U-Pb and U-series dating of silica: Growth rates in natural systems and the behaviour of uranium and its daughter isotopes during experimental silica aging

对元素迁移和地下水流动等部分受气候条件控制的近地表过程的调查，严重依赖于重建地质系统长期行为的能力。土壤和风化剖面提供了这种长期行为的记录。然而，允许在这些概要文件的演进中放置时间约束的时间顺序工具是有限的。在这方面，天然二氧化硅或蛋白石是一个独特的阶段，可以提供时间数据，允许重建近地表地质构造的演变。二氧化硅广泛分布在各种低温环境中（甚至在火星上），它从不同来源的溶液中析出，并经常浓缩大量的铀，使其适用于精确的U-Pb和u系列测年，即使是通过敏感的高分辨率离子微探针（SHRIMP）。因此，二氧化硅或蛋白石的定年给地表过程的定年带来了革命性的机会，就像锆石给岩浆和变质体系的定年带来了革命性的变化一样，也就是说，二氧化硅的定年可能有助于建立二氧化硅沉积速率与水状况变化和气候变化之间的联系。该项目旨在(i)通过调查和定年几个自然产生的二氧化硅的例子，包括硅化木材，进一步评估二氧化硅定年法测量低温过程速率的能力，以及（ii）通过实验研究铀及其子同位素在水-二氧化硅系统中的动员，确定二氧化硅定年法的局限性。使用稳定同位素示踪剂（18O, 30Si, 2H）对二氧化硅老化过程的机理进行实验研究。使用SHRIMP对天然样品和实验处理过的二氧化硅进行空间微尺度分辨率的同位素系统研究，应伴随着详细的结构和矿物学研究（通过拉曼和红外光谱、荧光显微镜、XRD、阴极发光和背散射电子成像）。

The investigation of near surface processes such as elements migration and groundwater flow, partly controlled by climate conditions, rely heavily on the ability to reconstruct the long-term behaviour of geological systems. Soil and weathering profiles provide a record of such long-term behaviour. However, chronological tools that allow placing time constraints on the evolution of these profiles are limited. In that respect natural silica or opal is a unique phase that can provide chronological data that permit the reconstruction of the evolution of near surface geological formations. Silica is widely distributed in variety of low temperature environments (even on Mars), where it precipitates from the solutions of different origin and it often concentrates significant quantities of uranium, making it applicable for precise U-Pb and U-series dating even by sensitive high-resolution ion microprobe (SHRIMP). Therefore, dating silica or opal gives an opportunity to revolutionise dating of surface processes like zircon has revolutionised dating of mag2 matic and metamorphic systems, i.e., dating of silica potentially facilitates establishing the link between silica deposition rates and variation in water regime and climate change. This project aims to (i) further evaluate the power of silica dating for measuring rates of low temperature processes by investigating and dating several examples of naturally occurring silica, including silicified wood, and (ii) to establish limitations of silica dating by experimentally investigating the mobilisation of uranium and its daughter isotopes in water-silica systems. Mechanistic aspects of silica aging processes shall be investigated experimentally using stable isotope tracers (18O, 30Si, 2H). The study of the isotope systematics of both natural samples and experimentally treated silica with a spatial micro-scale resolution using SHRIMP shall be accompanied by detailed textural and mineralogical studies (by Raman and IR spectroscopy, fluorescence microscopy, XRD, and cathodoluminescence and backscattered electron imaging).