Partitioning of excess argon between alkali feldspars and glass in a young volcanic system

► We observe large amounts of excess Ar dissolved in phonolite glass clasts of the Arico ignimbrite. ► Despite excess Ar in the magma, alkali feldspars yield accurate age of emplacement due to a low crystal/melt partition coefficient. ► Determined maximum partition coefficient, KD, for Ar between alkali feldspar and glass is less than 10− 3. ► Welding of the Arico ignimbrite does not facilitate diffusive loss of Ar from glass over the relevant timescales.