P–T–X evolution of the 1280 AD Quilotoa dacite

• Pre-eruptive AD 1280 Quilotoa magma P–T–X and volatile budgets were determined.
• The pre-eruptive melt contained high concentrations of dissolved H2O (> 6 wt.%).
• Melt inclusions and phenocrysts record volatile degassing from pressures > 300 MPa.
• Volatile-oversaturated magma ascended continuously at ~ 0.02–0.08 m/s in 2–4 days.
• The cyclically-homogeneous chemically-buffered magma erupts when oversaturated.

The pre-eruptive storage conditions and volatile budgets were constrained for magma erupted with Plinian intensity from Quilotoa volcano in Ecuador in AD 1280. Geothermobarometry, solubility modeling, and petrologic observations were combined to estimate magma temperature, pressure, oxygen fugacity and concentrations of H2O, CO2 and S in the melt phase. The moderately oxidized (NNO + 1.63) magma was found to have resided between 4 and 11 km, under associated pressures of ~ 130–340 MPa and a temperature of 780 ± 20 °C. Variability in estimated storage pressures, melt volatile concentrations (4.8 ± 1.5 to 8.8 ± 2.8 wt.% H2O and 0 to 447 ± 170 ppm CO2), Na2O–MgO–H2O trends and a decrease in vapor bubble volumes in the glass inclusions suggest that the magma under Quilotoa concomitantly degassed and crystallized in the early stages of magma ascent, with maintained high H2O suppressing crystallization thereafter. H2O concentrations may have been partly buffered with volatile input from a deeper reservoir of chemically similar magma prior to the AD 1280 eruption. The Quilotoa deposits are thus thought to represent a homogenous chemically-buffered zone within an even larger-volume intermediate body that periodically erupts geochemically uniform deposits in a volatile-oversaturated system.