Planet(oid) core crystallisation and fractionation—evidence from the Agpalilik mass of the Cape York iron meteorite shower

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Physics of the Earth and Planetary Interiors, 29 (1982) 218—232 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

Planet(oid) core crystallisation and fractionation—evidence from the Agpalilik mass of the Cape York iron meteorite shower Kim H. Esbensen and Vagn F. Buchwald Department of Metallurgy, Building 204, Technical University of Denmark, DK-2800 Lyngby (Denmark) (Received and accepted for publication, March 22, 1982)

Esbensen, K.H. and Buchwald, V.F., 1982. Planet(oid) core crystallisation and fractionation—evidence from the Agpalilik mass of the Cape York iron meteorite shower. Phys. Earth Planet. Inter., 29: 218—232. Metallographic and chemical study of the Agpalilik mass (20 t) of the Cape York iron meteorite shower (totalling> 58 t), which belongs to the most populous group IIIAB, reveals evidence of the mode of crystallisation and fractionation of key elements consistent with a dendritic solidification of at least part of the once fully molten meteorite parent body metallic core. We assess systematic chemical gradients displayed by Zr and Au across an 85 cm section that is inferred to be perpendicular to the parent body gravitational field; these gradients are too large to be part of the fractionation resulting from the general fractional crystallisation radially outward of the IIIAB core. They are interpreted as representing a dendritic growth mode also explaining the characteristic elongated and orientated sulphide nodules found in Agpalilik and which signify trapped liquid of the late(st) stage(s) of crystallisation. Detailed mineralogic and chemical characterisation of the Agpalilik liquid—solid transformation products allow modelling of the entire crystallisation history commencing with dendritic metal precipitation through an ultimate troilite—taenite—Cu eutectic, representing a crystallisation range spanning approx. 1350—700°C. This constitutes a model system of the maximum potential fractionation of the parent body core, being a ‘telescoped’ forecasting of possible later events in the solidifying supernascent core pool. Adequate description of the salient phase relations requires the quaternary Fe—Ni—S—P system; the essentials of the major element fractionation can be encompassed by the residual system Fe—Ni—S. Viewed as a study in analogo of possible ways of crystallisation—fractionation of the cores of other inner solar system bodies, a broader scope can be attached to these findings. Provided the case can be made for comparable chemistry with the modelled system, which is the case for most iron meteorite groups, for the Earth at least some evidence for a match is at hand, taking into consideration the inherent large ambient pressure differences. Some aspects of the present inner/outer core solidification/fractionation of the Earth’s core would appear to be susceptible to interpretation within the confines of the principles of the present planet(oid) core crystallisation—fractionation model.

1. Introduction Most iron meteorites are believed to be fragments of once molten cores of early solar system planetoids/asteroids, called parent bodies. These form the so-called magmatic iron meteorites (Scott and Wasson, 1975). Others, primarily the two groups JAB and IIICD (see below for group definition), are held not to have experienced a similar common liquid stage, but to represent individual shock-melt pools formed by impacts in the outer parts of chondritic parent bodies (Wasson et al., 003l-920l/82/0000—0000/$02.75

1980). A relatively small number of such parent bodies would appear to account for the diverse chemical fractionations present within the approximately 600 known individual iron meteorites (Scott, 1977). Some 86% of all individual irons group into 13 major resolved groups, of which JAB and IIICD, the non-magmatic groups, form a subset. The 11 remaining magmatic groups include the IJIAB, to which belongs the Cape York iron, with no less than 33% of the total number of grouped iron meteorites. Each such group is interpreted as reflecting a suite of samples stemming

© 1982 Elsevier Scientific Publishing Company

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from one and the same parent body (Wasson, 1974). With respect to the magmatic groups, the genera! consensus regards the highly systematic chemical fractionations observed, and which form the basis for the classification into ‘groups’, as a manifestation of two distinct genetic processes (Scott, 1972, 1979; Wasson, 1974; Scott and Wasson, 1975). (1) a primary solar nebular condensation event essentially fixing the bulk chemical composition of the metal; (2) a secondary, planetary, fractionation process responsible for the systematic chemical covariances in each group. This planetary fractionation is believed to represent fractional crystallisation of the once fullymolten metallic cores (or raisins) in the pertinent parent bodies. This process is likely to have taken place early in the history of the solar system, i.e. with respect to the radioactive half-life of 26A1, in order for differentiated asteroids/meteorite parent bodies to have developed. Hence the reason to study the iron meteorites in order to shed some light on the early evolution of these small(er) bodies of the solar system. Information as to the crystallisation mode of these bodies may then serve as a possible analogue with which to assess the fractionation of other inner solar system bodies of comparable chemistry and other ambient characteristics. The present study addresses this planetoidal core liquid—solid transformation process mainly from a geochemical/metallurgical point of view, hopefully as a help in further understanding of such aspects of the early solar system history as seen from the geophysical viewpoint.

2. The Agpalilik mass of the Cape York shower The Cape York iron meteorite shower is probably the largest of its kind in existence. The Ahnighito and Agpalilik masses weigh 30.8 and 20.1 t, respectively. The total recovered material is some 58 t (Buchwald 1975) in contrast to the estimated 60 t for the Hoba iron, Namibia. There is good reason to suspect that even now, by no means all the Cape York material has been re-

covered (Buchwald, 1975). Cape York belongs to the largest magmatic group, IJIAB, which makes up some 33% of all grouped irons. The Cape York meteorite is regarded as the type iron meteorite ‘par excellence’, its chemistry being very close to the overall average compound ‘iron meteorite’ (Buchwald, 1975, table 31) and its metallographical and mineralogical appearance are also typical of the dominant octahedric iron meteorites. Extensive studies have been carried out on Agpalilik for these reasons. The subsolidus Widmanstätten exsolution pat-

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I. A Il~U 125 5 cm slice of the Agpalilik mass or the Cape York shower. The layout of numerous elongated sulphide nodules (troilite—FeS) clearly display a preferred orientation parallel to the long(est) axis of the slab. This structure is strongly indicative of unidirectional growth of the enclosing metal, which forms a single crystal. Detailed metallographical investigation reveals that the elongation/orientation generally coincides with [100] , which is a well-known growth direction of the y-iron (taenite-(fcc)). Parallel and perpendicular cutting has revealed that the troilite inclusions typically have diameters smaller than -~ 5 cm. 1-ig.

