Dating blueschist metamorphism: A combined ⁴⁰Ar/³⁹Ar and electron microprobe approach

⁴⁰Ar ³⁹Ar and electron microprobe examination of blueschist samples from the Iceberg Lake schist, southern Alaska suggest that phengite inclusions are the source of ⁴⁰Ar in crossite. Because such finegrained inclusions may be susceptible to argon loss, caution should be exercised in interpreting K-Ar ages from this phase, and possibly other low-K amphiboles from blueschist suites. The estimated blocking temperature for phengite in the matrix (314° to 450°C), however, is close to the estimated peak metamorphic temperatures (325° ± 50°C), suggesting that phengite ⁴⁰Ar ³⁹Ar plateau dates may coincide closely with the time of blueschist metamorphism.