The IIIAB irons

Iron meteorites are readily distinguished from pretty much all terrestrial stones. Thus they are well-represented in collections. But they are not that common: the Meteoritical Bulletin, as of 17 June 2022, lists 69,733 characterized meteorites, of which 1,328 (1.90 percent) are irons. A total of 338 (25% of all irons) are classified as "IIIAB", the most abundant class of irons. In some collections, irons are rarer than the nominal two percent, or 1 in 50 finds and falls. The iron meteorite Shisr 043 was found in the south Oman desert, the first iron amongst >1,400 meteorites found in Oman (Al-Kathiri et al., 2006). Some of the IIIAB class are giants: the largest example is Cape York (Greenland, 58.2 tonnes), the third-largest the iconic Willamette (Oregon, 15.5 tonnes). Eleven in all have TKW of 1 tonne or more, while 24th and 25th in the list, by weight, are Boxhole (Northern Territory of Australia, at 500 kg) and St-Aubin.

IIIAB iron meteorites are magmatic irons, whose observed chemical variations are consistent with fractionation in a melt, analogous to many terrestrial silicate rocks that crystallized in magma chambers at different depths in the Earth's crust. IIIAB irons are the largest group of irons. They display wide chemical variation, including a 3-order-of-magnitude range in Ir contents, indicative of fractional crystallization of a metallic magma (Haack and Scott, 1993; Ulff-Moller, 1998; Wasson, 1999). Wasson observed that "because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid". The parent magma of the IIIAB group may have had a modest bulk S content, about 2 wt.%. Wasson made a visual estimate of the FeS (and so S) content of irons by modal estimation of troilite abundance in sawn faces between 136 and 19,000 cm² in area - the preferred minimum area is 200 cm² - the largest face was on the Agpal mass of Cape York. Wasson and Richardson (2001) also compared the chemical trends in IIIAB irons with data on many IVA irons, of which the largest is Gibeon.

Troilite-dominated sulphide inclusions, formerly known as Reichenbach lamellae, may be rimmed by schreibersite and swathed by kamacite. These formed from residual sulphide-dominant melt after crystallization of the metal at higher temperatures (Brett and Henderson, 1967). These should not be confused with large platy schreibersites in irons, known as Brezina lamellae, distinct from the smaller crystallites known as rhabdites, both formed during the cooling of the host iron (Doan and Goldstein, 1969). See Wasson (1999, p.2886) for a fine photograph of an etched slice of the Buenaventura IIIAB iron with both linear and branching Brezina lamellae.

Yang et al. (2010) made a detailed comparison of some 34 pallasites with 6 IIIAB irons. In some meteorites, the classes seem juxtaposed, e.g., metal veins cutting classic pallasite textures in Seymchan. The chemical composition of metal in most main group pallasites is close to that in Ni-rich IIIAB irons. However, the IIIAB irons all cooled significantly faster than main group pallasites, such that most pallasites could not have formed at the core-mantle boundary of that body, up to now the most commonly cited mode of pallasite genesis. Furthermore, over 80% of IIIAB irons show shock features not seen in main group pallasites.