Time-resolved interaction of seawater with gabbro: An experimental study of rare-earth element behavior up to 475 °C, 100 MPa
Time-resolved interaction of seawater with gabbro: An experimental study of rare-earth element behavior up to 475 °C, 100 MPa
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DOI：
10.1016/j.gca.2016.10.016
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发表时间：
2017-01-15
影响因子：
5
通讯作者：
Holzheid, Astrid
中科院分区：
文献类型：
--
作者：
Beermann, Oliver;Garbe-Schoenberg, Dieter;Holzheid, Astrid
在大西洋中脊、南纬5度和东斯科舍脊的活跃热液喷口流体中，记录了异常（“非典型”）归一化模式的高金属和稀土元素（REE）浓度。在球粒陨石归一化模式下，这些流体显示中重稀土相对富集，几乎没有Eu异常。为了了解产生这种非典型稀土模式的过程，我们进行了一系列实验，在300至475摄氏度、40和100 MPa的温度下，将天然海底海水或水溶液（NaCl、NaCl- mgcl2或NaCl- cacl2）与辉长岩和辉长岩矿物组合反应。这些P-T条件是热液根部和排放带水岩相互作用的代表性条件。在实验中，通过改变水岩质量比（w/r）在0.5-10之间，并使用3-720 h的不同运行时间，研究了流体通量的变异性和动力学，只有海水和含mgcl2的合成流体从辉长岩、斜辉石和斜长石中调动了大量的稀土、硅、钙、铁和锰。在425℃和40 MPa下，流体最初呈酸性，pH值（25℃）接近2，随着反应的进行，pH值逐渐增加到接近4到7之间。稀土元素和Fe含量在与辉长岩矿物颗粒（125 ~ 500 μ m）的w/r为5 （REE）和2-5 （Fe）时，在作用后3 ~ 6 h达到峰值，但随着反应的继续而下降，REE分异不明显。从原生矿物中浸出并早期溶解于流体中的稀土元素，大部分在720 h后重新沉积为固相反应产物。溶解SiO2的含量与压力有关，在100 MPa（425℃）条件下，溶解SiO2的含量约为40 MPa（425℃）条件下的2倍，且以辉长岩和斜辉石为固体起始物质时低于石英饱和度，以斜长石为反应物时接近石英饱和度。而在100 MPa含辉长岩（w/r 0.5-1）的岩石为主实验中，Si含量降至极低。伴随而来的氯含量下降表明，这些变化可能是由于橄榄石的分解和蛇纹石和铁羟基氯化物的形成。无论何种起始固体反应物，流体稀土元素模式主要受w/r控制。在高w/r（>= 5）条件下，流体REE模式不典型，流体REE含量较高。而在低w/r为0.5-1时，海中脊喷口流体的典型稀土元素模式显示出轻稀土元素的相对富集和正Eu异常。因此，我们的结果清楚地表明，排放物的REE含量和模式对w/r的变化很敏感。(C) 2016 Elsevier Ltd.版权所有。
High metal and rare-earth element (REE) concentrations with unusual ('atypical') normalized REE patterns are documented in fluids from active hydrothermal vent fields on the Mid-Atlantic Ridge, 5 degrees S and the East Scotia Ridge. Those fluids show relative enrichment of middle heavy REEs and almost no Eu anomalies in chondrite-normalized patterns. To understand the processes that produce such atypical REE patterns we ran a series of experiments, in which natural bottom seawater or aqueous solutions (NaCl, NaCl-MgCl2, or NaCl-CaCl2) were reacted with gabbro and gabbro mineral assemblages from 300 to 475 degrees C and 40 and 100 MPa. These P-T conditions are representative for water-rock interactions in hydrothermal root and discharge zones. Fluid flux variability and kinetics were addressed in the experiments by varying the water-to-rock mass ratio (w/r) from 0.5-10 and using different run durations from 3-720 h.Only seawater and synthetic MgCl2-bearing fluid mobilized significant amounts of REEs, Si, Ca, Fe, and Mn from gabbro, from clinopyroxene, and from plagioclase. At 425 degrees C and 40 MPa, fluids were initially acidic with pH (25 degrees C) of similar to 2 increasing to values between similar to 4 and 7 upon progressing reactions. Rare earth element and Fe contents peaked within 3-6 h after interaction with gabbroic mineral grains (125-500 mu m) at w/r of 5 (REEs) and 2-5 (Fe) but decreased with continuing reaction without strong REE fractionation. Most of the REEs that were leached from primary minerals and dissolved in the fluids early became redeposited into solid reaction products after 720 h.Contents of dissolved SiO2 were pressure-dependent, being about twofold higher at 100 MPa than at 40 MPa (425 degrees C) and were below quartz saturation with gabbro and clinopyroxene as solid starting material and close to quartz saturation with plagioclase reactant. However, Si in fluids from the rock-dominated experiments at 100 MPa with gabbro (w/r 0.5-1) dropped to very low contents. A concomitant decrease in chlorinity suggests that these changes may be due to the breakdown of olivine and the formation of serpentine and Fe-hydroxy chlorides.Regardless of the starting solid reactants, fluid REE patterns were dominantly controlled by w/r. Atypical fluid REE patterns and high fluid REE contents were obtained at high w/r (>= 5). Whereas typical REE patterns known from many mid-ocean ridge vent fluids, showing relative enrichments of light REEs and a positive Eu anomaly, were obtained at low w/r of 0.5-1. Our results hence clearly show that REE contents and patterns of vent fluids are sensitive to variations in the w/r. (C) 2016 Elsevier Ltd. All rights reserved.