This is a chemically varied group of meteorites composed mainly of FeNi metal with small amounts of other minerals and trace elements. The magmatic iron meteorites (e.g., IIAB, IIIAB, and IVA) formed within ~0.51 m.y. after CAI formation when short-lived radionuclides were extant. They formed in the cores of small, 6400-km-diameter, partially to completely differentiated asteroids, although some are more consistent with formation in small impact-melt pools distributed within very small parent bodies. The accretion and differentiation of these iron parent asteroids predated chondrule formation and accretion into chondritic asteroids.
Iron meteorites have been resolved into 13 distinct chemical groups, with an "anomalous" or "ungrouped" designation given to the approximately 15% of irons with certain elemental abundances that plot outside of the normal ranges of the main groups. The main groups are resolved based on a congruence in the percentages of nickel (Ni) and certain trace elements, mainly germanium (Ge) and gallium (Ga). These two are especially useful because of their wide range across the entire iron spectrum, but narrow range within specific iron groups. Some other elements used to resolve groups are iridium (Ir), copper (Cu), cobalt (Co), and gold (Au); each resolved group represents an origin from a unique asteroid accreted in a unique nebular region. The occurrence of either positive or inverse correlations among the ratios of Ge, Ga, Ni, and Ir serves as a useful grouping determinant. Likewise, the consistent variation of certain elemental concentrations in the irons serves as a grouping determinant. Another useful, but less precise, grouping method is based on similarities in the macrostructure of etched irons, and is influenced by the bulk nickel content of taenite in association with the cooling rate. This feature primarily categorizes irons as either hexahedrites, octahedrites, or ataxites.
This group is one of the most well represented groups of iron meteorites, including the majority that contain silicate inclusions, most of which have nearly chondritic compositions. As evidenced by an IXe age of ~4.56 b.y., a high retention of planetary-type noble gases, and the small size of the precursor taenite crystals, group IAB irons cooled rapidly below temperatures necessary for isotopic exchange a very short time after formation. This suggests a non-magmatic origin without undergoing fractional crystallization. The implication is that the parent body was never in a fully molten state, and the observed metalsilicate fractionation occurred during accretion, during an impact-melt event, or more likely, through crystal segregation processes in distinct parental melt pools. Cooling rate data and other studies suggest that this body may have experienced a catastrophic breakup and reassembly event (Benedix et al., 2005). Trace element studies show that Ge is positively correlated with Ga, and inversely correlated with Ni. Another resolving characteristic is the proportionally higher contents of Ge and Ga for a given Ni content compared to other meteorites. The Thomson (Widmanstätten) structure seen in this group is commonly coarse, but spans the range of structures from hexahedrites to ataxites. Inclusions of troilite, graphite, and cohenite can be abundant. The similarity of group I Ge/Ga ratios and O-isotopic compositions with those of carbonaceous chondrites suggests that this iron group was originally formed from metal-rich carbonaceous chondrite material similar to CR and CH material.
Current studies of the IAB iron group, utilizing the more definitive elementAu diagrams, have resolved five well-defined subgroups in addition to the large main group (Wasson and Kallemeyn, 2002). Subgroup 1 is the most closely related to the main group, while subgroups 2 and 3 correspond to the old groups IIIC and IIID. Subgroups 4 and 5 are more distantly related to the main group. In addition to these, they resolved two grouplets, five duos, and numerous IAB-related members. An outline of the taxonomic system of the IAB Complex that they have proposed follows:
main group (MG)
low-Au, low-Ni subgroup (sLL)
low-Au, medium-Ni subgroup (sLM); corresponding to the old group IIIC
low-Au, high-Ni subgroup (sLH); corresponding to the old group IIID
high-Au, low-Ni subgroup (sHL)
high-Au, high-Ni subgroup (sHH)
Udei Station grouplet, closely related to sLL
Pitts grouplet, intermediate between sLL and sLM
NWA 468 duo
Twin City duo
various single IAB-related irons
The close similarities that exist among all of these iron groupings suggest that they all formed from similar chondritic material on a common parent body through independent impact processes affecting distinct melt pools. Impact melting leading to the redistribution of volatile elements and reduction of metallic Fe could have produced the compositional differences found among all of them. In fact, the pairs of groups consisting of [sLH and sLM], [MG and sLL], and [sHH and sHL] may be merged into just three groups if future studies of these IAB-related subgroups cannot be better resolved (Wasson, 2011). Moreover, the sHL member Sombrerete has a much more negative Δ17O than typical IAB irons, suggesting an origin for this particular subgroup on a different asteroid. The fact that a large concentration of sHL members has been found near Erfoud, Morocco is consistent with the scenario that this group is derived from impact-melt pods on a single unique asteroid.
These irons probably formed from small melt pods of FeNi-metal within the silicate mantle of their parent body where complete melting did not occur. Silicate material was mixed with the viscous FeNi metal, cooling to form silicated irons. Most belong to the three nonmagmatic groups IAB, IIICD, and IIE. Group IIE silicated irons are related to the H chondrites, while the unique silicated iron Steinbach is related to group IVA irons.