Role of Carbonate Minerals in the Distribution of Trace Elements in Marine Clay Formations

Abstract

Anthropogenic radionuclides (RN) are generated by a wide range of industrial and medical activities. In the contexts of waste storage, the quantification of RN migration is of paramount importance. RN migration is partly ruled by the interaction of RN with the solid surfaces. Usually experiments are conducted at various scales from laboratory to the field in order to measure retention and retardation parameters of radiotracers. Whereas this experimental approach is mandatory to tackle the issue of RN migration, the understanding of the natural speciation of stable isotopes that are analog to RN brings additional useful information. In particular, the RN natural speciation sheds light on RN isotopic exchange and “irreversible” trapping mechanisms. This study aims at overviewing the association of natural trace elements (U, Th, Ni, I, Sr and Zn) with carbonate minerals in the Callovian-Oxfordian sedimentary formation that is under consideration for deep nuclear waste disposal in France. The combined use of sequential extraction techniques, microscopic and spectrometric techniques, as well as laser-ablation coupled to chemical analysis techniques made it possible to establish the distribution of I, Sr, U, Th and Ni in the various mineral and organic phases present in the clay rock. I and Sr and in a less extent U and Th are mainly carried by carbonates while Ni is distributed in a variety of phases including pyrite, sphalerite, chlorite, organic matter and muscovite.