Rare Earth Elements in the hydrothermal system at Okinawa Trough back-arc basin
Rare Earth Elements in the hydrothermal system at Okinawa Trough back-arc basin
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DOI：
10.2343/geochemj.41.1
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发表时间：
2007-02
影响因子：
0.8
通讯作者：
Yayoi Hongo;H. Obata;T. Gamo;Miwako Nakaseama;J. Ishibashi;U. Konno;Shunsuke Saegusa;Satoru B. Ohkubo;U. Tsunogai
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文献类型：
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作者：
Yayoi Hongo;H. Obata;T. Gamo;Miwako Nakaseama;J. Ishibashi;U. Konno;Shunsuke Saegusa;Satoru B. Ohkubo;U. Tsunogai
本文提出了冲绳海槽与那国丘4号热液喷口场稀土元素分布的新数据集。3个水柱的REE垂直剖面在1000 ~ 1200 m范围内呈现水平变化。根据甲烷、锰和透射率的异常值发现了热液柱。北太平洋深水（NPDW） （Nozaki et al., 1999）铕异常的归一化模式随着距离热液喷口位置的距离而减小，表明可以利用稀土元素追踪地幔柱中热液的稀释作用。负ce异常的水平变化反映了地幔柱中悬浮物对稀土元素的持续清除。此外，我们还测量了9个热液样品。在缺乏沉积物的洋中脊，热液喷口系统的稀土元素地球化学研究已经得到了深入的研究，但很少有研究对冲绳海槽等沉积为主的热液系统进行研究。Yonaguni Knoll IV收集的流体的球粒陨石归一化稀土模式显示出典型的较轻的稀土元素（LREE）和与中洋脊相似的Eu富集。Yonaguni Knoll IV流体模式的显著特征是其较重稀土元素（HREE）和La组成的浓度高于缺乏沉积物的东太平洋隆起和跨大西洋地道线的热液流体。这种特征可以用弧后盆地冲绳海槽覆盖沉积物的影响来解释。在热液喷口处，流体在烟囱内与海水混合过程中，较轻的稀土元素（LREE）被系统地还原。轻稀土元素消除类似于水柱内颗粒清除引起的分馏。然而，在Yonaguni Knoll IV流体中，缺乏典型的水柱稀土元素特征——Ce耗损，以及明显的Eu还原，这表明排放口部位的流体稀土元素分馏主要是由热液源矿物（如硫酸盐和碳酸盐）的共沉淀引起的，而不是由Fe和/或Mn氧化物颗粒的粘附去除引起的。以往的研究表明，热液系统只在矿床样品中观察到稀土元素的去除和分馏。本研究结果利用以往的分析数据阐明了流体样品中的稀土分馏。利用稀土元素模式特征，我们还能够区分出在喷口部位和水柱处发生的稀土元素去除机制。
We present novel data sets of rare earth element (REE) distributions in a hydrothermal vent field at Yonaguni Knoll IV in the Okinawa Trough. Vertical REE profiles in three water columns showed horizontal variation of REE concentrations within 1000‐1200 m. Hydrothermal plumes were discovered by anomalous values of methane, manganese and transmissometry at that site. Europium anomalies in the North Pacific deep water (NPDW) (Nozaki et al., 1999) normalized pattern decreased with distance from the hydrothermal vent site, indicating that the dilution of hydrothermal fluid in the plume can be traced using REE. The horizontal variation of negative Ce-anomalies represents the continuous scavenging of REE by suspended matter in the plume. In addition, we measured nine hydrothermal fluid samples. The REE geochemistry of hydrothermal vent systems had been investigated intensively at sediment-starved mid-oceanic ridges, but few studies had examined sediment-hosted hydrothermal systems like those of the Okinawa Trough. The chondrite-normalized REE patterns of the fluids collected at Yonaguni Knoll IV show typical lighter rare earth elements (LREE) and Eu enrichment similar to at the Mid-ocean Ridge sites. A remarkable characteristic of the Yonaguni Knoll IV fluid pattern is its higher concentrations of heavier rare earth elements (HREE) and La composition than the hydrothermal fluids of the sediment-starved East Pacific Rise and Trans-Atlantic Geotraverse. Such a feature is explainable by influences of covering sediments in the back-arc basin Okinawa Trough. At the hydrothermal vent, lighter REE (LREE) in the fluid was reduced systematically during fluid mixing with seawater within the chimney. Light REE elimination resembles fractionation caused by particle scavenging within the water column. However, the lack of Ce depletion, which is a typical REE feature in the water column, along with distinctive Eu reduction, were unique in the Yonaguni Knoll IV fluid, suggesting that fluid REE fractionation at the vent site was induced predominantly by coprecipitation with hydrothermally originated minerals (e.g. sulfate and carbonate), not by adhesive removal by Fe and/or Mn oxide particles. Previous studies had shown that REE removal and fractionation of the hydrothermal system were observed only in deposit samples. Results of this study elucidated REE fractionation in fluid samples using previous analytical data. We were also able to distinguish REE removal mechanisms occurring at the vent site and water column using REE pattern characteristics.