9304072 Hodges Rigorous inversion of petrologic and geochronologic data to estimate parameters such as unroofing rate in ancient orogenic belts requires precise estimates of pressure-temperature and time-temperature paths for a variety of structural levels. Uncertainties in Tt paths largely stem from uncertainties in the temperatures at which isotopic themochronometers become closed with respect to loss of radiogenic daughter nuclides. Factors that can result in variable closure temperatures for a given mineral- isotopic system include: 1) compositional differences; 2) the extent of interaction with infiltrating fluids; and 3) differences in the effective diffusion dimension for the radiogenic daughter due to reaction or deformational history. The relative importance of these factors or deformational history. The relative importance of these factors in governing closure behavior is not well understood for most commonly used thermochronometers. This project is intended to improve this situation through a systematic empirical study of thermochronometers based on the U-Pb and K-Ar (40Ar/39Ar) decay schemes in the Connemara region or Ireland. Connemara was chosen for this study because it provides an unparalleled opportunity to race a variety of lithologies, containing many thermochronometrically important minerals, continuously from garnet grade to second sillimanite grade. We intend to explore variations in mineral ages as a function of composition, microstructure, grain size, and metamorphic grade. An important component of this work will be the coupling of thermochronology with detailed stable isotopic work in an effort to evaluate the significance of fluids in the radiogenic isotopic evolution of thermochronologic systems. The research will improve our understanding of the geochemical behavior of these systems, as well as provide important insights into the reliability of assumptions commonly used in the construction and interpretation of time- temperature paths in orogenic settings.
