MAXIMIZING THE THERMOCHRONOLOGIC POTENTIAL OF MUSCOVITE THROUGH MULTIPLE DIFFUSION DOMAIN MODELING: EPISODIC 40AR* LOSS IN THE BURRO MOUNTAINS, SOUTHERN NEW MEXICO

Quantification of the thermal conditions experienced by rocks through time is essential to understanding the timing, duration, and rates of geologic processes. For over fifty years diffusion theory has been successfully applied to the ⁴⁰Ar/³⁹Ar system to extract thermal histories from minerals with known kinetic parameters. ³⁹Ar-based Arrhenius plots derived from step heating experiments on muscovite appear to conform to a diffusion controlled process, and are also suggestive of multiple diffusion domain (MDD) behavior. If shown to be correct, quantitative muscovite thermochronology will be a valuable tool to obtain rock thermal histories. Here we apply the MDD method to naturally degassed muscovite crystals from Mesoproterozoic granite in the Burro Mountains, southern New Mexico. Due to Eocene reheating, muscovite yield climbing age spectra consistent with diffusive loss, and have a form consistent with MDD behavior. Arrhenius plots derived from ³⁹Ar release depart from linearity and are also consistent with degassing from more than one domain. Furthermore, log(r/r₀) plots determined from ³⁹Ar correlate with age spectra, strengthening the assertion that laboratory degassing mimics the processes active in nature. A thermal history based on regional geochronologic and thermochronologic constraints requiring rapid cooling from magmatic temperatures through biotite closure at 1.46 Ga, followed by a thermal spike induced by Eocene magmatism and caldera formation was imposed on the data. Modeled age spectra based on this thermal history closely match those produced by step heating experiments. These data are part of a growing body of evidence that suggests the MDD method can successfully be applied to understand thermal histories in a variety of geologic settings (e.g. episodic loss due to magmatic overprinting, and slow cooling via orogenic exhumation). As a nearly ubiquitous mineral in a wide range of medium-to-low grade metamorphic assemblages and igneous rocks, MDD modeling of muscovite could elucidate the secular thermal structure of medium-to-low grade orogenic belts and magmatic provinces globally.