PETROLOGIC IMAGING OF THE ARCHITECTURE OF MAGMA RESERVOIRS FEEDING CALDERA-FORMING ERUPTIONS

Caldera footprints and erupted magma volumes provide a unique constraint on vertical dimensions of upper crustal magma reservoirs that feed explosive silicic eruptions. Here we define a Vertical Separation (VS) ratio in which we compare this geometric vertical extent with the range of depths indicated petrologically by melt inclusion water and CO₂ saturation pressures for fifteen caldera-forming eruptions spanning ~10⁰ km³ to ~10³ km³ in volume. We supplement melt inclusion saturation pressures with rhyolite-MELTS barometry and plagioclase-melt hygrometry to generate a petrologic image of magma reservoir architecture. We find that pre-eruptive upper crustal magma reservoirs range from contiguous bodies (where petrologic and geometric estimates match closely) to vertically dispersed structures. Vertically dispersed pre-eruptive reservoirs are more common among intermediate-volume eruptions than among the smallest and largest caldera-forming eruptions. We infer that the architecture of magma reservoirs tracks the thermomechanical evolution of large volcanic systems.