Neutron activation analysis of trace elements in quartz sands: Its possibilities in the assessment of provenance

Abstract

A study has been made of the possibility of classifying quartz sands by means of the relative γ-peak ratios of long half-life radioistotopes formed by adequate neutron activation. Sand samples of particle sizes from 105 to 250 μm were etched with HF in order to eliminate the outer layer of each grain so as to reach the unaltered core. An acid etching resulting in a weight loss of at least 20% was found to remove the more altered outer layers sufficiently. Then, after neutron irradiation and approppriate decay times, the treated samples were counted by means of a Ge(Li) detector. Activity ratios for 46Sc, 59Fe, 182Ta, 152Eu, 60Co, 124Sb and 95Zr were computed relative to 181Hf. A constant value of 25 (w/w) for the Zr/Hf ratio was found. The samples were then classified according to the observed activity ratios by graphical interpretation of data as well as by statistical analysis of Hotelling's T 2 and discriminant function. The method here described, being independent of neutron flux factors and not requiring standards, can be applied to large-scale studies. It has been successfully applied to differentiate quartz sands in Venezuela, where a transect running from southwest to northeast in the central plains was sampled. The sand dunes in Apure were shown to have clearly different trace element patterns from those belonging to the Mesa Formation in the northeast. Both groups showed strong internal consistency regarding their activity ratios. Other sand-dune samples in the intermediate area (Guárico) were identified as belonging to either the Apure aeolic plain or the Mesa Formation, suggesting thus the possibility of drier periods in the past when the two formations overlapped. According to paleowind directions during the Pleistocene glaciations as given by J.E. Damuth and R.W. Fairbridge (1970), the origin of the Apure eolic plain materials could be in the region south of the Orinoco.