220

tern of the Agpalilik mass is continuous, indicating that the precursor y-iron(fcc) crystal had dimensions of at least 1—2 m; this makes the Agpalilik mass the largest known non-silicate single crystal. Cuts parallel to the (100) plane of the cubic ycrystal reveal a large number of elongated troilite (FeS) nodules oriented parallel to one of the cubic axes, which is parallel to the longest axis of the slab, presented in Figs. 1 and 2. At one end of these inclusions chromite and phosphates are found. These minerals are insoluble in, and probably lighter than, a low-temperature troilite—metal liquid. Their preferential distribution at one end is inferred to indicate the direction of the gravitational field vector in the Cape York parent body

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(Buchwald, 1971). The core crystallisation vector is radial if the magma formed a central metallic core, an almost universally accepted paradigm for this exemplar group. The elongated and parallel sulphide nodules tell an important story as to the mode of crystallisation, which is described below.

3. Scope of present study As part of a broader study of the Cape York shower a detailed chemical study of the Agpalilik mass has been undertaken. The full results are presented elsewhere (Esbensen et al., 1982), where the analytical procedures are also described. Some salient results from an earlier study by Esbensen et a!. (1980) are presented here. The applicability of the present conclusions rests on the representative role of group IIIAB with respect to all other magmatic iron meteorite groups, as established by Scott Ct a!. (1973). With Cape York being a prominent member of the most populous group, the present study of the crystallisation—fractionation of the Agpalilik mass will allow broad generalisations, although it is by no means certain that the results on the Agpalilik mass can be necessarily transferred to other specific masses of the Cape York (Esbensen et al., 1982); we therefore report only those conclusions here which are not susceptible to such ambiguity.

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Fig. 2. Diagram of Agpalilik sections studied chemically. Sketch of the slab in Fig. I with trace of complementary end piece removed before cutting of slab. The coordinate system used in detailed chemical investigations, Esbensen et al. (1981), i~ denoted by axes X, Y and Z: Y—Y’ parallels [l00l~ z—z’ ~ the ‘transverse’ direction of prime interest in the present paper; the vector Y—Y’ represents an outward-radial vector of the meteorite parent body metallic core (Chromite in the troilite nodules tends to concentrate at the Y’ end); S—S’ is presented in detail in Fig. 3. Duplicate chemical samples analyzed by INAA all relate to the chemistry of the metal phase. Details of analytical procedure etc. to be found in Esbensen et al. (1982).

4. Chemical investigations Figure 2 gives details of the chemical samples employed in the present study. The section Y’—Y is the assumed outward radial vector. We here report data for the 85 cm section S—S’ and survey results for the Y— Y’ section. The elements Jr and Au are used because of their high .

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precision in the INAA employed and because Jr is the most ‘sensitive’ element (highest solid—liquid distribution coefficient of the elements analysed). No previous studies of compositional variations across magmatic irons have been published. .

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An earlier study of the Savik I specimen of the Cape York yielded an Jr content of 5.0 ppm (Scott et a!., 1973). A second sample of Savik J, analysed

221

together with the present Agpalilik series, yielded 4.9 ppm. As argued by Esbensen et al. (1982) we believe that compositional variations in Cape York span a factor of 2 as illustrated for these metallic compositions, Agpalilik (at 2.7 ±0.2 ppm Ir) and Savik I. These data represent the extremes of the entire Cape York assemblage presently known and analysed. Jn Fig. 3 the Jr and Au variations observed in the S—S’ section are presented. The trends consist of two types of segment, one near a troilite nodule in which large compositional gradients are present, and another comprising the remainder of the section, across which concentrations systematically decrease (Ir) or increase (Au). This last trend, which we have termed the ‘macrofractionation’ trend, forms the essential basis for the present

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dendritic crystallisation model. The segment up to troilite we term the ‘microfractionation’ trend. The anticorrelation between Jr and Au is similar to that observed for all magmatic groups and can be attributed to different equilibrium solid—liquid distribution coefficients, k gold having a kAU smaller than 1 with kir being greater than 1 (Scott, 1972). We also report that elemental variation in the Y—Y’ direction for both elements are below ca. 3%. Jn this radial direction we have found no systematic gradients similar to those for the Z-direction. ,~

4.1. Microfractionation around troilite Because the solubility of S in solid Fe—Ni is very low we can be sure that the troilite nodules do not signify solid-state precipitation. Rather, they must be viewed as reflecting the presence of trapped liquid. The large concentration gradients in the metal near the troilite nodule may have resulted from the crystallisation of this trapped liquid, but detailed considerations (Esbensen et al., 1982), do not sup-

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crystallisation have precipitated stage, from a taking cotecticinto metal—troilite consideration the possibility ofeven asymmetric crystallisation

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Fig. 3. Transverse chemical gradients in the Agpalilik metal. S—S’ section across the Agpalilik slab presented in Fig. 2 with detailed sample locations, dimensions etc. Two types of trends are present: (I) A ‘macrofractionation’ trend (filled circles— thick, solid trace) across the entire slab, and (2) a ‘microfractionation’ trend (open circles—light, solid trace) only close to one troilite nodule, where sampling is very closely spaced. Both

suggested by the asymmetric microfractionation patterns on each side of the troilite nodule in Fig. 3. We are forced to explain why Ir rises before the sharp ultimate drop right up to troilite, and we cannot refer this to cotectic crystallisation. A. Kracher and J. Wasson (unpublished data) make the suggestion that increasing concentration of S resulting from the fact that k5 0.1 may have been instrumental in a significant increase in the effective kir. Experimental work by Narayan and Goldstein (1980 and personal communication, 1980) indicates that elements depressing the melting point of Fe—Ni, for most P and 5, indeed work in exactly this fashion. Narayan (1980) quantified these relations in the case of P and found indications of quite similar behaviour for S. If the sharp gradients in Ir were produced dur‘~

Jr and Au show a significant, systematic macrofractionation trend of opposite character (depleted—enriched from left to right, respectively), interpreted in the text as representing transverse dendritic growth during passage of the contemporaneous crystallisation front, parallel to [l00l.~ (the Y-direction) through the volume element now occupied by the Agpalilik mass. The microfractionation trend is attributed to liquid entrapment betweenprimary and tertiary dendrite arms (see Fig. 8). Cotectic metal—troilite precipitation most probably corresponds only to the two innermost (opposite) samples right up to the troilite nodule.

