STRANGE RESULTS OR: HOW I LEARNED TO STOP WORRYING AND LOVE COMPLICATED 40AR/39AR APPARENT AGE SPECTRA

The ⁴⁰Ar/³⁹Ar technique is based on the radioactive decay of ⁴⁰K to ⁴⁰Ar, and is one of the most versatile geochronologic methods available to Earth Scientists. Applications range from determining absolute ages (e.g. the timing of a volcanic eruption) to constraining thermal histories (e.g. cooling during exhumation). Most consumers of ⁴⁰Ar/³⁹Ar data consider flat apparent age spectra (i.e. plateau ages) the hallmark of a successful analysis. However, complicated age spectra may be more sensitive indicators of transient heating and/or deformation than the bulk mineralogy or microstructure of the sample. The interpretation of ⁴⁰Ar/³⁹Ar apparent ages in a thermochronologic (temperature-time) context is based on the assumption of argon loss via thermally-activated volume diffusion until cooling though a closure temperature window. The premise of ⁴⁰Ar/³⁹Ar step-heating a sample is to sequentially outgas more retentive domains within a mineral via progressive heating at subsequently higher temperatures, assuming that the mineral behaves in the laboratory as it does in nature. Staircase-shaped spectra, often referred to as argon-loss profiles, in this context are inferred to be the result of slow cooling or partial resetting due to a thermal event. However, because minerals may grow below their closure temperature, one must understand the petrologic and structural context of a mineral in the sample in order to determine whether to interpret apparent age spectra in the context of cooling, recrystallization, or neocrystallization. Additionally, deformation can result in the formation of dislocations, subgrain boundaries, microfractures, etc., which may be sites facilitating fast-path diffusion and/or prone to subsequent recrystallization, alteration, or dissolution. While localized recrystallization and argon loss due to deformation may pose complications for recovering modelled thermal histories, complex apparent-age spectra have the potential to elucidate the timing of both ductile and brittle deformation events in fault zones that record evidence for reactivation during polyphase tectonic histories.