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Constraining the volatile content of magmas is critical to our understanding of eruptive processes and their deep Earth cycling essential to planetary habitability [R. Dasgupta, M. M. Hirschmann, Earth Planet. Sci. Lett. 298 , 1 (2010)]. Yet, much of the work thus far on magmatic volatiles has been dedicated to understanding their cycling through subduction zones. Further, studies of intraplate mafic volcanism have disproportionately focused on Hawaii [P. E. Wieser et al., Geochem. Geophys. Geosyst. 22 , e2020GC009364 (2021)], making assessments of the overall role of intraplate volcanoes in the global volatile cycles a challenge. Additionally, while mafic volcanoes are the most common landform on Earth and the Solar System [C. A. Wood, J. Volcanol. Geotherm. Res. 7 , 387–413 (1980)], they tend to be overlooked in favor of silicic volcanoes when it comes to their potential for explosivity. Here, we report primitive (olivine-hosted, with host Magnesium number – Mg# 78 to 88%) melt inclusion (MI) data from Fogo volcano, Cabo Verde, that suggest that oceanic intraplate silica-undersaturated explosive eruptions sample volatile-rich sources. Primitive MI (melt Mg# 70 to 71%) data suggest that these melts are oxidized (NiNiO to NiNiO+1) and very high in volatiles (up to 2 wt% CO 2 , 2.8 wt% H 2 O, 6,000 ppm S, 1,900 ppm F, and 1,100 ppm Cl) making Fogo a global endmember. Storage depths calculated from these high volatile contents also imply that magma storage at Fogo occurs at mantle depths (~20 to 30 km) and that these eruptions are fed from the mantle. Our results suggest that oceanic intraplate mafic eruptions are sustained from the mantle by high volatile concentrations inherited from their source and that deep CO 2 exsolution (here up to ~800 MPa) drives their ascent and explosivity.

Raman spectroscopy has become the tool of choice for analyzing fluid inclusions and melt inclusion (MI) vapor bubbles as it allows the density of CO2-rich fluids to be quantified. Measurements are often made at ambient temperature (Tamb ~18-25 °C), resulting in reported bulk densities between 0.2 and 0.7 g/mL despite that single-phase CO2 under these conditions is thermodynamically unstable and instead consists of a liquid (~0.7 g/mL), and a vapor phase (~0.2 g/mL). Here, we present results from experiments conducted at Tamb and 37 °C (above the CO2 critical temperature) on 14 natural CO2-rich MI bubbles from Mount Morning, Antarctica. Here, we show that at Tamb, laser power strongly affects the CO2 Raman spectrum of MI bubbles with bulk densities within the miscibility gap. High-power laser heating and low spectral resolution explain why published measurements have reported such bulk densities at Tamb even when using an instrument-specific calibration.