Evolution of Asteroidal Cores 747

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Chabot and Haack: Evolution of Asteroidal Cores 747

Evolution of Asteroidal Cores

N. L. Chabot The Johns Hopkins Applied Physics Laboratory

H. Haack University of Copenhagen

Magmatic iron meteorites provide the opportunity to study the central metallic cores of asteroid-sized parent bodies. Samples from at least 11, and possibly as many as 60, different cores are currently believed to be present in our meteorite collections. The cores crystallized within 100 m.y. of each other, and the presence of signatures from short-lived isotopes indicates that the crystallization occurred early in the history of the solar system. Cooling rates are generally consistent with a core origin for many of the iron meteorite groups, and the most current cooling rates suggest that cores formed in asteroids with radii of 3–100 km. The physical process of core crystallization in an asteroid-sized body could be quite different than in Earth, with core crystallization probably initiated by dendrites growing deep into the core from the base of the mantle. Utilizing experimental partitioning values, fractional crystallization models have examined possible processes active during the solidification of asteroidal cores, such as dendritic crystallization, assimilation of new material during crystallization, incomplete mixing in the molten core, the onset of liquid immiscibility, and the trapping of melt during crystallization.

1. INTRODUCTION ture of the metal and the presence of secondary minerals, are also considered (Scott and Wasson, 1975). In the first From Mercury to the moons of the outer solar system, attempt to classify iron meteorites, groups I–IV were de- central metallic cores are common in the planetary bodies fined on the basis of their Ga and Ge concentrations. As of our solar system. However, the only samples we have of more and better data became available, complex names such any of these planetary cores are some iron meteorites, be- as the IAB complex and IIIAB evolved. For a recent re- lieved to have originated from the cores of up to 60 differ- view of the nomenclature and history of iron meteorite clas- ent asteroids. As our only samples of any planetary cores, sification, see Haack and McCoy (2003). The ungrouped these iron meteorites provide a unique opportunity to study iron meteorites, those that have less than four other iron planetary cores and how cores evolve. This chapter reviews meteorites to which they seem to be related, may sample what we have learned about the evolution of asteroidal cores up to 50 different asteroids (Scott, 1979b; Wasson, 1990). from studies based on iron meteorites. Section 2 introduces The ungrouped irons were previously referred to as “anoma- iron meteorites, providing general compositional and struc- lous,” but the term “ungrouped” is preferred to avoid sug- tural information and establishing a basis for the following gesting a different history for the ungrouped and grouped sections. Section 3 presents different timescales that have meteorites (Scott, 1979b; Kracher et al., 1980). Table 1 lists been determined for the evolution of asteroidal cores. Sec- the number of meteorites per group for irons that have been tion 4 discusses the physical process of core crystallization analyzed at UCLA by J. T. Wasson and colleagues, which in an asteroid-sized parent body, and section 5 compares is estimated to be >90% of all iron meteorites that are avail- recent core crystallization models. able for such analysis (J. T. Wasson, personal communica- tion, 2004). Iron meteorites exhibit a wide range of chemi- 2. OBSERVATIONS FROM “MAGMATIC” cal diversity, as displayed in Fig. 1. IRON METEORITES The groups are further subdivided into two main types: those believed to sample central metallic cores and those 2.1. Statistics of Iron Meteorites that do not. Significant differences exist between the two types of groups. “Magmatic” iron meteorite groups exhibit All iron meteorites are classified as belonging to one of similar elemental fractionation trends within each group that 13 groups or as being “ungrouped.” A group is defined as are generally consistent with being formed by fractional crys- having a minimum of five members that appear to be ge- tallization of a large, single melt body, commonly agreed netically related and are consequently believed to have to be the central metallic cores of asteroid-sized parent bod- formed on the same parent body. The classification is based ies (Scott, 1972, 1979a; Scott and Wasson, 1975; Wasson, largely on the trace element concentrations in the metal of 1985; Haack and McCoy, 2003). In contrast, the “nonmag- the iron meteorites, although other factors, such as the struc- matic” or “silicate-bearing” groups do not have large frac-

747 748 Meteorites and the Early Solar System II

TABLE 1. Classification of iron meteorites. matic iron meteorites and the insights that they have provided about the evolution of asteroidal cores. Group No. of members* Magmatic 2.2. Metallic Structure and Composition IIIAB 217 (31.5%) IIAB 71 (10.3%) As the name implies, iron meteorites are dominantly IVA 59 (8.6%) composed of Fe-Ni metal. The slow cooling of Fe-Ni metal IID 14 (2.0%) allows the formation of the Widmanstätten pattern, which IIIE 12 (1.7%) develops as an intergrowth of low-Ni kamacite and high- IVB 11 (1.6%) Ni taenite crystals during cooling (see review by Buchwald, IIIF 9 (1.3%) 1975). The exact structure of the Widmanstätten pattern de- IC 7 (1.0%) IIC 7 (1.0%) pends on the composition and the cooling rate of the metal. IIF 6 (0.9%) Iron meteorites that exhibit a Widmanstätten pattern are IIG 5 (0.7%) labeled as octahedrites; octahedrites are further subdivided Silicate-bearing into five types, ranging from coarsest to finest depending IAB complex† 171 (24.8%) IIE 17 (2.5%) Ungrouped 83 (12.1%) *Number in parentheses is the percent relative to all irons. Statis- tics include all irons that have been analyzed by J. T. Wasson and colleagues at UCLA, which is estimated at >90% of all iron meteorites that are available for such analysis. Statistics furnished by J. T. Wasson, and are the most recent as of March 2004. † IAB complex discussed in Wasson and Kallemeyn (2002). tionations in many siderophile elements, such as Ir, and have more variable Ga, Ge, and Ni contents as compared to the ranges of these elements within any one magmatic group, as shown in Fig. 1 (Scott, 1972; Scott and Wasson, 1975). Additionally, the silicate-bearing iron meteorites commonly contain silicate inclusions, often of primitive, chondritic compositions (Bunch et al., 1970; Bild, 1977; Scott and Wasson, 1975; Wasson and Wang, 1986; Mittle- fehldt et al., 1998). The silicate-bearing irons are thus not believed to have experienced the same fractional crystallization process as the magmatic irons. The terms “magmatic” and “nonmagmatic” have been used as labels for the two group types (Wasson, 1985; Wasson and Wang, 1986), although such terms can be de- ceiving, as both types of iron meteorites are currently believed to have formed from metal that was molten at one time. The label “silicate-bearing” is consequently preferred to the term “nonmagmatic” by some (Mittlefehldt et al., 1998; Haack and McCoy, 2003). Regardless of the label, the distinction between the two types of groups is an important one, as the different types of groups appear to have formed by different processes. Theories for the formation of silicate-bearing iron meteorites are currently debated and include processes such as the crystallization of a S-rich metal (Kracher, 1982; McCoy et al., 1993), the breakup and Fig. 1. Data for all iron meteorites are plotted. Each “magmatic” reassembly of an incompletely differentiated parent body group exhibits limited variations in Ga and Ni, in contrast to the (Benedix et al., 2000), the partial separation of silicate and “nonmagmatic” irons. The ungrouped irons have a large range of metallic melts in the parent asteroid (Takeda et al., 2000), chemical compositions. Magmatic groups, believed to each rep- and the generation of molten metal in impact melt pools resent samples from a different asteroidal core, are labeled in the on chondritic asteroids (Choi et al., 1995; Wasson and figure. References for the iron meteorite compositional data are Kallemeyn, 2002). The rest of this chapter deals with mag- given in the text. Chabot and Haack: Evolution of Asteroidal Cores 749

Traditionally, element vs. element diagrams for iron meteorites were plotted against Ni (e.g., Scott, 1972), but recent studies have shown that due to the larger variation in Au within iron meteorite groups and the precision of Au measurements, Au is a better choice (Haack and Scott, 1993; Wasson et al., 1998; Wasson, 1999). Magmatic groups clus- ter in different areas on Figs. 1 and 3 due to the specific history each group experienced prior to crystallizing in an asteroidal core. Both nebular and planetary processes, such as condensation from the solar nebula, accretion of the asteroid parent body, oxidation, and core formation, may have affected the mean compositions of the groups (Scott, 1972; Haack and McCoy, 2003). The asteroidal cores sampled by magmatic iron meteorites thus had different initial bulk compositions. However, the similar elemental fractionation trends suggest a similar evolution of the asteroidal cores subsequent to their formation. The trends are attributed to the fractional crystallization of the initially fully molten metallic cores (Scott, 1972), and recent crystallization models are discussed in section 5.

2.3. Secondary Minerals

Fig. 2. The IID magmatic iron meteorite Carbo exhibits a well- Magmatic iron meteorites contain a number of second- formed Widmanstätten pattern characteristic of a medium octa- ary minerals, the most common of which are troilite (FeS), hedrite. Two troilite inclusions are also shown, the larger of which schreibersite ((FeNi)3P), graphite (C), cohenite ((FeNi)3C), is 38 mm long. Carbo specimen (#1990.143) was provided by the chromite (FeCr2O4), and daubréelite (FeCr2S4). These min- Geological Museum, University of Copenhagen. erals were recently described by Mittlefehldt et al. (1998). A comprehensive list of minerals in iron meteorites may also be found in Buchwald (1975). Mittlefehldt et al. (1998) on the width of the kamacite bands. Iron meteorites with lists a total of 36 minerals in iron meteorites, where 22 are high Ni contents (generally ≥15 wt%) do not begin to grow either rare or occur only in one or a few meteorites. With kamacite crystals until lower temperatures and consequently the exception of lonsdaleite and diamond, which probably any intergrowth texture is only microscopically visible; are a result of transforming graphite during crater-forming these meteorites have been termed ataxites. The metal in events on Earth, all these minerals are believed to have low-Ni iron meteorites (<6 wt%) also exhibits no visible formed within the core of the parent body. Several of the Widmanstätten pattern due to the metal consisting almost secondary minerals are common in some groups of iron mete- entirely of kamacite; the term hexahedrites refers to such orites but not in others. This is not only useful for classifica- meteorites. The growth of the Widmanstätten pattern is dis- tion purposes but also demonstrates that the bulk chemistry cussed in detail in section 3.4, as understanding its forma- differed from group to group. Two main trends are apparent. tion has led to information regarding the cooling rates of One is that S- and P-rich minerals, such as troilite and schrei- asteroidal cores. Figure 2 shows an example of the Widman- bersite, decrease in abundance from the original group I stätten pattern in the medium octahedrite IID magmatic iron through groups II, III, and IV. The second is that some meteorite Carbo. Numerous images and descriptions of the groups appear to have crystallized from a more-reduced different metallic structures observed in iron meteorites can core liquid than others. The best example is group IIAB, be found in Buchwald (1975). where graphite and cohenite are common but phosphates The vast majority of compositional information about the are never observed. The apparently most oxidized group is metal in iron meteorites comes from an ongoing series of IIIAB, whose meteorites contain phosphates (e.g., Olsen et papers by J. T. Wasson and colleagues regarding the chemi- al., 1999) but never graphite and only rarely cohenite. cal classification of iron meteorites (Wasson, 1967, 1969, 1970; Wasson and Kimberlin, 1967; Wasson and Schaudy, 2.4. Evidence from Cape York Irons 1971; Schaudy et al., 1972; Scott et al., 1973; Scott and Wasson, 1976; Kracher et al., 1980; Malvin et al., 1984; The IIIAB Cape York irons, with a recovered mass of Wasson et al., 1989, 1998). Additional compositional studies 58 tons, form the largest known meteorite shower and are are listed in the recent review of Haack and McCoy (2003). only exceeded in mass by the 60-ton IVB meteorite, Hoba. Figure 3 shows some elemental trends for the three largest The Cape York irons deserve special attention since they magmatic iron meteorite groups: IIIAB, IVA, and IIAB. are the only iron meteorites where chemical gradients of 750 Meteorites and the Early Solar System II

