Neon identifies two billion year old fluid component in Kaapvaal Craton
Neon identifies two billion year old fluid component in Kaapvaal Craton
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DOI：
10.1016/j.chemgeo.2011.01.028
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发表时间：
2011-04-22
期刊：
影响因子：
3.9
通讯作者：
Onstott, Tullis C.
中科院分区：
文献类型：
--
作者：
Lippmann-Pipke, Johanna;Lollar, Barbara Sherwood;Onstott, Tullis C.
南非 Witwatersrand 盆地的深层金矿最近引起了人们的关注，不仅因为在那里发现了深层裂隙水和相关的富含 CH4 和 H-2 的气体，而且因为最近有报道称，深层微生物群落持续存在于近 3 公里深处，这是地球深层生物圈的奇异前哨。虽然盆地较浅的流体（约 1 公里）被发现含有丰富的产甲烷菌和硫酸盐还原细菌，但盆地最深、最古老、含盐量最高的裂隙水却拥有迄今为止未被认识到的低生物量和低生物多样性的独立于光球层的化能自养生态系统。浅层和深层流体在气体和流体地球化学方面也表现出明显的差异。古气象水主要由碳氢化合物气体组成，其成分和同位素特征与利用二氧化碳还原途径产生的产甲烷菌一致。相比之下，最深、大多数含盐裂缝水含有的气体主要是高浓度的 H-2 气体、CH4 和高级烃气体，其同位素特征归因于水-岩石反应的非生物成因过程。这些裂缝水的高盐度（高达数百克/升）、高度改变的 Delta O-18 和 Delta H-2 特征，以及 Cl-36 和共生核惰性气体的测量结果，与这些高岩/水比环境中地质长时间尺度上广泛的水-岩相互作用一致。虽然这些流体的最终起源在某些情况下被交替归因于渗透结晶基底的咸水、地层水或热液，但它们的 Delta O-18 和 Delta H-2 同位素特征通常被长期水-岩石相互作用的影响深深地印记，以至于对于大多数含盐端元来说，几乎没有证据表明它们的主要成分。本研究的主要目标是通过首次整合溶解气体的详细氖同位素分析来进一步研究这些流体的来源。氦同位素分析证实，不存在与这些流体和气体相关的重要地幔衍生成分。氖同位素结果显示氖成分存在明显差异，对应于先前确定的不同流体地球化学端元。在富含产甲烷菌的古气象水域中发现了典型的地壳霓虹灯特征（A 型）。相比之下，更深层的含盐量较高的裂缝水含有丰富的核生成氖特征，这与之前报道的地壳流体中的特征不同。这些样品显示了地下水中有史以来报道的最高 Ne-21/Ne-22 比率 (0.160 +/- 0.003)。这些岩石中的流体包裹体产生更高的 Ne-21/Ne-22 比率，介于 0.219 和 0.515 之间，与 Ne-20/Ne-22=0 时外推的 Ne-21/Ne-22 值 3.3 +/- 02 一致。我们表明，这种富集的成核氖端元代表了一种流体成分，该成分是在贫氟太古宙地层中产生的，并被困在 >= 2 Ga 之前的流体包裹体中。在深层裂缝水中观察到富集的核源氖特征意味着这种十亿年前的氖成分从流体包裹体中释放出来，并在异常孤立的裂缝水系统中积累。观察到的这种太古宙氖特征与溶解在同一深层地下水中的拟议非生物来源的富含H-2-烃的地气的关联表明，裂缝系统也允许在整个地质时期水-岩反应的各种产物的积累。这些裂缝系统之一包含地球上特征最深的微生物生态系统——化学石营养生物在独立于阳光的维持水平上勉强生存。因此，丰富的核生氖同位素特征可能表明太古代地壳中的深层生物圈研究可能集中在这些区域。 (C) 2011 Elsevier B.V. 保留所有权利。
The deep gold mines of the Witwatersrand Basin, South Africa have gained recent attention not only because of investigations of the deep fracture water and associated CH4- and H-2-rich gases found there, but because of recent reports of deep microbial communities persisting to depths of almost 3 km an exotic outpost of the Earth's deep biosphere. While shallower fluids in the basin (to approximately 1 km) were found to contain abundant populations of methanogens and sulphate-reducing bacteria, the deepest, oldest, most saline fracture waters in the basin hosted hitherto unrecognised low biomass and low biodiversity chemoautotrophic ecosystems independent from the photosphere. Shallow and deep fluids also show distinct differences in gas and fluid geochemistry. Paleometeoric waters are dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilising the CO2 reduction pathway. In contrast the deepest, most saline fracture waters contain gases that are dominated by high concentrations of H-2 gas, and CH4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water-rock reaction. The high salinities (up to hundreds of g/L), highly altered delta O-18 and delta H-2 signatures, and both Cl-36 and measurements of co-occurring nucleogenic noble gases for these fracture waters are consistent with extensive water-rock interaction over geologically long time scales in these high rock/water ratio environments. While the ultimate origin of these fluids has been attributed alternately to saline waters that penetrated the crystalline basement, formation water, or hydrothermal fluids in some cases, their delta O-18 and delta H-2 isotopic signatures have typically been so profoundly overprinted by the effects of long-term water-rock interaction that, for the most saline end-members, little evidence of their primary composition remains. The key objective of the present study is to further investigate the origin of these fluids by integrating for the first time detailed neon isotope analyses on the dissolved gases. Helium isotopic analysis confirmed that there is no significant mantle-derived component associated with these fluids and gases. Neon isotope results show distinct differences in neon composition that correspond to the different fluid geochemical end-members previously identified. Typical crustal neon signatures (type A) are identified in the paleometeoric waters populated with abundant methanogens. In contrast, the deep more saline fracture waters contain an enriched nucleogenic neon signature unlike any previously reported in crustal fluids. These samples show the highest Ne-21/Ne-22 ratios (0.160 +/- 0.003) ever reported in groundwater. Fluid inclusions in these rocks yield even higher Ne-21/Ne-22 ratios between 0.219 and 0.515, consistent with an extrapolated Ne-21/Ne-22 value of 3.3 +/- 02 at Ne-20/Ne-22=0. We show that this enriched nucleogenic neon end-member represents a fluid component that was produced in the fluorine-depleted Archaean formations and trapped in fluid inclusions >= 2 Ga ago. The observation of enriched nucleogenic neon signatures in deep fracture water implies the release of this billion year old neon component from the fluid inclusions and its accumulation in exceptionally isolated fracture water systems.The observed associaton of this Archean neon signature with H-2-hydrocarbon-rich geogases of proposed abiogenic origin dissolved in the same deep groundwater suggests that the fracture systems have also allowed for the accumulation of various products of water-rock reactions throughout geologic times. One of these fracture systems contained the deepest characterised microbial ecosystems on earth - chemolithotrophs eking out an existence at maintenance levels independent from sunlight Consequently, the enriched nucleogenic neon isotope signature may indicate regions in the Archaean crust where investigations of the deep biosphere might be focused. (C) 2011 Elsevier B.V. All rights reserved.