Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions
Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions
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DOI：
10.1073/pnas.1907871116
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发表时间：
2019-09-03
影响因子：
11.1
通讯作者：
Seewald, Jeffrey S.
中科院分区：
文献类型：
--
作者：
Klein, Frieder;Grozeva, Niya G.;Seewald, Jeffrey S.
利用共聚焦拉曼光谱、光学和扫描电子显微镜、电子探针分析和热力学模拟等方法，对橄榄石次生流体包裹体中甲烷(CH4)的形成条件及其在各种地质背景的橄榄岩和辉长岩中的分布情况进行了评价。对160个从超低到快速扩张的大洋中脊、俯冲带和蛇绿岩样品的详细研究表明，氢(H-2)和CH4的形成与橄榄石赋存的次生流体包裹体中的蛇纹岩作用有关，这是一个普遍的过程。流体包裹体含量以蛇纹石、水镁石、磁铁矿以及不同比例的CH4(G)和H-2(G)为主，与强还原闭合系统条件下的蛇纹岩作用一致。热力学约束表明，进入上地幔或下洋壳的水溶液在高于400℃的温度下以次生流体包裹体的形式被捕获在橄榄石中。当温度降至类似340℃以下时，流体包裹体壁上的橄榄石蛇纹岩作用导致捕获的液体H2O的近乎定量的消耗。富Fe(III)的子体矿物通过沉淀生成分子H-2，有利于无机碳的还原和CH4的形成。一旦形成，CH4(G)和H-2(G)可以在地质时间尺度上储存，直到通过橄榄石主体的溶解或破裂而被提取。流体包裹体是地球上海底和地下喷口系统中非生物CH4和H-2的广泛而重要的来源，可能也是太阳系其他地方的来源。
The conditions of methane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow-to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H-2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H-2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than similar to 400 degrees C. When temperatures decrease below similar to 340 degrees C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H-2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4. Once formed, CH4(g) and H-2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H-2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.