Garnets within geode-like serpentinite veins: Implications for element transport, hydrogen production and life-supporting environment formation
Garnets within geode-like serpentinite veins: Implications for element transport, hydrogen production and life-supporting environment formation
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DOI：
10.1016/j.gca.2014.07.002
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发表时间：
2014-09
影响因子：
5
通讯作者：
O. Plümper;A. Beinlich;W. Bach;E. Janots;H. Austrheim
中科院分区：
文献类型：
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作者：
O. Plümper;A. Beinlich;W. Bach;E. Janots;H. Austrheim
蛇纹石化系统中的地球化学微环境可以非生物合成碳氢化合物，并提供维持生命所需的成分。在中大西洋海岭蛇纹岩中发现的microgeode-like水石榴石中的有机物的观察表明，这些石榴石可能是海洋岩石圈内微生物生态系统殖民化的独特巢穴。然而，人们对形成这种独特环境的矿物学和地球化学过程知之甚少。在这里，我们提出的工作露头规模的静脉网络，从超镁铁岩在挪威，包含大量的球晶石榴石（钙铁榴石），这有助于限制这样的过程。在低fO 2和低aSiO 2，aq地球化学条件下，脉状钙铁榴石球粒与多面体蛇纹石、水镁石、Ni-Fe合金（铁镍榴石）和指示低温（<200 °C）蚀变的磁铁矿共生。结合露头和微尺度分析，地球化学反应路径模拟表明，在大尺度上存在有限的物质输运和流体流动。一旦打开的静脉保持孤立（封闭系统），形成非平衡微环境，允许在阈值过饱和，快速结晶（秒至数周）的球晶钙铁榴石。多面体的蛇纹石球的存在表明，静脉最初充满了凝胶状原蛇纹石相。此外，与钙铁榴石形成相关的大量Fe氧化可能产生多达600 mmol H2，aq/100 cm 3矿脉。虽然没有检测到碳质物质，静脉网络满足所报告的地球化学标准，产生非生物成因的碳氢化合物和支持微生物群落。因此，类似于这里研究的系统是在深层地下寻找生命支持环境时的主要兴趣。
Geochemical micro-environments within serpentinizing systems can abiotically synthesize hydrocarbons and provide the ingredients required to support life. Observations of organic matter in microgeode-like hydrogarnets found in Mid-Atlantic Ridge serpentinites suggest these garnets possibly represent unique nests for the colonization of microbial ecosystems within the oceanic lithosphere. However, little is known about the mineralogical and geochemical processes that allow such unique environments to form. Here we present work on outcrop-scale vein networks from an ultramafic massif in Norway that contain massive amounts of spherulitic garnets (andradite), which help to constrain such processes. Vein andradite spherulites are associated with polyhedral serpentine, brucite, Ni–Fe alloy (awaruite), and magnetite indicative of low temperature (<200 °C) alteration under lowfO2and low aSiO2,aq geochemical conditions. Together with the outcrop- and micro-scale analysis geochemical reaction path modeling shows that there was limited mass transport and fluid flow over a large scale. Once opened the veins remained isolated (closed system), forming non-equilibrium microenvironments that allowed, upon a threshold supersaturation, the rapid crystallization (seconds to weeks) of spherulitic andradite. The presence of polyhedral serpentine spheres indicates that veins were initially filled with a gel-like protoserpentine phase. In addition, massive Fe oxidation associated with andradite formation could have generated as much as 600 mmol H2,aq per 100 cm3vein. Although no carboneous matter was detected, the vein networks fulfill the reported geochemical criteria required to generate abiogenic hydrocarbons and support microbial communities. Thus, systems similar to those investigated here are of prime interest when searching for life-supporting environments within the deep subsurface.