; Chabot and pressures in excess of 350 GPa on Earth. is given by (DE ~ 1)/(DNi ~ 1), where DE and Drake, 2000), and trapping of sulfur-rich liquid The steep pressure gradient in the Earth's core, DNi are the liquid metal/solid metal distribution (Haack and Scott, 1993; Scott et aI., 1996; which causes the core to crystallize from the coefficients for the element and nickel, respect­ Wasson, 1999; Wasson and Richardson, 2001). 5 inside out, was absent in the asteroidal cores. ively. Although most trends define almost Numerical models of the fractional crystal­ straight lines, differences in slope from group to lization process are broadly consistent with the Since the core is cooled through the mantle, Haack 2 and Scott (1992) argued that the onset of core group and curved trends for some elements observed trends. There are, however, several crystallization in asteroids was probably from the such as gallium and germanium show that features that remain poorly understood. base of the mantle. Crystallization of the aster­ the distribution coefficients cannot be constant. -; oidal cores was likely in the form of kilometer­ The slopes of the Ni-Ir trends correlate with the '"~ sized dendrites as light buoyant liquid, enriched in volatile concentrations (including sulfur) for the 1.12,5,1 Imperfect Mixing during Crystallization .3 0.5 the incompatible element sulfur inhibited crystal­ different groups. Experimental work has shown ", The compositional data scatter from the average lization of the outer core. that the distribution coefficients are functions of trend for each group are significantly greater than 0.2 The fractional crystallization trends preserved the phosphorus, sulfur, and carbon concentration the analytical uncertainty, in particular for compa­ in iron meteorites are unparalleled in terrestrial in the liquid (Goldstein and Friel, 1978; Narayan tible elements (Pernicka and Wasson, 1987; Haack magmatic systems. Trace-element variations span and Goldstein, 1981, 1982; Willis and Goldstein, 0.1 and Scott, 1992, 1993; Scott et at., 1996; Wasson, more than three orders of magnitude within group 1982; Jones and Drake, 1983; Malvin et at., 1999; Wasson and Richardson, 2001). The scatter IIIAB irons (Figure 5). The traditional way to 1986; Jones and Malvin, 1990; Chabot and 0.05 shows that the assumption of metal crystallizing display the bulk compositional data of iron Drake, 1997, 1999, 2000; Liu and Fleet, 2001; from a perfectly mixed liquid breaks down meteorites is in a log nickel versus log element Chabot and Jones, 2003). Using the experimen­ during the course of crystallization. An interesting 0.02 diagram. More recent work has used gold as tally determined distribution coefficients it is example of this scatter is observed among and the reference element (Wasson, 1999; Wasson possible to calculate fractional crystallization within the multi ton fragments of the IIIAB iron and Richardson, 2001). Gold is a better choice trends (Figure 5). Early numerical models 0.01 meteorite Cape York (Esbensen and Buchwald, because the distribution coefficient is further from assumed that the liquid remained perfectly 2 3 1982; Esbensen ef aI., 1982). The compositional 0.4 0.5 0.8 1 unity giving a natural variation that far exceeds mixed throughout crystallization (Willis and Au (~g g- I) trend defined by the different Cape York fragments the analytical uncertainty (Haack and Scott, Goldstein, 1982; Jones and Drake, 1983), diverges from the general trend defined by IllAE 1993). In a log E versus log Ni diagram, ideal whereas later models have included the effects Figure 6 Solid (left) and liquid (right) tracks shown iron meteorites (Figure 6). The observation that the on a plot of lr versus Au. Crosses at 0%, 30%, 50%, compositional variation within a single meteorite 70%, and 80% crystallization show solid- liquid mixing 24 shower covers most of the scatter within the entire curves. The compositions of the Cape York irons Savik 5 group suggests that the compositional scatter of and Agpalilik fall close to the mixing line calculated for group JUAB is due to processes operating on a 30% crystallization. Also shown are the unusual meter scale. Wasson (1999) noted that both the IIIAB meteorites Augusta County, Buenaventura, Cape York trends and the scatter in the IllAB Delegate, Ilinskaya Stanitza, Tieraco Creek, Treysa, and Wonyulgunna (source Wasson, 1999). compositions tend to fall between the compositions

