Vagn Fabritius Buchwald

The chemical classification of iron meteorites—VII. A reinvestigation of irons with Ge concentrations between 25 and 80 ppm

We report Ni, Ga, Ge and Ir concentrations for 193 irons. The compositional trends in groups IIIA and IIIB are redefined, and the suggestion by Wasson and Kimberlin that they represent a single fractionation sequence (group IIIAB) is confirmed. A new group, HIE, is similar in its properties to group IIIA but distinguished by lower Ga/Ni and Ge/Ni ratios, larger bandwidths and the formation of haxonite (Fe, Ni)23C6 in each of its members. A sixth member, Hassi-Jekna, has been added to group IIIC, extending its Ge range up to 70 ppm. The characteristics of these groups can be summarized as follows: Group Structure Ni% Ga(ppm) Ge(ppm) Ir(ppm) IIIA Om 7.1–9.3 7–23 32–47 0.17–19 IIIB Om 8.4–10.5 16–21 27–46 0.014–0.17 IIIC Off-Of 10.5–13.0 11–27 8.6–70 0.08–0.6 IIIE Og 8.2–8.9 17–19 34–37 0.05–0.6 The Ge-Ni correlation is positive in IIIA, negative in IIIB and IIIC, and there is no significant correlation in IIIE. San Cristobal is identified as a member of group IAB, thereby extending the Ge and Ni range of this group to 25 ppm and 25 per cent, respectively. Previous reports of wide cooling-rate variations in group IIIAB are not substantiated, and current evidence favors a core over a raisin-bread model for this group. There appears to be no genetic relationship between group IIIAB and either the pallasites or the mesosiderites Full-size table Table options View in workspace Download as CSV

Slag Analysis as a Method for the Characterization and Provenancing of Ancient Iron Objects

Abstract About 900 slags and iron objects from the period 700 b.c. to 1850 a.d. have been examined and analyzed by classical optical microscopy and by energy dispersive analytical methods. Although ancient iron objects are extremely heterogeneous, a definite correlation between the metal phase and its slag inclusions is shown; for example, wustite-rich slags are located in ferrite, whereas glassy slags are located in pearlite. The ratios SiO 2 /Al 2 O 3 , Al 2 O 3 /CaO, and so forth, of the slag inclusions are shown to be helpful in identifying the production site or provenance of an ancient iron object. These ratios are further shown not to be the same in irons produced by the direct process and in irons produced by fining or puddling. Emphasis is on iron production methods in Sweden, Norway, and Denmark, illuminating Scandinavian iron-making practices through 2500 years. The slag analysis method developed here may, however, be extended to yield information on other European sites.

Systematic compositional variations in the Cape York iron meteorite

Concentrations of Re, Ir and Au are nearly constant within individual masses of the Cape York IIIAB iron meteorite, but differences between masses can be as large as a factor of 2, the extremes being Savik (5.1 μg/g Ir) and Agpalilik (2.7 μg/g Ir). The S concentration shows a still larger range from 13 mg/g in Agpalilik to 1.4 mg/g in Savik. A relatively large compositional hiatus between Dog and Agpalilik probably reflects inadequate sampling of the original material. Concentrations of Ir vary by ~10% and Au by ~3% between the ends of an 85-cm section from the Agpalilik mass of Cape York, but other sections through Agpalilik show smaller variations. These concentration ranges are much larger than expected from radial crystallization of a moderately large (radius 10 s of km) core. These variations in the Agpalilik mass may reflect dendritic crystallization, or they may have resulted from the process that produced the large concentration range among the Cape York masses. Large gradients in Re and Ir and small gradients in Ni and Au were also observed in samples within 2 cm of a large (100 cm3) troilite nodule. These gradients may reflect rapidly changing solid/liquid distribution coefficients during the final crystallization of S-rich liquid. The compositional trends among the various masses can either be explained by mixing of disparate end members followed by diffusive homogenization on a scale of m, or by dendritic crystallization on the ceiling of the IIIAB magma chamber. The mixing of a solid similar in composition to Savik with a liquid in equilibrium with this solid yields a good match to the observed trends, in which case Agpalilik consists of a mixture of 64% liquid and 36% solids. The bulk S content of the IIIAB core is calculated to be 14 mg/g on the basis of this model.

