Iron, IAB complex, sLM subgroup
standby for lamesa photo
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) derived 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 Livingstone [Algarrabo duo]) to 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 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).

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

standby for iab iron formation diagram
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'

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.

Members of the medium- and high-Ni subgroups are best resolved from the low-Ni IAB members based on Ir/Ni relationships, since 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.