The 40 Ar/ 39 Ar and U/Pb dating of young rhyolites in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40 Ar in biotite

High‐precision dating of Quaternary silicic magmas in the active Kos‐Nisyros volcanic center (Aegean Arc, Greece) by both 40 Ar/ 39 Ar on biotite and U/Pb on zircon reveals a complex geochronological story. U/Pb ID‐TIMS multi and single‐grain zircon analyses from 3 different units (Agios Mammas and Zini domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10–20 Ma. The youngest dates provide the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust (Miocene and older). Step‐heating 40 Ar/ 39 Ar experiments on 1–3 crystals of fresh biotite yielded highly disturbed Ar‐release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise 40 Ar/ 36 Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3 He flux in the Kos‐Nisyros area, we suggest that biotite crystals retained some mantle 40 Ar that led to the observed, anomalously old ages. In contrast, sanidine crystals from the only sanidine‐bearing unit in the Kos‐Nisyros volcanic center (the caldera‐forming Kos Plateau Tuff) do not appear to store any excess 40 Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle volatiles, 40 Ar/ 39 Ar geochronology on biotite can lead to erroneous results due to the presence of excess 40 Ar and should be checked either against 40 Ar/ 39 Ar sanidine or U/Pb zircon ages.