Uranium isotope fractionation during coprecipitation with aragonite and calcite
Uranium isotope fractionation during coprecipitation with aragonite and calcite
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DOI：
10.1016/j.gca.2016.05.022
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发表时间：
2016-09-01
影响因子：
5
通讯作者：
Anbar, Ariel D.
中科院分区：
文献类型：
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作者：
Chen, Xinming;Romaniello, Stephen J.;Anbar, Ariel D.
海洋碳酸钙中U-238/U-235的自然变化可能提供一种有用的方式来限制古环境的氧化还原条件。为了评估这个代理的可靠性，我们进行了文石和方解石共沉淀实验，在pH值类似于7.5和类似于8.5，研究可能的U同位素分馏过程中纳入这些矿物。小，但显着的U同位素分馏观察文石实验在pH值接近8.5，较重的U同位素优先富集在固相中。这些实验中溶解U的U-238/U-235可通过瑞利分馏曲线拟合，分馏因子为1.00007 + 0.00002/-0.00003，1.00005 +/-0.00001和1.00003 +/-0.00001。与此相反，没有可分辨的U同位素分馏观察到在文石实验中，在pH值类似于7.5或在方解石实验中，在任何pH值。平衡同位素分馏之间的不同的水U物种是最有可能的解释这些发现。相对于不带电的U物种Ca 2UO 2（CO 3）（3）（aq），某些带电的U物种优先掺入碳酸钙中，我们假设其具有比大多数带电物种更轻的平衡U同位素组成。根据这一假设，铀同位素分馏的大小应与溶解铀中以Ca 2UO 2（CO 3）（3）（aq）形式存在的比例成比例。我们的实验的平衡形态建模证实了这一预期。理论计算不同形态铀之间的同位素分馏因子可以进一步验证这一假说和我们提出的分馏机制。这些研究结果表明，U同位素在古碳酸盐岩的变化可以控制海水U的水形态的变化，特别是海水pH值，P-CO2，Ca 2+，或Mg 2+浓度的变化。一般来说，这些影响可能很小（
Natural variations in U-238/U-235 of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH similar to 7.5 and similar to 8.5 to study possible U isotope fractionation during incorporation into these minerals. Small but significant U isotope fractionation was observed in aragonite experiments at pH similar to 8.5, with heavier U isotopes preferentially enriched in the solid phase. U-238/U-235 of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007 + 0.00002/-0.00003, 1.00005 +/- 0.00001, and 1.00003 +/- 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH similar to 7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)(3)(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)(3)(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism. These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P-CO2, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (