GUADALUPE Y CALVO

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 called Neumann lines, but has 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 high initial S and P contents in the molten core. These high abundances are in accord with high abundances for other volatile elements, and it 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.

Guadalupe y Calvo is among the earliest IIAB irons to crystallize from the parental magma, 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 which 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 at the upper regions of the magma chamber (see the Guanaco page for more complete details). Material from the upper, nonmetal-rich core region 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). The specimen of Guadalupe y Calvo shown above is a 14.4 g partial slice.

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