Fig. 3. The concentrations of (a) Co, (b) Ge, (c) Ir, and (d) W in the three largest magmatic iron meteorite groups, IIAB, IIIAB, and IVA, are plotted against the concentration of Au. Element vs. element plots, such as these and many others, are used to classify iron meteorites into different groups. The similar fractionation trends observed in the groups are attributed to the crystallization process experienced by all the groups in their parental asteroidal cores. Meteorite data provided by J. T. Wasson.

the parent core have been studied. Chemical analysis shows represented a mixing line between trapped melt and solid. that the Cape York irons are chemically diverse and form a Additionally, elongated troilite nodules with buoyant phos- chemical trend different from that of the other IIIAB iron phate at one end and dense chromite grains at the opposite meteorites (Esbensen and Buchwald, 1982; Esbensen et al., end seem to indicate the direction of the ancient gravity field 1982). The second largest recovered Cape York mass, the (Buchwald, 1971; Buchwald, 1975). The inferred ancient 20-ton Agpalilik, is itself significantly chemically zoned; a gravity field is perpendicular to that of the chemical gradi- 90-cm-long profile across a slice of Agpalilik has an Ir vari- ent and thus argues against a concentric growth pattern for ation of about 12% and a Au variation of about 4%. The the core akin to the crystallization of Earth’s core. These magnitude of the chemical gradient observed within the Ag- observations suggest that the structure of the crystallizing palilik mass suggests that the distance between the chemi- solid was complex, at least in the portion of the IIIAB core cal end members, Thule and Savik, in the original Cape where the Cape York irons crystallized. York meteoroid was on the order of 10 m, meaning that chemical gradients must have existed in the IIIAB core on 2.5. Relationships to Other Meteorite Classes a much smaller scale than the approximately 10-km radius of the entire core (Haack et al., 1990; Haack and Scott, For samples of an asteroidal core to reach Earth, the par- 1993). ent body was likely significantly disrupted. Unfortunately, For those elements where the Cape York trend deviates it seems that our meteorite collections contain surprisingly significantly from the general IIIAB trend, the scatter from few samples of the corresponding silicate portions of the the general trend by other IIIAB irons is also greater, sug- planetary bodies from which magmatic iron meteorites orig- gesting that the process responsible for the deviating Cape inated. York trends is also potentially responsible for the scatter in Pallasites, composed dominantly of mixtures of olivine the compositions of the other IIIAB irons (Haack and Scott, and metal, are commonly believed to be samples from the 1992; Haack and Scott, 1993). Wasson (1999) suggested core-mantle boundary of asteroid-sized parent bodies. The that the divergent trends observed among Cape York irons trace element composition of the metal in main-group pal- Chabot and Haack: Evolution of Asteroidal Cores 751 lasites overlaps with the concentrations measured in more- A few Hf-W studies have provided specific information re- evolved IIIAB irons (Scott, 1977; Wasson and Choi, 2003), garding the timing of the formation of metallic cores on the suggesting a genetic relationship. The O-isotopic compo- iron meteorite parent bodies. The short-lived 182Hf-182W iso- sitions of IIIAB irons and main-group pallasites are also tope system, with a half-life of 9 m.y., has been used to date consistent with being derived from a single parent body the relative timing of core formation in planetary bodies (Clayton and Mayeda, 1996). (Harper and Jacobsen, 1996). Due to its lithophile ten- Oxygen-isotopic abundances for mesosiderites and HED dency, Hf will remain in the silicate mantle during metal- (howardite, eucrite, diogenite) meteorites also fall into the silicate differentiation; in contrast, some portion of the mod- same grouping as main-group pallasites and IIIAB irons erately siderophile W will partition into the forming me- (Clayton and Mayeda, 1996). Mesosiderites, like pallasites, tallic core. If differentiation occurred when there was still are mixtures of metal and silicate, but the silicate compo- a substantial amount of live 182Hf, the metal would conse- nent is mineralogically more consistent with a crustal rather quently have a deficit of 182W relative to an undifferentiated than a mantle origin (Mittlefehldt et al., 1998). Analysis of chondritic value. the metal in mesosiderites revealed a composition similar All measurements of magmatic iron meteorites show a to that of IIIAB irons, but the small range of measured Ir deficit of 182W (Lee and Halliday, 1995, 1996; Horan et concentrations is inconsistent with the source of the metal al., 1998), indicating that the formation of the parent body for mesosiderites being a fractionally crystallized core (Has- cores occurred early in the history of the solar system, such sanzadeh et al., 1990). The HED meteorites are crustal igne- as within the first 30 m.y. (Yin et al., 2002; Kleine et al., ous rocks (Mittlefehldt et al., 1998), suggested to be samples 2002). Additionally, no resolvable W-isotopic variations from the large asteroid 4 Vesta (Consolmagno and Drake, between the groups have been detected. Thus, all the par- 1977; Binzel and Xu, 1993; Keil, 2002). If HED meteorites ent bodies underwent core formation within a very narrow are from Vesta, it seems that the main-group pallasites and time interval of <6 m.y. (Horan et al., 1998). Table 2 lists IIIAB irons must be from a different parent body than the the relative timing of core formation on four of the larger HED meteorites; having >200 samples of the central me- magmatic iron meteorite parent bodies, which show no re- tallic core of an asteroid would seem to require severe dis- solvable differences between the groups within the given ruption of the body. However, if Vesta is not the HED par- errors. Because early and late crystallizing members for ent body (Cruikshank et al., 1991), a genetic link between each group show no 182W differences, Horan et al. (1998) HED meteorites, main-group pallasites, and IIIAB irons concluded that less than 2% of late segregating metal could could be suggested by the O isotopes, although unrelated have been added to the core during crystallization without groups of meteorites with similar O isotopes do exist. The leaving a discernable effect on the W-isotopic ratio. This IVA irons have O-isotopic ratios that are distinct from any indicates that metal segregation to form the parent cores of known achondrite group (Clayton and Mayeda, 1996). iron meteorites was complete to very nearly complete prior Thus, while sampling at least 11 different asteroidal to the start of core crystallization. cores, our meteorite collections seem to be lacking the corresponding crustal and mantle materials for the same par- 3.2. Crystallization of Asteroidal Cores ent bodies. Intuitively, iron meteorites are more resistant to terrestrial weathering than stony meteorites, resulting in When metallic cores first formed, the cores were com- longer survival times on Earth. The strength of Fe-Ni metal pletely molten (e.g., McCoy et al., 2006). As the parent would also make it more durable than silicate materials in bodies cooled, the cores began to solidify by a fractional space, and the longer cosmic-ray exposure ages of irons crystallization process. Some isotopic systems have pro- than chondrites seem to support this statement (Voshage and vided information about how long it took for the cores to Feldman, 1979; Wasson, 1985). Bottke et al. (2000), study- crystallize. ing the Yarkovsky effect, found that stony meteorites are 3.2.1. Rhenium-osmium system. The best-understood transferred from the asteroid belt to the inner solar system information about the timing of core crystallization comes on much shorter timescales than iron meteorites, consistent from studies of the long-lived Re-Os isotopic system. Rhe- with the differences in exposure ages. Whether the Yarkov- nium-187 decays to 187Os with a half-life of 41.6 G.y. (Smol- sky effect, the greater strength in space, and/or the higher iar et al., 1996). Both elements are highly siderophile and resistance to terrestrial weathering is enough of an explana- are consequently concentrated in metallic cores during core tion to account for the overabundance of irons remains to formation. However, during core crystallization, Re and Os be thoroughly investigated. have slightly different tendencies for partitioning between the growing solid core and the remaining molten portion, 3. TIMESCALES OF CORE EVOLUTION creating a fractionation of the two elements that can be dated. 3.1. Formation of Asteroidal Cores The recent study of Cook et al. (2004) compiled and compared all the previous Re-Os work for the IIAB and The separation of metal from silicate to form the cen- IIIAB groups, finding good agreement between the results tral metallic cores of asteroid-sized bodies occurred early from the previous studies. Using the Mn-Cr age of IIIAB in the history of the solar system (e.g., Wadhwa et al., 2006). phosphates (discussed in section 3.3), Cook et al. (2004) 752 Meteorites and the Early Solar System II

TABLE 2. Summary of timescales of core evolution and implied parent body sizes.

IIIAB IIAB IVA IVB Relative timing of core formation, ∆t, 0 (+2.2, –1.9) 1.8 (+1.4, –1.3) –0.6 (+2.5, –2.1) 0 (+1.5, –1.4) relative to IIIAB formation (m.y.)* Timing of core crystallization (Ma) 4517 ± 28† 4546 ± 46† 4464‡–4606§ 4527 ± 29‡ Core cooling rate (K/m.y.) 49 (+38, –21)¶ 2** 200–1500†† 4300 (+4300, –2150)‡‡ Parent body radius (km)§§ 20–30 100 5–10 2–4 Disruption of the core (Ma)¶¶ 675 ± 100 450 ± 100 *Horan et al. (1998), 182Hf-182W chronometer, ±2σ. † Cook et al. (2004), 187Re-187Os chronometer, ±2σ. ‡ Smoliar et al. (1996), 187Re-187Os chronometer, ±2σ. § Shen et al. (1996), 187Re-187Os chronometer. ¶ Rasmussen (1989b), metallographic techniques, ±1σ. ** Randich and Goldstein (1978), modeling phosphide growth. †† Goldstein and Hopfe (2001) and Yang and Goldstein (2005), metallographic techniques. ‡‡ Rasmussen (1989a), metallographic techniques, ±1σ. §§ Haack et al. (1990), equation (1). ¶¶ Voshage and Feldmann (1979), cosmic-ray exposure ages, ±100 is the general estimated uncertainty of the method.

determined absolute ages of 4546 ± 46 Ma and 4517 ± body is not well established. However, although there are 28 Ma for crystallization of the IIAB and IIIAB cores re- contradictory results between the two studies, both suggest spectively. Thus, within the given errors, the timing of core that the IIAB and IVA cores crystallized within 100 m.y. of crystallization for the IIAB and IIIAB groups show no re- each other. solvable difference, although an earlier study by Smoliar et 3.2.2. Platinum-osmium system. Similar to the Re-Os al. (1996) on a more limited set of samples suggested other- system, Pt and Os are fractionated from each other due to wise. The IVB core is also found to have crystallized around differing partitioning behaviors during the crystallization of the time of the IIAB and IIIAB cores (Smoliar et al., 1996). the core. The long-lived isotope 190Pt decays to 186Os with Table 2 summarizes the crystallization ages. a half-life of about 470 G.y. (Begemann et al., 2001). Cook The ability for multiple meteorites from the same group et al. (2004) determined Pt-Os ages of 4323 ± 80 Ma for to define a single Re-Os isochron suggests that the amount the IIAB group and 4315 ± 29 Ma for the IIIAB group, ages of time for the core to evolve from completely molten to too young to be consistent with the corresponding Re-Os mostly solidified was relatively quick and not extended over data. Error in the value of the 190Pt decay constant was sug- tens of millions of years (Shirey and Walker, 1998). Isoch- gested as the most likely source for the discrepancy. In addi- rons determined from early-crystallizing IIAB irons and tion to the IIAB and IIIAB groups having Pt-Os ages unre- late-crystallizing IIAB irons were found to be indistinguish- solvable from each other, Pt-Os isochrons also showed no able within analytical uncertainties, suggesting crystalliza- evidence for age differences within either group. However, tion of the IIAB core was completed in less than 50 m.y. due to larger fractionations between Pt-Os than between Re- (Cook et al., 2004). Similarly, Cook et al. (2004) found no Os, future Pt-Os studies may place important constraints on evidence for resolvable age differences between the early- the timing of core crystallization. and late-crystallizing IIIAB members or for the IIIAB Cape 3.2.3. Palladium-silver system. In contrast to the Re- York shower. If resolvable Re-Os differences between mem- Os and Pt-Os systems, Pd-Ag is a short-lived isotopic sys- bers of the same group can be measured in the future, such tem: 107Pd decays to 107Ag with a half-life of 6.5 m.y. (Chen work has the potential to provide information about the rate and Wasserburg, 1996). The Pd-Ag chronometer reflects the of crystallization of the parent body cores. time of major chemical fractionation of Pd and Ag. How- In contrast to the studies of other iron meteorite groups, ever, it is unclear what event caused the major Pd-Ag frac- the work of Smoliar et al. (1996) and Shen et al. (1996) are tionation recorded in magmatic iron meteorites, with pro- in disagreement about the relative timing of the crystalliza- posed choices including nebular condensation, planetary tion of the IVA core. Of the ten IVA iron meteorites meas- body formation, and solidification of the parent body core ured by Smoliar et al. (1996), seven defined an isochron (Chen and Wasserburg, 1996). Regardless of the process, with a relative age about 70 m.y. younger than the IIAB all the measured groups showed evidence for excess 107Ag, irons; the three IVA meteorites that were inconsistent with indicating the presence of live 107Pd at some stage during the isochron suggested an older age. Shen et al. (1996) their formation and implying that the the Pd-Ag system is reported that the IVA irons recorded a crystallization age recording an early fractionation event (Chen and Wasser- that was older than the IIAB irons by about 60 m.y. Thus, burg, 1996, Chen et al., 2002). Additionally, Chen and Was- the relative timing of core crystallization on the IVA parent serburg (1996) noted that the time differences suggested by Chabot and Haack: Evolution of Asteroidal Cores 753 their Pd-Ag-isotopic data for the IIAB, IIIAB, IVA, and IVB and Olsen, 1991; Hutcheon et al., 1992). In the same IIIAB iron meteorite groups were within 12 m.y. of each other. meteorites, chromites, troilite nodules, and other types of Recently, a new technique involving multicollector plas- phosphates all had normal Cr-isotopic compositions, likely ma-ionization mass spectrometry has improved the preci- due to the Mn-poor nature of these phases. The presence sion of Ag-isotopic measurements, allowing such measure- of excess 53Cr indicates such phosphates formed early in ments to be made on samples with lower Pd/Ag ratios than the solar system, when 53Mn was still alive, and IIIAB Mn- previously possible (Carlson and Hauri, 2001). Currently, rich phosphates are estimated to have formed 7–27 m.y. the new technique has only been applied to one magmatic after the formation of Allende CAIs (Davis and Olsen, 1990; iron, the IIIAB iron Grant, with results that were consis- Hutcheon et al., 1992). tent with the previous work of Chen and Wasserburg (1996). With this advancement, future studies may yield more pre- 3.4. Cooling of Asteroidal Cores cise relative Pd-Ag ages. 3.2.4. Technetium-ruthenium systems. In these short- Information about the rate of cooling of metallic cores lived isotopic systems, 98Tc decays to 98Ru with a poorly following crystallization comes from understanding the constrained half-life of 4 to10 m.y., and 99Tc decays to 99Ru Widmanstätten pattern in magmatic iron meteorites. The with a half-life of just 0.21 m.y. Examining meteorites from Widmanstätten pattern develops as a two-phase intergrowth the IIAB and IIIAB groups, Becker and Walker (2003) of the low-Ni, body-centered cubic α-phase kamacite and found no signature from live 98Tc or 99Tc in any of the irons. the higher-Ni, face-centered cubic γ-phase taenite over the temperature range of about 800° to 200°C. 3.3. Formation of Secondary Minerals 3.4.1. Determining cooling rates. There are two main methods currently in use for determining metallographic The formation processes of secondary minerals in iron cooling rates. The taenite central Ni content method (Wood, meteorites are often not fully understood, and consequently, 1964) plots the central Ni content of measured meteoritic interpreting isotopic age studies of these inclusions can be taenite against the halfwidth of the taenite; this plot is com- difficult; it is not always clear what event the isotopic sys- pared to computer calculations, and meteorite data are ex- tem is recording. However, such studies can still provide pected to fall along one of the computed isocooling rate some constraints about the crystallization and post-crystalli- curves. The taenite profile-matching method (Goldstein and zation evolution of asteroidal cores. Ogilvie, 1965) works by matching the calculated Ni com- In addition to determining Re-Os isochrons for the metal, positional profiles in taenite obtained from computer simu- Shen et al. (1996) also dated a schreibersite-metal pair from lations with the Ni profiles measured in meteoritic taenite. a IIIAB iron and determined that the schreibersite became Other methods have also been previously used to deter- closed to the 187Re-187Os system 0.3–0.5 G.y. after the mine cooling rates, but subsequent work demonstrated that neighboring metal. Schreibersite grows through solid-state these methods were less accurate. The kamacite bandwidth precipitation and diffusion (e.g., Goldstein and Ogilvie, width method (Goldstein and Short, 1967; Narayan and 1963; Randich and Goldstein, 1975), and thus the Re-Os Goldstein, 1985) directly related the kamacite bandwidth age likely reflects nucleation and growth following crys- with the cooling rate, but the assumption that the diffusion tallization. distance between neighboring kamacite plates was infinite The 107Pd-107Ag system has been used to examine both was shown to be invalid in most cases (Saikumar and Gold- schreibersite and troilite nodules in iron meteorites (Chen stein, 1988). Similar to the methods involving taenite, cool- and Wasserburg, 1990, 1996). In IIIAB and IIAB irons, ing rates were also determined based on the Ni profile in excess 107Ag was measured in schreibersite but not in troi- kamacite or the kamacite central Ni content. However, the lite. IVA troilite showed measurable amounts of 107Ag, al- sensitivity of the methods based on kamacite rather than though in many of the IVA nodules, the isotopic ratios were taenite are much lower and can be insufficient to determine not consistent with 107Ag due to the in situ decay of 107Pd. cooling rates accurately to within even an order of magni- Cooling is not instantaneous for iron meteorites, and 107Pd tude (Hopfe and Goldstein, 2001). is a short-lived isotope. Consequently, Chen and Wasserburg Modeling the formation of the Widmanstätten pattern (1996) suggested that considerable diffusion could have requires knowledge about the relevant diffusion coefficients taken place that may have significantly affected the Pd-Ag and phase diagram. Interdiffusion coefficients for Ni in systematics, making any interpretation of the data difficult. kamacite and taenite were most recently determined by The short-lived isotope 53Mn decays to 53Cr with a half- Dean and Goldstein (1986). Over the temperature range of life of 3.7 m.y., and an advantage of the Mn-Cr system is 925°–550°C, the experimentally measured interdiffusion that it has been applied to CAIs in the primitive meteorite coefficients for Ni ranged from 10–12 to 10–18 cm2/s. The Allende as well as other specimens (Birck and Allegre, 1988). latest Fe-Ni phase diagram is shown in Fig. 4 (Yang et al., The Mn-Cr isotopic system thus can provide ages relative 1996). On the Fe-Ni phase diagram, the α + γ (kamacite + to the formation of the first material in the solar system. taenite) two-phase field was determined largely from the Evidence for excess 53Cr in Mn-rich phosphates of IIIAB experimental data of Romig and Goldstein (1980), who irons has been reported (Davis and Olsen, 1990; Hutcheon formed coexisting kamacite and taenite over the tempera- 754 Meteorites and the Early Solar System II

