OXYGEN AND HYDROGEN ISOTOPE GEOCHEMISTRY OF CLAYS - EXAMPLES OF APPLICATION TO DIAGENETIC AND GEOTHERMAL ENVIRONMENTS

The application of oxygen and hydrogen isotope geochemistry to the stu dy oi clays and clayey rocks has developed considerably over the past 30-40 years. Although much work remains to be completed, the knowledge of isotope fractionation between clays and water has progressed signi ficantly for the main groups of clay minerals. in addition, the increa sing number of case studies calling upon stable isotope geochemistry h as provided a lot oi information on the variability of isotopic compos itions of natural clays in low temperature environments, such as diage nesis and geothermal areas. The integration of isotopic data with thos e of mineralogy, petrography, major and trace elements geochemistry as well as site geology has resulted on many occasions, in a better char acterization of the conditions of clay formation and transformation in these environments. After reviewing the basic principles of oxygen an d hydrogen isotope geochemistry, we present several remarkable example s of application of this tool to the study of clay minerals in sedimen tary rocks, as an illustration of the significant contribution of this discipline to the characterization oi clay genesis media. A substanti al part of this article is devoted to the methods of clay separation a nd purification, which represent the most difficult step in many isoto pic studies. Our main ambition is to provide a synthetic snapshot of t he current state of knowledge in oxygen and hydrogen isotope geochemis try of clays, with the goal of introducing this discipline to students and researchers from other geoscientific fields.