Earth history in miniature: Zircon under the microscope

Dr Robert Sturm (Austria) The mineral zircon (more correctly, orthosilicate zircon or ZrSiO4) is an important accessory mineral in various rocks of the earth’s crust, but most of all of igneous rocks with the mineral composition of granite. An accessory mineral is a mineral comprising less than about 10% of a rock and which therefore plays little or no role in naming or classifying that rock. Fig. 1. The typical appearance and morphology of zircon crystals that have been separated from various granitic rocks. As well as its ubiquitous appearance in magmatic, metamorphic and sedimentary rocks, zircon is a remarkable mineral due to its high resistance to mechanical and chemical processes within the earth crust. Therefore, it is very useful as a protolith indicator in different types of crustal rocks (Speer, 1982). (A protolith indicator is a mineral in a metamorphic assemblage that provides information on the chemistry of the host rock, within which it had originally grown.) As a result of the repeated formation of magmatic overgrowths around older ‘inherited’ zircon cores (like the rings of an onion), evidence of several stages of earth history are preserved within a single grain and can be scientifically analysed (Sturm, 1999, 2004). Once these overgrowths have been identified, the phases of crystallisation included in accessory zircon can be attributed to geological time periods. This is achieved by radiometric dating, based on the mineral’s content of radioactive uranium and thorium, and the redistribution of these isotopes and their daughter products. Another important characteristic … Read More

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