APPENDIX

PART III

IRONS

This is a chemically varied group of meteorites composed mainly of FeNi metal with small amounts of other minerals and trace elements. Magmatic iron meteorites formed within 0.5–1 m.y. of CAI formation when short-lived radionuclides were extant. They formed in the cores of small, 6–400-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.

IAB
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 a Rb–Sr age of ~4.7 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 metal–silicate fractionation occurred in the solar nebula, during accretion, or more likely, during an impact-melt event. Studies suggest that this body underwent 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 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 with that 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 element–Au 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
Algarrabo duo
Mundrabilla duo
Britstown duo
NWA 468 duo
Twin City duo
various single IAB-related irons

The close similarities among all of these iron groupings suggest that they all formed from similar chondritic material, perhaps on a common parent body, through independent impact processes. Impact melting leading to the redistribution of volatile elements and reduction of metallic Fe could have produced the compositional differences found in all of them.

(IB)
Previously, an arbitrary boundary was established to separate the IA and IB subgroups; those members with low Ni contents and Ge contents above 190 ppm were designated group IA, while those members with high Ni contents and Ge contents below 190 ppm were designated group IB. Additionally, extrapolation of Ni and other elemental trends defined a continuum for the IAB and IIICD groups, suggesting they originated from different sections of a common parent asteroid. The silicate inclusions in this group are closely related to the group of meteorites known as winonaites, and they probably originated on a common parent body.

IC
This is a small, structurally diverse group that plots close to the IAB field, but with a lower elemental abundance of Au and As. Most members of this igneous group contain abundant cohenite.

IIABG
Group IIA consists of single-crystal hexahedrites in which Ni, Ge, and Ga are positively correlated with each other, and inversely correlated with Ir. The large range in Ir contents among groups IIA and IIB is consistent with an efficient fractional crystallization model, while the continuity in the bulk compositions of both groups is consistent with derivation from a common magma source. A common origin is further supported by their similar cosmic-ray exposure ages. An arbitrary boundary separating these two igneous groups was once proposed to be an Ir content of 1 PPM, but this artificial division is no longer considered viable. Low degrees of trapped melt are manefest as FeS nodules. The similarity of group IIA Ge/Ga ratios with that of carbonaceous and enstatite chondrites suggests that this iron group formed from similar precursor materials, but O-isotopic studies have not yet been conducted. Recently, group IIG iron members have been shown to be chemically similar to those of the IIAB iron group, forming extensions to IIAB trends on element–Au diagrams (Wasson and Choe, 2009). The IIG irons are thought to have crystallized in a lower, P-rich realm of the evolved IIAB core (see IIG description below).

(IIB)
Group IIB members exhibit the Thomson structure of a coarsest octahedrite. In contrast to the correlations in IIA, there is a positive correlation between Ge, Ga, and Ir in IIB, and an inverse correlation with Ni. There is both a continuity in composition and a similarity in cosmic-ray age among groups IIA and IIB, which suggests a genetic relationship exists. An inverse correlation exists between the cosmic-ray exposure (CRE) age and the Ir content. This fact can be explained through a scenario whereby Ir is concentrated at depth, and members of group IIB were closer to the surface where they were stripped from the asteroid first, thus establishing a correlation between a higher CRE age and a lower Ir content.

IIC
Group IIC consists of plessitic octahedrites in which Ni, Ge, and Ga are positively correlated with each other. Ni is inversely correlated with Ir. This group appears to have undergone igneous fractionation in the core.

IID
This is the fourth largest magmatic iron group. Ni, Ge, and Ga are positively correlated with each other, and inversely correlated with Ir. Members are enriched in refractory and volatile siderophiles, but have a low S content. This contradictory condition is best explained by a scenario in which this group underwent slow internal heating through the decay of short-lived radionuclides, which produced igneous fractionation in the core, resulting in a very high Ga/Ge ratio and a schreibersite-rich composition (Wasson and Huber, 2006). As temperatures slowly increased, all of the S was entrained within an FeS-rich melt, which separated to form a core. As temperatures continued to increase, a second P-rich melt formed and drained through the S-rich layer establishing a new inner core a few km wide, from which the group-IID meteorites would eventually originate. As temperatures finally began to cool, the inner and outer cores became immiscible, with a diffusional barrier developing between them. Based on cooling rate comparisons, the IID parent body was ~40–100 km in diameter, or about 10 times larger than the IIC parent body.

