GEORGETOWN (IRON)


Iron, IAB-complex, ungrouped
(sulfide-rich)
standby for georgetown photo
Found 1988

As described by R. Haag (1992), this meteorite was discovered by a prospector using a metal-detector to search for gold in a mining region of Queensland, Australia. As few as four small, weathered masses of this unusual iron were found scattered over an area of a quarter of a mile; the combined weight of the fragments was 5 kg. Georgetown contains an abundance of sinuous FeS (troilite) ribbons throughout the FeNi-metal host, thought to have been trapped during an impact-melt event. Alternative mechanisms for concentrating troilite in FeNi-metal have been suggested, such as by co-crystallization in a S-saturated magma, possibly involving volatile-driven migration of FeNi–FeS blebs into large channels (Kracher, 1985).

Although Georgetown was previously classified as an anomalous member of the IIICD group, it was reclassified into the IAB iron-meteorite complex by J. Wasson and G. Kallemeyn (2002) (see Appendix Part III: Irons for further information on this taxonomic scheme). By utilizing diagrams that compare various compositional elements with Au, it was revealed that Georgetown plots outside of the fields for the main group, the five subgroups, and the five duos. However, on most diagrams, it plots near an extension of the low-Au trend line of the sLM field, but still outside of the sLM trend for all elements studied. Based on this data, Georgetown was classified as an ungrouped solo member of the IAB complex. The name Georgetown (iron) was formally recognized by the NomComm in June of 2008.

Further investigation of the IAB complex irons with respect to HSE abundances was conducted by Worsham et al., 2016). Their data confirm the ungrouped classification for Georgetown (see diagram below).

standby for hse abundances diagram
Diagram credit: Worsham et al., GCA, vol. 188, p. 269 (2016)
'Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group'
(https://doi.org/10.1016/j.gca.2016.05.019)

The O-isotope data for Georgetown suggest that the precursor material was a metal-rich, FeS-rich, carbonaceous chondrite, perhaps similar to the CR chondrite parent body. Impact-melting was responsible for the segregation of sulfides from metal, and the initially high FeS content likely sustained the melting process at lower temperatures (Choi et al., 1994). The photo above shows a 47.2 g etched slice of Georgetown exhibiting abundant troilite.