Contrasting types of chromium-spinel peridotite xenoliths in basanitic lavas, western Grand Canyon, Arizona

Several hundred xenoliths of Cr-diopside-spinel peridotite from basanitic lavas in the western Grand Canyon along the eastern margin of the Basin and Range Province can be grouped into essentially three distinct textural categories with correlative contrasts in mineral composition. At least two of these textures seem unique to this locality. Xenoliths display varying degrees of depletion in basaltic constituents and last equilibrated between 900 and 1000°C. A few contain coarse exsolution intergrowths of diopside and enstatite whose reconstructed composition has about 15 wt.% CaO corresponding to an original temperature of crystallization of approximately 1400°C. The genetic relation between the three types — if such exists — is uncertain, but overall they indicate a vertically inhomogeneous upper mantle. The variety of textures found worldwide in mantle-derived Cr-spinel peridotite xenoliths of more or less equivalent composition suggests varied and probably complexly superposed histories.