MINERAL HOMOGENEITY IN PERIDOTITE XENOLITHS FROM MINETTE AT BUELL PARK, AZ: KINETICS, THERMOMETRY, AND THE COLORADO PLATEAU MANTLE

Temperatures in uppermost mantle of the Colorado Plateau at about 25 Ma are recorded by inclusions in two types of host rock in the Navajo field, minette and serpentinized ultramafic microbreccia (SUM). Although some pairs of minerals within inclusions in SUM record temperatures of about 500°C, peridotite xenoliths in minette at Buell Park record temperatures in the range 700-1000°C by a variety of thermometers. Compositional gradients within minerals of these Buell Park xenoliths have been characterized by detailed electron probe analysis to evaluate kinetics that dictate the temperature record in these and other inclusions from cool uppermost mantle. Zonation of grains in the Buell Park xenoliths is texturally dependent. Spinel grains with convolute margins are zoned in Al, Cr, Fe, Mg, and Ni, and olivine is zoned to lower Ni and higher Fe adjacent to spinel. The gradients within olivine record heating that accompanied and followed incorporation of the peridotite inclusions in minette. In contrast, gradients in pyroxenes primarily record cooling. Al and Ca in orthopyroxene typically decrease core-to-rim, although the gradients depend upon contacting phases. In orthopyroxene, the lowest concentrations of Al₂O₃ and CaO (0.88 and 0.16wt%, respectively) are found in unusual lamellar intergrowths with spinel. The compositional differences and gradients are consistent with the hypotheses that the temperatures near 700°C recorded by pyroxenes are blocking temperatures, and that the range to higher temperatures is partly an artifact of disequilibrium. The temperatures near 500°C recorded by some inclusions in SUM could have also characterized the mantle source of peridotite in Buell Park minette. Temperatures near and above 850°C are recorded by peridotite inclusions in Quaternary basalts at other Plateau localities: these higher values record prograde metamorphism, not cooling. The temperature increase from 500°C to above 800°C may be associated with Plateau uplift.