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

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• 作者：Andri Stefá ; nsson^a ; ^{as@hi.is" class="auth_mail" title="E-mail the corresponding author} ; Á ; rný ; E. Sveinbjö ; rnsdó ; ttir^a ; Jan Heinemeier^b ; Stefá ; n Arnó ; rsson^a ; Rí ; key Kjartansdó ; ttir^a ; Hrefna Kristmannsdó ; ttir^c
• 关键词：Carbon isotopes ; Thermal fluids ; Mantle CO2 degassing ; Isotope geochemical modeling ; Iceland
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 October 2016
• 年：2016
• 卷：191
• 期：Complete
• 页码：300-319
• 全文大小：1272 K

文摘

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 4100 ppm. 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 · 10¹⁰ 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 CO₂ may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO₂ partial pressures of ∼10⁻⁶–1 bar and to a mantle CO₂ flux of <5 · 10⁵ mol/km²/yr for most areas and up to 125 · 10⁵ and 1600 · 10⁵ for the Southern Lowlands and Sn&aelig;fellsnes Peninsula, respectively. The CO₂ flux from active volcanic geothermal systems in Iceland was estimated to be ∼500–3000 · 10⁵ mol CO₂/km²/yr, considerably greater than the highest values observed off-axis.