Native nickel in the TAG hydrothermal field sediments (Mid-Atlantic Ridge, 26°N):: Space trotter, guest from mantle, or a widespread mineral, connected with serpentinization?
Native nickel in the TAG hydrothermal field sediments (Mid-Atlantic Ridge, 26°N):: Space trotter, guest from mantle, or a widespread mineral, connected with serpentinization?
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DOI：
10.1029/2005jb003955
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发表时间：
2006-05-31
影响因子：
3.9
通讯作者：
Dekov, Vesselin
中科院分区：
文献类型：
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作者：
Dekov, Vesselin
[ 1]在大西洋中脊的跨大西洋地热区（TAG）热液区的沉积物中发现了微小的金属颗粒。金属颗粒几乎是纯镍，其成分和结构特征与其他陆地金属Ni-0矿点相似。首次在海洋沉积物中记录的天然镍颗粒出现在钙质软泥中，中间夹有含金属层、基岩碎石层和零星分散的宇宙微球体。金属Ni 0的成因有三种可能性：（1）宇宙成因，（2）核幔边界下地幔成因，（3）上地幔/下地壳成因。Ni-0颗粒的片状外观排除了宇宙消融起源。没有明确的证据表明Ni-0粒子可能是在类似于核幔边界的高压条件下形成的。上地幔/下地壳海水蛇纹石化形成Ni-0是最合理的机制。在缓慢扩张的MAR，海水向下渗透通过高度破碎的波峰带，并蛇纹化上地幔/下地壳超镁铁质岩/镁铁质岩的橄榄石和辉石。蛇纹石化导致生成高度还原的挥发性物质：H-2和碳氢化合物。在这种极端还原的环境中，当fS（2）接近0时，从橄榄石和辉石中释放的Fe和其他过渡金属（Co、Ni、Cu、Zn）倾向于以自然金属的形式出现。变轻的蛇纹岩块隆起并暴露出裂谷壁上的蛇纹岩化超镁铁岩/镁铁岩。我推测，构造运动和风化作用可以释放原生金属颗粒分散在蛇纹岩，然后可能已经分散在附近的沉积物中的近底电流。
[ 1] Tiny metallic particles have been found in the sediments of the Trans-Atlantic Geotraverse (TAG) hydrothermal field, Mid-Atlantic Ridge ( MAR). The metallic grains are almost pure Ni and have compositional and structural characteristics similar to those of other terrestrial metallic Ni-0 occurrences. Native nickel grains, documented here for the first time in oceanic sediments, appear in calcareous oozes intercalated with metalliferous layers, layers with bedrock rubble, and sporadically scattered cosmic microspherules. There are three possibilities for the metallic Ni 0 genesis: ( 1) cosmic, ( 2) lower mantle at the core-mantle boundary, or ( 3) upper mantle/lower crust. The flake-like appearance of the Ni-0 grains rules out cosmic ablation origin. No unequivocal evidence was found that the Ni-0 particles could have formed under high-pressure conditions similar to those at the core-mantle boundary. Ni-0 formation through seawater serpentinization of the upper mantle/lower crust is the most plausible mechanism. At the slow spreading MAR, seawater penetrates down through highly fractured crest zone and serpentinizes the olivine and pyroxene of the upper mantle/lower crust ultramafics/mafics. Serpentinization results in the generation of highly reduced volatile species: H-2 and hydrocarbons. In such an extremely reduced environment, Fe and other transition metals (Co, Ni, Cu, Zn) released from olivines and pyroxenes tend to appear as native metals if fS(2) approaches 0. The lightened serpentinized blocks uplift and expose the serpentinized ultramafics/ mafics at the rift valley's walls. I speculate that tectonic movements and weathering could liberate the native metallic grains scattered in the serpentinized rocks, which may then have been dispersed in the proximate sediments by near-bottom currents.