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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology