Trace Element Analysis in Geological Samples and Studies in Geochronology by Nuclear Techniques

Recent studies carried out in the Analytical Chemistry Division, BARC, in the field of elemental and isotopic analyses of samples of geological importance by the neutron activation methods are reviewed. The salient features of the analysis of a wide variety of samples for their uranium content by the delayed fission neutron counting technique is described. The technique is quite specific and sensitive down to 0.1 ppm of uranium, with the possibility of the extension of the limit down to 0.004 ppm with sample size of about 500 mg. Another technique of interest to geologists is the fission track counting method, which is quite simple and can easily be used by laboratories with limited facilities and situated away from the reactor centre. The lower limit of analysis by the solubilation and standard addition approach adopted in this work is governed by the purity, with reference to uranium, of the reagents used for sample decomposition.

In the field of geochronology, the uranium, and thorium contents of zircons have been determined by the neutron activation method and the lead content by anodic stripping voltametry. These measurements have permitted the determination of Th/U ratios and the chemical ages of zircons. The limitations of the technique are the same as applies to the Pb-α method, however, unlike this method, which normally assumes a Th/U ratio of 1, the present method uses the experimental Th/U ratios, which seems to vary with the genesis of the zircons. Using the neutron activation method, the isotopic analysis of Pb-204 and Pb-208 has been carried out and the data has permitted the dating of thorium minerals. Model ages of galenas have been determined using the data obtained. The limitation of techniques and the interpretation of the ages have been critically commented upon.