IIE
Just as for group IAB-IIICD, the IIE parent body was never completely molten. It was likely during impact-heating events that the globular, Fe-rich silicates were mixed with metal. These events produced rapid heating of the chondritic material, with temperatures reaching higher levels than in IAB-IIICD melts due to a lower content of the elements C and S (having melt-inducing properties) in IIE. Consistent with these higher melting temperatures for group IIE are the differentiated and completely segregated silicates that form droplets rather than clasts. The higher temperatures are also responsible for the much lower levels of planetary-type rare gases than are found in IAB-IIICD. A wide variation in cooling rates is also found among IIE members, indicative of cooling at a broad range of depths within the parent body. Consequently, there is no typical structure for the IIE group. The group was divided into two metamorphic types—primitive (Netshaëvo, Techado, Watson) and differentiated (Miles, Weekeroo Station, Kodaikanal, Colomera, Elga)(McCoy, 1995; Ikeda et al., 1997). Based on bulk metal compositions and O-isotopes, it is thought that the IIE irons may have originated on or near the same parent body as the H chondrites.

IIF
Resolution of this group is based on several factors including structure, which ranges from ataxitic to plessitic, an unusually high Ge/Ga ratio, a high Ni content, a high Co and Cu content, and a low P content. The compositional trends are most consistent with igneous fractional crystallization of a core. Contrary to accepted theory, the Ni content is positively correlated with the Thomson (Widmanstätten) structure bandwidth. This occurrence could be explained by either differences in the cooling rate or by the effects of bulk P content on bandwidth growth. Oxygen-isotope data indicates a relationship exists between the IIF irons, the Eagle Station pallasites, and the CO–CV carbonaceous chondrites, with formation occurring in the outer solar system.

(IIG)
The discovery in 2000 of the iron meteorite Guanaco, which has a chemical composition similar to that of a grouplet of four previously found iron meteorites known together as the Bellsbank Quartet—namely, Bellsbank, La Primitiva, Tombigbee River, and Twannberg—satisfies the requisite number of five members in order to establish a new iron chemical group, thereupon designated IIG (Wasson, 2004). Members of this group are structurally hexahedrites transitional to coarsest octahedrites (H–Ogg), representing the most Ni-poor and schreibersite/P-rich samples of any iron group. Group IIG members are chemically most similar to members of the IIAB iron group, forming smooth extensions of IIAB trends on element–Au diagrams consistent with fractional crystallization. It has been proposed by Wasson and Choe (2009) that formation of IIG irons occurred inside isolated cavities which remained after crystallization of an evolved IIAB magma. After the loss of significant immiscible S the P/S ratio of the IIAB magma approached parity, at which point the P-rich IIG magma was able to begin to crystallize. IIG iron crystallization likely occurred in the lower regions of the IIAB core, while an immiscible and buoyant S-rich melt collected at the upper region of the chamber. Certain elements like Au and Ge were likely removed along with the S-rich melt phase, while the low-Ni content of IIG irons may be attributed to diffusion and redistibution of Ni out of metal and into schreibersite during the core's extended cooling history.

IIIAB
Group IIIAB irons are the most represented group of magmatic iron meteorites, accounting for almost a third of all known irons. Compositional, structural, CRE age, and cooling rate data all provide convincing evidence in support of the theory that groups IIIA and IIIB originated on the same parent body. The Ni content of group IIIA and IIIB members forms a continuous sequence between the groups. The Ge content of IIIA members overlaps that of IIIB members. In spite of the fact that these two groups have different structures (IIIA has mostly coarse textures while IIIB has mostly medium textures), and that they are negatively correlated with respect to Ge/Ni and C/Ni, they are considered to be genetically related. The compositional differences are likely due to late-stage chemical interactions during fractional crystallization involving mixing of different zones of the core. The IIIAB group members share a similar cosmic-ray exposure age of ~650 m.y., signifying a single breakup event at this time.

