Lewis A. Banks *1,2, Matthew S.A. Horstwood 1, Simon R. Tapster 1, Simon R.C. Chenery 1, Tiffany Barry 2, Daniel Smith 2
1 British Geological Survey, Nicker Hill, Keyworth, Nottingham, Nottinghamshire, NG12 5GG, UK; lba@bgs.ac.uk 2 School of Geography, Geology and the Environment, University of Leicester, Leicester, LE1 7RH UK
In order to precisely measure the ratio of isotopes, typically a large mass of the element (thousands of crystals) being measured is required. In this case, the element in question is uranium. To gather the large mass of uranium required, it is common practice to dissolve lots of crystals together. The issue with this is that the ratio between the isotopes of uranium in the individual crystals might be different, therefore, dissolving them together will give an averaged uranium isotope composition.
A popular mineral that is used to date rocks is zircon. Isotopes of uranium decay to isotopes of lead at different, but known, rates. By measuring the lead isotopes relative to the uranium isotopes, the age of a crystal can be determined. Therefore, an accurate age of a crystal is a product of the initial uranium isotope composition. Due to the large amount of material required, it has been common use an assumed uranium isotope composition in age calculations, based on multi-crystal dissolutions. However, isotopic compositions between crystals can vary and therefore the determined age of a crystal may be inaccurate if the wrong uranium isotope composition is used. Measurement of the isotopes of uranium in single crystals is now possible and can be applied to this problem.
We show the ability to precisely measure the isotopes of uranium in single crystals, and the effect of this on the determined age of the crystal. This has significant implications for understanding ore deposit formation and the timing of volcanic eruptions.