'" of solid metal and its inferred coexisting liquid. IIlAB meteorites Using modified distribution coefficients, he was --.- simple fractional concentration by analyzing iron meteorites that --<>- boundary layer able to model the Cape York trend as a mixing line -I'r- constant zone between liquid and solid (Figure 6). He suggested represent the crystallized metal. Sulfur may only -0- sh.rinking zone be determined indirectly by treating it as a free -0- complex history that the observed scatter and the Cape York trend (b) parameter in the fractional crystallization models 12p======~~==~ are caused by diffusional homogenization of trapped melt pools and solid metal. and choose the sulfur composition that provides the best fit to the data (Jones and Drake, 1983; 10 Haack and Scott, 1993; Chabot and Drake, 2000). The concentration of sulfur in the liquid as 1.12,5,2 Late-stage Crystallization and crystallization proceeds would lead to changes in Immiscible Liquid the slope of the elemental trends that are not For most groups the modeled compositional matched by observations. Apparently, the distri­ trends tend to deviate from the observations bution coefficients are either incorrect (Wasson, toward the late stages of the crystallization 1999) or some mechanism prevented the concen­ (Figure 5). There are several possible reasons for tration of sulfur in the late-stage liquid (Haack and these discrepancies. The distribution coefficients Scott, 1993; Chabot and Drake, 1999). (d) 20 (e) 0.00lL-~ __~ __.!-~ __L- __-' are functions of the concentration of the incom­ As sulfur and phosphorus concentrations increase, liquid immiscibility may also play an 7 7 10 patible elements sulfur and phosphorus in the Ni (wt.%) Ni (Wt.%) lIqUId. The phosphorus concentration of the melt increasingly important role. Experiments show that at high sulfur and phosphorus concentrations Figure 5 Calculated fractional crystallization trends for group IIIAB iron meteorites using several different types of IS well constrained but the behavior of sulfur models. The simple fractional crystallization models assume a perfectly mixed liquid whereas the other four models during crystallization remains poorly understood. (and high oxygen fugacity), the liquid will enter a assume different types of imperfect mixing (reproduced by permission of the Meteoritical Society from Meteorit. SUlfur is almost insoluble in solid metal, and it is two-phase field (Jones and Drake, 1983; Chabot Planet. Sci. 1999, 34, 235-246). therefore not possible to estimate the liquid and Drake, 2000) (Figure 7). Ulff-M~ller (1998) 337 336 Iron and Stony-iron Meteorites Cooling Rates and Sizes of Parent Bodies Table 2 Metallographic cooling rates and correspond­ ing parent-body radii of the main iron and stony-iron 30 -- meteorite cryslallization paths • 2 liquids, 1 solid experiments meteorite groups. • I liquid, I solid experiments Group Cooling rate Parent-body Refs. -- Fe-P-S diagram I (K Myr- ) radius 25 (km)

lAB 25 >33 !lAB 6-12 45-65 20 IIIAB 15 - 85 20 - 40 7.5 - 15 42-58 IVA 19-3,400 >40 [VB 170- 230 12 - 14 [5 Pallasites 2.5 - 4 80- 100 Mesosiderites 0.5 200

References: (a) Yang el al. (1997). (b) Saikumar and Goldstein (1988), 10 (e) Rasmussen (1989), (d) Rasmussen el af. (1995), and (e) Rasmussen 2 liquid tield et al. (1984). A radius of the parent body can only be calculated for those groups where the meteorites are believed to have cooled in the , core. For other groups a minimum parent-body size may be calculated. 5 "- • "- Figure 8 Widmanstatten pattern in a polished and • - etched section of the lID iron meteorite, Carbo. 10 cm rates at temperatures below 400 °c (Haack ef aI., IVa --... I 1i4uiu fielJ •• field of view. The brownish tear shaped troilite nodule 1996b; Rasmussen ef al., 2001). Yang er al. 0 to the left has acted as precipitation site for kamacite (1997) showed that the size of the so-called island (Geological Museum, University of Copenhagen, 0 2 4 6 8 phase is inversely correlated with cooling rate specimen 1990.143). and may be used to infer the cooling rate at wt.!)