(IIIB)
An arbitrary boundary has been established dividing the IIIA and IIIB subgroups. Group IIIA members have a Ni content at the low range from 7.1 to 8.6%, while group IIIB members have a Ni content at the high range from 9.2 to 10.5%. The main-group pallasites have metal compositions and O-isotopes which are nearly identical to those of an evolved, high-Au, high-Ni IIIAB melt following ~80% crystallization of the core. It was once considered plausible that both main-group pallasites and IIIAB irons originated on a common parent body. However, metallographic cooling rates of these pallasites are not what one would expect given an origin at the core–mantle boundary, but instead, they are significantly slower than those of IIIAB irons. Moreover, the Re–Os chronometer demonstrates that main-group pallasites formed 60 m.y. later than IIIAB irons, raising further doubts about a IIIAB core–mantle origin for these pallasites.

IIICD
This group was incorporated into the IAB iron-meteorite complex (Wasson and Kallemeyn, 2002). Despite the wide range of Ge and Ga contents among members of the IIIC and IIID groups, plots of these two elements with Ni produce a smooth curve. In addition, similarities in their cooling rates make it likely that they originated on a common parent body. The trends for these two groups suggest a non-magmatic origin without fractional crystallization just as in the IAB group. Cosmic-ray exposure ages of IIIC members are ~700 m.y., while those of group IIID are ~200 m.y., suggesting a two-stage breakup event involving separate regions of a common parent body.

A defining characteristic of group IIICD members is the presence of the carbide haxonite. The members of the combined IIICD group are best resolved from the low-Ni IAB members based on Ni/Ir relationships since Ir shows more variation at lower Ni concentrations. This resolves the two groups well at higher Ni concentrations but fails to resolve them at the lower concentrations. Extrapolation of Ni and other elemental trends defines a continuum for the IAB and IIICD groups and it is probable they originated on a common asteroid. The occurrence of certain very high Ni members within the resolved IAB-IIICD group is explained by crystal settling in small, meter-sized, Ni-rich, impact-melt pools. However, most impact-melt pools cooled without experiencing this type of fractionation process. The most current taxonomic scheme proposed by Wasson and Kallemeyn (2002) for the IAB-IIICD irons can be found in the IAB section above.

(IIID)
The members of this small group are resolved from the IIIC group based on their high Ni contents (16.6% to 22.6%) compared to IIIC members (<13%). IIID members also have a more balanced ratio of Ge to Ga, with absolute values of Ge, Ga, and Ir much lower than those of IIIC members. Structurally, group IIIC and IIID members are all finest octahedrites with the exception of those IIID members having the highest Ni contents. In these meteorites, an ataxite structure is found.

IIIE
In spite of its similarities to the IIIAB group, resolution of this small group has been made based on its unique Ge,Ga/Ni and Ga,Co/Au ratios, swollen kamacite band structure, presence of inclusions of the carbide haxonite, and lower CRE ages. The two groups are marginally different in their C-isotopic composition and in their occurrence of the nitride carlsbergite.

IIIF
Members of this low-Ni, low-Ge group exhibit several characteristics that help to resolve this group from the others. They have an inverse correlation between Ni and Ge contents, very low Co and P contents, and uniform cooling rates. Inclusions are rare in this structurally diverse group.

IVA
There is a positive correlation in the concentrations of Ge with Ga, and Ge with Ni in this group—this supports its resolution as an independent group. The compositional trends are most consistent with fractional crystallization of a core. The low Ni content is thought to be a result of Fe dilution derived both from dissociation of FeS and by reduction of FeO (Wasson et al., 2006). A lower than normal Ge and Ga content for this iron group suggests their loss through high temperatures in an open system. Melting and differentiation of the iron and its subsequent drainage to form a core was possibly initiated by a large impact(s) onto a porous L–LL-type asteroid between ~20 and 160 km in diameter, rather than by radiogenic heating (26Al, 60Fe) alone (Wasson and Kunihiro, 2004; Kunihiro et al., 2004).

One scenario suggests that a catastrophic impact disrupted the IVA parent body, and large core fragments were mixed with cooler mantle fragments, producing the wide range of metallographic cooling rates. However, current studies by Wasson et al. (2006) dispute the possibility of this impact-scrambling model, and suggest instead that IVA irons experienced a typical cooling scenario with significant thermal gradients. This object was a molten metallic core 200–400 km in diameter, and likely had original dimensions of >600 km when accounting for all of the differentiated layers. Later collisional events led to its breakup, and some orthopyroxene was converted into clinopyroxene. Members of group IVA can be separated into high-Ni and low-Ni subsets, with an inverse correlation between Ni content and cooling rate. Almost all members of the IVA group exhibit a fine structure.

