40Ar/39Ar thermochronology is commonly used to constrain the timing, and rates, of cooling in exhumed metamorphic terranes. Metamorphic micas that crystallise at high temperatures are usually considered to lose 40Ar via thermally-activated volume diffusion, and the resulting age is commonly linked to temperature via Dodson’s closure temperature (Te) formulation. 40Ar/39Ar ages from many metamorphic terranes commonly do not fit within a chronological framework defined by other, higher T chronometers, such as U-Pb zircon, suggesting that linking age to temperature via pure open system diffusion may not always be a correct interpretation.
The Western Gneiss Region of Norway documents isothermal decompression from high pressures (from -3.0 GPa to -1.0 GPa) at 700°C. White mica, biotite, amphibole, and feldspar single grain fusion and laser ablation 40Ar/39Ar ages yield a broad range that spans the known timing of the metamorphic cycle and that cannot be readily reconciled with models of simple thermal diffusion.
Instead, the 40Ar/39Ar data record the cumulative effects of a complex interplay of processes that affected the rocks during decompression and exhumation. Recrystallization, deformation, partial melting, and fluid infiltration all acted to add or remove Ar from the system at different times in different lithologies. Samples that show significant recrystallization and deformation yield younger white mica but older biotite ages than more pristine samples. Furthermore, migmatisation acted to make biotite ages younger in the amphibolite-facies gneisses whereas fluid infiltration acted to increase biotite ages. Data from white mica breaking down to form a symplectite of biotite and plagioclase show that during recrystallization, Ar was not lost to the grain boundary, but instead becomes incorporated into biotite and plagioclase.
These physical processes appear to have dominated over thermally-activated volume diffusion as the means by which Ar was (re)distributed within, and between, different lithologies. Overall, these data support the observation that 40Ar/39Ar ages from K-bearing minerals in high-temperature and high-pressure metamorphic terranes may not always be interpretable as cooling ages and that metamorphic 40Ar/39Ar ages require careful assessment with respect to metamorphic stage and petrographic evolution
