Cape York

Iron, IIIA, octahedrite
standby for cape york photo
Found 1818
76° 8′ N., 64° 56′ W. At least 8 masses of this meteorite have been recovered from the glacial region of Greenland having a total weight of 58 tons, the largest combined mass of any other recovered meteorite. The most recent found mass was a 20-ton mass named Agpalilik found in 1963. It has undergone 0.5 mm of corrosion on the above ground portion with more than 2 mm of the surface lost on portions below the soil line. Chlorides are a significant invasive element contributing to the deterioration.

The troilite inclusions, a cross-section of which is pictured above, form parallel, sausage-shaped formations within the meteorite and are derived from a trapped melt component. These troilite inclusions represent 5.6 vol% of the total mass of Cape York. A simple fractional crystallization model for the IIIAB group gives an estimate for the initial S-content of the molten core of 12 (±1.5) wt%, and indicates that most of the core material formed from the later crystallized, S-rich residual liquid is not represented in our collections (N. Chabot, 2004). Low-density phosphates and high-density metal grains are present in the troilite nodules opposite each other, reflecting the direction of the gravitational field on the parent body. The nitride carlsbergite [CrN] was first discovered in the Cape York meteorite. Notably, an undifferentiated silicate inclusion, similar to the type that occur in IAB complex irons, was discovered in the IIIAB Puente del Zacate iron; it is unclear how this occurred (Ruzicka, 2014).

The IIIAB iron group constitutes ~33% of all iron meteorites recovered. Structural evidence shows that the meteorite went through a molten stage on its parent body. Based on cooling rate models, the diameter of the parent body is estimated to have been ~100 km, taking ~50 m.y. for the core to solidify. In a study by Yang et al. (2006, 2010), a wide range of cooling rates was found for IIIAB irons, from 60° to 340K/m.y. They propose that the IIIAB parent body had its silicate mantle mostly stripped off during one or more impact events prior to kamacite formation and while it was still molten, and that it crystallized from the surface towards the core resulting in lower cooling rates for the more highly insulated, high-Ni subset. The Hf–W chronometer indicates that the core formed ~1.2 m.y. after CAIs, while the Pd–Ag chronometer indicates that cooling below the closure temperature for this isotopic system occurred by ~4 m.y. after CAIs (Mathes et al., (2015). Therefore, they reasoned that given a large parent body size of ~100 km, the removal of the mantle by impacts would have necessarily occurred prior to ~4 m.y. after CAIs. Notably, a genetic relationship between groups IIIAB and IIIE has been posited based primarily on matching Cu isotopic systematics (Bishop et al., 2012).

The O-isotopic composition of chromites was ascertained for Cape York and other IIIAB irons (Franchi et al., 2013). Curiously, the values show that Cape York (–0.27‰) has a discrepant O-isotopic composition from the others studied (–0.18‰), while the main group pallasites have an identical O-isotopic composition to the others (–0.18 (±0.02) ‰). The different value for Cape York was ascribed by the investigative team to either multiple sources for the chromites, structural diversity of the IIIAB parent body, or an origin for Cape York on a separate parent body.

Trace-element studies along with metal and O-isotopic compositions of the main-group pallasites are consistent with those of the late-crystallized (high-Au, high Ni, reflecting ~80% core crystallization) residual melts of the IIIAB iron core. The enrichment of these pallasites in the volatile and refractory siderophiles Ga, Ge, and Ir relative to the IIIAB group could have occurred during the crystallization of a sulfide-rich liquid fraction at the core–mantle boundary. Still, it is now considered more likely that this enrichment observed in pallasites occurred during the condensation of a metallic melt gas phase concentrated within voids which formed by core contraction and mantle collapse during cooling; moreover, recent studies rule out a core–mantle boundary formation scenario for pallasites (Yang and Goldstein, 2006, 2010). Based on the size of the island phase in the cloudy zone of these pallasites, the metallographic cooling rates appear to have been significantly lower than those of IIIAB irons. In their measurement of high-Ni particles within the cloudy zone of several main-group pallasites and IIIAB irons, Yang et al. (2007) found that a correlation with Ni exists only in the IIIAB irons. Based on the significantly larger high-Ni particle size in the pallasites (105–188 nm) vs. the IIIAB irons (47–71 nm), they determined that the cooling rate was ~2.5–25× lower for the pallasites, with the wide range suggesting that a large thermal heterogeneity existed within the pallasite zone. In addition, the Re–Os chronometer suggests that pallasites formed 60 m.y. later than IIIAB irons, raising further doubts about a IIIAB core–mantle origin for main-group pallasites (E. Scott, 2007). Further information on the formation of the main-group pallasites can be found on the Imilac page.

Studies of the Pd–Ag systematics of the Cape York IIIA iron and the Grant IIIB iron by Matthes et al. (2013, 2014) indicate that the IIIAB core cooled rapidly, within the first ~2 m.y. of Solar System history using IVA Muonionalusta as the anchor. It is considered most likely that the core crystallized inwards, with crystallization of Cape York succeeding that of Henbury and preceeding that of Grant (Matthes et al., 2014). It was shown that each of these meteorites cooled below the Pd–Ag closure age within ~1 m.y. of each other. The corrected I–Xe isotopic systematics for Cape York yields a CRE age of 82 (±7) m.y. (Marti et al., 2004; Mathew and Marti, 2009).

To learn more about the relationship between this and other iron chemical groups, click here. The photo above shows a 57.2 g Cape York specimen. The 20-ton Agpalilik mass is pictured below.

standby for agpalilik photo
Photo courtesy University of Copenhagen—Geological Museum

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