Volcanic gases reflect magma stalling and launching depths

Many open-vent arc volcanoes display two modes in their continuous gas emissions, one with a characteristic CO₂/ S_T ratio typical of periods of quiescent degassing and another punctuated by high CO₂/ S_T gas emitted in the weeks before eruption, a recently recognized eruption precursor. In this study we explore the origin of the two modes of degassing revealed by time-series gas data at Turrialba volcano (Costa Rica) in the context of new melt inclusion (MI) data. To reconstruct the c[CO₂] of undegassed magma, we developed a rapid-quench piston-cylinder assembly to rehomogenize the vapor bubble commonly contained in MIs. We focus on olivine-hosted MIs from a mafic scoria sample erupted from Turrialba in 1864–1866. The reconstructed CO₂ contents in MIs decrease from ∼4000 to