HRTEM Study of Zircon from Eliseev Anorthosite Complex, Antarctica

Zircon is a common accessory mineral in magmatic, metamorphic and sedimentary rocks. The crystal structure is tetragonal with space group I41/amd. Zircon contains minor amounts of U and Th and can be dated by a variety of techniques yielding ages of crystallization, cooling, and redistribution of radioactive isotopes and their daughter products. The precision of the radiogenic age determination strongly depends on the ability of zircon to retain the daughter products which were produced by the radiogenic decay of U and Th.Zircon-bearing rocks of this study are metamorphic oxide-apatite gabbronorites (OAGN) from the Eliseev Anorthosite Complex, Wohlthat-Massif, East Antarctica (Kampf et al., 1995). These unusual rocks are strongly enriched in accessory minerals (apatite: < 10 vol.%; zircon: < 1 vol.%, Owens & Dymek, 1992). Three steps in the evolution of these rocks are distinguished: a magmatic formation, followed by a granulite facies metamorphism and finally a tectonomagmatic overprint. The zircon crystals of this study are brown colored, up to 12 mm in length and up to 3 mm wide (Fig.1). Petrological investigations show that zircon has formed during the granulite facies event. Optical microscopy and cathodoluminiscence microscopy reveal a rhythmic zoning and many microcracks. The concentrations of uranium and thorium are low (U: 34-89 ppm and Th: 3-9 ppm). The radiation damage by radioactive decay of U and Th is expected to be minor due to the low uranium and thorium content.