recently proposed that in low-P meteorites, the growth of kamacite follows the stabilization of the low-Ni bcc martensite phase. They suggested that the growth of kamacite, and hence the formation of the Widmanstätten pattern, will dominantly follow one of two reactions paths, to which they gave numerical labels: In Mechanism II, as the temperature drops, irons with higher P contents will reach P saturation and grow phosphides, leading to the nucleation of kamacite; in Mechanism V, irons with lower P contents will cross the martensite start temperature, forming martensite, which will subsequently transform to kamacite and taenite. Yang and Goldstein (2005) indicated that the critical P content that would determine which of the mechanisms was followed was dependent on the exact Ni content of the meteorite but generally ranged from about 0.2–0.3 wt% P. The taenite in iron meteorites is actually composed of multiple phases, due to a series of complex phase transformations that occur at or below 400°C (Yang et al., 1997a), Fig. 4. The subsolidus Fe-Ni phase diagram from Yang et al. as shown on Fig. 4. Yang et al. (1997b) observed an em- (1996) is shown. At high temperatures, taenite (γ) is the stable pirical relationship between the size of the “island phase,” phase. Given the bulk composition of most iron meteorites, as the found next to the outer taenite rim in meteoritic metal, and temperature drops, it is common for iron meteorites to encounter the metallographic cooling rate determined by the tech- γ α the + (kamacite) field, where the Widmanstätten pattern will niques just discussed. The “island phase” is composed of begin to form. Many of the phases shown below ~400°C are high-Ni ordered FeNi tetrataenite (γ" on Fig. 4) with a few metastable. The label MS refers to the low-Ni bcc phase marten- γ γ low-Ni bcc precipitates (martensite on Fig. 4), and Yang et site, 1 represents a low-Ni paramagnetic fcc phase, 2 represents γ al. (1997b) found the width of the island phase varied from a high-Ni ferromagnetic fcc phase, ' refers to ordered Ni3Fe, and γ 20 to 500 nm as a regular function of the cooling rate. Thus, "represents ordered FeNi. The label TC refers to the Curie temperature. the size of the island phase can potentially be used to estimate the cooling rate of a meteorite. Although not an inde- pendent method because it is fundamentally based on cooling rates determined by other metallographic techniques, ture range of 700°–300°C and measured the compositions such estimates are still valuable for assessing relative rates of both phases. In contrast, the other phase transformations within groups. of taenite below 400°C were determined by examining the 3.4.2. Cooling rates of magmatic irons. A metallic as- metallic regions of meteorites using high-resolution analyti- teroidal core is expected to be essentially isothermal, due cal electron microscopy techniques (Yang et al., 1996). to the much higher thermal conductivity of the Fe-Ni metal Although composed dominantly of Fe and Ni, magmatic than that of the overlying mantle. Consequently, if each iron iron meteorites also contain ~0.02–1 wt% P (Scott, 1972), meteorite group samples an asteroidal core, the cooling rates which can significantly affect the formation of the Widman- determined for all members of that iron meteorite group stätten pattern. The presence of P increases the diffusivity should be the same. The most recent estimate for the cooling of Ni (Dean and Goldstein, 1986) and decreases the extent rate of the IIIAB group is 49 K/m.y. (Rasmussen, 1989b). of the α + γ two-phase field (Romig and Goldstein, 1980). Although there is nearly an order-of-magnitude scatter However, the most important effect P has on the formation among the cooling rates of individual IIIAB irons, the lack of the Widmanstätten pattern could be in determining the nu- of a correlation with composition suggests that the results cleation process for the transformation. Experimental stud- are generally consistent with a single cooling rate and hence ies that began with taenite and were cooled to grow kama- with a core origin for the group. Additionally, Yang et al. cite found that the presence of phosphides was necessary to (1997b) found the island phases in four IIIAB iron meteor- form intragranular kamacite (Narayan and Goldstein, 1984, ites to be essentially the same size, also suggesting a com- 1985). In the presence of phosphides, the nucleation of ka- mon cooling rate. macite required little to no undercooling in the experiments. In IVB irons, due to the high Ni content, the Widman- Narayan and Goldstein (1985) suggested that intragranular stätten pattern did not start to form until a lower tempera- kamacite would not grow until the temperature had dropped ture than in other groups. Applying updated diffusion rate to a point where phosphides would form. and phase diagram data to the IVB study of Rasmussen et In iron meteorites with low P contents, P saturation and al. (1984), Rasmussen (1989a) concluded that the IVB data the subsequent formation of phosphides will not occur until were consistent within a factor of 2 with a cooling rate of quite low temperatures, if at all. Yang and Goldstein (2005) 4300 K/m.y. Although Rasmussen (1989a) used the kama- Chabot and Haack: Evolution of Asteroidal Cores 755 cite bandwidth method, which is generally considered inac- curate (Saikumar and Goldstein, 1988), the result may be correct, as the high Ni content results in large kamacite spacing, potentially making the assumption of infinite diffusion length between kamacite plates valid. Due to the low Ni content, taenite is only present in some IIAB meteorites, and the metallographic cooling rate methods can only be applied to these irons. Randich and Gold- stein (1978) developed a technique for estimating the cooling rate of low-Ni IIAB meteorites based on the width of phosphides and suggested that the measured IIAB irons were consistent with a single cooling rate of approximately 2 K/m.y. The higher Ni members of the IIAB group do have a Widmanstätten pattern to which metallographic cooling rate techniques can be applied, although no such study has been undertaken. In contrast to other magmatic iron meteorite groups, studies have debated whether different IVA meteorites experienced different cooling rates (Moren and Goldstein, Fig. 5. An application of the taenite profile-matching method for 1978, 1979; Willis and Wasson, 1978a,b; Rasmussen, 1982; determining the metallographic cooling rate is illustrated for the Rasmussen et al., 1995). Employing the latest Hopfe and IVA iron meteorite Bristol. The measurements by Massalski et al. Goldstein (2001) cooling-rate model, Goldstein and Hopfe (1966) show a characteristic “M”-shaped diffusion profile for Ni (2001) found that cooling rates for the IVA group varied across the taenite grain. Because the IVA meteorite Bristol is low from 200 to 1500 K/m.y. and, as found in previous studies, in P (0.06 wt%), the model of Yang and Goldstein (2005) is used to determine a cooling rate of 220 K/m.y. Model calculations per- were inversely correlated with the bulk Ni content of the formed by J. Yang and J. I. Goldstein. meteorites. Variable cooling rates within the IVA group are inconsistent with a simple core origin, although the chemical trends within the IVA group suggest such an origin. Because silicate inclusions in IVA iron meteorites show 3.5. Implications for the Size of the evidence of rapid cooling from high temperatures, Haack Parent Asteroids et al. (1996) proposed that the IVA parent body was catastrophically fragmented and subsequently reassembled after Modeling the thermal evolution of an asteroid-sized body the core was nearly crystallized but prior to the formation has led to estimates of the sizes of the parent asteroids of of the Widmanstätten pattern. Such an event would be con- different meteorite groups (e.g., Wood, 1979). Larger as- sistent with a variation in metallographic cooling rates, but teroids cool more slowly, resulting in slower cooling rates. offers no explanation for the apparent correlation between Compared to the overlying silicates, the metallic core has cooling rate and Ni content. In contrast, Yang et al. (1997b) an extremely high thermal conductivity. The outer silicate found that the size of the island phase in IVA iron meteor- layers thus exclusively control the cooling of the entire as- ites was nearly the same, implying a similar cooling rate teroid. Haack et al. (1990) demonstrated that the presence for all of the irons at temperatures around 300°C. Citing of an insulating regolith layer on the asteroid would espe- the constant island phase result, Wasson and Richardson cially influence the cooling rate of the asteroid’s interior. (2001) argued that the correlation with Ni content could Assuming a regolith thickness of 0.3% of the asteroidal ra- imply a systematic error in the cooling rate model rather dius, Haack et al. (1990) introduced an approximate rela- than a true variation in the IVA cooling rates. tionship between the radius of the asteroid and the cooling Applying the Yang and Goldstein (2005) model to the rate of its metallic core IVA meteorite Bristol, as shown in Fig. 5, a cooling rate of 220 K/m.y. is estimated, which is consistent with the 200– R = 149(CR)(–0.465) (1) 1500 K/m.y. range determined by Goldstein and Hopfe (2001). Currently, the Yang and Goldstein (2005) model has The radius of the asteroid, R, is in km, and the cooling rate, not been applied to any other IVA irons, so it is not pos- CR, is in K/m.y. Haack et al. (1990) also modified equa- sible to evaluate if the new calculation method will solve tion (1) to model the effects of varying mantle conductivi- the issue of variable cooling rates among IVA irons. Addi- ties, initial temperatures, concentrations of radioactive ele- tional work is needed before the cooling rates of IVA mete- ments, temperatures at which the cooling rate is measured, orites will be fully understood. Table 2 summarizes the most and, most crucial, regolith and/or megaregolith thicknesses. recent estimates for the cooling rates of the IIAB, IIIAB, Given that the potential thickness of an asteroidal regolith IVA, and IVB parental cores. layer is fairly unconstrained and that a number of factors 756 Meteorites and the Early Solar System II can influence the calculation, equation (1) provides just a 2002). The parent bodies of magmatic iron meteorites are rough estimate for the size of the parent asteroid. thus not compellingly linked to any asteroid type. Using equation (1) to estimate the parent body size to just one significant figure, the IIIAB cooling rate suggests 4. PHYSICS OF CORE CRYSTALLIZATION a parent asteroid with a radius of 20–30 km while the higher IVB cooling rate implies a smaller radius of just 2–4 km. 4.1. Heat Sources The limited IIAB cooling rate data suggest a parent body radius of slightly over 100 km. The current cooling rate data Although the presence of iron meteorites shows that for the IVA group are not consistent with a single cooling there was a heat source present in their parent bodies, no rate, and consequently equation (1) is not strictly applicable. evidence of the heat source(s) has been found in these mete- However, the range in IVA cooling rates suggests a parent orites. Three types of heat sources have been suggested: body with a radius of about 5–10 km. These estimates are short-lived radioactive heat sources like 26Al (Grimm and summarized in Table 2. The cooling rates of magmatic iron McSween, 1993, Bizzarro et al., 2004) and 60Fe (Tachibana meteorites are consistent with being cooled in the interiors, and Huss, 2003), electromagnetic heating (Herbert and not at the surfaces, of asteroids, as would be expected from Sonett, 1979), and impact heating (Rubin et al., 2001). a core origin, although a large range of parent body sizes is Long-lived radioactive heat sources, such as 235U, 238U, and suggested. 232Th, are not important for asteroid-sized bodies since their heat production within the geological lifetime of asteroids 3.6. Disruption of the Parent Asteroids is insignificant (Haack et al., 1990; Ghosh and McSween, 1998). The role of impact heating has also been questioned, The presence of material from asteroidal cores in our since impacts large enough to contribute significantly to the meteorite collections is evidence for the fragmentation and heating of a small body were also calculated to be suffi- break-up of the parent asteroids. The IIIAB core seems to ciently energetic to catastrophically disrupt it (Keil et al., be compositionally well-sampled by the >200 IIIAB irons, 1997). The only heat source that could potentially heat implying that the former asteroidal core has been com- the core directly is 60Fe, although with a half-life of only pletely disrupted. While in space, materials are exposed to 1.5 m.y., any live 60Fe had likely decayed prior to the onset cosmic rays, and therefore cosmic-ray exposure ages indi- of crystallization in the core. cate how long a sample has been in space within about 1 m of the surface (e.g., Eugster et al., 2006). Nearly all 41K/ 4.2. State of the Core Prior to Crystallization 40K cosmic-ray exposure ages of IIIAB iron meteorites clus- ter around 675 ± 100 Ma (Voshage and Feldmann, 1979). Under the assumption that asteroidal heat sources were The similar age implies that the IIIAB irons are from a short-lived and became extinct a few million years after single parent body that was catastrophically disrupted into accretion, heating times are short compared to the charac- meter-sized pieces about 650 m.y. ago by a single event teristic thermal diffusion time, t = L2/D, for a differentiated (Keil et al., 1994). Similarly, the large majority of IVA irons asteroid. If the characteristic length scale, L, is 20 km (i.e., have 41K/40K cosmic-ray exposure ages of 450 ± 100 Ma the depth of the mantle) and the thermal diffusivity, D, for (Voshage and Feldmann, 1979), also implying catastrophic the mantle is 4.8 × 10–7 m2/s (Haack et al., 1990), the char- fragmentation of the parental core in a single event. Table 2 acteristic thermal diffusion time becomes 25 m.y., which summarizes the cosmic-ray exposure ages. It is unknown is much longer than the heating time of short-lived heat if the IIIAB and IVA parent asteroids still retained any over- sources. Since thermal diffusion is a slow process compared lying silicate layers at the time of the catastrophic events to the heating timescale, the temperature distribution of that disrupted the metallic cores. asteroids was controlled by the distribution of heat sources. In contrast to the IIIAB and IVA groups, the IIAB and If 26Al was the dominating heat source, the surface may IVB irons exhibit a wide range of cosmic-ray exposure ages have been warmer than the interior of the asteroid, due to and hence were likely produced by numerous, separate col- the concentration of Al in crustal basaltic melts. The core lisions in the last billion years (Voshage and Feldman, may have been the coldest part of the asteroid since it con- 1979). It may thus be expected that remnants of central tained no 26Al and thus may not have been heated, unlike metallic cores are currently present in the asteroid belt. Due the mantle, following core formation. A partially or fully to the relatively featureless spectra of Fe-Ni, iron meteor- molten mantle would have been convecting, suppressing any ites have been commonly linked to M-type asteroids, al- mantle temperature gradients and accelerating cooling of though the M-type spectral signature is also consistent with the asteroid. The onset of mantle crystallization would in- that of other meteorites; additionally, M asteroids do tend hibit convection, and the thermal gradient across the mantle to have higher radar albedos, as would be expected from a would then increase. metallic composition, but groundbased measurements in- Cooling of the core could not begin until a thermal gradi- dicate that the bulk densities may be too low to be Fe-Ni ent had developed across the mantle in response to cooling metal without considerable porosity (e.g., Burbine et al., at the surface. Depending on the size of the parent body, Chabot and Haack: Evolution of Asteroidal Cores 757

