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

Abstract

The contribution of olivine-poor lithologies (pyroxenite, eclogite, etc.) in the mantle to the composition of mantle-derived melts is a growing topic of discussion in light of results suggesting that some magmatic characteristics are not easily explained by melt derivation solely from peridotitic sources. To investigate this issue, we have examined the Hf and Os isotopic composition of potassic magmas from the Navajo Volcanic Field of the Colorado Plateau. Most of the magmas have Os isotopic compositions much more radiogenic than observed for any uncontaminated oceanic basalt. The Os isotope compositions of these samples overlap those measured for crustal/upper mantle eclogitic xenoliths carried by nearby intrusions. Compositional and Os isotope systematics, however, suggest that the characteristics of the magmas are not determined by crustal contamination, but rather are the result of melting of pyroxene- and mica-rich veins/layers in the lithospheric mantle beneath the plateau. Once generated, the melts of olivine-poor lithologies interacted with surrounding metasomatized peridotite to create the compositional spectrum from katungite to minette. One component of this compositional variation is the appearance of strong relative depletions of high field strength elements in the minette end-member, suggesting that this chemical signature, common in convergent margin magmas, may be caused by melt-wall rock interaction.