222

ing crystallisation at a cotectic temperature of 1000°C (Kullerud et al., 1969), see below, the preservation of these gradients shows that diffusion at temperatures below 1000°Chas been very limited. This is indeed the exact result for Ir found by Narayan (1980) in experimental work on the solidification and diffusion in ternary alloys Fe— Ni—X matching the iron meteorites: Jr was found to be the least diffusive element of the suite investigated (Ni, Ir, P, Ge and Au). Au is somewhat more susceptible to solid state diffusion at these temperatures and consequently the Au-profile in Fig. 3 must be viewed in this context to be slightly levelled relative to the ‘magmatic’ trend, —

4.2. Macrofractionation across the Agpalilik mass The general Jr trend across the length of the S—S’ section is also shown in Fig. 3. Maximum gradient estimates at the extreme ends correspond to some 14% decrease (from left to right). The possibility that this smooth trend is part of the core-wide IJIAB fractional crystallisation trend that fractionated all elements, including Jr by a factor of 2000 (Scott and Wasson, 1975), cannot be accommodated with an analysis based on the well-known Rayleigh relationship of trace element fractionation X — X k ~ — ~ k x’l ‘1~ — ~ g, k ~ where: £~ = initial liquid concentration of trace element; g = fraction of melt solidified, at which stage X denotes the contemporaneous solid cornposition. With group IJIAB fractional crystallisation parameters as given by Willis (1980) (k Ir = 4.6 and initial solid Jr composition = 18.7 ppm) we can estimate that the maximum and minimum Jr concentrations of 2.88 and 2.54 ppm of section S—S’ respectively correspond to 40.82 and 42.86% crystallisation as measured with a core geometry of the entire JIIAB core. This is easily translated into a core radius of some 70 m as the maximum core diameter by algebraic manipulation of these crystallisation parameters in eq. 1. This would appear to be a very modest dimension of the JIIAB core; Wasson (1974), on the basis of the observed fall rate and the cosmic-ray age distribution of group IIJAB, calculated a minimum diameter of .~-‘.

300 rn. In addition, numerous cooling rate considerations indicate much larger core radii, one or more orders of magnitude greater (see discussion in Scott, 1977, 1979). Another geometric argument can be reported: as indicated above, the main core crystallisation direction and gravity vector were found to be parallel, yet the S—S’ section is perpendicular to this direction (see Figs. 2 and 3). An alternative explanation for the systematic macrofractionation pattern suggests that it reflects local imperfect mixing of the liquid phase during fractional crystallisation. The numerous troilite nodules in the Agpalilik testify to the existence of a significant proportion of trapped liquid in this mass. The meteorite cannot have formed simply by gradual movement through the Agpalilik volume of a plane crystallisation front. With respect to the diverse mechanisms possible for trapping of liquid Esbensen et al. (1982) wrestle at great length with still alternative models for the chemical origin of the entire Cape York fractionation patterns. We present our view on the interpretation of the Agpalilik evidence below. 4.3. Crystallisation modes Two contrasting types of crystallisation have been envisaged for the iron meteorites, the planefront crystallisation model (see, for example: Scott, 1972; Wasson, 1974; Kracher et al., 1977), and the dendritic crystallisation model (see for example: Goldstein et al., 1979; Narayan, 1980; Narayan and Goldstein, 1980). Whether or not a plane-front crystallisation is stable depends on the magnitude of the ratio G/R, where G is the thermal gradient in the liquid and R the rate of advance of the crystallisation front. Computer modelling of likely meteorite silicate parent bodies, complete with metallic cores (Goldstein et al., 1979), indicates that a plane crystallisation front would not be stable for these bodies, and thus, by this analysis, a dendritic crystallisation mode seems to be required. On the other hand, arguments mainly pertaining to the convecting environment of the liquid core (Esbensen et a!., 1982), are against any all-pervasive dendritic crystallisation mode of the entire core as envisaged

223

by Narayan (1980). We develop the dendrite mode! below. The Jr-profile in Fig. 3 fitted by the simple Rayleigh fractionation formula in eq. 1, represents a minimum diffusion subsequent to macrofractionation. By this methodology we can arrive at a rough estimate of the primary dendrite arm spacing of some 26 m. Taking liquid as well as solid diffusion into consideration, Narayan (1980) estimated corresponding interdendritic distances of the order of ~ 500 m. We need not take similar diffusion considerations ad notam for the Jr-profile modelled here; as stated above, Jr has a very low diffusivity, corroborated by the very same experimental results (Narayan, 1980). Accordingly, our empirical estimate will be compared with other laboratory results in the following in order to shed light on this order-of-magnitude discrepancy which we suspect at least partly to be attributable to the fact that very extensive extrapolation from the experimental cooling rates to the meteoritical cooling rates (typically in the order of 10K Ma1) is necessary in the experimental work. The dendrite spacing is critically sensitive to several parameters involved in these estimates (Narayan, 1980). With these reservations in mind, we may evaluate the results of Flemings et al. (1970), who presented both primary and secondary arm spacings vs. cooling rates for an Fe—26% Ni alloy unidirectionally solidified in the laboratory. Extrapolation of these data to the appropriate meteoritic cooling rates (six orders of magnitude) yields a primary arm spacing of 100 m, and a secondary arm spacing of 0.1 m. We emphasize that the present distribution of troilite nodules in the Agpalilik ‘need not represent the primary dendrite spacing; indeed, trapping of interdendritic liquid is more likely to have taken place between secondary and tertiary arms (see below). The detailed relationship between first-, second- and third-order dendrite arm spacings is not well established. Here we do not treat this hypothetical dendrite model at a more sophisticated quantitative level; we may note, however, that the present troilite distribution across the Agpalilik is rather non-uniform. A very rough estimate of the average troilite—troilite distance would be 5—15 cm, which —~

