Low-Mg rock debris in howardites: Evidence for KREEPy lithologies on Vesta?

The Northwest Africa (NWA) 1664 and 1769 howardites contain fragments of low-Mg lithologies. These clasts consist of plagioclase, Fe-rich pyroxenes (high-Ca pyroxene and an Fe-rich pigeonite (or pyroxferroite) partially to totally converted to a silica–fayalite–hedenbergite symplectite), K–Ba-feldspars (hyalophane), silica, troilite, phosphates (merrillite and apatite), ilmenite, baddeleyite and zircon. Textural considerations and phase compositions show that these clasts cannot be unrepresentative debris of mesostasis-rich areas from eucrites, but are instead remnants of evolved rocks. The occurrence of hyalophane and the modal abundances of phosphates in the largest of them suggest that their source rocks were much more K- and P-rich than regular HEDs. Moreover, the rare earth element abundances of the pyroxenes indicate that their parental melts were rich in incompatible trace elements (possibly at the level of 100 × CI or much higher for heavy REEs), with variable negative Eu anomalies. Although, their petrogenesis is uncertain, they demonstrate that magmatic activity on Vesta has been able to generate a diversity of rocks greater than generally thought, and local lithologies with possible KREEP affinities.