Insights into the evolution of the uppermost continental mantle from xenolith localities on and near the Colorado Plateau and regional comparisons

The interplay between the evolution of mantle and crust has been investigated by textural and electron probe analysis of Cr-diopside spinel peridotite xenoliths from six localities and by comparisons to a database of mineral compositions. The six localities are on a 300-km transect from the Colorado Plateau into the adjoining Rio Grande rift. Typical xenolith populations from the Colorado Plateau province have magnesian olivine (average Fo(90.5)) and Ti-and Na-poor clinopyroxene. In contrast, typical xenolith populations from the Rio Grande rift and the Basin and Range are characterized by more Fe-rich olivine (average Fo(89.6)) and clinopyroxene with a range of Ti and Na (average 0.44 wt % TiO2). The contrast between the xenoliths from these adjacent provinces documents that the Colorado Plateau has a depleted mantle root that helps to stabilize it. At Mount Taylor, however, a locality high on a composite volcano within the eastern Colorado Plateau, most xenoliths are not depleted, and the Fe-rich olivine (Fo(86) - Fo(88)) in them may characterize mantle affected by silicate melt metasomatism at high melt/rock ratios. Low-Al orthopyroxene occurs at two of the six transect localities, and part of a large porphyroblast contains only 0.13 wt % Al2O3; because of the unusual compositions and textures, water-rock reactions are inferred to have been important in parts of the refractory continental mantle. Zonation of Ca and Al in orthopyroxene provides constraints on the temperature histories of the unusual pyroxenes. Equilibration temperatures for xenoliths from Lunar Crater, Nevada, average 1270 degrees C, much higher than the average of 1012 degrees C for others from the Basin and Range; Lunar Crater xenoliths may be samples of a plume or asthenosphere emplaced recently into the uppermost mantle.