Trace element zoning as a record of chemical disequilibrium during garnet growth

Trace element concentrations in pelitic garnets from the Picuris Range of N ew Mexico dis play precipitous changes coincident with abrupt variations in Ca concentration. These patterns probably arise from the transitory partic ipation of different trace element-enriched phases in the garnet forming re action. Changes in the reactant and product assemblages occur at different times during the reaction history for crystals of different size, so they c annot be the result of any event affecting the entire rock, such as a chang e in pressure, temperature, or fluid composition. Instead, they reflect kin etic factors that cause Ca, Y, Yb, P, Ti, Sc, Zr, Hf, Sr, Na, and Li to fai l to achieve chemical equilibrium during garnet growth. Caution is needed t o avoid misinterpreting excursions in the concentration of these elements a s event markers recording simultaneous rockwide changes in intensive parame ters, when in fact they may record transient disequilibrium states that are local in scope, and not contemporaneous.