风化过程中234U优先溶解的控制机理

基于颗粒铀同位素组成的破碎年代学记录了颗粒自破碎以来经历的时间，具有示踪地表物质循环过程的巨大潜力。矿物颗粒表面α反冲弹射作用导致颗粒破碎后234U逐渐亏损是铀同位素破碎年代学的理论基础。然而，放射性破坏晶格中234U优先溶解会导致新鲜破碎岩石细颗粒组分的铀同位素值低于久期平衡值，从而引起颗粒“破碎年龄”的高估。室内溶解实验以及野外新破碎基岩铀同位素证据表明234U优先溶解会影响矿物颗粒的铀同位素。但是，铀同位素破碎年代学的成功运用表明优先溶解的影响可能十分有限。234U优先溶解与具有高能量的放射性破碎晶格有关。这些高能量的晶体表面的点位错容易发育溶蚀坑，可能导致优先溶解的发生。理论计算表明，高能破坏晶格溶蚀坑的发育受溶液饱和度控制。本项目拟通过室内风化溶解实验，并结合野外风化剖面矿物表面微形貌证据，探究溶液饱和度对234U优先溶解的影响，进而为铀同位素破碎年代学方法的应用提供理论支撑。

The comminution age of sediment particles records the time since the grains break apart, providing a useful measure of the timescales of weathering, erosion, and sediment transfer. The uranium isotope ratio, 234U/238U, has been used to quantify comminution age of fine particles, making it a promising tool in studying Earth surface processes. However, a major knowledge gap exists in the theories of the 234U/238U chronometer: the effect and controlling mechanisms of the preferential dissolution of 234U. Specifically, the current uranium isotope chronometer calculates the comminution age based on the 234U loss caused by α-recoil. This approach does not account for another mechanism, the preferential dissolution of 234U from radioactively-damaged lattices, that also causes 234U/238U fractionation. Theories predict that without accounting for the preferential dissolution effect, the communition age would be largely overestimated. Laboratory experiments and field observations also suggest that the preferential dissolution of 234U will affect the 234U/238U ratio of surficial water and fine grain particles. However, the successful application of the uranium chronometer and its consistency with other constraints on comminution age suggest limited influence from the preferential dissolution process. .To reconcile this discrepancy, this project proposes to conduct a comprehensive study of the preferential dissolution of 234U. Specifically, this project will test a theory which states that the preferential dissolution of 234U is related to the radioactively-damaged lattice with excess stress energy. Once in contact with a solution, those high-stress-energy dislocation areas on mineral surfaces tend to develop favorable sites for dissolution, and a microscopic hole (i.e., an etch pit) will form around the dislocation, likely causing the preferential dissolution of 234U. Theoretical calculations also predict the formation of etch pit around the dislocation depends mainly on the saturation index of solution, but lack laboratory and field evidences. With these understandings, this project will combine laboratory experiments (e.g. characterizing mineral surface morphology) and field studies to investigate the preferential dissolution of 234U, its controlling mechanisms (e.g. role of saturation state), and its influence on the uranium isotope-based comminution age. Overall, the outcomes from this project will advance our understanding of the uranium isotope chronometer, promising new applications of this powerful tool in Earth surface environments.