this would probably take a few tens of millions of years. lateral temperature variations at the base of the mantle could Although the gravity is a modest 0.1 m/s2 at the surface of have been coupled with the topographic relief. If such tem- a 50-km-radius asteroid, core convection is still expected perature variations were at least a few degrees (Haack and to be vigorous at this stage. For a 20-km-radius core, the Scott, 1992), they could have dominated over the tempera- Rayleigh number is 1019, which is 15 orders of magnitude ture variation in the core. above the critical value (Haack and Scott, 1992). This would lead to thorough stirring of the molten core for a very long 4.3. The Significance of Sulfur during time, and it is therefore reasonable to assume that the me- Core Crystallization tallic liquid was homogeneous prior to the onset of crystallization. The single most important element in controlling the With a well-stirred molten core, the core’s vertical tem- structure and chemistry of the growing solid in the core is perature gradient can be calculated. The temperature gra- S. Sulfur is almost insoluble in solid metal (Willis and dient can influence if the onset of crystallization is at the Goldstein, 1982) and consequently becomes concentrated center or at the top of the core. The only planetary body in the liquid during crystallization. As a result of their lower for which we can presently study the macroscopic charac- densities, S-bearing liquids are buoyant relative to Fe-Ni- teristics of ongoing core crystallization is Earth, where the rich fluids. The convection pattern in the core following the core crystallizes outward from the center. The reason for onset of crystallization was therefore likely controlled by the outward growth of Earth’s core is the increase in pres- the distribution of S rather than temperature differences. sure across the core from 135.7 GPa at the core-mantle As the liquid S concentration increases from a few weight interface to 363.9 GPa at the center of the core (Dziewonski percent to about 30 wt% at the eutectic composition, the and Anderson, 1981). The pressure-induced increase in the freezing point of the liquid decreases by more than 500 K. liquidus temperature of the metallic liquid is greater than Crystallization will thus be enhanced in regions with lower the actual increase in temperature from the core-mantle S concentrations relative to areas with higher S contents. boundary to Earth’s center. In comparison, the pressure at Again, this effect is much more significant than any effects the center of even a 100-km-radius asteroid is less than from the <1 K temperature variations that were likely pres- 0.1 GPa, and the pressure variation across the core is only ent in the core. The distribution of S in the core therefore ~0.025 GPa. Since the liquidus of iron increases by 0.027– controls where the crystallization takes place and the con- 0.030 K/MPa (MacLachlan and Ehlers, 1971), the total vection pattern in the remaining fluid. pressure-induced variation in liquidus temperature across Unfortunately, it is difficult to determine the S concen- an asteroidal core is less than 0.8 K. The actual temperature tration in the core prior to the onset of crystallization. Due variation across the core must follow an adiabatic profile to the low solubility in solid Fe-Ni, S in iron meteorites is if the liquid is well stirred. This gives a temperature gradi- almost exclusively found in heterogeneously distributed ent of 0.034 K/MPa (Haack and Scott, 1992) and a total troilite nodules. This makes it difficult to constrain the con- variation of up to 0.85 K in the core of a 100-km-radius centration of S in the original liquid. Since S can have a asteroid, which is slightly more than the pressure-induced significant effect on the partition coefficients between liq- variation in liquidus temperature. uid and solid metal, it is possible to indirectly estimate the Given the nearly isothermal state of the core, the first S content of the liquid by adjusting the S concentration until solid to crystallize was probably at the only place where an acceptable match to the iron meteorite elemental frac- nucleation sites were available: the base of the mantle. The tionation trends is achieved. This is discussed in section 5. first phase to nucleate for a typical core composition is the The models suggest that S was present in most cores at the γ-taenite phase. Phosphides and other minerals, which could weight percent level, making S one of the three most abun- have acted as nucleation sites for the metal, formed as sec- dant elements in the cores. ondary phases (Doan and Goldstein, 1970; Sellamuthu and A significant problem in our understanding of the role of Goldstein, 1983). The nucleation of metal at the base of the S is the large discrepancy between the amount of eutectic mantle could have been instantaneous when the liquidus liquid predicted by the models and the almost complete temperature was reached, whereas the absence of nucleation absence of samples of the solidified S-rich melt in the form sites in the central parts of the core may have required the of meteorites. Although S-rich meteorites are probably more metallic liquid to become undercooled before crystallizing. fragile and ablate more readily during atmospheric entry Another factor that could have controlled the initial crys- (Kracher and Wasson, 1982), it remains a mystery why we tallization is the shape of the core. The core cavity may not see so few in our collections. have had time to become perfectly spherical prior to core crystallization, in particular since the driving force, gravity, 4.4. Dendritic or Concentric Growth? was small. If core crystallization commenced a few million years after core formation, Haack and Scott (1992) found Dendritic growth is a consequence of insufficient mix- that the core-mantle boundary was likely to have topography ing of the liquid. During crystallization, the boundary layer on the order of tens of meters. If the mantle was convecting, liquid adjacent to the crystallizing solid becomes enriched 758 Meteorites and the Early Solar System II in incompatible elements, and if these elements are not crystallization acts as a buffer for the temperature of the removed, crystallization within the boundary layer will be core and results in a steady-state situation where the temper- halted. This effect is known as constitutional supercooling. ature drops off linearly with decreasing depth in the mantle. Liquid diffusion may prevent constitutional supercooling if Therefore, it does not seem possible for any metallic liq- the characteristic diffusion length, L = (D ∆t)1/2 (where D uid that accreted to the surface at a late time to subsequently is the liquid diffusivity and ∆t is a characteristic timescale travel all the way to the core. Also, since the mantle is cool- for crystallization), is shorter than the crystal growth scale ing, it seems unlikely that any metal in the mantle that had in the same time interval. Assuming a D of 10–9 cm2/s (Gei- not already drained down to the core should do so at this ger et al., 1975) and a timescale of 10 m.y. yields a char- stage. acteristic diffusion length of only 17 cm, which is orders of magnitude less than the characteristic length scale for 4.6. Trapped Melt crystal growth in the same time interval. Liquid diffusion is therefore too slow to prevent the formation of a consti- The ubiquitous presence of troilite nodules in even the tutionally supercooled boundary layer, although it cannot earliest irons to crystallize seems to require that pockets of be ruled out that convective mixing may suffice. The bound- melt were trapped (Esbensen et al., 1982; Haack and Scott, ary layer effect will be more pronounced in areas where the 1993; Wasson, 1999). If the trapped melt had the same crystallizing front is concave rather than convex. In con- composition as the bulk core liquid, it should crystallize just cave areas, the liquid volume is smaller relative to the sur- like the rest of the core and thus produce similar fraction- face area of the growing solid and removal of the boundary ation trends. However, if the dimensions of the trapped liquid by convection is less efficient. Only that portion of volume were small enough, solid state diffusion may have the solid that extends through the constitutionally super- erased these trends resulting in a volume of solid metal with cooled boundary layer, such as the tips, will be able to grow. the same composition as the original liquid. Assuming a This will eventually lead to a highly complex solid growth diffusivity of 10–4 m2/s for Ni at 1300°C (Dean and Gold- front known as a dendritic structure. stein, 1986), the characteristic diffusion length in a 10-m.y. Empirical relationships between dendrite dimensions and time interval is 5 m. Therefore, if the growing solid formed cooling rates suggest that kilometer-sized dendrites existed isolated pockets with dimensions <10 m, it seems likely that in asteroidal cores (Narayan and Goldstein, 1982). In the we should see geochemical effects from trapped melt. If the case of inward growth of the core, the S-enriched boundary core crystallized as a highly complex dendritic structure, it layer liquid would concentrate in the upper parts of the core may be expected that melt was trapped between dendrites, due to its buoyancy, further promoting dendritic growth. in particular during the later stages of crystallization. These Dendrites extending deep into the core would grow in liquid pockets of trapped melt could have spanned a wide range lower in S and consequently may have crystallized faster of dimensions, all the way from sub-meter- to kilometer- than solids higher up. The deepest parts of the core-mantle sized. Eventually, as the dimensions of the residual melt interface would have been the most likely nucleation sites pockets became sufficiently small, all traces of the crystal- for the earliest dendrites, especially if such locations were lization process could be erased by solid-state diffusion. associated with cool, mantle downwelling (e.g., Fig. 4 of The best examples of geochemical trends caused by Haack and Scott, 1992). During crystallization, the S-rich trapped melt are probably the Cape York irons. Esbensen liquids produced would rise and accumulate between these et al. (1982) and Wasson (1999) suggested that the diver- protruding parts of the mantle, thereby halting crystalliza- gent compositional trends, defined by the fragments from tion in the uppermost portions of the core. the Cape York shower, form a mixing line between coexisting solid and liquid in the crystallizing IIIAB core, the 4.5. Assimilation of Liquid during modeling of which is discussed in section 5.8. The obser- Crystallization vation that fragments plotting closer to the inferred liquid composition are also richer in troilite speaks in favor of the The idea of continued addition of metallic liquid to the model (Wasson, 1999). core during crystallization was originally introduced in order to explain an apparent leveling off of the Ir vs. Ni trend for 4.7. Different Modes of Crystallization groups IIAB and IIIAB (Pernicka and Wasson, 1987; Mal- vin, 1988). The model of Malvin (1988) is discussed in more Although we will argue that inward dendritic growth is detail in section 5.5. It is an open question whether the level- the most likely mode of crystallization, no alternative can ing of Ir concentrations in the more evolved iron meteorites be ruled out conclusively in the absence of any direct ob- of these groups represents a genuine trend representative servations of asteroidal cores. Given the evidence that iron of the entire core, an artifact of poor sampling, or a local meteorite parent bodies constituted a highly diverse group in sequence of meteorites similar to the Cape York irons. terms of size, peak temperature, bulk chemistry, or impact/ Physically, a problem with continued assimilation is that collision history (to name just a few parameters for which during core crystallization, the temperature in most of the we may have some constraints), it should also be pointed mantle had already dropped below the freezing point of the out that the mode of crystallization may not have been the metallic liquid. Release of latent heat from the core during same for all parent bodies. Two different approaches have Chabot and Haack: Evolution of Asteroidal Cores 759

