MEASURING GEOLOGIC TIME WHEN EPISODES LACK EVENTS: THE SIGNIFICANCE OF INTRACRYSTALLINE 40AR/39AR AGE GRADIENTS IN MICAS

Radiometric age dating studies commonly have a goal to determine ages of formation for geologic features that formed in specific, brief events. However, evolving orogens can experience intervals of tens of million years without major tectonic changes to lithologies at depth, whereas cratonization and development of a stable continental geotherm can result in histories of partial radiogenic daughter retention and loss that span hundreds of millions of years. The common, simplifying constructs for evaluating thermal history, such as a ‘closure temperature’ and calculation of a ‘cooling rate,’ loose meaning when considering lithologies with extremely non-linear thermal histories, particularly for systems that were partially retentive of isotopes for long periods of geologic time. Hodges, Hames and Bowring (Geology, 1993) presented one of the first studies to utilize in situ laser ⁴⁰Ar/³⁹Ar dating of subvolumes within single muscovite and biotite phenocrysts to constrain meaningful thermal histories spanning hundreds of millions of years. Bowring had long considered published bulk-sample ⁴⁰Ar/³⁹Ar ages for a Proterozoic orogenic belt along the southern Wyoming craton in the context of U/Pb ages for titanite and zircon, and he interpreted that the bulk sample mica ages only recorded the integrated retention of argon in samples that experienced cooling for hundreds of millions of years: the bulk sample mica ages of plutonic rocks in Crazy Basin were ‘meaningless’ if one tried to apply them to a single, specific thermal event. This study, and several similar investigations that followed, have significant and extensive implications for thermochronology and understanding the crystal-chemical factors that control mineral ages at the scale of and within single crystals (e.g., Blackburn et al., Cont. Min. Pet., 2011). Given their strong asymmetry for lattice diffusion and perfect cleavage, and a window of partial retention temperatures that can span ~ 500-300°C, in situ laser ⁴⁰Ar/³⁹Ar ages of micas are particularly well-suited to evaluate the transition in mid-crustal levels from early and more rapid cooling to with evolution in a stable geotherm. This presentation will focus on thermal modeling and the tectonic significance of such age profiles in micas from Proterozoic and younger tectonic settings.