Initial argon in amphiboles from the Chugach Mountains, southern Alaska

The ⁴⁰Ar/³⁹Ar step-heating analyses of three hornblende samples from the Border Ranges ultramafic-mafic assemblage yield plateaus with slightly discordant dates of 180.2 ± 0.7, 177.5 ± 0.7, and 181.1 ± 1.7 Ma. A plot of the plateau temperature data for all three hornblende samples on an ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar diagram reveals the presence of a non-atmospheric "initial' argon component with an apparent ⁴⁰Ar/³⁶Ar versus ³⁹Ar/⁴⁰Ar diagram reveals the presence of a non-atmospheric "initial' argon comonent with an apparent ⁴⁰Ar/³⁶Ar of 379 ± 16 and yields a date of 175.7 ± 1.0 Ma. A potassium-poor hastingsite from the same complex yields a complicated spectra with dates as high as ~900 Ma. These older dates reflect contamination by argon with an isotopic composition similar to that found for the initial argon in the hornblende and do not record a Paleozoic component in the age of the Border Ranges ultramafic-mafic complex. -from Authors