The principal investigator, in collaboration with Lang Farmer (University of Colorado), will study the Organ Mountain Batholith (south central New Mexico) using isotopic and petrologic techniques, with emphasis on establishing the vertical chemical and isotopic zonations of this igneous body. The study is made special by the ability to study the top of the magma chamber, a section that is commonly not accessible in other such bodies. Through this work geologists will gain important information about the processes of magmatic differentiation that result in large-volume silicic igneous rock bodies.