Low-Mg rock debris in howardites: Evidence for KREEPy lithologies on Vesta?
Résumé
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 unrepre- sentative debris of mesostasis-rich areas from eucrites, but are instead remnants of evolved rocks. The occurrence of hyalo- phane 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 ﰀ 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.