Iron, IAB complex, main group
silicated iron
standby for landes photo
Found ~1930; Recognized 1968
38° 54' N., 79° 11' W.

A 69.8 kg mass was plowed up in a cornfield about a mile east of Landes Post Office, West Virginia; however, it was not recognized as meteoritic until 1968. Landes was previously classified as an anomalous IAB member due to its high Cu content. The IAB iron-meteorite complex, a systematic model recently proposed by Wasson and Kallemeyn (2002), comprises iron meteorites from the former IAB–IIICD group, as well as numerous related irons. The following compositional values were determined for inclusion into the IAB complex:

Au >1.3 ppm
As >10 ppm
Co >3,900 ppm
Sb >0.18 ppm
Ge/Ga ratio between 0.4 and 7

Many of the IAB complex members contain silicate inclusions, including types which are sulfide-rich, silicate-rich chondritic, silicate-rich non-chondritic, graphite-rich, and phosphate bearing. Based on bulk composition and REE abundance data for Landes, silicate inclusions are generally found to be chondritic, comprising nearly 40 vol% of the meteorite. The precursor of the IAB complex irons is considered to have been a volatile-rich planetesimal related to carbonaceous chondrites (Ruzicka, 2014).

On a Ni–Au diagram, Landes plots within the IAB main group and shares an Ar–Ar retention age of ~4.43 b.y. with Copiapo and certain other members, somewhat younger than that of typical IAB meteorites. This younger age could be due to later impact-heating events, or to terrestrial weathering processes (Bogard et al., 2005). 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, 2017).

Based on similar silicate textures, reduced mineral chemistry, and O-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. In addition, the metal in this winonaite retains a near chondritic composition likely representative of the precursor material of the parent body. In view of these findings, they suggest that the sLL subgroup rather than the main group of the IAB complex represents the primitive metal of the IAB–winonaite parent body, with the main group possibly representing a partial melt of the sLL subgroup.

In their study of plagioclase separates derived from individual silicate grains composing different inclusions within Landes, Caddo County, Campo del Cielo, and Ocotillo, Vogel and Renne (2008) found that corrected Ar–Ar ages have a significant range—from as old as ~4.55 b.y., near the probable onset of differentiation from radiogenic heating (26Ar), to as young as ~4.43 b.y., presumably reflecting grains that experienced the latest closure of the K–Ar system following reassembly and/or late impact events. The slope of a Pd–Ag isochron corresponds to an age of ~14.6 (+6.7/–7) m.y. after CAI formation (4.559–4.545 b.y. ago), reflecting closure of the Pd–Ag system and the mixing of metal and silicates (Theis et al., 2010). Schulz et al. (2012) identified some silicates in IAB Caddo County indicating that metal–silicate segregation occurred as early as 0.86 (+0.93/–0.86) m.y. after CAIs. Other age measurements based on the I–Xe system for IAB iron silicates give a range of 4.564–4.558 b.y. (Pravdivtseva et al., 2013). Schulz et al. (2012) established the period for metal–silicate segregation in IAB irons at 5.06 (+0.42/–0.41) m.y. after CAIs, while modeling by Ruzicka (2014) based on the Hf–W chronometer for metal and silicate led to the determination of 3.6 (±3.1) m.y. after CAIs. Taken together, the Hf–W and I–Xe chronometer ages provided an average closure age of 4.5622 (±0.004) b.y. It was suggested that this wide range of ages represents silicate grains that were cooled at different rates and different depths within the IAB parent body, consistent with an origin from multiple impact melt accumulation pools that were buried deeply in a regolith (Ruzicka, 2014).

Only after collisional disruption leading to the mixing of solid-to-partially melted silicates with molten metal from diverse accumulation pools, and the subsequent gravitational reassembly of this planetesimal, were the individual silicate grains from different source regions intermixed to form the composite IAB inclusions we observe today (Benedix et al., 2000). Following this catastrophic disruption, which is calculated to have likely occurred ~4.47–4.54 b.y. ago, reassembly and initial cooling proceeded rapidly to preserve the pre-established Ar–Ar ages of individual grains. Reburial of this silicated iron material allowed for sub-solidus cooling at a slow rate over an extended period (Worsham et al., 2016).

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. Based on the constraints provided by the timing of metal segregation, they modeled the early history of the 120(+)-km-diameter IAB parent body as outlined in the following diagram:

standby for IAB formation history diagram
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'

Dey et al. (2019) made use of 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 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
standby for o-cr isotope diagram
click on diagram for a magnified view

Diagram credit: Dey et al., 50th LPSC, #2977 (2019)

Whereas the highest Ar–Ar age estimate for Landes would still make it younger than Caddo County, and given that the cooling rate of metal is lower for Landes than that for Caddo County, it was inferred that Landes was the more deeply buried of the two source lithologies, both pre-disruption and post-reassembly of the IAB planetesimal (Vogel and Renne, 2008). In contrast to these meteorites, they demonstrated that the Ar–Ar-based age of Campo del Cielo reflects resetting in a more high-temperature thermal environment, probably at a deeper burial location pre- and/or post-reassembly.

A refinement of IAB iron CRE ages is under development through a broad cosmogenic noble gas study of silicates and metal in IAB silicated irons (Vogel and Leya, 2007). Data indicate a CRE age of ~200 m.y. for Landes, which was calculated based on metal 38Ar, 21Ne, and 3He, and on silicate 38Ar; the lower CRE age obtained for Landes based on silicate 3He and 21Ne might indicate loss of cosmogenic He and Ne (Vogel and Leya, 2008). Their studies revealed a pre-atmospheric diameter for Landes of ~40 cm. The Landes specimen shown above is a 27.1 g etched partial slice.