Tracking Deep Magma Migration Through Diffuse CO ₂ Degassing: Insights From Piton de la Fournaise Volcano (2013–2023)

Abstract Volcano monitoring requires identifying and distinguishing physico‐chemical processes at various depths within magma plumbing systems and their links to volcanic activity. Long‐term monitoring of deep magma reservoirs, extending down to tens of kilometers, is crucial even though signals from these depths are often altered or attenuated before reaching the surface. Here, we investigate the architecture and dynamics of the Piton de la Fournaise plumbing system by integrating soil CO 2 flux mapping, continuous soil CO 2 monitoring, seismicity, ground deformation, and lava geochemistry recorded from 2013 to 2023. Environmental influences on CO 2 time series are corrected using advanced cross‐correlation techniques. The system's architecture is mapped by combining CO 2 flux data, self‐potential mapping, and seismic density analysis. Results confirm a complex plumbing system, laterally shifted relative to the summit, with a shallow reservoir (∼2 km) near sea level fed by melts from the Moho (∼10 km), branching along the N120 rift zone and extending to a deeper melt zone at ∼30 km depth. Periodic recharge of the Moho deep reservoir induces intermittent CO 2 degassing in soils and feeds a hydrothermal system. Two major recharge events are identified: (a) in 2013–2015, involving seismic activity, soil CO 2 increase, inflation, and initial eruptions; and (b) in 2019, marked by rapid magma transfer, degassing, and eruptions with distinct strontium isotopic signatures. These findings highlight the value of diffuse CO 2 monitoring for understanding deep volcanic processes and provide early indicators of magma recharge, improving volcanic hazard assessment for basaltic shield volcanoes like Piton de la Fournaise.