Volcanic gases reflect magma stalling and launching depths

Shuo Ding; Terry Plank; J. Maarten de Moor; Yves Moussallam; Maryjo Brounce; Peter J. Kelly

doi:10.1016/j.epsl.2025.119349

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Volcanic gases reflect magma stalling and launching depths

Shuo Ding·Terry Plank·J. Maarten de Moor·Yves Moussallam·Maryjo Brounce·Peter J. Kelly

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Abstract

Many open-vent arc volcanoes display two modes in their continuous gas emissions, one with a characteristic CO 2 / S T ratio typical of periods of quiescent degassing and another punctuated by high CO 2 / 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[CO 2 ] 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 CO 2 contents in MIs decrease from ∼4000 to <1000 ppmw as S contents decrease from 3500 to <1000 ppmw. The highest reconstructed S and CO 2 in the MIs resulted in an initial magmatic CO 2 / S T ratio (molar) of 0.83. Informed by the MI data, we modeled the decompression degassing of Turrialba magma and vapor composition using the Sulfur_X and EVo models. Instead of being controlled by initial magmatic CO 2 /S T ratio as suggested by previous studies, we find that the quiescent gas emitted from Turrialba during 2014–2018 (CO 2 / S T = 2.3 ± 0.8, molar) appears to reflectequilibrium with magmas stored at 4–8 km (Sulfur_X) or 2 km (EVo) depth, when H 2 O is degassing extensively from the magma. A magma storage region at 4–8 km is also supported by seismic tomography. The second gas mode is noted by spikes in CO 2 / S T ∼ 7.9 ± 2 in the weeks prior to eruption. This gas reflects equilibrium with a magma at 12–18 km (Sulfur_X) or 4–8 km (EVo), where the ascending magma is saturated with a CO 2 -rich vapor. Thus, there are two important trans crustal depths beneath the volcano: one where the rate of H 2 O loss from the magma and thus magma viscosity increases, and one at greater depths where high CO 2 /S T vapor forms and may facilitate dike propagation. We interpret the shallower, H 2 O-loss region as the main site of magma stalling and storage, where quiescent gas is generated continuously. We interpret the greater depth (12–18 km) as the source of the precursory gas that precedes eruption, and where the mafic melt lastly equilibrated with a mush zone before ascending and triggering eruption weeks later. This hypothesis is ripe for testing at other volcanoes that exhibit two modes in gas geochemistry.

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Earth and Planetary Science Letters Article

Source-reported bounding extent: 9.9167° to 10.0833° latitude; -83.92° to -83.667° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Shuo Ding, Terry Plank, J. Maarten de Moor, Yves Moussallam, Maryjo Brounce, Peter J. Kelly. 2025-04-15. Volcanic gases reflect magma stalling and launching depths. https://doi.org/10.1016/j.epsl.2025.119349

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