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Rejuvenation effects during plastic deformation of Zircon: geochronological implications

Autoren:Kovaleva, ElizavetaKloetzli, Urs
Abstrakt:Zircon is one of the most stable accessory minerals known on the Earth; it was believed that zircon isotopic agesmostly record primary igneous crystallization events. It is true until the mineral is not affected by plastic deformationor other disturbing events during its life after crystallization. Zircon may deform by the recovery/subgrainrotation recrystallisation that indicates formation and migration of dislocations under crustal conditions. Deformationoccurs at depth due to stresses associated with collision of the phases, and forms such microstructures aslow-angle boundaries (Reddy et al., 2007).Low-angle boundaries act as fluid migration paths and elements diffusion paths. Facilitating Pb, Ti, U, Th and traceelements mobility in the crystalline zircon, these structures can change chemical, REE and isotopic compositionof certain grain’s parts (Reddy and Timms, 2010) and lead to isotopic resetting in the mineral domains. Since theisotopic age of the domains of single crystal can vary, it should be possible to recognize and interpret this variationand distinguish the timing of different high-temperature deformation events. Zircon can preserve low-angleboundaries and associated age disturbance under lower-crust temperatures for billions of years (Moser et al.,2009).Electron backscatter diffraction (EBSD) allows us to make microstructural-crystallographic analyses in orderto measure the crystallographic orientations in crystalline material. EBSD mapping is supposed to be able toconstrain potential diffusion pathways in minerals. It can indicate areas of damaged crystalline structure, helps toexamine substructures of minerals used in radiometric dating and to assess the potential for resetting of ages bydeformation events (Reddy et al., 2007).In this research we are trying to answer a list of questions, related to isotopic resetting due to deformation:What is the behavior of zircons which were plastically deformed during metamorphic-deformation events andhosted by the shear zones? How to evaluate the significance of isotopic-system resetting in deformed domains?How to estimate the possibility of dating the deformation events and what is the reliability of such dating?A complex of methods is used for this research: transmitted and reflected light microscopy, CL (cathode luminescence)imaging for growth history, BSE (backscattered electrons) imaging for orientation and chemical contrast,SE (secondary elements) pictures for surface relief, SHRIMP (Sensitive High Resolution Ion Microprobe) for agedating, EBSD for crystallographic orientations. We suppose that careful combination of all these methods willlead to better understanding of some internal processes which happen with mineral during its evolution in hostrocks.References:Moser D.E., Davis W.J., Reddy S.M., Flemming R.L., Hart R.J., 2009. Zircon U–Pb strain chronometry revealsdeep impact-triggered flow. Earth and Planetary Science Letters 277, 73–79.Reddy, S.M., Timms, N.E., Pantleon, W., Trimby, T., 2007. Quantitative characterization of plastic deformation ofzircon and geological implications. Contributions to Mineralogy and Petrology 153, 625–645.Reddy, S.M., Timms, N.E., 2010. Deformation of zircon and implications for geochemistry and geochronology.Source Abstracts with Programs - Geological Society of America 42 (5), 634.
Publikations Typ:Beitrag in Konferenzband
Buchtitel:Geophysical Research Abstracts
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