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首页> 外文期刊>Isotopes in environmental and health studies >U-238/U-235 isotope ratios of crustal material, rivers and products of hydrothermal alteration: new insights on the oceanic U isotope mass balance
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U-238/U-235 isotope ratios of crustal material, rivers and products of hydrothermal alteration: new insights on the oceanic U isotope mass balance

机译：地壳物质，河流和热液蚀变产物的U-238 / U-235同位素比：关于海洋U同位素质量平衡的新见解

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摘要

In this study, the U isotope composition, n(U-238)(U-235), of major components of the upper continental crust, including granitic rocks of different age and post-Archaean shales, as well as that of rivers (the major U source to the oceans) was investigated. Furthermore, U isotope fractionation during the removal of U at mid-ocean ridges, an important sink for U from the oceans, was investigated by the analyses of hydrothermal water samples (including low- and high-temperature fluids), low-temperature altered basalts and calcium carbonate veins. All analysed rock samples from the continental crust fall into a limited range of delta U-238 between -0.45 and -0.21 parts per thousand (relative to NBL CRM 112-A), with an average of -0.30 +/- 0.15 parts per thousand (2 SD, N = 11). Despite differences in catchment lithologies, all major rivers define a relatively narrow range between -0.31 and -0.13 parts per thousand, with a weighted mean isotope composition of -0.27 parts per thousand, which is indistinguishable from the estimate for the upper continental crust (-0.30 parts per thousand). Only some tributary rivers from the Swiss Alps display a slightly larger range in delta U-238 (-0.29 to +0.01 parts per thousand) and lower U concentrations (0.87-3.08 nmol/kg) compared to the investigated major rivers (5.19-11.69 nmol/kg). These findings indicate that only minor net U isotope fractionation occurs during weathering and transport of material from the continental crust to the oceans. Altered basalts display moderately enriched U concentrations (by a factor of 3-18) compared to those typically observed for normal mid-ocean ridge basalts. These, and carbonate veins within altered basalts, show large U isotope fractionation towards both heavy and light U isotope compositions (ranging from -0.63 to +0.27 parts per thousand). Hydrothermal water samples display low U concentrations (0.3-1 nmol/kg) and only limited variations in their U isotope composition (-0.43 +/- 0.25 parts per thousand) around the seawater value. Nevertheless, two of the investigated fluids display significantly lower delta U-238 (-0.55 and -0.59 parts per thousand) than seawater (-0.38 parts per thousand). These findings, together with the heavier U isotope composition observed for some altered basalts and carbonate veins support a model, in which redox processes mostly drive U isotope fractionation. This may result in a slightly heavier U isotope composition of U that is removed from seawater during hydrothermal seafloor alteration compared to that of seawater. Using the estimated isotope compositions of rivers and all U sinks from the ocean (of this study and the literature) for modelling of the isotopic U mass balance, this gives reasonable results for recent estimates of the oceanic U budget. It furthermore provides additional constraints on the relative size of the diverse U sinks and respective net isotope fractionation during U removal.
机译：在这项研究中，上大陆壳主要成分的U同位素组成n（U-238）/ n（U-235），包括不同年龄的花岗岩，后Archaean页岩以及河流（主要的U来源海洋）进行了调查。此外，通过分析热液水样（包括低温和高温流体），低温蚀变玄武岩，研究了在洋中脊（U是海洋中重要的U汇）去除U期间的U同位素分馏。和碳酸钙静脉。所有分析的来自地壳的岩石样品都落在U-238的有限范围内（相对于NBL CRM 112-A为-0.45至-0.21千分之一），平均为-0.30 +/- 0.15千分之一（2 SD，N = 11）。尽管集水区岩性存在差异，但所有主要河流均在-0.31至-0.13千分之几的相对狭窄范围内，加权平均同位素组成为-0.27千分之几，与上陆壳的估算值没有区别（-千分之0.30）。与被调查的主要河流（5.19-11.69）相比，仅瑞士阿尔卑斯山的一些支流河流的三角洲U-238范围稍大（-0.29至+0.01千分之一），铀浓度较低（0.87-3.08 nmol / kg）。 nmol / kg）。这些发现表明，在风化和物质从大陆壳到海洋的运输过程中，仅发生了微量的净U同位素分馏。与正常的大洋中脊玄武岩通常观察到的相比，蚀变的玄武岩显示出适度富集的U浓度（3-18倍）。这些以及碳酸盐岩脉在改变的玄武岩中均显示出大的U同位素分馏，朝着重和轻的U同位素组成（从-0.63到+0.27千分之几）。热液水样品显示出低的U浓度（0.3-1 nmol / kg），并且其U同位素组成在海水值附近仅有有限的变化（-0.43 +/- 0.25份/千份）。尽管如此，两种被研究的流体显示出的U-238δ值（-0.55和-0.59千分之一）明显低于海水（-0.38千分之一）。这些发现以及观察到的某些玄武岩和碳酸盐岩脉变的较重的U同位素组成共同支持了一个模型，在该模型中，氧化还原过程主要驱动U同位素分馏。与水相比，这可能导致在热液海底改造期间从海水中除去的U的U同位素组成稍重。使用河流和所有来自海洋的U汇的同位素组成（本研究和文献资料）对同位素U质量平衡进行建模，可以为最近的海洋U预算估算提供合理的结果。此外，它还对去除U期间各种U阱的相对大小和相应的净同位素分馏提供了额外的限制。

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