Magma recharge dynamics before the 2021 eruption at Pacaya volcano (Guatemala) as revealed by textures and zoning of plagioclase and olivine crystals

The February-May 2021 eruptive activity of Pacaya volcano, Guatemala, represents one of the most impactful phases of its recent history, producing basaltic lava flows, ashfall, and explosive activity that exposed nearby communities to multiple volcanic hazards. We present a petrological study aimed at understanding the dynamics that led to the onset of such activity by combining compositional and textural features of plagioclase and olivine crystals. We used rock and crystal chemistry to constrain thermodynamically the crystallization conditions of plagioclase and olivine and to reproduce the evolution of magmas feeding the eruption. The analysis of compositional variations in olivine crystals allowed the identification of two main compositionally distinct magmatic environments where crystals grew at stable pressure, temperature and redox conditions. The deeper magmatic environment (M1) was constrained at depht of similar to 4 km below the summit and the shallowest (M2) at a depth of 1-2 km. Magma residing in these environments were found to interact at shallow depth before the eruption. Plagioclase textures and zoning patterns confirm the evidence of shallow magmatic interaction and provide further insights on pre-eruptive magma ascent and degassing processes. According to Fe-Mg diffusion timescales in olivine, two subsequent recharging phases occurred. An early magmatic input was recorded between December 2020 and January 2021 during a period of mild eruptive activity, followed by a second phase of recharge and mixing coinciding with the most intense phase of volcanic activity in late February-March. This study highlights the mechanism by which the Pacaya plumbing system is capable of mobilizing magma to the surface over timescales of weeks to months, producing both explosive events and sustained effusive activity.