Limitations and Improvements in Methods for Precise U-Pb Isotopic Dating of Precambrian Zircon.

摘要

This thesis addresses various issues in U-Pb zircon geochronology, proposing new experimental protocols in conventional chemical abrasion-isotope dilution thermal ionization mass spectrometry or CA-(ID)-TIMS and developing a new method for Pb evaporation-condensation from zircon that allows high precision Pb-Pb age determination on Precambrian samples. Various experiments are also done on zircon to extract U-Pb information by in situ flux aided fusion methods and to optimize a better silica gel Pb-ionization activator.;Radiation damage caused by U decay in zircon disrupts its ‘closed system’ behavior leading to the loss of daughter radiogenic Pb and resulting in inaccurate ages. A high temperature thermal annealing procedure has been proposed to prevent such Pb loss. Studies presented here have been carried out using Laser Raman Spectroscopy and Scanning Electron Microscopy to characterize radiation damage and effects of laboratory induced thermal annealing on such damage. Backscattered electron images reveal a variety of textures for ZrO 2 overgrowths on zircon annealed at 1450°C. Highly damaged zircon produces finer polycrystalline aggregates (<5µm) than zircon with less damage. Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000°C for 2–3 or 20 days. Annealing at 1450°C for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon showing that when highly damaged zircon is recrystallized, it becomes a polycrystalline aggregate that pseudomorphs the original single crystal.;The whole grain Pb evaporation-condensation method is based on 206Pb-207Pb age analyses where zircon grains are pre-treated at 1450°C to drive out all disturbed Pb and then they are kept at 1600°C for an hour or two during which Pb atoms are evaporated out of the grain and deposited directly into a clean Savillex teflon vial or a wide Re filament. This technique allows the use of a 202Pb- 205Pb double spike for precise isotopic fractionation correction. Examples are shown in which application of this technique to zircon from Precambrian samples has successfully yielded sub-million year age precisions.