The effect of crystal-plastic deformation on isotope and trace element distribution in zircon: Combined BSE, CL, EBSD, FEG-EMPA and NanoSIMS study
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Date
2016
Authors
Kovaleva, Elizaveta
Klotzli, Urs
Habler, Gerlinde
Huet, Benjamin
Guan, Yun-bin
Rhede, Dieter
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier
Abstract
Plastically-deformed zircon grains from granulite-facies (Ivrea-Verbano Zone, Southern Alps, Italy)
and amphibolite-facies (Tauern Window, Eastern Alps, Austria) shear zones have been investigated. The
main focus was on the effects of crystal-plastic deformation on the distribution of trace elements and their
isotopes. Zircon grains reveal crystal-plastic deformation patterns in form of (I) gradual bending of the
lattice (high density of free dislocations), (II) highly-deformed margins revealing a combination of low- and
high-angle boundaries and gradual bending of the lattice, and (III) a low-angle boundary (LAB) network.
Chemical and isotopic maps and profiles show that trace elements in zircon are re-distributed in all
plastically-deformed domains. Changes in trace element composition along LABs are sometimes revealed
by brighter CL and darker BSE signal. LABs and domains of high free-dislocation density have depletion in U,
Y, Yb and enrichment in Ce, La and Nd, while Ti and P are either enriched, depleted or remain unaffected,
and Hf demonstrates stability. Y and Yb are decreased in concentration across LABs, and have oscillating
concentration in domains of high free-dislocation density. Our observations confirm that crystal-plastically
deformed domains in zircon act as effective pathways for trace cations.
The Pb isotopic system is disturbed by crystal-plastic deformation microstructures, as indicated by
relative 207Pb/206Pb ages showing significant discordance in plastically-deformed zircon domains. In
deformed domains, a positive correlation between dislocation density and the degree of isotopic age
distortion is observed. Fractured or porous domains and domains with high density of (sub)grain
boundaries are enriched in common Pb from the matrix, and thus show significantly older relative
207Pb/206Pb ages than pristine domains. In contrast, Pb loss occurs in domains with high free-dislocation density at a distance of 5-10 μm from the grain boundary, caused by out-diffusion of radiogenic Pb.
This study adds to a growing database on the consequences of crystal-plastic deformation for trace
elements and isotopic systems in zircon. We provide additional evidence that such deformation cannot be
neglected and may have important implications for zircon geochronology and geochemistry.
Description
Keywords
Zircon, Trace elements, Isotopic distortion, EBSD, NanoSIMS, Crystal-plastic deformation
Citation
Kovaleva, E., Klötzli, U., Habler, G., Huet, B., Guan, Y., & Rhede, D. (2016). The effect of crystal-plastic deformation on isotope and trace element distribution in zircon: Combined BSE, CL, EBSD, FEG-EMPA and NanoSIMS study. Chemical Biology. Advanced online publication. DOI:10.1016/j.chemgeo.2016.12.030