ARE THE HEDS AND THE IIIABS KEEPING IT ALL IN THE FAMILY?

There is strong evidence linking the howardites, eucrites, and diogenites (HEDs) to the asteroid 4Vesta (McSween et al., 2013); however, the origin of HEDs from Vesta is not universally accepted. It has been proposed that the HEDs, IIIABs, and main-group pallasites (MGPs) originate from the same parent body. This model would require: a) a parent body that is no longer intact, ruling out a vestan origin and b) that the HEDs, IIIABs, and MGPs share isotopic characteristics consistent with one parent body. Oxygen isotopes argue against a common parent body. Wasson (2013) suggested that the oxygen isotopic differences between the MGPs-IIIABs and the HEDs can be explained by rapid melting of unequilibrated chondrite material. He concluded that isotopic evidence, including both oxygen isotopes and ε⁵⁴Cr data, for a common non-Vestan parent body for the IIIABs and HEDs is stronger than spectral evidence against it. 

Puente del Zacate, a IIIAB iron meteorite, contains a primitive silicate clast. If the HEDs and the IIIABs are genetically linked, the chondritic compositions of Puente del Zacate silicates, when partially melted, should produce HED-like basaltic materials. MELTS modeling using the bulk composition of Puente del Zacate determined by Olsen et al. (1996) was conducted to test the proposed IIIAB-HED relationship. The Fe/Mg of the melts produced from Puente del Zacate is around 0.1. While some diogenites contain orthopyroxenes with Fe/Mg as low as 0.19, no HED silicates have values as low as the Puente del Zacate melts. In fact, the eucrites have even higher Fe/Mg values that vary from 0.4 to 2, with the lower end of the range representing the cumulate eucrites. This indicates that FeO-poor chondritic starting materials, like Puente del Zacate, cannot produce melts as FeO-rich as the HEDs, eliminating them as possible source materials. This further strengthens the argument that the IIIABs and HEDs do not originate from the same parent body.