TY - JOUR
T1 - Mantle CO2 degassing through the Icelandic crust
T2 - Evidence from carbon isotopes in groundwater
AU - Stefansson, Andri
AU - Sveinbjornsdottir, Arny E.
AU - Heinemeier, Jan
AU - Arnorsson, Stefan
AU - Kjartansdottir, Rikey
AU - Kristmannsdottir, Hrefna
PY - 2016/10
Y1 - 2016/10
N2 - 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 (∑CO
2) between 1.8 and 4100 ppm. The measured range of δ
13CO
2 and
14CO
2 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 CO
2 were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO
2 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 CO
2 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 CO
2 sources. The CO
2 flux of the Icelandic crust was estimated to be ∼5–10 · 10
10 mol/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 CO
2 may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO
2 partial pressures of ∼10
−6–1 bar and to a mantle CO
2 flux of <5 · 10
5 mol/km
2/yr for most areas and up to 125 · 10
5 and 1600 · 10
5 for the Southern Lowlands and Snæfellsnes Peninsula, respectively. The CO
2 flux from active volcanic geothermal systems in Iceland was estimated to be ∼500–3000 · 10
5 mol CO
2/km
2/yr, considerably greater than the highest values observed off-axis.
AB - 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 (∑CO
2) between 1.8 and 4100 ppm. The measured range of δ
13CO
2 and
14CO
2 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 CO
2 were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO
2 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 CO
2 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 CO
2 sources. The CO
2 flux of the Icelandic crust was estimated to be ∼5–10 · 10
10 mol/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 CO
2 may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO
2 partial pressures of ∼10
−6–1 bar and to a mantle CO
2 flux of <5 · 10
5 mol/km
2/yr for most areas and up to 125 · 10
5 and 1600 · 10
5 for the Southern Lowlands and Snæfellsnes Peninsula, respectively. The CO
2 flux from active volcanic geothermal systems in Iceland was estimated to be ∼500–3000 · 10
5 mol CO
2/km
2/yr, considerably greater than the highest values observed off-axis.
KW - Carbon isotopes
KW - DEGREES-C
KW - DIOXIDE
KW - EASTERN ICELAND
KW - EMISSIONS
KW - FRACTIONATION
KW - GAS
KW - GEOTHERMAL FLUIDS
KW - Iceland
KW - Isotope geochemical modeling
KW - Mantle CO2 degassing
KW - NATURAL-WATERS
KW - SUBGLACIAL BASALTS
KW - SYSTEMATICS
KW - Thermal fluids
UR - http://www.scopus.com/inward/record.url?scp=84983491136&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2016.06.038
DO - 10.1016/j.gca.2016.06.038
M3 - Journal article
SN - 0016-7037
VL - 191
SP - 300
EP - 319
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -