Co₂ Sequestration During Interactions Between Fluid and Mafic to Intermediate Intrusive Rocks on Vannøya Island, West Troms Basement Complex, North Norway

Abstract

The island of Vannøya, which is part of the West Troms Basement Complex, North Norway, is dominated by 2885 Ma tonalitic gneisses. The gneisses were intruded by a mafic dyke swarm about 2400 Ma ago. Unconformable above the tonalitic gneisses and the mafic dykes, the Vanna Group metasedimentary rocks were deposited. The metasedimentary rocks were subsequently intruded by a unit of mafic to intermediate rocks (SiO₂ = 50–60 wt%) 2220 Ma ago. The intrusive rocks have undergone at least two stages of deformation and metamorphism, possibly during a Svecofennian (ca. 1.7 Ga) tectonothermal event: the first stage was associated with infiltration of a hydrous fluid, while the second stage was associated with infiltration of a CO₂ bearing fluid. The latter fluid was forcefully injected into the host rock, leading to fragmentation of the host rock and formation of explosion breccia. The primary igneous mineral assemblage of the intrusive rock was plagioclase, hornblende, and Fe–Ti oxides. During the first stage of fluid infiltration, which was associated with a hydrous fluid, the rock underwent various degree of spilitization (Na-metasomatism). This resulted in metamorphic alteration of the primary igneous mineral assemblage to an amphibole facies assemblage composed of plagioclase, amphibole, Fe–Ti oxides, biotite, epidote/zoisite, and chlorite. The mineral content and chemical composition of the rock that reacted with the CO₂ bearing fluid indicate that the fluid was extremely reactive. All Fe–Mg silicates and Fe–Ti oxides of the earlier mineral assemblages were completely broken down during the reactions. Furthermore, all primary plagioclase was transformed to albite, and significant amounts of carbonate minerals formed. The result of the reactions was a brown weathering rock composed of albite, carbonate minerals (Fe-bearing dolomite and calcite), and minor amounts of quartz and rutile. Whole rock compositional data shows that the rock that interacted with the CO₂ bearing fluid was strongly depleted in iron: the primary igneous rock and its amphibole facies equivalent show Fe₂O₃ content in the range 12–18 wt%, while the CO₂ reacted rock shows Fe₂O₃ content in the range 1–4 wt%. Furthermore, the CO₂ reacted rock was depleted in MgO, K₂O, LREE, and MREE, but enriched in SiO₂, Al₂O₃, Na₂O, CaO, and TiO₂. In addition, a significant amount of CO₂ was sequestrated in the rock as a result of carbonate (dolomite and calcite) formation. The rock shows CO₂ concentrations up to 15 wt%, which corresponds to more than 400 kg of CO₂ per cubic meter of rock.