Storage conditions of a caldera-forming volcanic eruption: Insights from the Pudahuel rhyolitic ignimbrite in central Chile (32° 10′S)

The Pudahuel Ignimbrite is a rhyolitic, crystal poor deposit associated with the formation of the Diamante caldera in the Southern Volcanic Zone of the Andes. This Quaternary ignimbrite is the result of a massive eruption in what is now a densely populated area. The goal of this study is to understand the origin of this caldera-forming eruption by determining the pre-eruptive magmatic conditions through analytical and experimental approaches. The FeTi oxide geothermometer and the plagioclase hygrometer were used to determine the temperature-XH2Ofl conditions that produce the glass, feldspar, and FeTi oxide compositions observed in the natural sample. The results were combined with a composition-dependent water solubility model to constrain the magma storage pressure. This methodology was extended to temperatures up to 850 °C to define a band of P-T-H2O conditions that satisfy the natural plagioclase and glass compositional pairs, from 700 °C at 310 MPa to 850 °C at 85 MPa. Phase equilibrium experiments were performed at conditions within the band at H2O-vapor saturated and undersaturated conditions using a representative pumice sample as the starting material. The purely analytical approach, based on FeTi oxide geothermometry and plagioclase hygrometry of the pumice phases, suggests pre-eruptive temperatures of 717 ± 7 °C, PH2O of 200–360 MPa and between 6.9 and 7.6 wt% H2O. The natural phase assemblage and the glass composition was best reproduced by experiments performed at low temperatures (700–750 °C) and high PTotal (>275 MPa, more likely 300–350 MPa), with water contents of at least 6 wt%. A compositional match between synthetic and natural glasses occurred in some H2O-undersaturated experiments, suggesting that the melt was not necessarily saturated with H2O-rich fluid. Overall, our work highlights that the source region related to the caldera-forming Pudahuel ignimbrite was relatively deep (> 10 km). This depth, combined with the low crystallinity of the magma, and the possibility of an H2O-undersaturated melt, suggest that caldera formation was likely initiated by an external trigger rather than internal pressurization due to volatile buildup in the reservoir. The storage conditions determined in this study lay the groundwork for continuing work to understand the Maipo volcanic complex and the future hazards it poses.