Geochemical fluxes related to alteration of a subaerially exposed seamount: Nintoku seamount, ODP Leg 197, Site 1205
Geochemical fluxes related to alteration of a subaerially exposed seamount: Nintoku seamount, ODP Leg 197, Site 1205
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DOI：
10.1029/2006gc001400
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发表时间：
2007-02
期刊：
影响因子：
3.7
通讯作者：
S. Révillon;D. Teagle;P. Boulvais;J. Shafer;C. Neal
中科院分区：
文献类型：
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作者：
S. Révillon;D. Teagle;P. Boulvais;J. Shafer;C. Neal
1205 A孔是在仁德海山钻探的，仁德海山位于天皇海山链的中部。这座海山在1.56亿年前出现，但在54 Ma前被淹没，因此熔岩在陆上和海底环境中都经受了风化。我们已经研究了岩石学，矿物学和地球化学的夹层蚀变玄武岩，角砾岩，并在孔1205 A恢复土壤样品，以量化海山和海水和/或大气流体之间的化学交换。整个剖面的次生矿物学相对均匀，包括蒙皂石、铁氢氧化物、伊丁石和碳酸钙。土壤由高岭石、蒙皂石、蛭石和少量针铁矿、赤铁矿和磁铁矿基质中的蚀变玄武岩碎屑组成。在整个基底部分，蚀变玄武岩、角砾岩和土壤中的Si、Mg、Ca、Na、Sr、Rb和Ba都是亏损的，而Fe是富集的。Fe ~（3+）/Fe ~（3+）T（高达1 ‰）、δ ~（18）O（高达1 ‰ +20‰）和~（87）Sr/~（86）Sr比值相对于原生岩浆岩值有明显的升高。87 Sr/86 Sr比值的差异确定了87 Sr/86 Sr接近56 Ma海水（0.7077）的上蚀变带和87 Sr/86 Sr升高较小（0.704）的下蚀变带。低蚀变带可能反映了低温下与陆上氧化流体的相互作用。这一地带可能保留了大部分原始的陆上风化特征。上蚀变带是通过大量冷氧化海水的循环而改变的，这些海水部分地覆盖了陆上风化化学特征。改变的样品进行了比较，估计原岩组成，以计算化学收益和损失。采用不同的岩性比例模型和不同的洋岛就位率，计算了整个基底剖面的全球化学通量。虽然与大洋中脊的火成岩增生相比，海洋岛屿的全球建造速度很小，但化学变化的幅度表明，海洋岛屿和海山可能是海洋化学收支的重要贡献者。
Hole 1205A was drilled on Nintoku Seamount, which lies in the midportion of the Emperor Seamount Chain. This seamount was emergent ∼56 Myr ago but was submerged by 54 Ma, so the lavas have endured weathering in both subaerial and submarine environments. We have studied the petrology, mineralogy, and geochemistry of intercalated altered basalts, breccias, and soil samples recovered at Hole 1205A to quantify the chemical exchanges between the seamount and seawater and/or meteoric fluids. The secondary mineralogy is relatively uniform throughout the section and comprises smectite, Fe‐oxyhydroxides, iddingsite, and Ca‐carbonates. Soils are composed of variably altered basaltic clasts in a matrix of kaolinite, smectite, and vermiculite with minor goethite, hematite, and magnetite. Throughout the basement section, altered basalts, breccias, and soils are depleted in Si, Mg, Ca, Na, Sr, Rb, and Ba and enriched in Fe. Fe3+/FeT (up to ∼1), δ18O (up to ∼+20‰), and 87Sr/86Sr ratios are strongly elevated relative to primary igneous values. Differences in the 87Sr/86Sr ratios define an Upper Alteration Zone with 87Sr/86Sr close to 56 Ma seawater (∼0.7077) from a Lower Alteration Zone where 87Sr/86Sr are less elevated (∼0.704). The Lower Alteration Zone likely reflects interaction with a subaerial oxidizing fluid at low temperature. This zone probably retained most of the original subaerial weathering signature. The Upper Alteration Zone was altered through circulation of large quantities of cold oxidizing seawater that partially overprinted the subaerial weathering chemical characteristics. Altered samples were compared to estimated protolith compositions to calculate chemical gains and losses. Global chemical fluxes are calculated for the entire basement section using different lithological proportions models and different rates of oceanic island emplacement. Although the global construction rate of ocean islands is small compared to igneous accretion at mid‐ocean ridges, the magnitude of the chemical changes indicates that ocean islands and seamounts may be a significant contributor to the chemical budget of the oceans.