Lamesa

Iron, IAB complex, sLM subgroup

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 kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word 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 bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today.. A defining characteristic of these two subgroups are the presence of the carbide haxoniteIron-nickel carbide, (Fe,Ni)23C6, found in iron meteorites.. The fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. trends for the IAB iron meteorites suggest a nonmagmatic origin without fractional crystallizationA crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a key process in the formation of igneous rocks during the process of magmatic differentiation. Also known as crystal fractionation.. 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 regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).   of a carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than parent body, which then experienced variable degrees of impact mixing and crystal segregation/fractional crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. 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 neutronCharge-neutral hadron with a mass of 1.6748 x 10 kg, equivalent to 939.573 MeV, and an intrinsic angular momentum, or spin, of ½ (in units of h/2π). The neutron is a nucleon, one of the two basic constituents of all atomic nuclei (apart from H, which consists of a single capture by 182W due to galactic cosmic raysHigh-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one of the few direct samples of matter from outside our solar system and travel through space at nearly the speed of light. These charged particles – positively charged protons or, Hunt et al. (2018) derived the timing of metal–silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] and the ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they. 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 metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn 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).
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’
(https://doi.org/10.1016/j.epsl.2017.11.034)

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 meteoriteIron meteorites consist mostly of metallic iron alloyed with typically between ~5 to ~30 wt% nickel. The main metal phases are kamacite α-(Fe, Ni) and taenite y-(Fe, Ni). Based on their group classification, they may also contain a small weight percentage of one or more of the following minerals: • complex’
(https://doi.org/10.1016/j.epsl.2017.02.044) Most of the IAB complex members contain reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants, near-chondritic silicates containing planetary-type rare gases, and these are closely related to the chondritic winonaites. Based on the similar silicate textures, reduced mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous 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. AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The/Ni coupled diagram, Hidaka et al. (2015) determined that FeNi-metal in the winonaitea partially differentiated asteroid that was disrupted just as it began to form an Fe core and a silicate-rich crust. This disrupting impact mixed silicates into molten Ni-Fe metal forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites. A few winonaites 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 isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: H, H (deuterium), and H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. 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) employed 17O 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 coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. and mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and groups (e.g., IAB with winonaites, IIE with H chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. 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 cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce 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. 17O vs. ε54Cr for Irons and Pallasites

click on photo for a magnified view

Diagrams 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, 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 oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of  4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below.. This specimen was previously part of the Oscar Monnig Collection at Texas Christian University, Fort Worth, Texas.