} P temperatures ~ 320 0c. The island phase is high­ Figure 7 Two-liquid field in the Fe-S- P system as determined by Raghavan (1988) (hatched line) and Chabot and parent taenite crystals had dimensions larger than nickel tetrataenite that forms irregular globules Drake (2000) (white area). Chabot and Drake perfonned their experiment at more oxidizing condition- assumed typical meteorites, a continuous pattern of four with dimensions up to 470 nm in the nickel-nch relevant for group IIIAB iron meteorites. Also shown are the trends followed by the major iron meteorite groups. Note different sets of kamacite plates may be observed rims of taenite. that all groups eventually enter the two-liquid field (reproduced by permission of the Meteoritical Society from on etched surfaces of typical iron meteorites. The With the exception of group IVA iron meteor­ Meleoril. Planel. Sci., 2000, 35,807-8[6). limiting factor for the growth of kamacite is the ites, the metallographic cooling rates tend to be slow diffusion of nickel through taenite. Nickel similar within each group but different from group profiles across taenite lamellae may, therefore, be to group (Table 2). This is consistent with the idea iron meteorite parent bodies are astonishingly modeled the crystallization of the IIlAB core used to determine the so-called metallographic that each iTon meteorite group cooled in its own rare. Clearly, processes that selectively remove taking liquid immiscibility into account. He found cooling rate of the parent-body core at ~500 'C separate metallic core surrounded by an insulating more fragile materials en route to the Earth are of that the effect of liquid immiscibility changes the (Wood, 1964; Rasmussen, 1981; Herpfer el al., mantle. The cooling rates for the different groups significant importance. compositional trends resulting from late-stage 1994; Yang el al., 1997; Hopfe and Goldstein, of iron meteorites are generally in the range l crystallization significantly. 2001 ; Rasmussen el al., 2001). A number of 10- 100 T Myr- revisions of the metallographic cooling rate The metallographic cooling rates combined method have been implemented since its original with numerical models of the thermal evolution 1.12.5.3 The Missing Sulfur-rich Meteorites 1.12.6 COOLING RATES AND SIZES formulation (Wood, 1964; Goldstein and Ogilvie, of the parent bodies may be used to constrain the OF PARENT BODIES 1965). Revisions have resulted from improved sizes of the parent bodies (Wood, 1964; Goldstein Models predict that a considerable volume of ternary (Fe-Ni-P) phase diagrams (Doan and and Short, 1967; Haack el al., 1990). The main Fe,Ni - FeS eutectic melt should be produced at After the cores of the differentiated asteroids Goldstein, 1970; Romig and Goldstein, 1980; uncertainties in the numerical models are the the end of core crystallization. However, the had crystallized. a slow cooling period com­ Yang ef al., 1996) and, in particular, the discovery states of the mantle, surface, and crust during number of sulfur-rich meteorites faUs far short of menced. During this period the most prominent that small amounts of phosphorus may increase cooling. Observations of present-day asteroids the predicted abundances. For the sulfur- and feature of iron meteoTites evolved-the Widman­ the diffusion rate of nickel through taenite by an show Ihat they tend to be covered by a thick phosphorus-rich group IIAB, the models predict statten pattern (Figure 8). Several characteristics order of magnitude (Narayan and Goldstein, 1985; insulting regolith and that they may be heavily that the unsampled volume of eutectic melt is of the Widmanstatten pattern may be used to Dean and Goldstein, 1986). brecciated. A brecciated mantle andlor a highly ~70% (Chabot and Drake, 2000). Although constrain the thermal evolution and the sizes of the porous regolith cover on the surface of an asteroid arguments may be made that the sulfur-rich iron meteorite parent bodies. Several other characteristics of the Widman­ could potentially slow the cooling rate by a factor material is weaker and, therefore, more easily In a typical parent-body core with nickel sUi.tten pattern have provided additional infor­ of 5- 10 (Haack er al., 1990). An approximate broken down in space and ablates more rapidly concentrations in the range 7-15 wt. % Ni, the mation on the thermal history of the metal. relationship between metallographic cooling rates during atmospheric entry (Kracher and Wasson, low-nickel phase kamacite will grow at the Diffusion controlled