we interpret as indicating trapping between tertiary arms (Fig. 4 and below). Apparently, our empirical and very simple analysis of the Agpalilik data matches with the lower end of the rather large range of experimental results. From the above we deduce that the elemental fractionation pattern across the Agpa!ilik mass can be reconciled with a dendritic crystallisation mode! whose scale parameter (average primary dendrite spacing) can be estimated at the order of magnitude level to be a few tens of meters. This tentative physical correspondence between experimental and empirical dendritic fractionation patterns can be taken one step further in Fig. 4, where the full three-dimensional experimental dendritic pattern can be used to ascertain the characteristic primary—tertiary pattern, which we interpret for the Agpalilik section. Here we tentatively contend that such pattern similarities reflect genetic similarities of the type envisaged: the chemical pattern across the Agpalilik section would imply a dendritic crystallisation mode of the volume now occupied by this IJIAB core fragment. However, a severe problem regarding the application of such dendrite models for the Cape York meteorite in toto stems from the fact that other fragments have radically dissimilar Jr and more important, S contents and corresponding other elemental abundances, e.g. as was demonstrated above for the Savik J mass. The Jr-value of 4.9 ppm corresponds to an overall crystallisation stage of 30% measured identically to the 42% calculated for the Agpalilik above. For the reasons outlined in the Section Ofl macrofractionation we must reject the possibility of any simple Rayleigh mechanism producing this large difference (corresponding to more than 10% difference w.r.t. the JIJAB core crystallisation) within the confines of the Cape York, a volume totalling only some 10—20 m3. In a companion paper to Esbensen et a!. (1982) we explore the full scale physical model needed in order to account for these discontinuous elemental patterns. The dendritic crystallisation mode, then, need not pertain to the entire Cape York meteoroid, far less to the entire IIIAB core. Jrrespective of these ambiguities, the main lesson to be learned from both these chemical investigations, as well as the metallographical observa‘-~

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Fig. 4. Comparison between experimental and natural dendritic fractionation patterns: (a) A l96xmagnified section parallel to secondary dendrite arms (see Fig. 8) modified from Singh et al. (1970, fig. 5). Note how the eutectic is preferentially concentrated between primary and tertiary arms in this experimental system. Contours (wt% Al): (7) 98.00; (6) 97.75; (5) 97.50; (4) 97.25; (3) 97.00; (2) 96.50; (1) 96.00; (black) eutectic. (b) Profile A in (a) for comparison with (c) Zr-profile parallel to primary and tertiary arms in the Agpalilik (from Fig. 3). Note identical shape of trends in (b) and (c), disregarding the primary dendrite segment, present as the leftmost third of profile A in (b); the corresponding segment was not sampled by the present Agpalilik volume, due to the very large interdendritic distance inferred to be one order of magnitude greater than the sampled section. In the text we argue for a genetic significance of this match, both trends reflecting dendritic fractionation.

tions reported below, is the finding that significant trapping of contemporaneous liquid accompanied the solidification of the JIJAB volume now OCcupied by the Agpalilik mass. It is exactly this trapping that will allow us to explore the ultimate crystallisation—fractionation of this system.

Since the solubility of sulphur in y-iron is low (less than 0.08%) the behaviour of S in the course of solidification of the Fe—Ni—S—P system is extremely simple, at temperatures above 700°C (Fig. 5). The sulphide (the main phase to take up 5) will be concentrated where the last liquid solidifies. The Fe—S system solidifies in a eutectic

the cotectic boundary AB. An ambiguity exists, however. An alternative suggestion has been put forward regarding the meteoritical setting of this Fe—Ni—S system, that of a second immiscible, S-rich liquid preferentially trapped by the growing solid(s) (Kracher et al., 1977). There does not appear to be any experimental evidence for such immiscible relationships in metallic melts of meteontic composition, however (Scott, 1977; Narayan, 1980). In the following we outline our arguments for the cotectic configuration of the phase relationships presented in Fig. 5. Detailed metallographic investigations of the troilite inclusions have been carried out, together with the immediately surrounding metal. Schreibersite, (Fe, Ni)3P, and a very Ni-rich taenite, both

manner (see point A in Fig. 5). The Fe—Ni system displays a minimum melt configuration at 68% w/w Ni. In Fig. 5 these two systems bound the ternary Fe—Ni—S liquidus relations relevant to the discussion of the metallographic observations. Note

lining the metal/troilite interface, have been characterised by the microscope and the microprobe. The results are all consistent with an interpretation of crystallisation from a progressively differentiating Fe—Ni—S—P liquid.

5. Metallographical investigations

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of the dendritic crystallisation of the metal. Schreibersite as well as a high-Ni taenite are observed to line the metal/troilite interface (Fig. 6). The occurrence of this high-Ni taenite has not been reported before. Jt has the highest recorded Ni-content (53—61% from themodal magmatic groups. It only w/w) occurs of in any 0.01 of the amount of the schreibersite present. Although volumetrically quite insignificant, genetically it is

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Fig. 5. Phase relationships in the Fe—Ni—S system above -~ 700°C.This system pertains to the entire fractional crystallisation of the Agpalihk meteorite, spanning a range of liquidus temperatures from -~ 1350 to 700°C.The corresponding liquid differentiation is routed from the initial bulk Cape York cornposition via DL through LL (‘last liquid’). Composition LS represents the ‘last solid’ crystallisation product, the troilite— taenite eutectic further elucidated in Figs. 6 and 7. B represents the lowest possible temperature/composition of the cotectic metal/troilite boundary AB before appearance of ternary phases (not observed in the Agpaliik meteorite). The initial dendritic crystallisation of the Fe—Ni alloy takes place in the interval — 1350—975°Cas the liquid differentiates to composition DL. Interdendritic entrapment (see Fig. 8) occurs just before and penecontemporaneously with this onset of cotectic metal—troilite precipitationjudging from textural and chemical

of high importance (seeobserved, below). i.e. Twoa lamellar distinct habit modes have been and a distinct eutectic component, both illustrated in Fig. 6. Microprobe investigations reveal that both modes have identical chemistry, having 53— 61% Ni. Zoning in the lamellar mode, from the __________________________________________________ relationships. ‘Sedimentation’ of chromite and phosphate also must have taken place at this stage, before the complete crystallisation of troilite. Note that all subsolidus relationships have been ignored in this diagram. Study of iron meteorites usually employs either the front Fe—Ni system (augmented by -P) or the left backside Fe—S system. Our metallurgical studies of the entire fractionation present in this model system of the IIIAB core fractionation allow us to conclude that the essentials of the later stages fractional crystallisation/chemical fractionation of the major elements can be displayed in this residual Fe—Ni—S system. Figure 7 depicts two isothermal sections of this system corresponding to liquid compositions DL and LL, respectively.

