In situ sequestration of atmospheric CO 2 at low temperature and surface cracking of serpentinized peridotite in mine shafts

The investigation of carbonate formation at low temperature during weathering of ultramafic rocks has become increasingly important as it represents an analog study for a cost-efficient carbon disposal strategy. Here we present new insight into carbonate formation under subarctic surface conditions obtained from extensively carbonated chromite mine shafts in the Feragen ultramafic body, E Norway. Carbonation proceeded by formation of mm- to cm-thick carbonate surface coatings on the serpentinized peridotite host rock. The surface coatings consist in most cases of pure lansfordite (MgCO 3·3H 2O). Heavy O- and C-isotopes are strongly enriched in the carbonate coatings relative to drip water samples from the mines and water samples from subaerial ponds. The major element composition of the water samples reveal that infiltrating rainwater reacts rapidly with the peridotite mainly due to brucite dissolution. Within the mines, discharging alkaline water forms thin films on walls and ceilings that evaporate upon circulation of cold, dry air. In mines with only one opening the carbonation is limited to the entrance area. In mines with a more efficient air circulation due to multiple entrances carbonation is found throughout the mine. Evaporation of the mine water results in lansfordite saturation and precipitation and is accompanied by a Rayleigh-type distillation effect causing enrichment of the heavy C- and O-isotopes in the liquid from which the carbonates precipitate. Formation of the carbonate coatings involved fragmentation of the underlying serpentinite substrate. Fragmentation proceeded by subcritical cracking, i.e. by a combination of substrate weakening due to brucite dissolution and stress build-up due to lansfordite precipitation in fractures close to the surface. Reaction-induced fracturing represents a positive feedback mechanism as it creates additional reactive surface area during the carbonation. Mining operations in the area ceased in the mid 1920s indicating that the described carbonation occurs on a time-scale relevant for the disposal of anthropogenic CO 2. © 2012 Elsevier B.V