Crystals and Melt Inclusions Record Deep Storage of Superhydrous Magma Prior to the Largest Known Eruption of Cerro Machín Volcano, Colombia.

This article focuses on the petrological and geochemical investigation of Cerro Machín volcano (CMV) in Colombia, emphasizing the presence of superhydrous dacitic magma (>8 wt % H2O) stored in a mid-to-lower crustal reservoir at pressures around 700 ± 250 MPa (~25 km depth). Direct measurements of water content in quartz- and plagioclase-hosted melt inclusions, supported by amphibole barometry and mineral chemistry, reveal exceptionally high water concentrations up to 10.7 wt % H2O, indicating a remarkably water-rich magmatic system. The magmas exhibit an adakite geochemical signature, likely derived from melting of hydrated basaltic slab material and subsequent fractionation of garnet and amphibole, with evidence for minor andesitic magma recharge triggering the largest known plinian eruption (~3600 years BP). These findings contribute to understanding the magmatic architecture, volatile budget, and eruption dynamics of CMV, a volcano considered highly hazardous due to its explosive history and proximity to populated areas.