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

对部分受气候条件控制的元素迁移和地下水流动等近地表过程的调查，在很大程度上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，允许对这些概况的演变施加时间限制的时间工具是有限的。在这方面，天然硅石或蛋白石是一种独特的相态，可以提供年代数据，从而可以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），在那里它从不同来源的溶液中沉淀出来，并且经常浓缩大量的铀，使其适用于精确的U-Pb和U系测年，甚至通过灵敏的高分辨率离子微探针（SHRIMP）。因此，二氧化硅或蛋白石的定年提供了一个机会，彻底改变表面过程的定年，如锆石彻底改变了岩浆和变质系统的定年，即，测定二氧化硅的年代可能有助于确定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）通过调查和测定几个自然发生的二氧化硅（包括硅化木材）的实例，进一步评估二氧化硅测年法测量低温过程速率的能力，以及（ii）通过实验调查水-二氧化硅系统中铀及其子体同位素的活动，确定二氧化硅测年法的局限性。应使用稳定同位素示踪剂（18 O、30 Si、2 H）对二氧化硅老化过程的机械方面进行实验研究。使用SHRIMP对天然样品和经过实验处理的硅石进行空间微尺度分辨率的同位素系统学研究时，应同时进行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、X射线衍射、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).