Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment
Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment
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DOI：
10.1016/j.epsl.2008.10.025
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发表时间：
2009-01
影响因子：
5.3
通讯作者：
A. Schmitt;S. Galer;W. Abouchami
中科院分区：
文献类型：
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作者：
A. Schmitt;S. Galer;W. Abouchami
我们报告了一项使用双峰技术（DS-TIMS）的热电离质谱法测量自然质量依赖的镉同位素分馏的调查。本文分析了来自不同环境的60多个天然陆源Cd样品，包括MORB、OIB、陆相黄土、氢源和热液锰铁矿床以及闪锌矿（包括海洋和主要陆相矿床）。我们的结果以ε112/110Cd表示，这是112cd /110Cd与我们内部JMC Cd标准（parts per 104）的偏差。总的ε112/110Cd变化相对较小，只有5个ε-单位，比之前在陨石中发现的变化要小一到两个数量级。MORB、OIB和黄土的ε112/110Cd值相似，可以很好地估计硅酸盐土（BSE）的总体值，相对于我们的Cd标准为- 0.95±0.12（相对于m<s:1> nster JMC Cd ε112/110Cd=+0.16）。综上所述，这些数据表明在壳幔分离过程中很少发生Cd同位素分馏。世界范围内大陆闪锌矿（ZnS）和高温海洋热液硫化物的Cd同位素组成ε112/110Cd值具有显著的相似性，与我们对BSE的估计一致。相比之下，来自单个绝灭热液烟囱的中温海洋硫化物表现出超过4个ε-单位的变化，并且大多数为负值。这些变化很可能是由热液系统中硫化物相的沉淀/再溶解引起的。全球海相铁锰矿床的ε112/110Cd变率反映了来自周围海水降水的海水Cd同位素信号。ε112/110Cd在由浅水铁锰沉积向深水铁锰沉积过渡过程中呈下降趋势。这种转变可以用与上层水柱浮游植物对溶解海水Cd的吸收有关的生物分馏来解释。ε112/110Cd值在深部较为均匀，接近于零，与Cd在深部的再生和再矿化作用相一致。我们的数据表明，Cd同位素——很像有孔虫的Cd/Ca比值——可能作为过去生物生产力变化的代表。东北大西洋2000 m深度Fe-Mn地壳的时间Cd同位素记录表明，在过去的8 Ma中，Cd循环没有发生明显的长期变化。
We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε112/110Cd, which are deviations in112Cd/110Cd from our in-house JMC Cd standard in parts per 104. The total ε112/110Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites. The MORB, OIB and loess ε112/110Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is −0.95±0.12 relative to our Cd standard (ε112/110Cd=+0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε112/110Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/redissolution of sulphide phases en route within the hydrothermal system. The ε112/110Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε112/110Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (>2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε112/110Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma.