Mantle CO 2 degassing through the Icelandic crust: Evidence from carbon isotopes in groundwater
Mantle CO 2 degassing through the Icelandic crust: Evidence from carbon isotopes in groundwater
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地幔CO 2 通过冰岛地壳脱气：地下水中碳同位素的证据
DOI：
10.1016/j.gca.2016.06.038
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发表时间：
2016
影响因子：
5
通讯作者：
H. Kristmannsdóttir
中科院分区：
文献类型：
--
作者：
A. Stefánsson;A. Sveinbjörnsdóttir;J. Heinemeier;S. Arnórsson;R. Kjartansdóttir;H. Kristmannsdóttir
研究了冰岛地下水的碳同位素，以确定与活跃火山系统无关的不同板块边界处的碳源和反应。所有水均来自大气，温度为 1–130 °C，pH 值为 ∼4.5–10.5，溶解无机碳 (ΣCO2) 为 1.8 至 4100 ppm。这些水域中 δ13CO2 和 14CO2 的测量范围很大，分别为 -27.4 至 +2.0‰ 和 0.6–118 pMC。通过将测量的化学和同位素组成与使用同位素地球化学模型模拟的化学和同位素组成进行比较，研究了溶解无机碳的来源和反应。确定了二氧化碳的三个主要来源：（1）在地表或浅层形成的部分脱气玄武岩的溶解，（2）通过地表空气-水交换产生的大气二氧化碳，以及（3）深层气体输入地下水系统，其碳和同位素组成与冰岛下方上地幔和下地壳喷发前的熔体相似。在地下水系统中，由于碳酸盐矿物沉淀以及水-岩相互作用中水物质分布的变化，二氧化碳化学和同位素含量发生了变化；需要对这些变化进行量化，以揭示各种二氧化碳来源。冰岛地壳的 CO2 通量估计为 ∼5–10 ·1010mol/年，其中高达 50% 的通量与活动火山中心无关，但位于轴外，其中很大一部分 CO2 可能源自地幔。离轴地下水的地幔输入对应于 ~10−6–1 bar 的 CO2 分压，大部分地区的地幔 CO2 通量 <5 · 105mol/km2/年，南部低地和斯奈山半岛分别高达 125 · 105 和 1600 · 105。冰岛活火山地热系统的二氧化碳通量估计为 ∼500–3000·105mol CO2/km2/年，远高于离轴观测到的最高值。
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 δ13CO2and14CO2in 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 CO2were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO2through 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 CO2chemistry 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 CO2sources. The CO2flux 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 CO2may originate from the mantle. The mantle input of the groundwater off-axis corresponds to CO2partial pressures of ∼10−6–1 bar and to a mantle CO2flux of <5 · 105mol/km2/yr for most areas and up to 125 · 105and 1600 · 105for the Southern Lowlands and Snæfellsnes Peninsula, respectively. The CO2flux 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.