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American Mineralogist; July 2007; v. 92; no. 7; p. 1213-1224; DOI: 10.2138/am.2007.2458
© 2007 Mineralogical Society of America
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Brittle fracturing and fracture healing of zircon: An integrated cathodoluminescence, EBSD, U-Th-Pb, and REE study

A. Rimsa1,2,*, M.J. Whitehouse1, L. Johansson2 and S. Piazolo3

1 Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden
2 GeoBiosphere Science Centre/Lund University, Department of Lithosphere and Paleobiosphere Science, Sölvegatan 12, SE-22362 Lund, Sweden
3 Department of Geology and Geochemistry, Stockholm University, SE-106 91, Sweden

Correspondence: * E-mail: andrius.rimsa{at}nrm.se

The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjärnesjö intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 ± 51 Ma (lower intercept age, 2{sigma}, MSWD = 1.09) with a limit for the youngest possible age of 960 ± 16 Ma (207Pb/206Pb, 2{sigma}, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.

Key Words: Zircon • hydrothermal • cathodoluminescence • U-Pb • Sveconorwegian • SIMS • EBSD • fracturing • hydraulic






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