Experimental determination of barium isotope fractionation during diffusion and adsorption processes at low temperatures
Experimental determination of barium isotope fractionation during diffusion and adsorption processes at low temperatures
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DOI：
10.1016/j.gca.2016.04.049
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发表时间：
2016-08
影响因子：
5
通讯作者：
K. Zuilen;T. Müller;T. Nägler;M. Dietzel;T. Küsters
中科院分区：
文献类型：
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作者：
K. Zuilen;T. Müller;T. Nägler;M. Dietzel;T. Küsters
近年来，在低温和高温环境中测量的钡（Ba）稳定同位素丰度的变化受到越来越多的关注。然而，控制Ba同位素分馏的实际过程仍然难以捉摸。在这项研究中，我们提出了第一个实验方法来量化在水中Ba 2+离子通过多孔介质传输过程中扩散和吸附对质量依赖的Ba同位素分馏的贡献。在实验中，已知同位素组成的氯化钡溶液在10°C和25°C的u形玻璃管中扩散，长达201天。扩散的Ba在δ 137/134 Ba中分馏率高达- 2.15‰，尽管原子质量的相对差异很小。Ba同位素有效扩散系数（d137ba / d134ba =(m 134/m 137) β）的质量相关差异，可以通过扩散输运模型成功再现时间相关同位素分异。输运模型提取的β值在0.010 ~ 0.011之间。独立进行的批量实验表明，Ba在硅凝胶表面的吸附有利于较重的Ba同位素（α= 1.00015±0.00008）。然而，在扩散实验中，吸附对总体同位素分馏的贡献很小。我们的结果有助于理解Ba同位素分馏过程，这对于解释自然同位素变化和评估Ba同位素比率作为地球化学指标至关重要。
Variations in barium (Ba) stable isotope abundances measured in low and high temperature environments have recently received increasing attention. The actual processes controlling Ba isotope fractionation, however, remain mostly elusive. In this study, we present the first experimental approach to quantify the contribution of diffusion and adsorption on mass-dependent Ba isotope fractionation during transport of aqueous Ba 2+ ions through a porous medium. Experiments have been carried out in which a BaCl 2 solution of known isotopic composition diffused through u-shaped glass tubes filled with silica hydrogel at 10° C and 25° C for up to 201 days. The diffused Ba was highly fractionated by up to− 2.15‰ in δ 137/134 Ba, despite the low relative difference in atomic mass. The time-dependent isotope fractionation can be successfully reproduced by a diffusive transport model accounting for mass-dependent differences in the effective diffusivities of the Ba isotope species (D 137 Ba/D 134 Ba=(m 134/m 137) β). Values of β extracted from the transport model were in the range of 0.010–0.011. Independently conducted batch experiments revealed that adsorption of Ba onto the surface of silica hydrogel favoured the heavier Ba isotopes (α= 1.00015±0.00008). The contribution of adsorption on the overall isotope fractionation in the diffusion experiments, however, was found to be small. Our results contribute to the understanding of Ba isotope fractionation processes, which is crucial for interpreting natural isotope variations and the assessment of Ba isotope ratios as geochemical proxies.