Studies of trace-element zoning in mantle-derived minerals can provide important constraints on the nature and time scales of metasomatic processes and thermal events in the upper mantle, and on diffusion rates. Zoning data on garnet in peridotite xenoliths restrict some metasomatic processes to geologically short time-spans (10-10 4 years) prior to the eruption of the host magmas, and suggest that the metasomatism is genetically connected to the magmatic events or their immediate precursors. Such metasomatism effects major changes in both the major-element and trace-element composition of the rocks, and these xenoliths should not be used to represent large volumes in the mantle. Modeling of heating and cooling rates, based on Ni zoning in garnet, suggests that the heating observed in high-T xenoliths is a transient phenomenon associated with local intrusion of magma bodies. The relative lengths of zoning profiles for various elements in single crystals of garnet suggest that diffusion coefficients, over the temperature range 1200-1400 degrees C, decrease in the order Ni> or =Fe, Mn>Ti, Zr> or =Y (and heavy REE) nearly equal Ca>Cr, Ga, V. Analyses of zoning profiles in garnet cooled from a high temperature suggest that rates of Ni diffusion are equivalent to those of Fe-Mg down to temperatures near 600 degrees C. Pyroxene equilibration during thermal and metasomatic events probably involves volume diffusion over very short distances only, between high-mobility paths such as cleavage planes.