Fig. 6. Ultimate liquid—solid transformation product of Agpalilik fractional crystallisation, the troilite—high-Ni taenite eutectic: (a) 240 X magnified microphotograph. Three occurrences of the eutectic structures found close to metal/troilite boundary (running east-west); top half of picture is pure troilite, bottom half is kamacite (n-iron(bcc)), a subsolidus exsolution product from the precursor, magmatic -y-iron). Also shown is lamellar high-Ni taenite (left centre). Bulk composition of eutectic proper corresponds to LS in Figs. 5 and 7. Both lamellar and eutectic taenite has the average Ni-content of 58% (range of zoning 53—61%) the highest of any taenite met in magmatic iron meteorites. In centre eutectic note tarnished patch stretching NW representing elemental copper, a third component of the ultimate eutectic that may or may not be present in the occurrences investigated. (Centre ‘crater’ is a defect from the polishing of this specimen). (b) 380 X magnified microphotograph. A similar lamellar—eutectic occurrence of high-Ni taenite as in (a), but nested inside a ‘bay’ in schreibersite, (Fe, Ni) 3P. stretching NW—SE along the metal/troilite boundary. Kamacite to the left, troilite to the right. The schreibersite is characteristically cracked; some of these cracks must have been produced due to later crystallisation of metal—troilite as they have been observed to be filled with the elemental copper elsewhere only present in the eutectic structures (cracks at upper left). These textural relationships clearly demonstrate that schreibersite crystallised before the high-Ni taemte and the bulk of the troiite. This is interpreted to signify a penultimate purging of the full Fe—Ni—S—P system of P subsequent to the ultimate solid precipitation of troilite, lamellar taemte and the ultimate eutectic in the residual Fe—Ni—S system.

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compositionally similar Oktibbeha County meteorite by Wasson et al. (1980), is shown schematically by the stippled line joining troilite with the average high-Ni taenite composition, 58% Ni. The Oktibbeha County iron, however, is not a magmatic iron, but belongs to the non-magmatic group lAB of radically dissimilar genesis, see introduction, and our textural observations do not support equilibrium crystallisation.) (b) At 700°C(735°C),corresponding to liquid composition LL(LL’) in Fig. 5, respectively. This differentiation interval is reflected in the zoning observed in the high-Ni taenite. The y + liquid + FeS three-phase triangle now covers a much wider area, the

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S SYSTEM 6

quite distinct from those of schreibersite at metal—metal interfaces outside the troilite nodules. These latter are held to have nucleated and grown in the solid state (Buchwald, 1975). We interpret the distinctly different chemical characteristics of the former schreibersite as signifying precipitation

‘ISp area used for the modal analysis. This change in composition with definition of the ‘closed system’ reflects the ongoing the shift from FL to LS is primarily a function of this change progressive liquid differentiation following entrapment. Thus of physical definition of system; the increased S-content, from 1.3 to 6% as well as the Ni-fractionation, however, reflects the combined effect of liquid differentiation due to fractional crystallisation as well as progressive entrapment of liquid. Note that the subsolidus y —‘ a + y transformation producing the Widmanstgtten exsolution structure, is indicated. Though it is

‘I.

Fig. 7. Isothermal sections of the Fe—Ni—S system in Fig. 5. Modified from Kullerud ci al. (1969): (a) At 975°C, corresponding to the liquid composition DL in Fig. 5. Composition FL corresponds to the initial bulk Cape York composition. The monosulphide-field is shown hatched. Liquid is black. The three-phase triangle Fe—Ni alloy—liquid—troilite is denoted as y + L+ FeS. Tielines show coexisting compositions of two-phase

of no consequence for the present discussion, it takes place in the same temperature range as the ultimate crystallisation. (The relation between the boundary troilite—liquid and the monosulphide,~-fieldis simplified. In this area complications arise due to the appearance of the ternary phase (Ni, Fe) 3 ~S2, at approx. 700°C. This phase has not been observed in the paragenesis investigated; hence we conclude that these relations are of no interest to the present study.)

228

that schreibersite precipitated from the Fe—Ni—S— P liquid prior to the crystallisation of the highly fractionated taenite, effectively purging the system for P. Subsequent crystallisation took place in the residual Fe—Ni—S system. The taenite—troilite eutectic, additionally carrying elemental copper as a third component (Fig. 6) represents the ultimate liquid—solid transformation product of this isolated system. Modal analysis of this eutectic allows estimation of the lowest crystallisation temperature, approx. 700°C(Fig. 7). In the light of these findings, an integrated understanding of the late(st) stages of the Agpalilik natural Fe—Ni—S—P system can be solely ascribed to a liquid—solid transformation process which successfully accounts for all chemical and metallographical relationships. With respect to Fig. 5, we can appreciate that the fractional crystallisation spans the range 1350—700°C. In the pure Fe—Ni system, the typical liquidus—solidus temperature range is only of the order of 5°Cfor meteoritical bulk Ni-concentrations of 10%. Narayan (1980) found that addition of P (and 5) to this system dramatically extended the freezing range, e.g. for the addition of 1.25% P the resulting freezing range was 500°C. Only preliminary results were reported for S; these indicated similar, perhaps even greater, freezing ranges. This is of course the exact results of inspection of the Fe—Ni—S system (see Fig. 5) where the S-dependence on this fractional crystallisation can be appreciated in its total layout, cf. the ca. 650°C freezing range we have estimated on the basis of our metallographical observations compared with the experimental Fe—Ni—S data of Kullerud et al. (1969). Of course, what we actually observe is the combined effect in this natural Fe—Ni—S—P system. Agpalilik’s bulk P-content is 0.2%, bulk S is 1.3% (Buchwald, 1975, p. 416). —~

