Knowledge of the mechanisms and time scales for the production and differentiation of silicic magmas is critical to our understanding of fundamental geologic problems such as the formation and growth of the Earth's continents and predicting volcanic hazards. Newly developed microsampling techniques will be used to obtain high precision Sr and Nd isotopic data from individual phenocrysts associated with two well-characterized, middle-Tertiary silicic magmatic centers in the southwestern United States. When coupled with crystal size distribution studies this information will provide greater temporal resolution of the compositional evolution of silicic magmas than obtainable in studies utilizing bulk samples alone. The first field area provides a unique opportunity to study the link between ash-flow tuffs and granitic batholiths in the Organ Mountains, southern New Mexico. The second study area, a group of well-mapped and dated lava domes in southern-central New Mexico, will allow examination of the timing and dynamics of contamination and recharge during open-system evolution of silicic magmas. Together, studies of these two localities will provide new insights into both the large- and small-scale processes operating in complex continental silicic magmatic systems.
