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Xenolith constraints on the thermal history of the mantle below the Colorado Plateau

¹ Department of Geological Sciences, University of Texas, Austin, Texas 78712

Analyses of minerals in xenoliths from the Grand Canyon volcanic field (1 Ma) and published results from the Navajo field (20–30 Ma) constrain the history of the mantle below the Colorado Plateau. The Navajo xenoliths establish that the mantle was cool to a depth of at least 140 km, until heated by magma just before eruption, and so no frictional heating was recorded from shallow subduction below the plateau in Cenozoic time. The temperature record is consistent with a low elevation of the plateau until uplift consequent to igneous events at 25 Ma. Rims of minerals of Grand Canyon spinel peridotite xenoliths record mantle temperatures in the range 800–1000 °C at 1 Ma. Orthopyroxene grains in two rocks, however, contain interior domains with Ca like that of grain rims, but with much lower Al and Cr. The zonation of orthopyroxene records heating and annealing over a period of at least several million years, and this history is consistent with the isotopically unusual Grand Canyon xenoliths being from mantle lithosphere that persisted through a period of shallow subduction. Temperatures recorded by rims of minerals typically are cooler by about 100 °C than those of xenoliths from similar localities in the Basin and Range province, consistent with seismic studies that indicate relatively faster velocities for the uppermost mantle of the plateau.

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