Compositional trends and cooling rates of group IVB iron meteorites

Based on new neutron activation data for group IVB we find that log-element — log-Ni trends are best understood in terms of core formation and fractional crystallization. The limited compositional range found in group IVB seems to reflect the fact that, because of the low concentrations of S, P and C and the high concentration of Ni, kχ values are nearer unity than are those in other magmatic groups. Mean volatile abundances in group IVB are much lower than those found in any group of chondritic meteorites, suggesting that these low abundances were not entirely the result of nebular processes, but that planetary outgassing was also involved. We calculated cooling rates on the basis of a computer simulation of the growth of kamacite crystals; these calculations are particularly straightforward for the high-Ni irons since no local bulk Ni enrichment is involved. We estimate a mean IVB cooling rate of 170–230 K/Ma, the lower values based on 20 K undercooling, the higher on no undercooling. There is no dependence of cooling rate on chemical composition. The mean cooling rate of the low-volatile groups IVB and IVA are both much higher than those typical of iron-meteorite groups. This indicates small parent bodies, and reinforces the above suggestion that the low volatile contents resulted from planetary outgassing. There is a small compositional hiatus in group IVB, but since the sets on both sides of the hiatus form continuous trends on log-element — log-Ni diagrams and have the same cooling rates, it appears that both sets originated in a single oxidized, refractory-rich parent body. This sampling hiatus corresponds to 26% of the original core, a value shown to be typical for a random sequence sampled 11 times.

Phosphate Minerals in Meteorites and Lunar Rocks

Phosphorus is in meteorites and lunar rocks a minor, but in many respects important element. It occurs chemically bound as phosphates and phosphides, and it occurs in solid solution in the iron-nickel alloys kamacite and taenite. Broadly speaking, the phosphates are characteristic of the stone meteorites and of the inclusion-rich parts of the iron meteorites, while the phosphides, i.e., schreibersite and rhabdite, (Fe, Ni)3P, are mainly present in iron meteorites. In pallasites, phosphates, and phosphides occur together in significant amounts.

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

Abstract 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 dentritic solidification of at least part of the once fully molten meteorite parent body metallic core. We assess systematic chemical gradients displayed by Ir 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 Earths core would appear to be susceptible to interpretation within the confines of the principles of the present planet(oid) core crystallisation-fractionation model.

On the use of iron by the Eskimos in Greenland

Abstract Seventy-four iron objects were randomly selected from the archaeological items found in Greenland and have been stored in Copenhagen since about 1850. The objects consist of knives, ulos (knife used by Eskimo women for skinning), and harpoon blades, but also several nonworked fragments and some “hammerstones” were included. The objects were subjected to microscopical examination and x-ray microanalysis to determine their nature and mode of fabrication. The objects may be sorted into three groups. The majority of tools found north of Melville Bay were produced from small fragments of the Cape York iron meteorite shower that fell near Savigsivik over 2000 years ago. Half of the objects found in the Disko Bay area may be traced to occurrences of iron-bearing basalt, while the other half were produced from wrought iron. Some of these wrought-iron tools originated at Norse settlements and were apparently carried as far north as 77° by Norse ships as early as the 12th Century. Other wrought-iron tools were introduced by whalers, mainly of Dutch, Spanish and British origin, after about a.d. 1575. Some tools may derive from iron nails and hoops from wrecked ships. No signs of indigenous iron production have been detected.

The iron meteorite “Thule”, North Greenland

Abstract The paper describes a 48.6 kg siderite, which was found in 1955 on a nunatak near Thule, North Greenland. The Thule meteorite can be classified as a medium octahedrite with 8.13% Ni and other normal constituents. The micro-hardness of the phases are given. The examination of the well-preserved crust yielded data, which gave a value for heat ablation during the passage through the atmosphere of about 1.6 mm/sec.