6. Summary of the Agpalilik crystallisation history (I) Initial crystallisation of the bulk Cape York composition yields zoned dendrites, most effectively shown by Jr, starting with major element composition D at 1350°C. Progressive entrap-~

ment of interdendritic liquid starts at or slightly before liquid composition DL (975°C), the onset of the metal—troilite cocrystallisation stage. In this 400°C interval chromite and phosphate must have nucleated (or alternatively they can have nucleated even before the metal precipitation starts—they probably effectively will have been slurried into the liquid), grown and ‘sedimented’ as solid phases trapped in the isolated pockets before complete troilite crystallisation has taken place. We are still in the Fe—Ni—S—P system. (2) The cotectic stage along the DL—LL boundary. Somewhere along this liquid differentiation route the effective phosphorus depletion, due to schreibersite precipitation, must have taken place. We then enter the residual Fe—Ni—S system proper, where Fig. 5 does good justice. (3) We can isolate the ultimate liquid—solid transformation stage characterised by the precipitation of zoned taenite corresponding to the LL’— LL last liquid differentiation. The ultimate troilite—taenite eutectic carries the additional component Cu (see Fig. 6), which does not influence this major element system in any discernable way, however. The details of the phase diagrams in Figs. 5 and 7 capture the essentials of the major element fractionation. Figure 7b depicts the extreme situation of the crystallisation of the most differentiated liquid obtainable in the trapped system. This liquid is charactensed by a S-content of some 25%. -~

7. Interpretations—a crystallisation model and implications The chemical patterns across the Agpalilik can be interpreted as reflecting the passage of a dendritic crystallisation front through the volume element of the IIIAB core now represented by this mass. The modelled Jr-profile we interpret as reflecting ‘transverse’ thickening, growth perpendicu!ar to the primary dendrite arm direction. Primary dendrite growth is along the Y-direction, but the attendant zoning in the Z-direction was much more pronounced. We can even fix the polarity of this primary growth knowing the chromite/phosphate ‘sedimentary’ evidence from the troilite nod-

229

ules: since in Fig. 3 growth must have occurred from left to right, kir> 1, the ‘downward’ (Y’—Y direction in Fig. 2) must coincide with the advancement of the primary dendrite growth, which

is represents identicaldendritic to the direction crystallisation of advancement at the of the of general Esbensen et ceiling al. (1982) present crystallisation a suggestion, front. that perhaps the Agpalilik the core. With reference to Figs. 4 and 8 we interpret the present elongated and orientated troilite nodules as distributed in accordance with trapping between tertiary dendrite arms; these are parallel to the primary dendrite arms, but with a characteristic arm spacing some two orders of magnitude lower, very roughly estimated. The microfractionation pattern around these troilite nodules we ascribe to the chemically not very well-known mechanism of trapping of contemporaneous liquid, Only a thin rind of the enclosing metal can be ascribed to cotectic troilite—metal precipitation. This cotectic crystallisation stage plays a very important role in explaining the mineralogical habit of these troilite nodules and their characteristic associated phases, schreibersite and high-Ni taemte, however. Relating these findings to the well-known Fe—Ni—S phase diagram allows a quantitative estimation of the entire Agpalilik crystallisation history (see Figs. 5 and 7). These chemical/metallographical considerations form a basis for visualizing this system as a ‘telescoped’ model for the possible maximum fractionation of the entire IJIAB core, if this can be described as related to a similar fractional crystallisation. The latter premise has been amply substantiated by numerous studies of group IIJAB (e.g. Scott, 1972; Scott et a!., 1973; Wasson, 1974). It is important to note that any trapping of contemporaneous liquid will reduce the effective fractionation in the supernascent liquid core poo1. If the proportion of trapped liquid = a, then in eq. 1 a correction factor to the exponential term is necessary to quantify this reduction of fractionation efficiency (k>~—I)(1 ~ X= ~k~(l g) (2) The overall fractionation of Jr in group IIIAB is the next largest of all magmatic groups (Wasson, —

SECONDARY ARMS

TERTINAY ARMS

a

~IM~Y~...

50%

ENTRAPMENT -

b

90%

Fig. 8. Schematic illustration of dendritic crystallisation leading to liquid entrapment. At progressively later crystallisation stages physical interaction (and hence chemical transport in the liquid state) with the superjacent magma is hampered, finally resulting in complete isolation of residual liquid pockets. Because of the inequilibrium crystallisation mode of the metal alloy this entrapment is accompanied by progressive differentiation of the liquid, cmp. Figs. 5 and 7: (a) 50% crystallised (with respect to volume); 90% crystallised entrapment is entering theinitial effective stage.(b)Further crystallisation leads to isolated pockets. Note how the interdendritic interstices serve to ‘dongate’ and orientate the liquid entrapments. At approximately stage (b) liquid differentiation reaches composition DL in Figs. 5 and 7: troilite then appears on the liquidus until the crystallisation is the complete. troilite nodules in Fig. assemblage, I must be viewed as sulphideThe component of this cotectic the immediately surrounding metal as the complementary phase. Drawing modified from Singh et al. (1970) (Fig. 4). Esbensen ci al (1982) discuss other plausible models for the origin of fractionation patterns as well as liquid entrapment in the entire Cape York meteorite, not all based on a dendritic crystallisation model.

1974). Such effective fractionation necessitates quite ideal trace element distribution, which means that, in general, a must have been low. We note that in general the proportion of trapped liquid in group JIJAB irons, as reflected in troilite contents, would appear to be significantly lower than for the Agpalilik (Buchwald, 1975, table 30) and indeed this holds true for all magmatic groups. Esbensen et a!. (1982) note the possibility that Agpalilik might turn out to be somewhat anomalous in this respect, being by far the most S-endowed IIIAB iron known. There are reasons to believe that other S-rich irons may be recognized when sufficiently large sections are studied, however, and further study is needed to clarify this. Jrrespective of these uncertainties, the model of late crystallisation/fractionation that we have been able to sketch, because of this (un)fortunate trap-

230

ping, stands as a valid yardstick with which to measure the potential maximal fractionation of both the IJIAB core in particular and the magmatic iron meteorite groups in general. Hence, the main result of the present study relates this three-component model of the major element system Fe—Ni—S to the core composition and crystallisation mode of these planetoid solar system bodies.

