Volcanic eruptions and their associated gas emissions significantly impact Earth's volatile cycles and climate. Precise quantification of magma volatile content, particularly CO2 (due to its low solubility and early exsolution), is crucial for volcanological research. While volatile solubility is well-understood, CO2 contents measured in melt inclusions often underestimate initial magmatic CO2 due to pre-entrapment exsolution and post-entrapment loss. Syn-eruptive CO2 emission measurements, combined with magma eruption rate, SO2 fluxes, and initial magma sulfur content, offer a more robust approach to determine pre-eruptive CO2. Recent discoveries suggest magmatic CO2 contents are higher than previously thought, with implications for the global geological CO2 budget and climate change. Alkaline mafic magmas, particularly those in intra-plate oceanic islands, are believed to be especially CO2-rich. The 2021 Tajogaite eruption on La Palma Island, Canary Islands, provided a unique opportunity to quantify volatile emissions and magma volatile contents for alkaline mafic magmatism in this archipelago, as previous eruptions lacked such measurements.

Prior research established the importance of quantifying volatile content in magmas, focusing on CO2 due to its low solubility. Studies utilizing melt inclusions have yielded insights into volatile solubility and behavior under various pressure-temperature conditions. However, limitations exist due to CO2 loss during melt inclusion formation and ascent. Alternative methods employing trace element ratios (e.g., CO2/Nb, CO2/Ba) have been explored to constrain initial magmatic CO2 content but are subject to uncertainties from mantle heterogeneity and melt mixing. Syn-eruptive measurements offer a more direct and accurate way to assess volatile contents when combined with other eruption parameters. Existing literature highlights the potentially high CO2 content of alkaline mafic magmas, especially those found in intra-plate oceanic settings, with suggestions that these magmas originate from low-degree partial melting of enriched mantle sources. Previous studies on the Canary Islands indicated high CO2 content in basanites but lacked direct syn-eruptive CO2 emission measurements.

The 2021 Tajogaite eruption offered an ideal opportunity for comprehensive gas emission measurements. The study employed multiple approaches to quantify emitted gas compositions and fluxes: 1. **Multi-GAS:** In-situ measurements were conducted using ground-based (mobile and fixed stations) and drone-mounted Multi-GAS systems, providing data on CO2, SO2, H2S + H2, pressure, temperature, and relative humidity during the eruption's initial phase (September 27-October 4, 2021). These measurements focused on different vents exhibiting various eruptive styles (fountaining, spattering, and effusive). 2. **Open-path FTIR (OP-FTIR) spectroscopy:** Remote sensing measurements from October 3 to December 2, 2021, were used to analyze gas compositions from various vents, using both passive and solar absorption modes. These measurements provided data on H2O, CO2, SO2, and HCl. 3. **Satellite-based SO2 flux measurement:** TROPOMI data from the Sentinel-5P satellite was analyzed using the PlumeTraj approach to determine SO2 fluxes and plume heights, providing an overall estimate of SO2 emission throughout the eruption. 4. **Melt inclusion analysis:** Volatile contents (H2O, CO2, S) in olivine-hosted glassy melt inclusions were measured via electron microprobe analysis to constrain the initial volatile abundance of the parental magma. This provided key data on initial sulfur abundance in the magma. 5. **Degassing modeling:** A C-S-O-H saturation model was used to compute the evolving fluid composition during decompression of the magma. Model runs were conducted under various initial conditions (CO2 content, redox state) and degassing scenarios (closed-system, open-system) to reconstruct the magma's degassing path. The researchers combined these measurements with volcanological and petrological data (magma eruption rate, SO2 fluxes, initial magma sulfur content) to determine the CO2 content of the basanite magma and the total CO2 emission during the eruption.

The study revealed significant findings: 1. **High CO2/SO2 ratios:** A stark contrast in CO2/SO2 ratios was observed between the explosive upper vents (FV) and the spattering/effusive flank vents (SV/EV), indicating preferential outgassing of CO2 through the upper vents. FV displayed exceptionally high CO2/SO2 ratios (mean of 33 ± 9.7 molar, or 23 ± 6.7 by mass) throughout the eruption, among the highest ever recorded. 2. **Magmatic CO2 content:** By combining syn-eruptive gas measurements with magma eruption rates and initial sulfur content, researchers estimated a high initial magmatic CO2 content of 4.5 ± 1.5 wt% for the 2021 Tajogaite eruption. This estimation accounts for gas-melt separation at a shallow branch point in the conduit system. 3. **Total CO2 emission:** The total CO2 emission during the 86-day eruption was calculated to be 28 ± 14 Mt CO2. 4. **Magma degassing path:** The gas composition data, combined with seismic evidence, suggests a near closed-system ascent of magma to a shallow branch point where gas-melt separation occurred, with most exsolved gas channeled to the FV and magma predominantly erupted at the SV/EV. 5. **Melt inclusion data:** Melt inclusion analysis yielded a mean sulfur content of 3290 ± 390 ppm, indicating high initial sulfur abundance in the parental basanite. The H2O and CO2 content in these inclusions suggested entrapment pressures consistent with an intermediate magma ponding zone at around 10-12km depth. 6. **Extrapolation to La Palma formation:** Extrapolating the estimated CO2 content and eruption rates to the entire subaerial and submarine formation of La Palma island yielded an estimated total CO2 emission of around 1.1 x 10^16 moles over 11 Ma, a substantial amount comparable to 20% of the CO2 released during a typical large igneous province eruption. This suggests that the formation of the Canary Islands contributed significantly to the geological carbon cycle.

The exceptionally high CO2 content of the 2021 Tajogaite eruption magma contrasts with previous assumptions about intraplate alkaline volcanism. This high CO2 content, primarily released through the upper vents, points to a magma source strongly enriched in CO2 due to low-degree partial melting of a metasomatized mantle source, possibly involving recycled oceanic crust. The efficient channeling of the gas phase to the upper vents during the eruption, as well as the relatively high initial sulfur content of the magma, played a significant role in shaping the eruption's dynamics and the final gas compositions. The significant CO2 released during the eruption and extrapolated to the formation of La Palma, as well as the whole Canary archipelago, highlights the substantial contribution of this type of volcanism to the Earth's long-term carbon cycle. The high CO2 content at depth likely contributed to the magma's explosivity and its ability to sustain high-intensity eruption activity for many weeks.

This study provides the first direct syn-eruptive measurements of CO2 emissions from a volcano in the Canary Islands. The findings demonstrate exceptionally high CO2 emissions from the 2021 Tajogaite eruption, highlighting the significant contribution of intraplate alkaline ocean volcanism to the geological carbon cycle. The study's results challenge previous assumptions regarding CO2 content in such magmas and provide new insights into the dynamics of magma degassing and eruption. Future research could focus on more detailed modeling of the complex conduit system and broader analysis of other volcanoes in the Canary archipelago to better understand the variability and extent of CO2 emissions from this volcanic chain.

While the study provides valuable insights into CO2 emissions from the 2021 eruption, some limitations exist. The mass-balance approach for calculating initial magma CO2 content relies on several assumptions (e.g., proportion of gas channeled to the explosive vents, fraction of sulfur exsolved at the branch point), which introduce uncertainties. Additionally, the extrapolation of CO2 emissions to the entire formation of La Palma (and the Canary archipelago) is based on several simplifications (e.g., constant eruption rate, uniform magma composition throughout the edifice's history) that could affect the accuracy of the overall estimate. Further, limited melt inclusion analyses for CO2 and H2O introduce additional uncertainties into the modelling of the magma degassing path.