AGE DATING OF INDIVIDUAL GRAINS OF URANINITE IN ROCKS FROM ELECTRON-MICROPROBE ANALYSES

It is shown that an electron microprobe analysis of the U, Th and Pb content of a single uraninite (UO2) grain may be used to calculate the age of formation of that grain. This is known as a chemical age. Examples are shown for a range of natural samples with a wide range of ages. Comparison with ages derived from isotopic measurements shows the results to be valid. Approximate formulae which have been proposed for the calculation of the chemical age are not sufficiently reliable, especially where the Th content is high and for older ages. A method for calculating the age by iteration is presented; this uses the full and exact formula and is shown to produce a reliable age for the uraninite compared to isotopic measurements. The method is particularly useful for samples where the minerals and mineral textures record information on more than one event. Two examples of samples recording information on dual ages are discussed to show how these ages can be related to events in the formation of the host rocks. In one example an alluvial heavy-mineral concentrate is shown to contain uraninites from two distinct sources with significantly different ages. In the other example two phases of U mineralisation are distinguished in the same rock sample; one phase relates to the genesis of the sediments from which the rock was formed; the second phase records remobilisation of the U during later tectonic movement. © 1990.