Petrogenesis of silicic magmas in Iceland has fundamental significance for understanding the relative importance of fractional crystallization of mantle-derived basalt and partial melting of hydrothermally altered basaltic crust in formation of the earliest continental crust. First results of in situ oxygen isotope investigation of zircons in large-volume silicic eruptive products of three volcanoes in Iceland (Askja, Torfajo ̈ kull, and Hekla) demonstrate isotope diversity and disequilibria and long U-Th zircon pre-eruptive residence of 103-104 year. This suggests that zircons did not grow from their host melts but instead were inherited from older magma batches and leftover cumulates with generally low and variable d18O values. This study demonstrates that segregation of cubic kilometres of silicic magma is faster than mineral-diffusive or recrystallization time-scales (estimated at ∼103 years), and it suggests that partial melting of hydrothermally altered and oxidized oceanic crust is the mechanism that best explains silicic rocks in Iceland and early earth environments.