growth of schreibersite has and parent-body radius for regolith covered 1982), it remains a mystery why the number of expense of the high-nickel phase taenite between been used to determine cooling rates of hexa­ asteroids was given by Haack el al. (1990): sulfur-rich meteorites is so low. It is, however, 800 °c and 500 0c. Several other elements are hedrites (Randich and Goldstein, 1978). Although R = 149 X CR - 0.465, where R is the radius in worth noting that although the mantle and crust partitioned between the two phases during growth nIckel diffusion through kamacite is much faster km and CR the cooling rate in K Myr- I Table 2 probably comprised more than 85 vol.% of of the kamacite phase (Rasmussen ef al .• 1988). th~n through taenite, the decreasing temperature differentiated parent bodies, achondrites that The kamacite grows as platelets in four possi ble WIll eventually result in zoned kamacite as well. gives a compilation of cooling rates and corre­ could represent mantle and crust materials of the orientations relative to the taenite.host. Since most Zoned kamacite has been used to infer cooling sponding parent-body sizes. Parent Bodies of Iron and Stony-iron Meteorites 339 338 Iron and Stony-iron Meteorites et al., 1998). Phosphates show REE patterns, disparate features be reconciled and what are the 2.0,------~ 1.12.7 PALLASITES implications for the nature of asteroidal cores inherited from the olivines, largely consistent with 8 Chulofinnoe(ffiAB) Pallasites are the most abundant group of stony­ the olivines forming as cumulates at the base of during cooling and solidification. Scott (1977c) S ~~=--- suggested that the main group pallasite metal is a Aboe(Eli) . " irons. Their features are easily reconciled with the mantle (Davis and Olsen, 1991; Davis and ~ 1.5 _" •••• • .•• '16~;"he simple models of asteroid differentiation, yet Olsen, 1996). The metal composition is similar to reasonable crystallization product after - 80% .,,'. ibb·$M,1l Un crystallization of IIIAB metallic melt. However, "§ Hvittil(E1..6) challenge some of our basic assumptions about high-nickel lIlAB irons, but with a greater scatter o core formation and crystallization (Mittlefehldt in nickel (-7 -13 wt. %) and some other sidero­ asteroidal cores probably crystallized from the et al.. 1998). The 50 known pallasites are divided phile elements (Davis, 1977; Scott, 1977b). The core-mantle boundary inwards and we should r expect pallasites to have the signature of early­ into main group pallasites, the Eagle Station pallasite-IlTAB iron link is supported by the 0.5_LL.~-'--:'~~L.L~~-'--:'~~L.L~~ grouplet (three members) and the pyroxene similarity in oxygen isotopic compositions of crystallizing, low-nickel melts, not late-crystal­ 0.0 0.5 1.0 1.5 2.0 2.5 pallasite grouplet (two members). These are their silicates (Clayton and Mayeda, 1996). lizing, high-nickel melts. Haack and Scott (1993) Wavelength (!-Lm) postulated that residual metallic melt migrated to distinguished based on oxygen isotopic, mineral, The Eagle Station trio (Eagle Station, Cold Bay, Figure 10 Normalized reflectance versus wavelength and metal compositions. Main group pallasites are ltzawisis) is comprised dominantly of iron and the core- mantle boundary between dendrites and (f.Lm) for M-type 16 Psyche versus iron and enstatite comprised of subequaJ mixtures of forsteritic nickel metal and olivine with lesser amounts of Ulff-M011er ef al. (1997) proposed mixing of late­ chondrite meteorites (lines) (Gaffey, 1976; Bell et ai., olivine and iron, nickel metal, often heteroge­ clinopyroxene, orthopyroxene, chromite, and stage metallic melt with earlier solidified metal to 1988; Bus, 1999). All spectra are normalized to unity at neously distributed. Clusters of olivine grains phosphates. Olivines in the Eagle Station produce the range of pallasite compositions. Late­ 0.55 f.Lm. Spectra for all three are relatively featureless stage intrusions might explain the presence of with red spectral slopes and moderate albedos. While reach several centimeters (Ulff-M011er et al., (FO _ Sl) are more ferroan than in the main SO some M-class asteroids may be metallic core material, 1998), while Brenham has metal cross-cutting group and differ substantially in their Fe/Mn ratio angular olivines, which were fractured during the intrusion (Scott, 1977b), and the hetero­ existing spectral and density data are inconsistent with zones with more typical pallasitic texture (Mittlefehldt et al., 1998). Metal in the Eagle this explanation for all M asteroids. (Figure 9). Oli vine morphology also differs Station pallasites has higher iridium and nickel geneous textures seen in pallasites like Brenham. significantly, from angular-subangular in Salta compared to main group pallasites and is closer to Ulff-M011er ef al. (1997) suggested that high­ pressure injection of metal, perhaps caused by to rounded in Thiel mountains (Buseck. 