Arsenic in marine hydrothermal fluids
Arsenic in marine hydrothermal fluids
复制标题
DOI：
10.1016/j.chemgeo.2012.10.044
复制
发表时间：
2013-06-19
期刊：
影响因子：
3.9
通讯作者：
Pichler, Thomas
中科院分区：
文献类型：
--
作者：
Breuer, Christian;Pichler, Thomas
已知在大洋中脊（莫尔）附近或弧后盆地（BAB）和岛弧（IA）环境中出现的热液流体含有大量溶解的金属（如Fe、Zn、Cu、Cd），这是由于温度升高时的水-岩相互作用和潜在的岩浆脱气作用。来自这些流体的化学输入对海洋化学和元素循环起着重要作用。尽管砷（As）作为一种环境毒素而声名狼借，并且在热液矿床中与金结合丰富，但它是很少被研究的少数元素之一，与公海海水中约1.7 μ g L-1的As含量相比，热液流体的浓度可能要高得多。流体从莫尔热液系统，如东太平洋海隆可以包含高达80.5微克L-1的作为和在大西洋中脊的最高值约为24微克L-1。这些值，虽然升高，但被BAB和IA托管热液系统中的As浓度超过，在BAB环境中As浓度可高达1386 μ g L-1，甚至更高，高达5850 μ g L-1（类似于3900倍的海水）在IA浅水环境中，在海岸热液系统附近发生。控制热液流体中As含量的最重要因素是不同的-主要是物理化学-海底和海底之下的条件。这些因素包括温度（控制流体的相分离和主岩中的浸出过程）、作为深度函数的压力、直接影响浸出过程的pH值、As流动性和形态、反应时间和系统成熟度以及氧化还原反应和各种化学反应（如吸附和解吸）。下伏寄主岩石中As的浓度也可能起作用，尽管其矿物学关联可能比大块岩石浓度更重要。As的额外输入可能是由岩浆中富含金属的挥发分脱气或沉积物覆盖的热液系统中的沉积物-流体相互作用引起的。岩浆挥发分的贡献很难量化，因此在计算和讨论中经常被忽略。砷的氧化还原物种，砷酸盐（As-V）和亚砷酸盐（As-III），亚砷酸盐更容易在气相中传输。然而，砷的形态并不是传统化学分析的一部分。在海底排放之前，热液流体中As的浓度可能会迅速下降，如果沉淀的As轴承和As清除矿物，如黄铁矿，雌黄，claudetite和水合铁/锰氧化物，发生在浅层地下的热力学计算。这是一致的观察，这些热液流体中高的Fe和Mn往往是低的As。砷浓度从浅BAB/IA热液系统的流体中高于那些在深位于莫尔热液系统最有可能是由于其不同的物理化学条件。玄武岩（莫尔）和英安岩/安山岩（BAB/IA）的As含量基本相同，不能解释流体As含量的巨大差异。As是海水中含量高于河水的少数微量元素之一，这可能是由热液系统中As的通量造成的。据估计，莫尔省每年可排放3.0 x 10（3）至1.25 x 10（8）千克砷，与河流每年输送的53.9 x 10（6）千克砷相比，这是一个很大的数量。在BAB和IA设置下，预计通量较高，但数据稀疏。一个单一的IA系统，局限于一个非常小的面积为60 × 100米（图图姆湾，巴布亚新几内亚），排放约5.5 × 10（2）公斤砷每年。(C)2012爱思唯尔有限公司版权所有。
Hydrothermal fluids emerging at the seafloor near mid-ocean ridges (MOR) or in back-arc basins (BAB) and island arc (IA) settings are known to contain a considerable amount of dissolved metals (e.g. Fe, Zn, Cu, Cd) due to water-rock interaction at elevated temperatures and potentially magmatic degassing. The chemical input from these fluids plays an important role for ocean chemistry and the cycling of elements. Despite its notoriety as an environmental toxin and its abundance in hydrothermal ore deposits in combination with gold, arsenic (As) is one of the few elements, which has been rarely investigated.Compared to the amount of As in open ocean seawater of around 1.7 mu g L-1, hydrothermal fluids can have significantly higher concentrations. Fluids from MOR hydrothermal systems such as the East Pacific Rise can contain up to 80.5 mu g L-1 As and at the Mid-Atlantic Ridge the highest values were around 24 mu g L-1. Those values, although elevated are surpassed by As concentrations in BAB and IA hosted hydrothermal systems, which can be as high as 1386 mu g L-1 in BAB settings and even higher with values up to 5850 mu g L-1 (similar to 3900-times seawater) in IA shallow-water settings, occurring near shore hydrothermal systems.The most important factors controlling the amount of As in hydrothermal fluids are the different - mostly physicochemical - conditions at and beneath the seafloor. These include temperature (controlling phase separation of the fluids and leaching processes in the host rock), pressure as a function of depth, pH directly influencing leaching processes, As mobility and speciation, reaction time and maturity of the system in combination with redox reactions and diverse chemical reactions like adsorption and desorption. The concentration of As in the underlying host rock may also play a role, although its mineralogical association may be more important than bulk rock concentration. Additional input of As could be caused by degassing magmatic metal-rich volatiles or sediment-fluid interaction in sediment covered hydrothermal systems. The contribution from magmatic volatiles is hard to quantify and therefore often neglected in calculations and discussions. Of the two As redox species, arsenate (As-V) and arsenite (As-III), arsenite is easier transported in the vapor phase. However, As speciation has not been traditionally part of chemical analyses. Prior to discharge at the seafloor, the concentration of As in hydrothermal fluids may decrease rapidly, if precipitation of As-bearing and As-scavenging minerals, such as pyrite, orpiment, claudetite and hydrous ferric/manganese oxides, occurs in the shallow subsurface as shown by thermodynamic calculations. This is consistent to the observation that those hydrothermal fluids high in Fe and Mn are often low in As.Arsenic concentrations in fluids from shallow BAB/IA hydrothermal systems are higher than those in deep situated MOR hydrothermal systems most likely due to their different physicochemical conditions. Basaltic host rocks (MOR) and dacitic/andesitic host rocks (BAB/IA) have As concentrations which are more or less identical and cannot account for the huge differences in fluid As concentration.Arsenic is one of the few trace elements whose concentration in seawater is higher than in river water, which could be caused by the flux of As from hydrothermal systems. Estimating that between 3.0 x 10(3) and 1.25 x 10(8) kg As could be discharged at the MOR annually, which is a substantial amount compared to 53.9 x 10(6) kg transported annually by rivers. Higher flux can be expected at BAB and IA settings, however data is sparse. A single IA system, confined to a very small area of 60 by 100 m (Tutum Bay, Papua New Guinea), discharges around 5.5 x 10(2) kg As annually. (C) 2012 Elsevier B.V. All rights reserved.