Iron, IIC
plessitic octahedrite
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Found August 1906
37° 44' N., 89° 53' W.

An angular mass of 17.5 kg was found on the farm of Patrick Monaghan about one mile west of Perryville, Missouri. The mass was sticking out of the soil as if it had been previously disturbed by plowing. The meteorite has suffered from terrestrial weathering and lost an average of 1–2 mm from the surface, but it still preserves sections of the heat-affected zone and retains evidence of ablation pits (see photo below).

A Ni content of ~9.6% creates a µm-sized Thomson (Widmanstätten) structure of fine-grained plessite composed of kamacite spindles having widths of ~0.06 mm and lengths up to ~0.9 mm, most all of them containing nuclei of schreibersite. Perryville was formed at an early-stage (~10%) of fractional crystallization of an evolving liquid associated with core formation (Tornabene et al., 2019). The iron began as a single taenite crystal that formed a progressively finer structure as it cooled. This uncommon plessitic structure is transitional between ataxites and finest octahedrites.

The Re–Os age for the IIC group was determined by Tornabene et al. (2019) to be 4.538 (±0.140) b.y. Perryville is closely related to the other six irons constituting the IIC group: Ballinoo, Cratheús (1950), Darinskoe, Kumerina, Salt River, and Unter Mässing. The previously included iron Wiley has been eliminated from the group based on its disparate HSE values and the relative enrichments in its 94,95,97Mo and 183W isotopes (Tornabene et al., 2018, 2019). Compared to all other meteorite groups, both IIC irons and CR chondrites have significant δ183W excesses and elevated δ15N, and they share similar Mo isotope systematics; therefore, a genetic link is inferred (Kruijer et al., 2017; Budde et al., 2018) (see diagram below).

Ru vs. Mo Isotopes
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Diagram credit: Tornabene et al., 50th LPSC, #1236 (2019)

Based on corrected µ182W values, Tornabene et al. (2020) ascertained the timing of core formation on the IIC parent body which corresponds to a metal–silicate segregation (core formation) age of 3.2 (±0.7) m.y. after CAIs. Based on the metal–silicate segregation age together with some model assumptions, they were able to infer an accretion age of 1.4 (±0.5) m.y. after CAIs. It was demonstrated by Tornabene et al. (2020) that the seven known members of this group are related through varying degrees of fractional crystallization from a common parental melt, with Darinskoe and Unter Mässing representing 10% and 26% fractional crystallization, respectively (see diagrams below). The solidification of Perryville and Cratheús (1950) occurred at a similar point in the fractionation sequence (~11%), and similarly for Ballinoo and Kumerina following in sequence thereafter (~17%).

HSE Abundances and Fractionation Sequence for IIC Irons
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Diagrams credit: Tornabene et al., GCA, vol. 288, pp. 36–50 (2020)
'Genetics, Age and Crystallization History of Group IIC Iron Meteorites'

Utilizing coupled µ94Mo vs. µ95Mo and µ97Mo vs. µ100Ru diagrams, Tornabene et al. (2020) demonstrated that the IIC iron group plots along the CC trend line indicating that it accreted in the outer Solar System (see diagrams below). However, the IIC group is relatively enriched in 94,95,97Mo and µ183W isotopes and plots away from the other CC iron groups towards a greater enrichment of p- and r-process isotopes with a greater depletion of s-process isotopes. Notably, the ungrouped iron Wiley has the largest Mo and W nucleosynthetic isotope anomalies of any iron meteorite analyzed to date. Tornabene et al. (2020) concluded that the IIC group and ungrouped Wiley could represent planetesimals with a unique thermal processing history, or perhaps planetesimals that accreted in a distinct nebular reservoir.

Nucleosynthetic Isotope Compositions of IIC Irons and Wiley
(µ notation denotes deviation from terrestrial standards in parts per million)
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Diagrams credit: Tornabene et al., GCA, vol. 288, pp. 36–50 (2020)
'Genetics, Age and Crystallization History of Group IIC Iron Meteorites'

Further information about the IIC and other iron chemical groups can be found on the Appendix III page. The photo above shows a 11.5 g etched partial slice that displays kamacite spindles in a µm-sized Thomson (Widmanstätten) pattern. An ~1 mm shocked troilite nodule is visible on the slice. These 0.1–2 mm nodules are prevalent in this meteorite, on average one per 25 cm², and will rarely even attain a size as large as 8 mm. The photo of the original mass of Perryville is shown below, courtesy of the US National Museum.

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