Rejuvenation effects during plastic deformation of Zircon: geochronological implications
- Author(s)
- Elizaveta Kovaleva, Urs Kloetzli
- Abstract
Zircon is one of the most stable accessory minerals known on the Earth; it was believed that zircon isotopic ages
mostly record primary igneous crystallization events. It is true until the mineral is not affected by plastic deformation
or other disturbing events during its life after crystallization. Zircon may deform by the recovery/subgrain
rotation recrystallisation that indicates formation and migration of dislocations under crustal conditions. Deformation
occurs at depth due to stresses associated with collision of the phases, and forms such microstructures as
low-angle boundaries (Reddy et al., 2007).
Low-angle boundaries act as fluid migration paths and elements diffusion paths. Facilitating Pb, Ti, U, Th and trace
elements mobility in the crystalline zircon, these structures can change chemical, REE and isotopic composition
of certain grain’s parts (Reddy and Timms, 2010) and lead to isotopic resetting in the mineral domains. Since the
isotopic age of the domains of single crystal can vary, it should be possible to recognize and interpret this variation
and distinguish the timing of different high-temperature deformation events. Zircon can preserve low-angle
boundaries 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 order
to measure the crystallographic orientations in crystalline material. EBSD mapping is supposed to be able to
constrain potential diffusion pathways in minerals. It can indicate areas of damaged crystalline structure, helps to
examine substructures of minerals used in radiometric dating and to assess the potential for resetting of ages by
deformation 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 and
hosted 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 age
dating, EBSD for crystallographic orientations. We suppose that careful combination of all these methods will
lead to better understanding of some internal processes which happen with mineral during its evolution in host
rocks.
References:
Moser D.E., Davis W.J., Reddy S.M., Flemming R.L., Hart R.J., 2009. Zircon U–Pb strain chronometry reveals
deep 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 of
zircon 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.- Organisation(s)
- Department of Lithospheric Research
- Volume
- 15
- Publication date
- 2013
- Austrian Fields of Science 2012
- 105101 General geology
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/11dcb5aa-5d13-4ae1-8d8c-ba9a05f7041e