8. The Earth’s core—a possible match? For one terrestrial planet at least, the Earth itself, some evidence exists for a bulk chemistry not too dissimilar to that of the iron meteorites in general, though a great deal of discussion is still going on. Gubbins et al. (1979) developed a quite sophisticated geophysical model for the inner/outer core solidification/fractionation of the Earth, being able to erect a two-component model: the liquid core is assumed to be an ideal mixture of a ‘light’ and a ‘heavy’ component, the latter making up the solid(ifying) inner core. The light component is considered to be present in the outer core exclusively. Masters (1978) found no good evidence for a stratified outer core, thermally or chemically. A fundamental ambiguity as to the nature of this ‘light’ component exists. The two main contenders would appear to be sulphur and oxygen, with silicon being another perhaps less likely candidate (see reviews by Brett, 1976; Gubbins et al., 1979; Ringwood, 1979). Here we do not enter into this discussion, we only present some possible analogies that may be applicable to the case of the core of the Earth, provided one considers the case of S as most likely (Gubbins et al., 1979; Ahrens, 1979). If the light component is indeed sulphur, the results of Brett and Bell (1969) augmented by Usselman (1975a,b) may indicate that a reasonably close correspondence to an ideal mixture of Fe and S might be the case for the outer core from high pressure experimental work on the Fe—S system. If the light component is Si, no modelling such as that of Gubbins et al. (1979) can hold true (Braginsky, 1963). If this component is taken to be oxygen, as advocated by Ringwood (1979), the

Fe—FeO system at very high pressures is likely to behave analogously to the Fe—FeS system at low pressure. Assuming S then, from the physical modelling of Gubbins et a!. (1979), the solidus—liquidus freezing range must also be taken into consideration. Freezing of the heavy component drives a progressive differentiation of the outer core, effectively homogenised by thermally- and/or chemically-driven convection. As can be seen, this situation has very much in common with the physical and chemical setting of the interpreted crystallisation—fractionation of the smaller meteorite parent body cores outlined above, the main difference relating to the large ambient pressure differences. For planetoidal/asteroidal cores, pressure effects on the liquidus—solidus relationships are likely to be of negligible significance due to the relatively small size of these objects. For the case of the Earth’s core no experimental evidence can be brought to bear directly on this question, but most probably the very much larger ambient pressures will make for a substantial decrease of the freezing interval; Gubbins et al. (1979) also assumed that this interval was ‘small’. We offer a three component model in this context, and tentatively suggest that the principles of interpretation of the Fe—Ni—S model of the planetoidal cores might also pertain to a refined understanding of the fractionation taking place attending the present inner/outer core solidification and can add one more component to this picture. The Ni-fractionation of the planetoidal bodies, treated above, has always been the fractionation parameter ‘par excellence’. Of course applying these planetoidal concepts on a truly planetary scale is dangerous, but hopefully not fatally over-optimistic (see for example Fearn and Loper, 1981) * Interestingly a key lecture on the origin of the solar system comes somewhat obliquely to the rescue in this respect. Geo-

*

The notion of inner core growth via the dendritic mode has also suggested by geophysical workers (e.g. (Brown Loper, 1981).been Latest high pressure work on the Fe system and McQueen, 1981) hints at an S-content for the outer core of 10% or less, and an estimate at the core—mantle boundary in the range of 5-9% (at 3700°C).

231

physical arguments in favour of planetary accretion of protoplanets complete with already differentiated cores and surrounding silicate shells were put forward by Tozer (1976). In such a context the present findings would appear to be slightly more at home; perhaps they can be of value in making testable deductions as to parts of the detailed chemical picture of the make up of such protoplanetary objects, a point certainly in line with the philosophical message in Tozer’s paper, to which we strongly subscribe. Hence, the title of this paper. As a glimpse of such prospects we can compare some independent estimates of the bulk concentration of this light, 5, component in the outer core of the Earth with an estimate of the initial S-content of the IIIAB core in a broader planetological setting. Gubbins et al. (1979) consider a not too definite range from 2—18% 5 with a value of 10% used in further studies of the thermal evolution of the Earth’s core. Usselman (1975a) studied the Fe—Ni—S system from 30 to 100 kb and developed a model for the Earth’s core thereon, Usselman (197Sb). He derived a range of possible contents of 9—13% S at the mantle—outer core boundary, with 5—8% S at the inner/outer core interface. Unfortunately only the most Fe-rich portion of the Fe—Ni—S system was investigated, at Ni-contents below 6.S%, where he found no great deviations from the Fe—S system. This Ni-concentration range barely touches on the iron meteoritical realm, only encompassing the magmatic group JJA. Most magmatic irons are more Ni-rich than this, in the range 5.5—20%. However, the findings of Usselman indicate that the cotectic relationships between troilite and Fe—Ni metal may still be qualitatively comparable to Fig. 5 at ambient core pressures, at least close to the A-end of the AB boundary. These concentration ranges are probably reasonable in relation to the bulk core composition, perhaps slightly less so for the presumably 5- and Ni-enriched outer core, Esbensen et al. (1982) inferred a value of 1.4% S as the initial S-content of the bulk JJIAB core. Above it was noted that the maximum possible S-enrichment corresponds to some 25% S. The Ni-fractionation of group JJIAB is modest (73 10.6%) so that although trace element fractiona-

tion, e.g. for Jr can be severe (factor of several thousands), the major element fractionation is only very modest, corresponding to distribution coefficients close to unity, e.g. for Ni Narayan (1980) found kNI = 0.88. Perhaps we are entitled to suggest a similar trace element/major element relative fractionation for the protoplanets going into the make up of what we now call the planet Earth; perhaps the analogy can be drawn to encompass such aspects of the presently ongoing fractionation of the inner/outer core of the Earth. By calculating the systematic covariances between such key elements as used here, Ni, S and Jr, etc., we will be able to quantify such relative fractionation behaviour with the aid of the threecomponent core composition model, Thus, a possibility exists for integrating the geophysical picture of (proto)planet(oidal/ary) cores with the chemical setting of their crystallisation/fractionation, though the subject is only hinted at here.

