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 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).

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).

Since the highest Ar–Ar-based age estimate for Landes is younger than the highest measured for Caddo County, and since the cooling rate of metal was determined to be lower for Landes than that for Caddo County, it was inferred that Landes was the more deeply buried of the two, 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.