December Editorial      




Zircon (ZrSiO4) is an interesting mineral for a number of reasons. For a start it is evenly distributed within the Earth's crust. Zircons can be found in igneous rocks, metamorphic rocks and also in sedimentary deposits. They are really large crystals. A typical zircon crystal in granite for example is 100 - 300 micrometers across, although some can grow to several centimetres in size - especially in pegmatites (a very coarse-grained rock similar in composition to granite). Another interesting property of Zircon is its durability. Zircons seem survive most geologic processes, including erosion, transport, and even high-grade metamorphism. A very useful property of Zircons is their ability to accept high-field-strength elements. So Zirconium, which is one of the high-field-strength elements, can be substituted for another element, for example: Hafnium (Hf), Thorium (Th) or Uranium (U). Therefore, because Zircons always contain traces of Uranium and Thorium they are used extensively for radiometric dating.

Those radimetric dating techniques have been invaluable in a recent study. Here researchers showed that Zircons can form in the upper mantle and persist there for millions of years before entering the Earth's crust through volcanic processes. The Zircon crystals used for this latest study (recently published in Nature Geoscience (1)) came from recently discovered basaltic fields in north-eastern Bavaria. The gem-quality crystals from this area were larger than average, being about 1-3 mm in diameter.

Using the Uranium-thorium-helium dating technique the authors of the Nature Geoscience paper showed that the Zircon crystals in question were transported to the Earth's crust by basalt eruptions.

Uranium-thorium dating, also called thorium-230 dating technique, is unlike other ratiometric dating techniques, in that it calculates age from the degree to which the equilibrium between Thorium-230 isotope and Uranium-234 has been restored within the sample. Thorium-230 is the product of Uranium-234 decay. But the Thorium-230 is also radioactive with a half-life of 75 thousand years. For this study both Zircons and the host basalt were analysed.

By estimating the Thorium-230/uranium-234 ratio in the sample it proved possible to estimate time the sample had been within the Earth's crust. By comparing the age of the basalt with the age of the zircons it was possible to show that the Zircons were transported to the Earth's surface not earlier that 29 million years ago.

Next the researchers used uranium-lead dating technique to estimate the actual age of the Zircon crystals. Uranium-lead dating, unlike the uranium-thorium dating described above, looks for an end product of the decay of uranium-238 or lead-207 from uranium-235, this product being lead-208. Because Zircon does not naturally incorporate lead atoms into its structure, it can be assumed that the lead content in Zircon is the product of uranium decay. Taking the half-life of uranium and the amount of lead in the Zircon crystals, one can estimate the age of the Zircon. This, in the case of the Bavarian sample, was between 51 and 80 million years. The difference between the uranium-thorium dating and the uranium-lead dating suggests that the Zircon remained intact in the Earth's upper mantle for up to 60 million years. Although Zircon is well known for its durability, this research further demonstrates the ability of Zircon to exist despite extreme geological conditions. As this analysis has shown, Zircons can even survive the pressure and temperatures of the Earth's mantle.

1. Wolfgang Siebel, Axel K. Schmitt, Martin Daniík, Fukun Chen, Stefan Meier, Stefan Wei and Sümeyya Erolu. Prolonged mantle residence of zircon xenocrysts from the western Eger rift. Nature Geoscience 2009, 2, pp. 886 - 890.


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