Collaborative Research: Little Devils Postpile Revisited: Intercalibration of Thermochronometer Kinetics in a Contact Aureole

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Thermochronometer Kinetics in a Contact AureoleDriven by the demand for constraints on thermal histories of rocks in the upper few kilometers of the Earth's crust, intermediate- and low-temperature thermochronology has proliferated in tectonic and geomorphic applications in the last two decades. Despite this growth, every thermochronologic application is limited by uncertainties in fundamental kinetic calibrations and intra-sample variation that to one degree or another raise questions about geologic interpretations derived from them. These uncertainties arise from many sources, but perhaps most importantly from the interpretation and extrapolation of empirical laboratory step-heating diffusion and annealing experiments that are usually performed at rates and temperatures many orders of magnitude different from relevant natural conditions. Several lines of evidence, including ab initio kinetic models, deviations of natural samples from idealized configurations, and simply observations of "intriguing complexities" in many cooling-age, -spectra, -profile and track-length data sets, suggest the existence of significant gaps in our understanding of routinely used fundamental thermochronologic kinetic calibrations. A careful and deliberate benchmarking and intercalibration study of multiple thermochronometers from a well-controlled natural laboratory is needed to illuminate these issues and to ultimately establish more confidence in kinetic models and geologic interpretations derived from them. It is proposed to resample and analyze the detailed behavior of 13 different noble-gas and fissiontrack thermochronometers in profiles adjacent to Little Devils Postpile, a classic natural laboratory studied by Calk and Naeser in 1973, where a ~100 m basalt plug intruded ~80-Ma granitoid rock at ~8 Ma. Although possibly complicated by hydrothermal circulation and other effects, the relatively simple configuration of this natural experiment will allow construction of realistic models of the thermal histories associated with the intrusion, hence prediction of profiles of age and other thermochronometric properties as a function of distance from the contacts. The abundance of many different minerals dateable by both noble gas and fission-track methods in the country rock will then allow comparison between and predicted the observed thermochronologic patterns based on many parameters (incl. bulk ages, profiles, spectra, and track lengths, etc.). Inter-combinations of well-calibrated dating systems will serve as benchmarks to identify interpretive problems and to infer potential refinements needed to improve calibrations of other dating systems.Intellectual Merit: This focused and collaborative study will provide a relatively rare and needed opportunity to test, refine, and in some cases establish kinetic calibrations used by many Earth and planetary scientists for hundreds of applications each year. This will be accomplished using a natural experiment performed under conditions not achievable in the laboratory, through a relatively simple contact relationship that allows for straightforward modeling, but with reasonable opportunity for revealing complexities (such as intrasample variations) that are likely to be commonly encountered in many geologic applications.Broader Impacts: Besides contributing to thermochronologic calibrations widely used in the geologic community, and potentially establishing a thermochronologic "type locality" useful for testing other systems, this work will provide training and support for several undergraduate and graduate students, establish new fission-track dating capabilities (in zircon, titanite, and epidote) at UT, and will provide for outreach opportunities to visitors at Yosemite National Park, through our collaboration with Park Geologist Dr. Gregory Stock.