Cooled magma reservoir conditions between explosive eruptions at Mount Pinatubo, Philippines

Mount Pinatubo has been active since 4 Ma, discharging dacite magma during at least seven eruptive periods. Despite our improved understanding of the 1991 eruption, little is known about the reservoir conditions and explosivities of the ancient volcano. We investigated zircon, Fe-Ti oxide, glass and bulk rock data from dacite eruption products (1991AD, 35000YBP and > 2.7-2.5 Ma). The mineral assemblage remains largely the same: Pl + Hbl ± Bt ± Qz ± Cum ± Anh. Fe-Ti oxides are low in Ti (X_Ulv 0.10, X_Ilm 0.51) and yield equilibrium temperatures ranging 708-752°C, 725-762°C and 710-748°C using the model of Ghiorso and Evans (2008), respectively. All Fe-Ti oxides yield high f_O2 ranging NNO + 1.5 to +1.6. In contrast, the Ti-in-zircon geothermometer of Ferry and Watson (2007) yields crystallization temperatures ranging 635-734°C, 666-758°C and 634-726°C, respectively. The calculation used a_TiO2 = 0.9 based on Fe-Ti oxide equilibrium and measured glass composition. Zircon crystallization temperatures are in good agreement with saturation at 738-754°C, obtained using glass and bulk rock composition and the method of Watson and Harrison (1983). Furthermore, applying the Ti-in-zircon pressure correction of -5°C/0.1 GPa suggests that magma may have been as cool as 594°C. Fe-Ti oxide temperatures record conditions shortly after mafic injection into the dacite reservoir, as they can re-equilibrate rapidly. On the other hand, we envision that low zircon crystallization temperatures (< 700°C) reflect cool, quiescent periods. Therefore, the Mount Pinatubo reservoir may have cooled to as low as 600°C between eruptions, resulting in highly viscous magmas. This may suggest comparable or more powerful eruptions in the volcano's past.