Assessing permanent CO2 storage volume in a buried volcano offshore West Iberia

In-situ mineral carbonation is a process that occurs naturally, through the interaction of CO₂ rich fluids and minerals of mafic and ultramafic rocks, towards the formation of new stable carbonate materials. Formed spontaneously in peridotites and serpentinites (e.g., the Samail Ophiolite of Oman), the concept has been successfully replicated through industrial applications with mineral trapping demonstrated to occur on Icelandic basaltic lava flows within a period of 2 years.

A Late Cretaceous volcanic edifice located on central West Iberian Margin, offshore Portugal, is investigated as a conceptual site for in-situ mineral carbonation, and a study case for the application of this model on similar volcanic edifices on continental margins worldwide.

Using seismic reflection surveys, the volcano is revealed as a 2800 m high edifice, with an estimated total rock volume of 327 km³. Dredges collected from an exposed crest of the volcano revealed vesicular olivine-rich sub-alkaline basalts, infilled with naturally formed carbonate minerals. Analysis of the internal architecture of the edifice reveals outward dipping reflectors, with its magmatic features assigned to alternating successions of lava flows and explosive debris, that have grown progressively to form a composite volcano. Lava flows directly associated with the final stages of volcanic build-up, comprise dendritic and lobate lava flows (pahoehoe or submarine flows) blanketing the flank of the edifice. Accounting for the auspicious architecture, nature, and rock properties of the edifice, a deterministic volumetric model is used to estimate different scenarios of the amount of CO₂ that can be safely and permanently stored in the volcano. Combining comprehensive inputs from bulk rock volume, effective porosity, and sequestration ratios, our estimations indicates that on a base case, the volcanic edifice has the potential to capture nearly 1.2 Gt CO₂ into new stable mineral phases. On a high case scenario, this single edifice could permanently capture up to 8.6 Gt of CO₂. Considering that during the 2015-2018 period, the Portuguese energy sector emitted an average volume of about 48 Mt CO₂ eq per year, our estimates suggest that the volcanic reservoir is capable of storing an equivalent of 24 years of the country’s industrial emissions. Compared with oceanic magmatic sequences worldwide, buried volcanic edifices on continental margins materialise as notable locations for in-situ mineral carbonation.

This assessment provides timely insights on the overall process of in-situ mineral carbonation, on ancient buried volcanoes, to reveal critical geological controls that can lead this technique to be applied on a pilot phase and envisage further concepts at economic scale. Moreover, geohazards associated with the proximity to populated areas are significantly minimised. Ultimately, results suggest that volcanoes on passive continental margins can be considered for safe and permanent carbon storage, by accommodating 100’s of Gt of CO₂ from energy intensive industry sources and contribute to mitigate the impacts of anthropogenic carbon emissions.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) UIDB/04035/2020-GeoBioTec and UIDB/50019/2020-IDL. D. Gamboa thanks FCT funding for project MAGICLAND (PTDC/CTA-GEO/30381/2017).