A classification of British Caledonian granites based on uranium and thorium contents

A method of epithermal neutron activation analysis for uranium and thorium is discussed in relation to other rapid analytical methods for these elements. Caledonian granites are divided into four main groups on the basis of U and Th contents: (a) a group of evolved late granites with high U and Th contents and high Th/U ratios, (b) a group of uranium-enriched mineralized granites, (c) granites containing approximately average levels of U and Th, and (d) a group of granites with low thorium content. THE data presented here were obtained in a reconnaissance survey of the Caledonian granites (sensu lato) of the British Isles, aimed at identifying variations in U and Th content between intrusions. The data set comprises 260 analyses of granite from northern England, Scotland, and Ireland; sample coverage is incomplete particularly in northeast Scotland and southern Ireland and many intrusions are represented by only a small number of sample values. Where possible fresh samples were collected away from surface weathering, hydrothermal alteration and mineralization but no attempt was made to distinguish between tonalites, granodiorites, and more evolved adamellites and granites. Nevertheless, the data enable a preliminary classification of Caledonian granites to be made, although this may be refined by further studies. A method of epithermal neutron activation analysis is briefly discussed in relation to other rapid analytical methods for uranium and thorium. Analysis for uranium and thorium. In this study instrumental neutron activation with a beam of epithermal neutrons (energies about 1 ev) is used to induce the following reactions: ##238U(n,~)239 U --~ 2a9Np ._~ 239pu ##232Th(n,y)233Th ~ 233pa ._. 233 U The first nuclides produced have half-lives of a few_ minutes and, if the samples are counted some days after irradiation, it may be assumed that complete conversion to neptunium and protot~ Copyright the Mineralogical Society actinium has occurred. Detection and counting of gamma photons from the decay of the second nuclides, which have half-lives of 2.35 and 27 days respectively, is relatively simple. Furthermore, other activated species may be detected, allowing simultaneous determination of such elements as the rare earths. A limitation of the method is that a high epithermal/thermal neutron flux ratio is needed to minimize the amount of uranium fission products which would complicate the gamma spectrum. This is normally achieved by irradiating samples in cans lined with Cd foil, and by utilizing an irradiation position with a high energy flux. The method has advantages over indirect methods of analysis such as gamma, beta, and alpha spectrometry which can be used in the field and to obtain precise results in the laboratory. These methods depend on the determination of intermediate nuclides rather than the parents or stable daughters of the radioelement decay chain, and assumptions about the state of secular equilibrium of the decay series are required. This may be particularly difficult if data are obtained by field gamma spectrometry because surface weathering processes may have destroyed the equilibrium, resulting in the over or under estimation of the uranium and thorium contents. Field gamma spectrometry has the advantage however of detecting or resolving small scale features in the distribution of radioelements which would easily be missed using laboratory-based analysis of rock samples. Epithermal neutron activation analysis also has advantages over the delayed neutron method (Amiel, 1962) in which a separate irradiation is required for thorium and which depends for its accuracy on careful control of the neutron flux and travel times of the sample to the counting devices. Experimental method. Finely ground samples each weighing 0.5 g are packed in high purity silica tubes and enclosed in cans lined with Cd foil; these are irradiated for approximately eight hours in the 300 kw reactor at the Liverpool and Manchester