Iron, IIAB, hexahedrite
(Compositionally related to group IIG)
Found 1971, recognized 1990
26° 6' N., 106° 58' W.
This dish-shaped 58.63 kg meteorite was found beside a mountain road 0.5 km south of Guadalupe y Calvo, Mexico. The meteorite was used as a dog bowl on a ranch in Chihuahua, Mexico until it was recognized as meteoritic and traded for a new pickup truck. Having low nickel contents of 4.5%6.5%, hexahedrites consist of large cubic crystals of kamacite. Upon etching, this meteorite class exhibits shock-induced, parallel twinning planes named "Neumann" lines, but exhibits no Thomson (Widmanstätten) structure. Previously, a division of the similar magmatic irons into the IIA and IIB groups was made based on this structure, but because there is no compositional hiatus between these groups, such a division is considered arbitrary and is no longer recognized (Wasson et al., 2007).
A simple fractional crystallization model for the IIAB group gives an estimate for a high initial S and P content in the molten core. These high abundances are in accord with high abundances for other volatile elements, and is consistent with the trends observed for Ga, Ge, and Ir vs. Au (N. Chabot, 2004). This fractionation model also indicates that the upper core material, which was derived from the late-crystallizing S-rich residual liquid, is not represented in our collections. Rather, IIAB irons sample the lower P-rich core region formed after no more than 48% crystallization of the parental magma.
In the fractional crystallization sequence of the IIAB parental magma, Guadalupe y Calvo represents the earliest phase, forming after only 2% crystallization (Sikhote Alin is among the latest, forming at the 48% crystallization stage). It has been proposed by Wasson and Choe (2009) that formation of IIG irons occurred inside isolated cavities that remained after crystallization of a more 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 in the upper regions of the magma chamber. The Ge-isotopic data obtained by Luais et al. (2014) support the formation history proposed by Wasson and Choe (2009) (see the Tombigbee River page for further details). Material from the upper, nonmetal-rich core region which was derived from the more highly evolved magmas is not present in our collections, and it is presumed that it does not survive the journey to Earth (Wasson et al., 2007).
In studies of a number of IIAB irons, Mei et al. (2020) applied HfW chronometry employing more precise W isotope data corrected for neutron capture effects. By using this technique they calculated the core formation age for the IIAB parent body to be ~0.6 m.y. after CAIs. The specimen of Guadalupe y Calvo shown above is a 14.4 g partial slice.