LASER 40AR/39AR EVALUATION OF SLOW COOLING AND EPISODIC LOSS OF 40AR FROM A SAMPLE OF POLYMETAMORPHIC MUSCOVITE

Volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the Earth's crust. Direct measurements of the zoning of radiogenic argon-40 in single muscovite porphyroblasts, from a complex terrain in the Vermont Appalachians, record multiple crustal events that span 150 million years. The crystal radius was the effective dimension for argon diffusion (approximately 1000 micrometers). Late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon-40. Zoning patterns of radiogenic isotopes, as observed in this study, are an increasingly important diagnostic tool for studying the thermal record of tectonic processes.