Time scales of silicic magma storage and differentiation beneath caldera volcanoes from uranium-238-thorium-230 disequilibrium dating of zircon and allanite

How quickly huge volumes of magma accumulate and evolve beneath caldera volcanoes and whether they are stored for long periods before eruption are fundamental to understanding the potential for destructive volcanic eruptions, how voluminous magmas differentiate, and how vast tracts of plutonic rocks are emplaced. I present the results of a new approach to determining the time scales of magma storage and differentiation by in situ dating of zircon and allanite crystals using 238U-230Th disequilibria. The ages and compositions of allanites and zircons from the Youngest Toba Tuff, Indonesia, record the prelude to Earth's largest Quaternary eruption and retain a faithful record of at least 150,000 years of magma storage and differentiation in a voluminous caldera-forming reservoir. The dominant subvolcanic magma was relatively homogeneous and nearly idle for ∼110,000 years. In the 35,000 years before eruption, the diversity of melts increased significantly as the system grew in size, finally merging for the climactic eruption at 75,000 years ago. The ages of zircons and the geochemistry of the voluminous Central Plateau Member rhyolites at Yellowstone caldera suggest that post-caldera magma was differentiated within 10's of thousands of years of eruption from a single reservoir that evolved for >100,000 years. Differentiation of the post-caldera reservoir was punctuated by episodes of effective crystal-melt separation at ∼200,000 years ago and ∼125,000 years ago. Small-volume and possibly pre-caldera rhyolites from the Coso volcanic field, California, contain abundant zircon xenocrysts and yield zircon and allanite ages indicating crystallization ≤30,000 years before eruption, despite field and petrologic evidence for a >200,000 year old reservoir of rhyolite that is growing in size.