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).

It was ascertained by Yang and Goldstein (2006) and Yang et al. (2010) that the cooling rates for IIIAB irons were widely variable at 56–338°C/m.y., while the specific rate for Cape York was given by Mathes et al. (2021) as 67–202°C/m.y. It is proposed that the silicate mantle of the IIIAB parent body was mostly stripped off during one or more impact events prior to kamacite formation while still in a partially molten state. It is also thought that crystallization occurred from the surface towards the core resulting in lower cooling rates for the more highly insulated, high-Ni subset. Based on Hf–W chronometry, Kruijer et al. (2017) determined that initial core formation occurred 1.2 (±0.5) m.y. after CAIs. High-precision data obtained by Mathes et al., (2015, 2021) from Pd–Ag chronometry indicates that core solidification occurred 2.6 (±1.3) m.y. after CAIs, while cooling below the closure temperature for this isotopic system occurred by 5.0 (±0.4) m.y. after CAIs. It is considered likely that given a large parent body size of ~100 km, the removal of the mantle by impacts would have necessarily occurred within ~2 m.y. of Solar System formation (Mathes et al., 2021). 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 investigative team attributed the different value obtained for Cape York to either multiple sources for the chromites, structural diversity of the IIIAB parent body, or an origin of 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. Nevertheless, some consider it more likely that this siderophile enrichment in pallasites occurred during the condensation of a metallic melt gas phase concentrated in voids that had been formed by core contraction and mantle collapse during cooling. Moreover, some 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) compared to the IIIAB irons (47–71 nm), they determined that the cooling rate was ~2.5–25× lower for the pallasites; such a wide range suggests a large thermal heterogeneity existed within the pallasite zone. Furthermore, the Re–Os chronometer indicates 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 about the formation of main-group pallasites can be found on the Imilac page.

An investigation of the Pd–Ag systematics of the Cape York IIIA and the Grant IIIB irons 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 preceding 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