1977; metal in IIF irons (Kracher ef aI., 1980). They also impact, might provide the most plausible mech­ high-nickel IYB) is probably not possible. His­ Scott,1977b). differ dramatically in oxygen isotopic compo­ torically, M-class asteroids, of which -40 are Olivines in the main group pailasites cluster sition from main group pallasites (Clayton and anism for producing pallasites. Finally, pallasites have experienced a long history of subsolidus known (Tholen, 1989), have been linked to iron around Fos8::!: I, although some reach FOSb and Mayeda, 1996). meteorites. These asteroids exhibit moderate exhibit core-ta-rim zoning of aluminum, chro­ Pyroxene pallasites are represented by only two cooling and annealing. Both minor-element zon­ ing in olivine and rounded olivines might be best visual albedos, relatively featureless spectra and mium, calcium, and manganese (Zhou and Steele, members: Vermillion and Yamato 845l. The two explained by subsolidus annealing and diffusion. red spectral slopes, similar to that of iron 1993; Hsu ef aI., 1997). Chromite, low-calcium meteorites share the common feature of contain­ The small Eagle Station and pyroxene-bearing meteorites. Radar albedos-an indirect measure pyroxene and a variety of phosphates comprise ing pyroxene, but differ substantially from one pallasite grouplets remain enigmatic, but suggest of near-surface bulk density- have been used as < 1 vol.% each (Buseck, 1977; Ulff-M011er another. Yamato 8451 consists of - 60% olivine, that similar processes operated on other parent supporting evidence for the link between iron 35% metal, 2% pyroxene, and 1% troilite (Hiroi bodies and may have been common among highly meteorites and M-class asteroids, as M asteroids et aI., 1993; Yanai and Kojima, 1995). In contrast, differentiated asteroids. tend to have higher radar albedos than C or S Vermillion contains around 14% olivine, and less asteroids (Magri et al., 1999) and the highest than 1% each of orthopyroxene, chromite, and asteroid radar albedos are known from the M phosphates (Boesenberg ef aI. , 2000). The iron asteroids 6178 (1986 DA) and 216 Kleopatra concentration in olivine (Foss _90) is similar to 1.12.8 PARENT BODIES OF IRON AND (Ostro et aI., 1991, 2000). There is reason to main group pallasites, although pyroxene palla­ STONY·IRON METEORITES believe that not all (perhaps not many) M-class sites have lower FelMn ratios (Mittlefehldt ef al. , asteroids are either core fragments or largely 1998). The two pyroxene pallasites do not share a While iron and stony-iron meteorites provide important snapshots of the origin and evolution of intact stripped cores of differentiated asteroids. common metal composition with each other or, in The enstatite chondrites also exhibit nearly detail, with other pallasites (Wasson ef al.. 1998). highly differentiated asteroids, they lack geologic featureless spectra (owing to the nearly Fe2+-free Pallasites are both intriguing and perplexing. context. From which type of asteroid do iron and composition of the enstatite; Keil, 1968) with red The two lithologies, Fe,Ni metal and olivine, are stony-iron meteorites originate? Are there enough spectral slopes and moderate albedos (Gaffey, reasonable assemblages expected at the core­ metallic asteroids to account for the enormous 1976) and have been suggested as possible mantle boundary of a differentiated asteroid. The diversity among iron meteorites, particularly the meteoritic analogues to M asteroids (Gaffey and lower mantle should be primarily olivine, either a ungrouped irons? Have we sampled correspond­ McCord, 1978) (Figure 10). Rivkin et al. (2000) residue from high degrees of partial melting or an ing abundances of crustal and mantle material in found that more than one-third of observed M-class early cumulate phase from a