Clay isotope geochemistry: A contribution beyond dating
Norbert Clauer, Centre de Géochimie de la Surface (CNRS/ULP), University of Strasbourg, CGS-UMR7517, 1 rue Blessig, 67084 STRASBOURG Cedex NA, France, Fax: +33 (0)3.90.24.04.02, nclauer@illite.u-strasbg.fr, Phone: +33 (0)3.90.24.04.33

Most clay minerals of supergene environments are potentially datable by isotopic means, while stable-isotope geochemistry enforces tracing of source or genesis. Most reported difficulties are not due to the methods, but to technical aspects or natural polygenic mixings. Isotopic disequilibria meaning scattered “ages” do not result from varied closure temperatures, as in high temperature minerals, but from mixing of particles with varied origins and in different evolution states. Clay “ages” depend mainly on mineral heterogeneity, but also on fluid chemistry, crystal-growth kinetic, reaction temperature, basin history. Interpretation of clay isotope dating needs also mineralogical, geochemical and field information. These different aspects will be evaluated in this presentation, especially why and how isotope studies of clay material can be reliable, what the data mean relative to independent information, and what can be expected beyond isotope dating: a powerful tracing tool of physical and chemical conditions during nucleation and crystal growth.