Surface occurrence of dawsonite and natural CO2 emanation in Covasna, in the Eastern Carpathians: A stable isotope study

Abstract

Dawsonite (NaAlCO3(OH)2) is widely known as a CO2 trapping mineral during the geological storage of CO2 (Carbon Capture and Storage). Dawsonite can occur naturally in various geological environments, however only few surface occurrences are known. The Covasna area (Eastern Carpathians) gives an excellent opportunity to study dawsonite in surface outcrop in the Hankó Valley and extend our knowledge about the possible precipitation environments of dawsonite. Based on the detailed petrographic and geochemical investigations of the rock and water samples from this area, formation environment of dawsonite and the concomitant mineral assemblage (e.g. alumohydrocalcite) were constrained. Based on our results, dawsonite and alumohydrocalcite formed at low temperature (7.6–20 °C). Utilizing the stable carbon, oxygen and hydrogen isotope composition of dawsonite separates (δ13CDaw: +8.0 − +10.1 ‰, δ18ODaw: +40.7 − +46.7 ‰, δ2HDaw: −55 - -42 ‰), the isotopic composition of the parental fluid was determined. Our results indicate that the dawsonite formed from a CO2 source, which has mantle origin. In addition, H2O which was present during the formation of dawsonite, had similar hydrogen isotope composition to recent spring waters, whereas the oxygen isotope composition is characterized by more positive values. This oxygen shifts relative to Global Meteoric Waterline might be related to water-rock interaction or contribution of metamorphic/magmatic water and oxygen isotope exchange between H2O and CO2.