Etude de la repartition de l'uranium dans les roches par la methode des traces de fission. Application a l'echantillonage en vue du dosage de l'uranium

The distribution of uranium in two different rock species: basalt from the Fangataufa atoll (located in Polynesia) and granit from the Hoggar mountains (located in Sahara) was studied by the fission track method. The two distributions observed appear quite different: uranium is uniformly distributed in the vitrous part of the basalt while in granit it is located on the intergranular joints. Applying the sampling model proposed by A. D. WILSON to the observed distributions of uranium, the weight of the analytical sample corresponding to a required sampling error can be estimated. Results show that representative sample weights of the bulk powdered material, as usually obtained by milling, are about some tenths of milligrams for basalt and more than the grams for granit (for uranium analysis). So, in trace element analysis, the sampling error could be more important than the analytical one. This fact could explain the relatively poor accuracy of trace determinations in rock samples (including the reference samples).