Iron, IAB complex, sLM subgroup
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Found 1981
32° 53' N., 101° 53' W.

A single mass of 16.9 kg was found 16 km north and 3 km west of Lamesa, Dawson County, Texas. It is was previously resolved to be a member of the small IIIC iron group based on its lower Ni content and higher contents of Ga, Ge, and Ir. Recently, a taxonomic revision was proposed by Wasson and Kallemeyn (2002) that includes iron meteorites from the IAB–IIICD group, along with numerous IAB-related meteorites. On a Ni-Au diagram, Lamesa and the other former IIIC members resolve a low-Au, medium-Ni subgroup (sLM), while the former IIID members resolve a low-Au, high-Ni subgroup (sLH).

Structurally, Lamesa has a fine Thomson (Widmanstätten) structure with a kamacite bandwidth of 0.3 mm. The cosmic-ray exposure ages of sLM members are ~700 m.y., while those of sLH members are ~200 m.y., suggesting a two-stage breakup event involving separate regions of the IAB parent body. A defining characteristic of these two subgroups are the presence of the carbide haxonite. The fractionation trends for the IAB iron meteorites suggest a nonmagmatic origin without fractional crystallization. A model involving crystal segregation was found to be most consistent with the HSE patterns in this subgroup (Worsham et al., 2016). Extrapolation of Ni and other elemental trends defines a continuum for the IAB complex, and it is considered that most members originated on a common asteroid. The different trends found among IAB complex irons are most consistent with separate impact melt pools within the regolith of a carbonaceous chondrite parent body, which then experienced variable degrees of impact mixing and crystal segregation/fractional crystallization as well as different cooling rates and equilibration conditions. It remains unresolved whether or not some IAB subgroups (e.g., sLM, sLH) share a genetic relationship with the IAB main group, while another subgroup (sHL) has been shown to be most consistent with formation on a separate parent body; the sHH subgroup has not yet been isotopically resolved (Worsham et al., 2016, 2017).

Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) determined the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungrouped Caddo County [Udei Station grouplet] and Livingston [Algarrabo duo]) to be 6.0 (±0.8) m.y. after CAIs. They contend that a catastrophic breakup and reassembly occurred during which different silicate lithologies were mixed. Based on the CRE-corrected W data, Worsham et al. (2017) derived a metal–silicate segregation age corresponding to 3.4 (±0.7), 5.0 (±1.0), and 5.1 (±0.6) m.y. after CAIs for the MG, sLL, and sLM subgroups, respectively. They argue that a breakup and reassembly event would have also mixed different metal lithologies together, and would have equilibrated the W systematics of the MG and sLL subgroup. The top schematic diagram below is the model of Hunt et al. (2018), which shows the early history of the 120(+)-km-diameter IAB parent body based on constraints provided by the timing of metal segregation. The schematic diagram beneath that shows the impact-generated melt model of Worsham et al. (2017).

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Diagram credit: Hunt et al., EPSL, vol. 482, p. 497 (2018, open access link)
'Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling'

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Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
'Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex'

In their chemical composition analysis of metal in primitive achondrites, which included the two winonaites NWA 725 and Y-8005, Hidaka et al. (2019) demonstrated that these two meteorites have significant differences in abundances of the moderately volatile siderophile elements Cu, Sb, Ga, Ge, and Sn. Moreover, utilizing coupled diagrams for various elemental abundance ratios in metal, they demonstrated that NWA 725 does not plot together with Y-8005 in the sLL subgroup field, but rather it generally plots in a unique space between the sLL and the overlapping sHL and sLM fields; notably, in the Ga/Ge coupled diagram NWA 725 plots at the very border of the sHL and sLM fields (see diagrams below). It was concluded that the metal in NWA 725 and Y-8005 had separate origins, and that NWA 725 possibly represents a reassembled daughter body that was contaminated by metal from the projectile which disrupted the IAB parent body.

Chemical Composition of Winonaite Metals and IAB Iron Subgroups
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Diagrams credit: Hidaka et al., MAPS, vol. 54, #5, p. 1158 (2019)
'Siderophile element characteristics of acapulcoite–lodranites and winonaites: Implications for the early differentiation processes of their parent bodies'

Most of the IAB complex members contain reduced, near-chondritic silicates containing planetary-type rare gases, and these are closely related to the chondritic winonaites. Based on the similar silicate textures, reduced mineral chemistry, and O and Mo isotopes, it is presumed that the winonaites and the IAB complex irons originated on a common parent body. Utilizing a Ge/Ni vs. Au/Ni coupled diagram, Hidaka et al. (2015) determined that FeNi-metal in the winonaite Y-8005 plots in the field of the sLL subgroup of the IAB complex irons. Moreover, the metal in Y-8005 retains a near chondritic composition likely representative of the precursor material of the parent body. In view of these findings, Hidaka et al. (2015) suggest that the sLL subgroup rather than the MG represents the primitive metal of the IAB–winonaite parent body, with the MG possibly representing a partial melt of the sLL subgroup. In a subsequent analysis of the IAB iron complex, Worsham et al. (2017) demonstrated that the Mo isotope data for the two winonaites they studied, Winona and HaH 193, also attest to a common parent body for winonaites and the MG/sLL irons.

Dey et al. (2019) made use of Δ17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were used in the study, and it was demonstrated on an O–Cr coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, the silicate component plots in a distinct region at higher values (see diagram below). From these results they ascertained that the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-like parent body and silicate from an unrelated and otherwise unsampled parent body. It may also be reasonably inferred that winonaites derive from a separate parent body (Goldstein et al., 2021). Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long duration cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, which produced the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages.

ε54Cr vs. Δ17O for Irons and Pallasites
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Diagram credit: Dey et al., 50th LPSC, #2977 (2019)

Members of the medium- and high-Ni subgroups are best resolved from the low-Ni IAB members based on Ir/Ni relationships because Ir shows more variation at lower Ni concentrations. This resolves the two subgroups well at higher Ni concentrations but fails to resolve them at the lower concentrations. The occurrence of certain very-high-Ni members within the resolved IAB complex can be explained by limited fractional crystallization in small, meter-sized, Ni-rich impact-melt pools.

New members of the sLM subgroup of the IAB complex have been found in Northwest Africa, including NWA 968, 2680, 4024, and 5980, the latter two classified as winonaites (Wasson, 2011). The specimen of Lamesa shown above is a 40.6 g partial slice with oxidized fusion crust. This specimen was previously part of the Oscar Monnig Collection at Texas Christian University, Fort Worth, Texas.