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 (Worsham et al., 2016).

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. Most of the IAB complex members contain reduced, near-chondritic silicates containing planetary-type rare gases. These silicates are closely related to the chondritic winonaites, and likely formed on the same parent body during different phases of impact events.

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.