Olivine-poor sources for mantle-derived magmas: Os and Hf isotopic evidence from potassic magmas of the Colorado Plateau

The contribution of olivine-poor lithologies (pyroxenite, eclogite, etc.) i n the mantle to the composition of mantle-derived melts is a growing topic of discussion in light of results suggesting that some magmatic characteris tics are not easily explained by melt derivation solely from peridotitic so urces. To investigate this issue, we have examined the Hf and Os isotopic c omposition of potassic magmas from the Navajo Volcanic Field of the Colorad o Plateau. Most of the magmas have Os isotopic compositions of these sample s overlap those measured for crustal/upper mantle eclogitic xenoliths carri ed by nearby intrusions. Compositional and Os isotope systematics, however, suggest that the characteristics of the magmas are not determined by crust al contamination, but rather are the result of melting of pyroxene- and mic a-rich veins/layers in the lithospheric mantle beneath the plateau. Once ge nerated, the melts of olivine-poor lithologies interacted with surrounding metasomatized peridotite to create the compositional spectrum from katungit e to minette. One component of this compositional variation is the appearan ce of strong relative depletions of high field strength elements in the min ette end-member, suggesting that this chemical signature, common in converg ent margin magmas, may be caused by melt-wall rock interaction.