Geochemistry of Light Hydrocarbons in Subduction-Related Volcanic and Hydrothermal Fluids
Geochemistry of Light Hydrocarbons in Subduction-Related Volcanic and Hydrothermal Fluids
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俯冲相关火山和热液流体中轻质烃的地球化学
DOI：
10.5382/sp.10.04
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发表时间：
2003
期刊：
影响因子：
5.3
通讯作者：
Werner F. G Iggenbach
中科院分区：
文献类型：
--
作者：
Y. Taran;Werner F. G Iggenbach
在他杰出的职业生涯的最后20年里，维尔纳·吉根巴赫收集并分析了来自新西兰怀特岛火山和地热系统以及世界各地许多火山和地热系统的数百个火山和热液气体样本。数百个样品被分析为C1-C6碳氢化合物，包括苯。根据White Island火山获得的数据集以及其他可用数据，基于W. Giggenbach开发的用于解释高温环境下地壳流体组成的技术，CH4、C2-C4碳氢化合物和苯的行为的几个一般趋势是明显的。这些趋势可分为两种主要类型，即温度依赖平衡和岩浆源与沉积源的碳混合。通常认为火山气体中CH4浓度随温度升高而降低的说法是不正确的，因为在达到岩浆温度之前，CH4/CO2的行为没有温度依赖的趋势。热液流体中甲烷的浓度主要受烃源输出控制，烃源输出由沉积岩中埋藏的有机质组成。这些有机质在地壳上层的热分解产生CH4和轻烃以及氮，并伴有很高的N2/Ar比。因此，热液流体富ch4端元往往具有较高的N2/Ar比值。相比之下，俯冲相关岩浆流体由于俯冲富有机质海洋沉积物的降解而具有较高的N2/Ar比，但几乎不含甲烷。因此，火山气体和热液流体的CH4浓度与N2/Ar比值具有两种不同的关系。两种体系与火山气体取样温度有良好的相关性：具有相同碳原子数的烷烃-烯烃对和乙烯-苯对。它们在火山气体中的浓度比（C2H6/C2H4, C3H8/C3H6， ΣC4H10/ΣC4H8, C2H4/C6H6）强烈依赖于喷气孔的温度，并且这些浓度比沿亚稳平衡路径随温度变化。这意味着烷烃+ H2 =烷烃和C6H6 + 3H2 = 3C2H4的化学氧化还原反应是快速的，但动力学控制，可能是通过氧化物和硫的催化。从2Cn = Cn‐1 + Cn + 1平衡的角度来看，无论是火山气体还是热液流体，烷烃-烷烃比值的变化都没有系统的趋势，即使在岩浆温度（>800°C），观测到的Cn - 1 /Cn - 1比值也往往对应于负平衡温度。CO2 + 4H2 = CH4 + 2H2O反应被一些地球化学家错误地称为费托反应，在热液和岩浆环境的氧化还原和温度条件下不太可能适用。唯一可能减少二氧化碳（和碳）的自然无机过程是富镁基性岩石的蛇纹石化。在地壳中普遍存在的条件下，导致CH4-CO2系统平衡的唯一过程是甲烷的氧化。这可以通过天然催化剂来促进，天然催化剂通常是氧化物，而不是像二氧化碳还原那样的天然金属。
During the last 20 years of his outstanding career, Werner Giggenbach collected and analyzed hundreds of samples of volcanic and hydrothermal gases from White Island volcano and geothermal systems in New Zealand, as well as many volcanoes and geothermal systems over the world. Hundreds of samples were analyzed for C1-C6 hydrocarbons, including benzene. On the basis of the data set obtained for the White Island volcano, together with other available data, several general trends in the behavior of CH4, C2-C4 hydrocarbons, and benzene are apparent, based on application of techniques developed by W. Giggenbach for the interpretation of crustal fluid composition in a high-temperature environment. The trends can be divided into two main types involving temperature-dependent equilibrium and mixing of carbon from magmatic and sedimentary sources. The common statement that the CH4 concentration in volcanic gases decreases with increasing temperature is not true, as there are no temperature-dependent trends in the CH4/CO2 behavior until magmatic temperatures are reached. The concentrations of methane in hydrothermal fluids are controlled mainly by the source output, comprising organic matter buried with sedimentary rocks. Thermal decomposition of this organic matter at upper crust levels produces CH4 and light hydrocarbons as well as nitrogen accompanied by a very high N2/Ar ratio. Therefore, the CH4-rich end member of hydrothermal fluids tends to have a high N2/Ar ratio. By contrast, subduction-related magmatic fluids have almost no methane despite having a high N2/Ar ratio due to degradation of subducted organic-rich oceanic sediments. Hence, volcanic gases and hydrothermal fluids are characterized by two different relationships between CH4 concentration and N2/Ar ratio. Two systems show a good correlation with sampling temperature in volcanic gases: alkane-alkene pairs with the same number of carbon atoms and ethene-benzene. Their concentration ratios (C2H6/C2H4, C3H8/C3H6, ΣC4H10/ΣC4H8, C2H4/C6H6) in volcanic gases are strongly dependent on the temperature of the fumarole, and these ratios change with temperature along metastable equilibrium paths. This means that the chemical redox reactions alkane + H2 = alkane and C6H6 + 3H2 = 3C2H4 are fast but kinetically controlled, probably through catalysis by oxides and sulfur species. Variations in alkane-alkane ratios in terms of the 2Cn = Cn ‐ 1 + Cn + 1 equilibrium, either for volcanic gases or for hydrothermal fluids, show no systematic trends, and even at magmatic temperatures (>800°C), the observed Cn/Cn ‐ 1 ratios often correspond to negative equilibrium temperatures. The reaction CO2 + 4H2 = CH4 + 2H2O, mistakenly called the Fischer-Tropsch reaction by some geochemists, is unlikely to apply under redox and temperature conditions in the hydrothermal and magmatic environment. The only natural inorganic process in which reduction of CO2 (and carbon) could be possible is the serpentinization of Mg-rich mafic rocks. Under conditions prevailing in the crust, the only process that results in equilibration within the CH4-CO2 system is the oxidation of methane. This can be facilitated by natural catalysts, which are usually oxides, but not native metals as in the case of the reduction of CO2.