The Chemical Dating Method

Chemical dating by EPMA is a specialised type of trace element analysis, in which the concentration of lead produced by the radioactive decay of uranium and thorium is measured and an age calculated

Chemical dating requires that there is no common lead in the mineral, and that there has not been gain or loss of U, Th or Pb by geochemical processes. These requirements limit the number of minerals to which the method can be reliably applied. To date, work has concentrated in particular on monazite, with successful work also on xenotime and thorite.

Minerals such as zircon cannot be dated by this method, for which isotopic measurements by ion microprobe are required. The other constraint on chemical dating by EPMA is the instrumental sensitivity. At ages below 90-100 Ma, the concentration of Pb is too low to measure by EPMA and ages cannot be determined by chemical analysis. For the same reason, EPMA is also less sensitive than SHRIMP or SIMS to very small differences in age.

However for suitable samples, chemical dating by EPMA offers geochronological precisions equivalent to those obtained using SHRIMP and SIMS. In addition, EPMA has the important advantage of greater spatial resolution than either of these other methods (typically 6-8 microns), of being non-destructive, and of being both faster and cheaper. Dating by EPMA is thus capable of producing data from smaller grains and from smaller zones within individual grains than the other methods.