Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
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DOI：
10.1016/j.gca.2014.09.001
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发表时间：
2014-11
影响因子：
5
通讯作者：
R. Coggon;M. Rehkämper;C. Atteck;D. Teagle;J. Alt;M. Cooper
中科院分区：
文献类型：
--
作者：
R. Coggon;M. Rehkämper;C. Atteck;D. Teagle;J. Alt;M. Cooper
热液循环是全球地球化学循环的基本组成部分。然而，高温轴向热液流体通量的大小仍然存在争议，而较低温度的脊侧翼流体通量难以量化。铊（Tl）同位素的行为不同，在轴向相比，脊侧翼系统，与Tl近定量剥离侵入地壳的高温热液反应，但加入到熔岩在低温反应与海水。这种对比行为提供了一种独特的方法来确定与轴向和脊侧面环境相关的流体通量。不幸的是，我们对铊同位素质量平衡的理解受到海洋地壳对铊吸收的矿物学、物理和化学控制的知识不足的阻碍。在这里，我们使用来自胡安·德富卡海岭侧翼的综合大洋钻探计划孔U1301 B的玄武质火山上地壳的分析，结合已发表的504 B和896 A孔的疏浚海底玄武岩和上地壳玄武岩的分析，研究大洋中脊玄武岩对铊吸收的控制作用，并评估在大洋中脊侧翼热液系统的演化过程中何时发生铊吸收。海底玄武岩表明玄武岩从冷海水中吸收铊与Cs和Rb的吸收之间存在关联，已知Cs和Rb可分为富钾相。虽然海底玄武岩的铊和钾含量之间没有明确的关系，但数据并不排除至少有一些铊与碱性元素结合在同一矿物中。与此相反，我们没有发现之间的关系，无论是次生层状硅酸盐矿物的丰度，或在上地壳玄武岩中的K，Cs或Rb含量的Tl含量。我们的结论是，在热液蚀变的上地壳的Tl和碱性元素的摄取涉及不同的过程和/或矿物相相比，那些统治海底风化。此外，U1301 B、504 B和896 A孔上地壳玄武岩中Tl和S浓度的相关性表明，Tl主要以次生硫化物的形式存在。由于微生物硫酸盐还原的结果，这些次生硫化物中的一些形成，微生物的作用至少是间接负责铊-uptake. Thallium富集的脊侧翼玄武岩需要一个铊轴承流体和物理，化学和微生物的条件，有利于次生硫化物的形成。Tl的摄取发生在还原环境中的背景岩石远离流体流动路径在早期的“开放”循环的氧化海水，但更普遍的整个系统在后来的“限制”循环的还原流体。因此，铊同位素系统是一个有用的示踪剂的流体通量通过“开放”和“限制”脊侧翼热液制度。
Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust.Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs.Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake.Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early ‘open’ circulation of oxidizing seawater but more pervasively throughout the system during later ‘restricted’ circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the ‘open’ and ‘restricted’ ridge flank hydrothermal regimes.