A natural laboratory for carbon capture and storage: listvenites along regional fault zones (Zermatt Saas Unit, Western Alps, Italy)

In the last decades many studies focussed on carbon capture and storage (CCS) to find a possible remedy to reduce the large increase of anthropogenic carbon dioxide (CO₂). CCS can potentially sequester billions of tonnes of CO₂ per year using the Earth as the widest laboratory available for long-term storage. In geological reservoirs, CO₂ can be trapped by physical and chemical mechanisms. Among chemical mechanisms, mineral carbonation plays a crucial role in CCS, being almost irreversible, involving the chemical reaction in aqueous environment between CO₂ and Mg- and/or Ca-rich minerals, where CO₂ is converted into solid carbonates.

In nature, listvenite, a rock mainly composed of Mg-Ca-bearing carbonates, quartz and Cr-bearing mica (fuchsite), documents natural CO₂ sequestration. Indeed, listvenites are the result of the extensive alteration of ultramafic rocks by CO₂-bearing fluids, which involved the substitution of olivine, pyroxene and serpentine by Ca- and Mg-carbonates. To date, very little is known about the kinetics and rate of this reaction, spanning from weeks (serpentinites) to thousands of years (peridotites).

We studied carbonated serpentinites from the Zermatt-Saas Zone (Corno del Camoscio, Western Alps, Italy) which underwent fluid-mediated natural carbonation under hydrothermal conditions. Hydrothermal carbonation is spatially associated to Oligo-Miocenic brittle faults of the Aosta-Ranzola system (Bistacchi et al., 2001). Field structural surveys identified two main strike-slip fault sets (N-S and NW-SE striking) controlling fluid flow, with voluminous carbonation observed mainly along the NW-SE-striking set. We collected a series of structurally-controlled samples along a reaction front from serpentinite to listvenite close to a major fault zone, aiming to relate the CO₂-rich fluid/rock interaction with mega and meso-structures, along with detailed microstructural and chemical analyses.

The petrographic study, along with X-ray maps and microprobe chemical analyses, identify the following mineral associations, from serpentinite to listvenite: (i) serpentine + chlorite and minor quartz + fuchsite, talc, calcite and dolomite, (ii) serpentine + brucite + chlorite and minor quartz, talc, calcite and dolomite-siderite, (iii) dolomite, quartz, chlorite, serpentine and minor fuchsite associated with quartz-chlorite layers, (iv) quartz, dolomite and fuchsite with relict brucite. Interestingly, samples collected close to the serpentinite show microfolds where dolomite is stable, subsequently cut by brittle deformation related to the large-scale faults, suggesting a previous stage of fluid-mediated carbonation under a ductile deformation regime.

Qualitative and quantitative X-ray powder diffraction data enabled us to calculate a mass balance to model the rate of reaction and the composition of the original fluids. Preliminary results indicate a structural control on the fluid drainage and the role of brucite to dominate the carbonation reaction, as reported by experimental results of Campione et al. (2024), along with fuchsite.

 

 

Bistacchi, A., Dal Piaz, G., Massironi, M., Zattin, M., Balestrieri, M. (2001). The Aosta–Ranzola extensional fault system and Oligocene–Present evolution of the Austroalpine–Penninic wedge in the northwestern Alps. International Journal of Earth Sciences, 90, 654-667

 

Campione, M., Corti, M., D’Alessio, D., Capitani, G., Lucotti, A. Yivlialin, R., Tommasini. M., Bussetti, G., Malaspina, N. (2024). Microwave-driven carbonation of brucite. Journal of CO₂ Utilization, 80, 102700