Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge
Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge
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DOI：
10.1007/s00126-005-0040-8
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发表时间：
2006-03
影响因子：
4.8
通讯作者：
Ana Filipa A. Marques;F. Barriga;V. Chavagnac;Y. Fouquet
中科院分区：
文献类型：
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作者：
Ana Filipa A. Marques;F. Barriga;V. Chavagnac;Y. Fouquet
岩石学，地球化学和Nd同位素分析已经进行了岩石样品从彩虹喷口区，以评估热液系统的演变。Rainbow喷口区是位于大西洋中脊的一个超镁铁质热液系统，其特征是剧烈的高温喷口（约365°C）和独特的流体化学成分：高氯度、低pH值和非常高的Fe和稀土元素（REE）含量（Douville等人，ChemicalGeology184：37-48，2002）。蛇纹石化作用发生在低温（<270°C）退变质状态下，后来被高温硫化物矿化事件覆盖。单靠逆蛇纹石化反应无法重现彩虹热液流体的热量和特定化学特征。在存款中识别出以下单元：（1）未矿化蛇纹岩，（2）矿化蛇纹岩-网脉，（3）滑石，（4）半块状硫化物，和（5）块状硫化物，包括富Cu块状硫化物（高达28wt% Cu）和富Zn块状硫化物烟囱（高达5wt% Zn）。硫化物矿化产生了显着的变化，含硫化物的岩石，包括富集过渡金属（铜，锌，铁，钴）和轻稀土元素，增加的Co/Ni比的镁铁质富铜火山岩托管块状硫化物矿床和不同的143 Nd/144 Nd同位素比值。喷口流体化学数据表明海底反应区的酸性、还原性和高温条件，在此条件下，流体很可能在亚临界条件（沸腾）下发生相分离。高氯度的解释是不简单的，除非与高盐度的盐水混合或直接贡献的岩浆富含氯的含水流体被认为是。这项研究增加了新的数据，结合目前对彩虹喷口区的了解，提供了令人信服的证据，证明在深处存在一个岩浆体，最有可能是辉长岩，为系统提供热量和金属。钴/镍比被证明是很好的工具，用于区分岩石单元，硫化物矿化度，并定位在热液系统。较深的单元具有Co/Ni <1，并且地下和表面单元具有Co/Ni >1。
Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (∼365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al.,Chemical Geology184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different143Nd/144Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1.