global magma ocean. both the meteorite and asteroid populations? asteroids have 3 J.Lm absorption features sugges­ However, the marked density contrast between These are the questions that we explore in this tive of hydrated silicates. Although this conclusion metal and silicates should lead to rapid separation. section. Further, some features of pallasitic olivine, such The major tool that allows us to relate classes of remains the subject of significant debate (Gaffey as the angular shapes and marked minor element meteorites to classes of asteroids is spectral ef al., 2002a; Rivkin et al., 2002), it would be zoning, seem inconsistent with formation at a reflectance (Burbine ef al., 2002). Unfortunately, inconsistent with core material. Finally. recent relatively quiescent, deep-seated core- mantle Ifon meteorites, with their paucity of silicate ground-based measurements of bulk densities for Figure 9 The Brenham pallasite contains areas of boundary. As Mittlefehldt ef al. (1998) note, the phases, have relatively featureless spectra with red M asteroids 16 Psyche (Viateau, 2000; Britt ef aI., both olivine-free regions and areas more typical of spectral slopes and moderate albedos (e.g., Cloutis 2002) and 22 Kalliope (Margot and Brown, 2001) core-mantle boundary origin remains the most 3 pallasites. In this specimen, pallasitic material is cross­ plausible, despite these difficulties. ef al .• 1990) (Figure 10). There appears to be no are ~2 g cm- , far below that expected for even cut by a metallic region, suggesting that silicate-metal If we assume a core-mantle boundary origin SImple correlation between nickel abundance and highly fragmented core material. mixing at the core-mantle boundary was a dynamic (see Mittlefehldt ef al., 1998, for references spectra redness and, thus, distinguishing different Equally puzzling is the lack of olivine-rich process. Length of specimen is - 13 em (Smithsonian chemical groups (e.g., low-nickel IIAB from asteroids corresponding to the large numbers of specimen USNM 266). suggesting alternative models), how can these 340 Iron and Stony-iron Meteorites References 341 asteroidal cores sampled by iron meteorites. If we A second field ripe for future work is experimental deeper-seated (?) pyroxenitic material, and oli­ asteroid of comparable diameter would release have sampled - 65 asteroidal cores, we might studies designed to increase our understanding of vine-rich (7) mantle, including their lateral and -2.5 times the energy of a stony asteroid. Further, expect corresponding abundances of olivine-rich the processes of core crystallization. Although we horizontal relationships. its physical properties would certainly be quite meteorites and A-type (olivine-rich) asteroids now have more than a rudimentary understanding Asteroid spectroscopy has provided possible different from that of a stony asteroid. The (Chapman, 1986; Bell ef al., 1989; Burbine ef al., of core crystallization, uncertainties in both links between iron and stony-iron meteorites and aforementioned m.ission might not only provide 1996; Gaffey ef al., 2002b). The near absence of partition coefficients (particularly as a function their possible parent as discussed in the previous substantial insights into the geologic history of dunite meteorites and paucity of A-type asteroids of sulfur, phosphorus, and carbon concentration) section. Iron meteorites provide evidence that asteroids, but also provide essential data for under· has come to be known as "The Great Dunite and liquid immiscibility in sulfur-, phosphorus-, numerous metal objects exist but the lack of standing the physical properties (e.g., material Shortage." The most likely explanation centers on and carbon-rich systems limit our understanding spectral features and the surprising low density distribution, global cracks) that would allow us to the age of disruption of differentiated parent of the nature of late-stage core crystallization. determined for some M-type asteroids makes it ensure the continuation of human history. bodies and the durability of metallic meteorites Recent experimental studies have made signifi­ impossible to establish a firm link between iron and asteroids relative to their stony counterparts. cant headway in these areas, and parametrization meteorites and M-type asteroids. 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