There are three arguments against this type of crystallization. Outward concentric growth requires that the solid nucleates at the center of the core where there are no nucleation sites available. Without nucleation sites, this would likely not be possible until the core was significantly undercooled, and central crystallization may therefore not have taken place until significant dendritic growth from the base of the mantle had already occurred. However, it cannot be ruled out that, due to impacts or other means, dendrites or other material may have been dislodged from the ceiling and settled to the center of the core, providing potential nucleation sites. Second, pure outward crystallization could stir the liquid so efficiently that we should expect that the liquid remained well-mixed and thus that the solid crystallized along a well-defined chemical trend. This is inconsistent with the observation that all iron meteorite groups have members that plot far from the average trend. Also, due to efficient mixing, crystallization at the colder, outer regions of the core would become more favorable. This would probably lead to a situation where crystallization was partially in the form of both outward concentric and inward dendritic growth. Third, concentric crystallization is inconsistent with Fig. 6. Cross sections of differentiated asteroids undergoing dif- the large gradient of the chemical zoning observed in the ferent modes of core crystallization are depicted: (a) Outward Cape York irons. The Cape York chemical gradient is orders concentric growth akin to the ongoing crystallization of the Earth’s of magnitude greater than that expected for concentric crys- core (section 4.7.1.). (b) Inward concentric growth (section 4.7.3.). tallization of an asteroidal core; in addition, the chemical (c) Outward dendritic growth (section 4.7.2.). (d) Inward dendritic gradient is horizontal, not vertical as expected for concen- growth (section 4.7.4.). Mantle and crust are shown as hatched tric growth. regions, while liquid metal is white and solid metal is black. Re- 4.7.2. Outward dendritic growth. Although it is pos- drawn from Haack and Scott (1992). sible that a fraction of the growth was outward from the center of the core, it seems unlikely that it would be dendritic. The buoyant boundary layer liquid produced at the been used in the past to constrain the crystallization mode center of the core would readily rise, which would lead to of asteroidal cores. One approach is to use the iron meteorite enhanced convection, thus creating good conditions for data to construct models that are able to match the observed concentric growth. It may be argued that at the center of compositional features. Another approach is to model the the core, the gravity is zero and the buoyant liquid was thus physics of asteroidal core crystallization and then evaluate unable to rise. However, the gravity increases linearly with how well the resulting model reproduces the observed prop- distance from the center, and the volume with near-zero erties of iron meteorites. In this section, we take the latter gravity is insignificant. Also, convection patterns driven by approach and base much of the following discussion on a thermal and compositional differences likely incorporated study of the physics of core crystallization (Haack and the central liquid as well. Scott, 1992). Figure 6 illustrates the different modes of crys- 4.7.3. Inward concentric growth. As already argued, tallization that we discuss. the initiation of crystallization was likely at the base of the 4.7.1. Outward concentric growth. Although we have mantle. At this stage, the base of the mantle probably pos- argued that the onset of core crystallization was probably sessed topography as well as lateral temperature variations not at the center of the core, we cannot rule out that out- related to the convection pattern in the mantle. Therefore, ward crystallization became important later. This could be crystallization may not have resulted in a uniform layer of the case if the liquid in the outer regions of the core was solid metal covering the entire inner surface of the mantle enriched in S during the course of crystallization. The but rather may have resembled isolated patches of metal boundary layer liquid next to the crystallizing solid would crystallizing on the coldest parts of the mantle. The onset be enriched in S and thus rise toward the base of the mantle. of crystallization would increase the S concentration of the In the case of outward concentric growth, the rising S-rich liquid under the base of the mantle. Since this liquid is liquid would lead to enhanced convection and very efficient buoyant, efficient mixing with the bulk core liquid may have stirring of the liquid. Since the fractional crystallization been difficult and inward concentric growth is therefore in- trends observed within iron meteorite groups indicate that consistent with the observation that all magmatic groups the metallic liquid was fairly well stirred during crystalli- crystallized from initially well-mixed magmas. zation, this has been used as an argument for outward crys- 4.7.4. Inward dendritic growth. This has been sug- tallization (Esbensen et al., 1982). gested as the most likely mode of crystallization (Esbensen 760 Meteorites and the Early Solar System II and Buchwald, 1982; Sellamuthu and Goldstein, 1983; drites, mixing of early crystallized metal with late stage Haack and Scott, 1992). As discussed, the initial crystalli- melt, and dragging minor pieces of the mantle into the core zation was likely at the base of the mantle and resulted in by dendrites broken off from the base of the mantle. an increase in the S content of the local liquid, which may have been difficult to efficiently remix with the bulk core 4.8. Late-Stage Crystallization: The Role liquid. Thus, the increasing S concentration would lower of Liquid Immiscibility the liquidus of the melt, potentially quickly inhibiting further crystallization in the upper parts of the core. Crystalli- The last stages of crystallization of the core are by far zation at the deepest parts of the solid would therefore be the least understood. The large scatter in the compositional enhanced relative to the upper parts, leading to a situation trends suggest that crystallization was quite far from ideal where the growth was dominantly at the tips of dendrites fractional crystallization during this time, we lack S-rich extending deep into the core. The liquid in the upper parts meteorites from this late period, and liquid immiscibility of the core, between the dendrites, would be enriched in S could have played a significant but poorly understood role and thus lighter than the central liquid below the deepest during this stage. The Fe-S-P phase diagram predicts that dendrite. As the dendrites reach the central parts of the core, at high S and P concentrations, the metallic liquid will enter liquid enriched in S would rise along the dendrites, forcing a two-phase field where the liquid splits into a S-rich liquid core-wide mixing (Haack and Scott, 1992). At a later stage, and a P-rich liquid (Raghavan, 1988). It has been hypoth- the entire core may have been transformed into a network esized that the size of the liquid immiscibility field may be of dendrites separated by residual liquid. Eventually, these influenced by the oxygen fugacity (Malvin et al., 1986), and dendrites would inhibit core-wide mixing and create pools experiments have suggested that for conditions relevant to of trapped melt. the crystallization of iron meteorites, the size of the liquid Inward dendritic growth seems consistent with the iron immiscibility field may be decreased from that given by the meteorite data. The observed increasing scatter of the Fe-S-P phase diagram (Chabot and Drake, 2000). However, chemical trends as crystallization proceeds is to be expected if a complex network of dendrites develops across the core, inhibiting core-wide mixing and facilitating the trapping of melt in pockets between dendrites. The large horizontal chemical gradients in the Cape York irons are also consistent with inward dendritic growth. The larger surface area of the solid in the case of dendritic growth results in chemical gradients over shorter distances; the entire chemical evolution of the core could have been recorded in a single dendrite cross section if the dendrite had grown slowly since the onset of crystallization. However, the compositional trends recorded across a growing dendrite are not expected to differ significantly from the average trend recorded throughout the crystallizing core, and the deviation of the Cape York compositional trends from those of the average IIIAB group has been suggested to be due to melt trapping during core crystallization (Wasson, 1999). Although growing dendrites are not the only way to trap melt, a complex solid structure, such as the one expected from inward dendritic growth, could lead to increased amounts of trapped melt. If main-group pallasites are samples of the core-mantle boundary of the IIIAB core, their metal composition should allow us to determine when solid metal formed at the base of the mantle. Assuming that the core crystallized inward from the mantle, we would expect the metal in main-group Fig. 7. A large liquid immiscibility field exists in the Fe-Ni-S- pallasites to be similar to the IIIAB iron meteorites repre- P system, shown by the unshaded region. The size of the liquid senting early crystallization. In actuality, main-group pallas- immiscibility field determined by experiments examining the crys- ites contain Ni-rich metal that is consistent with forming tallization of iron meteorites [solid line (Chabot and Drake, 2000)] is smaller than the two immiscible liquids field given by the Fe- after about 80% of the IIIAB iron meteorites had crystallized S-P phase diagram [dashed line (Raghavan, 1988)]. However, in (Scott, 1977; Haack and Scott, 1993; Wasson et al., 1999; either case, the IIAB, IIIAB, and IVA groups may have eventu- Wasson and Choi, 2003). Possible scenarios that have been ally encountered liquid immiscibility during their crystallization, suggested to reconcile inward growth with the pallasite depending on the S-content estimates used for the modeling of compositions include crystallizing the pallasites at a late each group. References for the experimental data and iron mete- stage from pockets of S-rich liquid trapped between den- orite crystallization paths are given in Chabot and Drake (2000). Chabot and Haack: Evolution of Asteroidal Cores 761 as shown in Fig. 7, with either of the two-phase field bound- small fraction and determining the composition of the solid aries, the IIAB, IIIAB, and IVA groups may still be ex- metal (CS) and the remaining metallic liquid in the core pected to have encountered liquid immiscibility during their (CL). The composition of the remaining metallic liquid then evolution, although this is dependent on the S contents of becomes the initial liquid composition (Ci) for the next solid their cores, estimates of which are discussed in section 5. metal to form, and the calculations are repeated, resulting Liquid immiscibility may have eventually occurred in the in fractional crystallization trends. large majority of asteroidal cores, as P and S are both incompatible and thus become increasingly concentrated in 5.2. Experimental Partition Coefficients the liquid as crystallization proceeds. The effects from liquid immiscibility were potentially quite important for the As is clear from equations (2) and (3), how an element IIAB group, which may have been rich in S and P and very will fractionate during crystallization of an asteroidal core reduced. is dependent on the element’s partition coefficient, D. The The resulting effects of liquid immiscibility depend partition coefficient is simply the ratio of the concentration strongly on how well the core liquid remained mixed dur- of the element in the solid metal divided by the concentra- ing continued crystallization. If the P- and S-rich liquids tion of that element in the liquid metal. Early modeling of remained in equilibrium with each other, the solid crystal- iron meteorite crystallization trends used values for the lizing from the two liquids would be different from the solid partition coefficients that were constant throughout the crys- crystallizing in a situation where there was not liquid im- tallization process (e.g., Scott, 1972). However, experimen- miscibility, but the effect would be fairly minor. However, tal determinations of partition coefficients demonstrated that if the two liquids, which have very different densities, be- the concentrations of S and P in the metallic liquid can dra- came isolated from one another, the solids crystallizing from matically affect the value of D (Willis and Goldstein, 1982; the two different liquids would follow divergent trends. The Jones and Drake, 1983). effects of liquid immiscibility on the IIIAB trend, as mod- Numerous experimental solid metal-liquid metal parti- eled by Ulff-Møller (1998), are discussed in section 5.7. tioning studies have been conducted in the last 25 years. Having disequilibrium between the two immiscible liquids All the equilibrium experimental data for Ag, As, Au, Co, may not have been unlikely toward the end of crystalliza- Cr, Cu, Ga, Ge, Ir, Ni, Os, P, Pd, Pt, Re, and W were re- tion, especially if the structure of the solid was highly com- cently compiled in Chabot et al. (2003) and Chabot and plex and core-wide mixing was inhibited. Lacking good Jones (2003). Malvin et al. (1986) demonstrated that some constraints in the form of iron meteorites representing the early experiments that employed a temperature gradient to last stages of crystallization, we seem to be quite far from a attempt to simulate fractional crystallization (Sellamutha good understanding of the last, complex portion of the crys- and Goldstein, 1983, 1984, 1985a,b) did not produce parti- tallization process. tioning results consistent with the equilibrium studies; although consequently excluded from the compilations, these 5. MODELING CORE CRYSTALLIZATION studies also illustrated the important influence of metallic liquid composition. Additionally, experimental values for 5.1. Fractional Crystallization D(Ru), D(Rh), D(Pb), D(Tl), D(Sb), D(Mo), and D(Sn) have also been determined (Fleet and Stone, 1991; Jones Asteroidal cores seem to have solidified by fractional et al., 1993; Jones and Casanova, 1993; Liu and Fleet, crystallization, accounting for the well-defined elemental 2001). trends observed within the magmatic iron meteorite groups, During the fractional crystallization of an asteroidal core, such as shown in Fig. 3. The process of fractional crystal- some P will partition into the solid metal, but S, which is lization can be modeled using simple mass balance equa- effectively excluded from the crystallizing solid, will be tions concentrated in the metallic liquid. Consequently, the S content of the molten portion of the core will increase as fractional crystallization proceeds. It is thus necessary to account [D(E)][C (E)] C (E) = i (2) for the compositional dependency of D when modeling the S + [1 – f fD(E)] crystallization of iron meteorites. Jones and Malvin (1990) developed a parameterization method for expressing D mathematically as a function of the S and P contents of the = CS(E) CL(E) (3) metallic liquid, based on the theory that the availability of D(E) Fe domains in the metallic liquid is the dominant influence on the partitioning behavior. Recently, this parameterization Ci(E) is the initial concentration of an element E in the method was revised slightly by Chabot and Jones (2003). molten portion of the core, while CS(E) and CL(E) are the Figure 8 shows how the S content of the metallic liquid concentrations in the crystallizing solid and remaining liq- affects D(Au), D(Ge), and D(Ir), three elements commonly uid respectively. The variable f refers to the fraction of solid measured in iron meteorites. The partition coefficient for metal formed, and D(E) is the solid metal-liquid metal par- Ir changes by about 3 orders of magnitude from the S-free tition coefficient. Modeling can be done by setting f to a system to a composition near the Fe-FeS eutectic. Although 762 Meteorites and the Early Solar System II

