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

• Applicant: Professor Dr. Thorsten Geisler-Wierwille
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 211049121

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).

• DFG Programme: Research Grants