Scanning Electron Microscope Study of Zircon Grains in Gneisses and Pegmatites from the Western Gneiss Region

April 29th, 2008

Metamorphic rocks, gneisses and pegmatites, from the Western Gneiss Region in southwestern Norway were analyzed to determine when the area was metamorphosed at ultrahigh pressures during the Caleonidonian orogeny. Cathodoluminescence images (CL) of zircon grains separated from the samples show older oscillatory-zoned igneous cores and younger metamorphic rims. The zircon samples were analyzed using a Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) to determine the age of metamorphism.
Multiple minerals are included within the zircon grains used for CL and LA-MC-ICP-MS. We analyzed the shape, size and elemental composition of the inclusions using a scanning electron microscope (SEM). The shapes and sizes of the inclusions varied, however the color of the inclusions are all dark except for monazite which is white. The location of the inclusions from these data specific minerals was identified. The location of each mineral inclusion was also compared to the chemical zonation of its host zircon grain to determine what role the mineral
inclusion played in the growth of the zircon. Results show that quartz and apatite inclusions are commonly found in the oscillatory core of the zircon, suggesting that the minerals were included during the growth of the zircon from a magma. For the monazite inclusions, they were commonly found in the younger metamorphic rims suggesting that the minerals were included during the final growth of the zircon.

Mike Quinn, ’08 Hanover Park, IL
Major: Geology

Sponsor: Emily Walsh

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