ments of Ga and Ge exhibit uniquely curved crystallization trends. Consequently, here, different crystallization models for the IIIAB parent body core are compared by examining trends involving Ir, Ge, and Au. Other elements, such as Pt, Re, and Os, also offer worthwhile challenges for modeling (Cook et al., 2004). In simple fractional crystallization, the fundamental assumption is that even though the composition of the molten portion of the core changes during crystallization, the metallic liquid quickly restores a single homogeneous composition through vigorous convection. The simple fractional crystallization model has been applied to the IIIAB group by Willis and Goldstein (1982), Jones and Drake (1983), Haack and Scott (1993), Chabot and Drake (1999), and Chabot (2004), with the difference being simply the amount of experimental partitioning data available at the time of each study. As shown in Fig. 9, the simple fractional crystallization model is able to explain the 3-orders-of-magnitude Ir fractionations and the curved Ge trend. However, the simple fractional crystallization model fails to reproduce the observed Ge vs. Ir IIIAB trend. Specifically, the later crystallizing members of the IIIAB group appear to have higher Ir contents than predicted by the simple fractional crystallization model. The S content of the parent body core cannot be determined through direct measurements of iron meteorites, Fig. 8. Experimental solid metal-liquid metal partition coeffi- since S is excluded from the crystallizing solid. However, cients (D) for three siderophile elements commonly measured in the partition coefficients depend strongly on the choice of iron meteorites, Au, Ge, and Ir, are shown as an example of the the S content of the metallic liquid. For the IIIAB group, significant effects the S content of the metallic liquid can have the best fits are obtained with a S content of about 12 wt% S on the partitioning values. The fitted curves are from the param- (Chabot, 2004). With an initial S content of 12 wt%, the eterizations of Chabot and Jones (2003). Experimental partition- trends in Fig. 9 are matched by about the first 55% of the ing data are also compiled in Chabot and Jones (2003). core to crystallize. After 61% of the core has solidified, the composition of the remaining metallic liquid is that of the Fe-FeS eutectic. The simple fractional crystallization model the absolute change in D(Ge) is less than that of D(Ir), thus implies that IIIAB iron meteorites only sample slightly D(Ge) exhibits the interesting behavior of switching from over half of the parent body core. It has been suggested that incompatible (D < 1) to compatible (D > 1) in solid metal the weaker nature of any S-rich meteorites would result in with increasing S content of the metallic liquid. The parti- a sampling bias in the meteorite collections (Kracher and tion coefficients in Fig. 8 all increase with increasing S con- Wasson, 1982), although such an idea is difficult to test. tent, but some elements, such as Ag, Cr, and Cu, are known Simple fractional crystallization cannot explain the ob- to be chalcophile and have experimental partition coeffi- served scatter around the general IIIAB trends in Fig. 9, cients that decrease with increasing S content. scatter that is greater than the analytical uncertainty in the measurements (Wasson, 1999). The IIIAB elemental trends 5.3. Simple Fractional Crystallization Model are generally consistent with being due to fractional crystallization of an asteroidal core, but the crystallization pro- With >200 members and a wide range of compositions, cess appears to have been more complex than just simple the IIIAB group is a good choice for comparing different fractional crystallization. models of asteroidal core crystallization. Data are available for many elements in iron meteorites, and any successful 5.4. Dendritic Crystallization Model crystallization model must account for all the elemental trends. However, some elements are not as severely frac- As discussed in section 4, it has been proposed that aster- tionated by core crystallization as others, and a wide range oidal cores solidified through the growth of large dendrites of conditions can explain their trends. Highly siderophile rather than from a planar crystallization front. The dendritic elements, such as Ir, have concentrations that span more crystallization model of Haack and Scott (1993) suggested than 3 orders of magnitude in the IIIAB group. The ele- that light S-rich liquid could accumulate preferentially at Chabot and Haack: Evolution of Asteroidal Cores 763 the top of the core or in structural traps formed by the growing dendrites, effectively removing that liquid from the crystallizing system. Mathematically, the model of Haack and Scott (1993) is identical to simple fractional crystallization except for the value used for D(S). To mimic the loss or trapping of S-rich liquid due to the growing dendrites, Haack and Scott (1993) used values for D(S) that ranged from 0.6 to 0.8; a D(S) of ~0.01 is generally used to reflect the very limited solubility of S in Fe-Ni metal. Haack and Scott (1993) did not suggest that the solubility of S in metal was larger, but rather, altering D(S) was a simple way to investigate the effects of removing S from the system. Figure 9 shows that the trends produced by the dendritic crystallization model do not differ much from those of the simple fractional crystallization model, especially considering the use of slightly different parameterizations for D(Ir), D(Ge), and D(Au) by the two models. Like the simple fractional crystallization calculations, the dendritic crystallization model also cannot match the IIIAB Ge vs. Ir trend. However, due to the different D(S) value, Haack and Scott (1993) determined an initial S content of 6 wt% for the IIIAB core, half the value determined by the simple fractional crystallization model. Furthermore, with a lower initial S content and by removing S-rich material from the system, the dendritic crystallization model is able to solidify 94% of the core to create the observed IIIAB elemental trends. It is much more satisfying to think that the >200 IIIAB irons sample 94% of the parent body core, as compared to only 55% as implied by the simple fractional crystallization model. Yet, the concentration of troilite in IIIAB irons is too low to account for all the S-rich material removed in the dendritic crystallization model, leading to the conclusion that S-rich material from the IIIAB core is still underrepresented in our meteorite collections. The growth of large dendrites in the IIIAB core could explain the scatter around the general crystallization trends. Large dendrites could impede thorough mixing in the metallic liquid, leading to local heterogeneities. Although the scatter can possibly be explained by the growth of dendrites in general, the model of Haack and Scott (1993) was unable to match the specific elemental trends observed in the Cape York irons.

Fig. 9. Four different crystallization models are applied to the IIIAB (a) Ir vs. Au, (b) Ge vs. Au, and (c) Ge vs. Ir trends. All four models can produce the large, 3-orders-of-magnitude variations measured in Ir and the general curved Ge trend. The assimilation fractional crystallization model (Malvin, 1988) and the mixing in the molten core model (Chabot and Drake, 1999) can also explain the leveling of the late stage Ir values, which the simple fractional crystallization model (Chabot, 2004) and the dendritic model (Haack and Scott, 1993) cannot. IIIAB iron meteorite data provided by J. T. Wasson. 764 Meteorites and the Early Solar System II

5.5. Assimilation Fractional Crystallization Model tion process is able to buffer the Ir concentration in the forming solid metal and consequently match the Ge vs. Ir In the assimilation fractional crystallization model, fresh IIIAB trend. The bulk compositional differences between metal is continually assimilated into the asteroidal core even the two zones are small, but even small heterogeneities can as the core crystallizes. Malvin (1988) suggested that such affect the resulting crystallization trends. Based on their a scenario could occur if the asteroid parent body was con- mixing model, Chabot and Drake (1999) suggested that as- tinuing to accrete material during the time the core was teroidal cores crystallized from the core-mantle boundary crystallizing. The physical plausibility of segregating me- inward, since the liquid near the crystallization front would tallic liquid through a solid mantle can be questioned, as be less dense and consequently above the second zone of discussed in section 4.5. Additionally, adding liquid to the liquid not actively involved in crystallization. However, it is molten core would be much more difficult if the outermost questionable how possible it is to maintain compositional core was solid Fe-Ni metal due to crystallization by inward differences in an essentially isothermal metallic core. Hetero- dendritic growth. The model calculations of Malvin (1988) geneities in the molten core could potentially explain the are the same as for simple fractional crystallization except scatter in the IIIAB trend, but Chabot and Drake (1999) that during each crystallization step, some amount of origi- were not able to match the Cape York trends in any mixing nal composition liquid is added into the remaining metal- model scenarios. The mixing model of Chabot and Drake lic liquid, modifying the composition of the molten core. (1999) also does not propose a unique solution, since mul- Malvin (1988) found that assimilating material into the tiple scenarios examined in their study resembled the IIIAB core at a rate of just 1% of the rate of crystallization gave trends. a better fit to the IIIAB Ir trend than simple fractional crystallization, as shown in Fig. 9. At that low rate, the small 5.7. Liquid Immiscibility Model amount of new liquid added to the core has little effect on the fractionation trends of most elements. However, if the As discussed in section 4.8, as an asteroidal core crys- concentration of an element, such as Ir, becomes severely tallizes, the molten portion of the core becomes enriched depleted in the molten core due to its large partitioning in S and P, and its composition may consequently fall into value, the continual addition of undepleted metallic liquid, the Fe-Ni-S-P liquid immiscibility field. The liquid immis- even at a low rate, becomes the primary source for that cibility model of Ulff-Møller (1998) is just simple fractional element in the molten core and the crystallizing solid. Con- crystallization until the composition of the metallic liquid sequently, the crystallization trend of that element will level encounters the liquid immiscibility field on the Fe-S-P off at an amount consistent with the amount of new mate- phase diagram (Raghavan, 1988). Ulff-Møller (1998) then rial being supplied to the molten core. allowed the molten core to separate into two immiscible liquids and examined the resulting crystallization trends 5.6. Mixing in the Molten Core Model from three scenarios: crystallization from the P-rich liquid only, crystallization from the S-rich liquid only, and crys- A fundamental assumption of simple fractional crystal- tallization if the two immiscible liquids remained in intimate lization is that the metallic liquid remains well-mixed equilibrium. Ulff-Møller (1998) investigated these three throughout the crystallization process. Chabot and Drake scenarios because they represented the most extreme cases (1999) developed a crystallization model to explore the pos- in equilibrium and disequilibrium conditions; any other sible effects that heterogeneities in the molten core could solution would fall in between these extremes. have on the resulting IIIAB crystallization trends. Math- The IIIAB Ir vs. Au trend is well matched by the sce- ematically, the model is a small variation on the simple frac- nario involving equilibrium between the two immiscible tional crystallization calculations. Chabot and Drake (1999) liquids, as shown in Fig. 10. Additionally, the Ir vs. Au trend divided the molten core into two zones of liquid, one zone is bounded by the two extreme disequilibrium cases of the actively involved in the crystallization process and another model, suggesting the scatter in the trend can be explained zone too far from the crystallization front to be immedi- by various degrees of local disequilibrium between the ately involved. In the mixing model, the two zones were immiscible liquids in the molten core. The study of Ulff- allowed to interact by mixing between the two zones and Møller (1998) does not show the Ge vs. Au trend specifi- by changing the size of the two zones. If mixing between cally, but it shows the similar Ga vs. Ni trend and states the two zones was set to a high value, the resulting elemen- that the results for Ge were comparable. Like the Ir vs. Au tal trends were identical to those from simple fractional trend, the IIIAB irons that compose the Ga vs. Ni trend are crystallization. If, however, mixing was not efficient enough bounded by the two extreme disequilibrium cases, but in to homogenize the compositions of the two zones, the com- contrast, the equilibrium scenario does not match the curved position of the crystallizing solid metal could differ from Ga trend. The curved nature of the Ga (and Ge) trend is the metal produced by simple fractional crystallization. attributed to the onset of liquid immiscibility and some Shown in Fig. 9 are the results from a scenario in which amount of disequilibrium associated with that event. the molten core is initially homogeneous but becomes less As shown in Fig. 10, the liquid immiscibility model well-mixed as crystallization proceeds. In the mixing model, cannot explain the IIIAB Ge vs. Ir trend. Ulff-Møller (1998) having some liquid not actively involved in the crystalliza- was also unable to produce any trends consistent with those Chabot and Haack: Evolution of Asteroidal Cores 765