Only two IVA members, Steinbach and São João Nepomuceno, contain significant amounts of silicates, these being orthopyroxene and the rare silica mineral tridymite. Two other IVA irons, Gibeon and Bishop Canyon, contain veins of tridymite formed by vapor deposition of a silica-rich source. The metal–orthopyrene–silica composition of Steinbach and SJN was formed by impact injection through fractures of the hot, solidified metal core. Formation of this core occurred within ~1 m.y. after CAI formation (Burkhardt et al., 2008).

IVB
The IVB group has the lowest abundances of the volatile siderophile elements such as Ga and Ge. This group shows a positive correlation in the Ga and Ge concentrations, as well as Ge with Ni, which supports its resolution as an independent group. The members of group IVB have an ataxite or plessitic composition with micron-sized kamacite spindles (forming a microscopic Thomson or Widmanstätten structure) and associated phosphides, both of which are more abundant in the members with higher Ni contents. Irons of group IVB have a very high enrichment in refractory siderophile elements, attributable to both nebular and parent melt fractionation processes. This group experienced very high internal temperature conditions (1760°K), resulting in the strong depletions of volatile siderophile elements in the melt. Approximately 72% of the Fe has been lost from the metal to the silicate through oxidation processes, which is responsible for the observed high Ni content. The compositional trends are most consistent with fractional crystallization from a low-S (~1 wt%) melt involving mixing of different zones of a core. The absence of silicate inclusions is also consistent with a formation within the core of the parent body.

Yang et al. (2009) determined that the varied CRE history of the IVB group, as well as the wide range of cooling rates measured for members of the group, suggest that a multiple breakup of the parent body and/or removal of the insulating mantle occurred. Goldstein et al. (2010) found that Ni concentration profiles measured along the kamacite–taenite interface attest to one of the fastest cooling rates among iron groups, but also encompass a wide range of cooling rates in a similar manner to IVA and IIIAB irons. However, in contrast to IVA and IIIAB irons, which crystallized inwards following mantle removal, low-Ni IVB irons cooled slower than high-Ni IVB irons consistent with concentric crystallization from the center outwards while temperatures were buffered by an insulating silicate mantle. That being said, to establish the wide variation in cooling rates that exists among IVB irons, the mantle would have to have been stripped prior to cooling below 600°C. Goldstein et al. (2010) and Yang et al. (2010) proposed that this impact event occurred while 25% of the outer core was still molten, and assert that IVB irons were derived from the 75% of the inner core that was solid, measuring 35–160 km. The inner core completed its final solidification after mantle removal (and possibly removal of the remaining liquid core), attested to by the wide variation in cooling rates among IVB irons. The core in which IVB iron crystallization occurred was calculated to have been 110–170 km in diameter on a pre-disrupted asteroid that had measured 220–340 km in diameter. The low-Ni IVB subgroup with the slowest cooling rate (475°K/m.y.) was located near the center of the core, while the high-Ni subgroup that crystallized late and had the fastest cooling rate (5000°K/m.y.) was located 62 km from the center. Interestingly, based on the length of time that the measured magnetic field persisted on the angrite parent body (8 m.y.), and the concomitant crystallization period of the IVB irons, it has been speculated (Campbell and Humayun, 2005) that the angrites may represent the silicate portion of the IVB parent body; more research is needed.

UNGROUPED
Many other irons that occur singly or in assemblages of less than five members remain "ungrouped". About 85% of the ~1,000 irons in our collections fall into one of the 13 main chemical groups, while another ~8% constitute 20 small grouplets. The remaining ~7% of the irons may have originated on their own unique parent body. Of these ~100 unique parent bodies, ~20 represent irons of impact-melt origin on chondritic asteroids, while the others represent irons of igneous origin in the cores of differentiated asteroids.

IRONS, SILICATED

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 non-magmatic 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.

CONTINUE TO
[PART I] Chondrites
[PART II] Achondrites
[PART IV] Stony-Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification
[APPENDECTOMY]

© 1997–2010 by David Weir