Iron, IIAB
Formerly considered a probable transported mass of Tombigbee River (IIG)
(Compositionally related to group IIG)

standby for auburn photo
Found before 1867
32° 37' N., 85° 30' W., approx.

An iron mass was found in 1859 in western Alabama, USA, followed in subsequent years by the recovery of five additional masses; these six iron masses, named Tombigbee River, had a combined weight of 43.8 kg. In 1867, an extensively oxidized 3.63 kg mass was plowed up on the Daniel Plantation located ~250 km east of the Tombigbee River find and ~1 mile west of East Alabama College in Auburn. The severely weathered mass of Auburn was broken up with a sledge hammer in a blacksmith's shop and possibly artificially heated before being described by several different researchers (V. Buchwald, 1975). Although Auburn has historically been considered to be a transported piece of Tombigbee River, it is now demonstrated by Hilton and Walker (2019) to more likely represent a separate iron belonging to the IIAB group. The classification of the Auburn iron meteorite has a long and varied history as described in the following entry from Grady's Catalogue:

"A mass of about 8lb (3.63kg) was ploughed up near East Alabama College, C.U. Shephard (1869). Described, with an analysis by O. Hildebrand, 4.67 %Ni, E. Cohen (1905). A second analysis, by A.A. Moss, gave 5.9 %Ni. Classification and analysis, E.R.D. Scott et al. (1973). Description, V.F. Buchwald (1975). Auburn has had a turbulent history, first recognised as an individual meteorite, then demoted to a transported piece of Tombigbee River ( q.v._ ). Correspondence between R.S. Clarke Jr., V. Buchwald and J.T. Wasson in 1994 (copies in Min. Dept., NHM, London) has prompted the reinstatement of a Catalogue entry for Auburn. V.F. Buchwald, pers._ commun._ (1994), notes that the octahedral structure and absence of schreibersite differentiates Auburn from Tombigbee River, but J.T. Wasson, pers._ commun._ (1994), takes the view that Auburn is a fragment from Tombigbee River. Until the relationship, or otherwise, between Auburn and Tombigbee River is established beyond doubt, then there is more to be gained than lost by keeping separate entries for the two specimens."

Wasson and Choe (2009) argued that group IIG irons are chemically similar to those of the IIAB iron group, forming extensions to IIAB trends on element–Au diagrams. It has been proposed by Wasson and Choe (2009) that formation of IIG irons occurred inside isolated cavities which remained after crystallization of an evolved IIAB magma. The IIG irons eventually crystallized in a P-rich region of the lower layer of the IIAB core, while an immiscible and buoyant S-rich magma collected at the upper regions of the magma chamber. Elements such as Au and Ge were likely removed in the S-rich melt phase, while the low-Ni content of IIG irons is attributed to diffusion and redistribution of Ni out of metal and into schreibersite during an extended cooling history. The Ge-isotopic data were obtained by Luais et al. (2014), and they found it to be almost identical for both IIG and IIB metal, while a Ge content of 1.3 ppm and a δ74Ge of –3.4‰ was ascertained for schreibersite in Tombigbee River. Their Ge data support the formation history proposed by Wasson and Choe (2009).

Hilton and Walker (2019) conducted a chemical and isotopic study of each of the IIG irons including a sample of the Auburn mass. They realized that Auburn has a significantly higher Ir concentration than all members of the IIG group, but it is consistent with some IIAB irons. In addition, they demonstrated that IIG and IIAB irons have similarities with respect to their Re–Os isotopic systematics. Furthermore, they found that Auburn has HSE abundances that are different from the IIG irons, but are consistent with some IIAB members such as Coahuila. These data suggest a likely genetic relationship between Auburn and the IIAB group irons, and they plan to use Mo isotopes in future studies to determine whether or not a genetic connection exists between the IIG and IIAB group irons (see diagrams below).

standby for auburn chemical and isotopic diagrams
Diagrams credit: Hilton and Walker, 50th LPSC, #1240 (2019)
(see abstract text for a full explanation of the diagrams)

A common metal melt origin for IIAB and IIG irons during core crystallization is supported by chemical–AU trends consistent with fractional crystallization (Wasson and Choe, 2009). A genetic connection is further supported by Cr and O isotope data newly obtained by Anand et al. (2022 #1891). On an ε54Cr vs. Δ17O coupled diagram, these irons plot in both the ureilite field and in the acapulcoite–lodranite field extended to lower ε54Cr values. Based on metallographic cooling rate data, the IIABG group parent body diameter was inferred to be 330 (±130) km (McSween, 1999; Kaminski et al., 2020), which is significantly smaller than that calculated for the ureilite parent body. The ε54Cr and Δ17O values for IIAB Sikhote Alin (chromite), IIG Twannberg (troilite), and Choteau (olivine) are identical within error. Although the two O–Cr isotope diagrams below are shown at different scales, together they depict that Choteau and IIABG irons plot very close to each other in ε54Cr vs. Δ17O space, possibly reflecting some form of a genetic relationship between them all. It is interesting that, as noted by Schrader et al. (2022 #6132), both the silicate compositions of acapulcoites are similar to those of IIAB irons (see McCoy et al., 1996, GCA, vol. 60, #14, Table 1. p. 2684; Schrader et al., 2017, GCA, vol. 205, p. 11), and the oxygen fugacity (ƒO2) of acapulcoites are similar to those of IIAB irons (see Schrader et al., 2017, GCA, vol. 205, p. 12, 23).

ε54Cr vs. Δ17O Diagram for IIABG Irons and Choteau
note: the verticle blue bar in the left diagram represents the ε54Cr variation in IIAB irons
standby for iiabg cr-ox isotope diagram standby for choteau cr-ox isotope diagram
Diagrams credit:
left: Anand et al., 53rd LPSC, #1891 (2022)
right: Dey and Yin, 53rd LPSC, #2428 (2022)

The 13.3 g angular specimen pictured above is a corroded fragment with label provenance from the Auburn mass, previously part of the Thomas M. Bee Collection. Most specimens of the Auburn meteorite are similar in size to this one or smaller, consisting of small angular fingers of kamacite after undergoing significant terrestrial corrosion and disintegration over the intervening years. The photo below shows two large fragments of Auburn weighing 1.7 and 0.7 kg.

standby for auburn large fragments photo
Photo credit: Vagn F. Buchwald, Handbook of Iron Meteorites Volume 2
University of California Press, p. 276 (1975) [online link]