Mantle CO2 degassing through the Icelandic crust: Evidence from carbon isotopes in groundwater

Abstract

Carbon isotopes of groundwater in Iceland were studied in order to determine the source and reactions of carbon at divergent plate boundaries not associated with active volcanic systems. All the waters were of meteoric origin, with temperatures of 1–130°C, pH of ∼4.5–10.5 and dissolved inorganic carbon (∑CO2) between 1.8 and 4100ppm. The measured range of δ13CO2 and 14CO2 in these waters was large, −27.4 to +2.0‰ and 0.6–118 pMC, respectively. The sources and reactions of dissolved inorganic carbon were studied by comparing the measured chemical and isotope composition with those simulated using isotope geochemical models. Three major sources of CO2 were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO2 through air–water exchange at surface, and (3) input of gas at depth into the groundwater systems that has similar carbon and isotope composition as the pre-erupted melt of the upper mantle and lower crust beneath Iceland. In the groundwater systems the CO2 chemistry and isotope content are modified due to carbonate mineral precipitation and changes in aqueous species distribution upon progressive water–rock interaction; these changes needed to be quantified in order to reveal the various CO2 sources. The CO2 flux of the Icelandic crust was estimated to be ∼5–10·1010mol/yr with as high as 50% of the flux not associated with active volcanic centers but placed off-axis where a significant proportion of the CO2 may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO2 partial pressures of ∼10−6–1bar and to a mantle CO2 flux of <5·105mol/km2/yr for most areas and up to 125·105 and 1600·105 for the Southern Lowlands and Snæfellsnes Peninsula, respectively. The CO2 flux from active volcanic geothermal systems in Iceland was estimated to be ∼500–3000·105mol CO2/km2/yr, considerably greater than the highest values observed off-axis.