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Contributions to Mineralogy and Petrology

, Volume 166, Issue 1, pp 21–41 | Cite as

Lattice distortion in a zircon population and its effects on trace element mobility and U–Th–Pb isotope systematics: examples from the Lewisian Gneiss Complex, northwest Scotland

  • John M. MacDonaldEmail author
  • John Wheeler
  • Simon L. Harley
  • Elisabetta Mariani
  • Kathryn M. Goodenough
  • Quentin Crowley
  • Daniel Tatham
Original Paper

Abstract

Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, 207Pb/206Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent 207Pb/206Pb isotopic ages (−3.0 to +0.3 % discordance) in the range of ~2,420–2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated.

Keywords

Zircon Lattice distortion Trace elements and isotopes EBSD 

Notes

Acknowledgments

This work was carried out under UK Natural Environment Research Council DTG NE/G523855/1 and British Geological Survey CASE Studentship 2K08E010 to JMM. Carmel Pinnington and Eddie Dempsey are thanked for assistance with SEM analysis. Ion microprobe analysis at the Edinburgh Ion Microprobe Facility was carried out with funding from NERC grant IMF384/1109; Richard Hinton, Cees-Jan De Hoog and John Craven are thanked for ion microprobe support and Mike Hall for assistance with sample preparation. Detailed reviews by Martin Whitehouse and an anonymous reviewer, plus discussions with Alan Boyle, Craig Storey and Nick Roberts, considerably improved this manuscript. KMG publishes with the permission of the Executive Director of the Geological Survey.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  • John M. MacDonald
    • 1
    • 5
    Email author
  • John Wheeler
    • 1
  • Simon L. Harley
    • 2
  • Elisabetta Mariani
    • 1
  • Kathryn M. Goodenough
    • 3
  • Quentin Crowley
    • 4
  • Daniel Tatham
    • 1
  1. 1.Jane Herdman Laboratories, School of Environmental SciencesUniversity of LiverpoolLiverpoolUK
  2. 2.Grant InstituteSchool of GeoSciencesEdinburghUK
  3. 3.British Geological SurveyEdinburghUK
  4. 4.Department of Geology, School of Natural SciencesTrinity CollegeDublin 2Ireland
  5. 5.Carbonate Research, Royal School of MinesImperial College LondonLondonUK

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