Fig. 10. The effects from the onset of liquid immiscibility in the Fe-Ni-S-P system on the IIIAB (a) Ir vs. Au and (b) Ge vs. Ir trends are plotted (Ulff-Møller, 1998). Three extreme endmember cases are explored: The two immiscible liquids remain in intimate equilibrium during crystallization, crystallization proceeds from the P-rich immiscible liquid only, or crystallization proceeds from the S-rich liquid only. All other cases should fall in between these extremes. IIIAB iron meteorite data provided by J. T. Wasson.

observed in the Cape York irons. However, any shortcom- calculations. The difference is just the choice of partition ings of the liquid immiscibility model do not mean that coefficients. The constraints, or lack thereof, that experi- liquid immiscibility did not occur in asteroidal cores or that mental studies can place on the partition coefficients applic- its effects can be neglected while modeling core crystalli- able to iron meteorite crystallization is a subject of debate zation. (Wasson, 1999; Wasson and Richardson, 2001; Chabot et al., 2003; Chabot, 2004). 5.8. Trapped Melt Model In the crystallization model of Wasson (1999), the scatter in the IIIAB trends is due to different amounts of trapped During the crystallization of iron meteorites, some melt in the IIIAB irons, as illustrated in Fig. 11. In the amount of metallic liquid was trapped by the growing solid, model, the IIIAB irons are bounded by the compositions as is evidenced by the presence of troilite nodules. Wasson of the crystallizing solid metal and the molten core; mete- (1999) examined the effects that the trapping of liquid dur- orites that fall to the right of the solid trend on the Ir vs. ing crystallization could have on the observed IIIAB el- Au plot do so because they contain some amount of melt emental trends, the details of which were later modified that was trapped during crystallization. The solid and liquid slightly by Wasson and Richardson (2001) and Wasson and paths similarly bound the Ge vs. Au trend and explain the Choi (2003). The approach of Wasson (1999) in modeling scatter. Thus, the trapped melt model is the first to offer a the IIIAB trends differs from the other models discussed quantitative explanation for the composition of each indi- in two ways. First, while other models attempted to fit all vidual IIIAB iron. The trapped melt model is also the first the IIIAB data with just the crystallizing solid track, Wasson to quantitatively explain the Cape York trends; the positions (1999) argued that iron meteorites represented mixtures of the Cape York irons are consistent with various degrees between the solid and liquid compositions. Second, other of mixing between solid metal and trapped melt when the models all used fitted parameterizations to the experimen- IIIAB core was about 30% solidified. tal partition coefficients and varied the initial S content to The trapped melt model of Wasson (1999) is thus a very fit the IIIAB trends. Wasson (1999) based the choice of the attractive model for its ability to quantitatively account for initial IIIAB S content on measurements of the amount of the scatter and the Cape York trends in the IIIAB group. troilite in large Cape York specimens and by attributing the Additionally, with a low initial S content of 2.5 wt%, the troilite to melt trapped during crystallization. By determin- IIIAB irons are concluded to represent about 80% of the ing the total amount of trapped melt each sample contained, parent body core, and the possibility of liquid immiscibil- the partition coefficients were varied to produce a match to ity is greatly diminished. The trapped melt model has not the IIIAB data, using an initial S content of 2.5 wt% (Was- been applied to the Ge vs. Ir trend, but it is possible that son and Richardson, 2001). Mathematically, the model of the leveling off of the Ir concentrations may be accounted Wasson (1999) is identical to simple fractional crystallization for by a mixing line between the solid metal and coexist- 766 Meteorites and the Early Solar System II

does have its positives and negatives, but at present, none of the models discussed can meet all the criteria. It is important to keep in mind that some models are just rough attempts at approximating a process mathematically, such as the dendritic crystallization (Haack and Scott, 1993) and mixing (Chabot and Drake, 1999) models. Any failure of these models does not necessarily mean that the IIIAB core did not solidify by growing large dendrites or that mixing between local heterogeneities did not occur in the molten core. These models were designed to explore the possible effects such processes might have on the crystallization of an asteroidal core. Similarly, the effects from liquid immiscibility must be included in any model of the IIIAB core if the core composition enters the Fe-Ni-S-P two-liquid field, regardless of the success or failure of the liquid immiscibility model of Ulff-Møller (1998). Currently, the trapped melt model of Wasson (1999) is the only model that attempts to match the composition of each individual IIIAB iron, which a successful model must be able to do. Additionally, the models in Table 3 are not necessarily mutually exclusive. Multiple processes could have affected the crystallizing IIIAB core. Conceptually, the models also have overlap. Incomplete mixing in the molten core may result from the crystallization of large dendrites or the onset of liquid immiscibility. Dendritic crystallization of the core may enhance the amount of melt trapped by the solid. Although the models in Table 3 are different, they all arise from the common conclusion that a process more complex than just simple fractional crystallization led to the solidification of the IIIAB parent body core.

5.10. Modeling Other Groups

Most modeling studies have focused on the IIIAB group, but studies of other iron meteorite groups provide the op- Fig. 11. The trapped melt model (Wasson, 1999; Wasson and portunity to compare how the cores from different asteroi- Choi, 2003) is applied to the IIIAB (a) Ir vs. Au and (b) Ge vs. dal parent bodies crystallized. Au trends. In the model, meteorites that fall on the solid metal A simple fractional crystallization model is able to match path contain no significant amount of trapped melt, while mete- the curved IIAB Ge trend and the large fractionation in Ir orites that fall in between the solid and liquid paths contain vary- concentrations using an initial S content of 17 wt% (Chabot, ing fractions of melt trapped during crystallization. Mixing lines 2004). Such a high S content implies that well over half between coexisting solid and liquid compositions at different de- the IIAB parent body core is unsampled. Additionally, with grees of crystallization of the core are shown. IIIAB iron meteorite data were provided by J. T. Wasson. an initial S content of 17 wt%, the onset of liquid immiscibility is predicted to occur early in the crystallization process (Chabot and Drake, 2000). Similar to the IIIAB group, there is evidence for leveling off of the Ir concentrations in the later-crystallizing IIAB irons; the assimilation frac- ing liquid. The largest issue for the trapped melt model is tional crystallization (Malvin, 1988) and mixing (Chabot whether all the experimental partitioning data must be used and Drake, 1999) models have both had success at explain- as a constraint, specifically for the choice of D(Ir), which ing the IIAB Ir fractionation. as used in the trapped melt model is inconsistent with the In contrast to the IIIAB and IIAB groups, simple frac- experimental data. tional crystallization is not able to explain both the IVA Ge vs. Au and Ir vs. Au trends with a single S content (Chabot, 5.9. Model Comparisons 2004). Scott et al. (1996) applied the dendritic crystallization model of Haack and Scott (1993) to the IVA group but Different crystallization models for the IIIAB parent were also unable to match all the IVA trends. Wasson and body core are compared in Table 3. The ideal model would Richardson (2001) applied the trapped melt model to the be able to meet all the criteria listed in Table 3. Each model IVA group and were able to match the Ir vs. Au trend. Chabot and Haack: Evolution of Asteroidal Cores 767

TABLE 3. Comparison of crystallization models for the IIIAB iron meteorite group.

Simple Liquid Trapped Crystallization Models (IIIAB) Fractional* Dendritic† Assimilation‡ Mixing§ Immiscibility¶ Melt** Initial wt% S 12 6 12 12 10.8 2.5 % of core represented 55 94 55 55 60 80 Match Ir vs. Au? yes yes yes yes yes yes Match Ge vs. Au? yes yes yes yes yes yes Match Ge vs. Ir? no no yes yes no (n.d.) Consistent with all experimental data? yes yes yes yes yes no Explain scatter in IIIAB trend? no maybe no maybe yes yes Explain Cape York trend? no no no no no yes * Chabot (2004). † Haack and Scott (1993). ‡ Malvin (1988). § Chabot and Drake (1999). ¶ Ulff-Møller (1998). ** Wasson (1999), Wasson and Choi (2003). n.d. = not determined by the study.

However, Wasson and Richardson (2001) did not attempt to from the core-mantle boundary inward or concentric growth fit the curved IVA Ge trend, and currently, no crystallization from the center outward, has implications for the chemical model has fit both the IVA Ir vs. Au and Ge vs. Au trends evolution of the core, since the physical structure of the simultaneously. crystallizing solid will likely control mixing of the liquid The elemental trends in the IVB group are consistent and the possible trapping of melt during crystallization. A with simple fractional crystallization of an asteroidal core better understanding of trace element partitioning behav- with 0–2 wt% S (Chabot, 2004). With only just over 10 ior and its sensitivity to the metallic liquid composition has members, crystallization trends of the IVB group are not led to more complex fractional crystallization models for as well defined as the larger iron meteorite groups, and asteroidal cores. As discussed, each of the current models consequently the IVB group is currently not well suited for has its strengths as well as weaknesses. Along with the de- more-detailed crystallization studies. velopment and testing of improved models, applying the models to other groups in addition to the IIIAB irons will 6. CONCLUSIONS allow a comparison between the processes involved in core crystallization on different parent bodies. As future research Reviewing our current knowledge of the evolution of promises further insights into the evolution of asteroidal asteroidal cores calls attention to the need for additional cores, iron meteorites will remain important and unique as research and future work. Current isotopic studies indicate our only samples of any planetary cores. that cores formed and evolved early in the history of the solar system, but there are discrepancies regarding the rela- Acknowledgments. This chapter benefited from thoughtful tive timing of core crystallization of the different iron me- and thorough reviews by J. I. Goldstein, A. Kracher, R. J. Walker, teorite groups. Future work may not only better resolve the and J. T. Wasson, and we greatly appreciate their time and effort. relative timing between groups but may perhaps even de- Additionally, we thank J. T. Wasson for generating the statistics termine the relative timing of crystallization within a single shown in Table 1 and for the use of his compiled iron meteorite datasets, some of which contained substantial amounts of as yet group. The model for determining the cooling rates of iron unpublished data. We also thank J. Yang and J. I. Goldstein for meteorites is still undergoing revision. Ongoing and future providing Figs. 4 and 5. Support for this work was provided by research in this area will hopefully clarify if the apparent NASA grants NAG5-12831 to N.L.C. and NAG5-11122 to R. P. correlation between cooling rates and Ni concentration Harvey. among group IVA iron meteorites is real and if the current contradiction between variable IVA cooling rates and a core REFERENCES origin can be resolved. An improved understanding of the formation of the Widmanstätten pattern is not only of in- Becker H. and Walker R. J. (2003) In search of extant Tc in the terest for studies of the enigmatic group IVA, but also of early solar system: 98Ru and 99Ru abundances in iron meteor- general interest to all studies that make use of one of the ites and chondrites. Chem. Geol., 196, 43–56. key techniques used to study the thermal evolution of aste- Begemann F., Ludwig K. R., Lugmair G. W., Min K., Nyquist roidal cores: metallographic cooling rates. The mode of L. E., Patchett P. J., Renne P. R., Shih C.-Y., Villa I. M., and asteroidal core crystallization, whether by dendritic growth Walker R. J. (2001) Call for an improved set of decay con- 768 Meteorites and the Early Solar System II

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