Acknowledgements We would like to draw attention to a paper on the role of S in the evolution of the parental cores of the iron meteorites, Kracher and Wasson (1982), which in almost any conceivable way forms a cosmochemical complement to the present paper. We thank our coauthors in Esbensen et al. (1982), John Wasson and Dan Malvin, for permission to publish these our very own thoughts. While the data earlier were presented by all four of us, the present interpretations rest on our shoulders exclusively. Technical assistance with respect to the INAA was expertly given by John Willis and Greg Kallemeyn, with A. Zane doing the irradiation work. One of us (K.E.) was supported by a Danish State Natural Science Research Council grant, which was greatly appreciated.

References Ahrens, T.J., 1979. Equations of state of iron sulphide and constraints on the sulphur content of the Earth. J. Geophys. es.,. —

232 Braginsky, SI., 1963. Structure of the F layer and reasons for convection in the Earth’s core. Dokl. Akad. Nauk. SSSR, 149: 1311—1314. Brett, R., 1976. The current status of speculations on the composition of the core of the Earth. Rev. Geophys. Space Phys., 14: 375—383. Brett, R. and Bell, P.M., 1969. Melting relations in the Fe-rich portion of the system Fe—S at 30 kb pressure. Earth Planet, Sci. Lett., 6: 479—482. Brown and McQueen, 1981. Earth’s core iron. Eos, Trans. Am. Geophys. Union, 62: 578. Buchwald, V.F., 1971. The Cape York shower, a typical group lilA iron meteorite, formed by directional solidification in a gravity field. Meteoritics, 6: 252—253 (abstr.). Buchwald, V.F., 1975. Handbook of Iron Meteorites, Vol. 1—3. University of California Press, Berkeley, CA, 1418 pp. Esbensen, K.H., Wasson, J.T., Malvin, D.J. and Buchwald, V.F., 1980. Detailed chemical investigations of sections through a large Cape York iron. Meteoritics, 15: 288—289 (abstr.). Esbensen, K.H., Buchwald, V.F., Malvin, D.J. and Wasson, J.T., 1982. Systematic compositional variations in the Cape York iron meteorite. Submitted to Geochim. Cosmochim. Acta. Fearn, D.R. and Loper, D.E., 1981. The evolution of an iron-poor core. I. Constraints on the growth of the inner core. Submitted to Geophys. Astrophys. Fluid Dyn. Flemings, MC., Poirier, DR., Barone, R.V. and Brody, H.D., 1970. Microsegregation in iron-base alloys. J. Iron Steel Inst., 208: 371—381. Goldstein, ii., Narayan, C. and Soroka, W., 1979. Solidifica(ion of metallic cores in meteoritic parent bodies. Meteoritics, 14: 403 (abstr.). Gubbins, D., Masters, T.G. and Jacobs, J.A., 1979. Thermal evolution of the Earth’s core. Geophys. J.R. Astron. Soc., 59: 57—99. Kracher, A. and Wasson, J.T., 1982. The role of S in the evolution of the parental cores of the iron meteorites, Submitted to Geochim. Cosmochim. Acta. Kracher, A., Kurat, G. and Buchwald, V.F., 1977. Cape York: the extraordinary mineralogy of an ordinary iron meteorite and its implication for the genesis of IIIAB irons. Geochem. J., 11: 207—217. Kullerud, G., Yund, R.A. and Moh, G.H., 1969. Phase relations in the Cu—Fe—S, Cu—Ni—S, and Fe—Ni—S systems. In: H.D.B. Wilson (Editor), Magmatic Ore Deposits. Econ. Geol. Monogr., pp. 323—343. Loper, D.E., 1981. News flash. Eos, Trans. Am. Geophys. Union, 62: 609.

Masters, G., 1978. Observational Constraints on the chemical and thermal structure of the Earth’s deep interior. Geophys. J.R. Astron. Soc., 57: 507—534. Narayan, C., 1980. Ternary partition coefficients in Fe—Ni—X alloys—implications on the solidification of iron meteorites. Thesis, Lehigh Univ., 124 pp. Narayan, C. and Goldstein, J.I., 1980. Experimental model for chemical fractionation of iron meteorites. Meteoritics, 15: 335—336 (abstr.). Ringwood, A.E., 1979. Origin of the Earth and the Moon. Springer, New York, NY, 295 pp. Scott, E.R.D., 1972. Chemical fractionation in iron meteorites and its interpretation. Geochim. Cosmochim. Acta, 36: 1205—1236. Scott, E.R.D., 1977. Parent bodies of iron meteorites. In: A.H. Delsemme (Editor), Comets, Asteroids, Meteorites—Interrelations, Evolution and Origins. University of Toledo Press, pp. 439—444. Scott, E.R.D., 1979. Origin of iron meteorites. In: T. Gehrels and MS. Matthews (Editors), Asteroids. University of Arizona Press, Tucson, AZ, pp. 892—925. Scott, E.R.D. and Wasson, iT., 1975. Classification and properties of iron meteorites. Rev. Geophys. Space Phys., 13: 527—546. Scott, E.R.D., Wasson, J.T. and Buchwald, V.F., 1973. The chemical classification of iron meteorites. VII. A reinvestigation of irons with Ge concentrations between 25 and 80 ppm. Geochim. Cosmochini. Acta, 37: 1957—1983. Singh, SN., Bardes, B.P. and Flemings, MC., 1970. Solution treatment of cast Al-4.5 pci Cu alloy. Metal Trans.. I: 1383—1388. Tozer, D.C., 1976. Terrestrial planet evolution and the observational consequences of their formation. In: S.F. Dermott (Editor), Origin of the Solar System. Wiley. New York, NY, pp. 433—462. Usselman, TM., 1975a. Experimental approach to the state of the core. I. The liquidus relations of the Fe-rich portion of the Fe—Ni—S system from 30 to 100 kb. Am. J. Sci.. 275: 278—290. Usselman, T.M., 1975b. Experimental approach to the state of the core. II. Composition and thermal regime. Am. J. Sci., 275: 29 1—303. Wasson, J.T., 1974. Meteorites. Springer-Verlag, Heidelberg, 316 pp. Wasson, J.T., Willis, J., Wai, C.H. and Kracher, A., 1980. Origin of iron meteorite groups lAB and IIICD. Z. Naturforsch., 35a: 781—795. Willis, J., 1980. The bulk composition of iron meteorite parent bodies. Ph.D. thesis Univ. California, Los Angeles, CA, 208 pp.