Isotope Geochemistry as a Tool for Deciphering Kinetics of Water-Rock Interaction

What have isotopes added to our understanding of the kinetics of water-rock interaction processes at the surface of the Earth? Since the development of routine techniques of mass spectrometry, an impressive number of studies have used isotopes to constrain water-rock interaction processes. A number of these initial studies were not purposefully designed to determine the kinetics of water-rock interaction, but they demonstrated that isotopic kinetic effects were both sufficiently large and specific so that they could be used to constrain the kinetics of water-rock interactions. Here we provide a brief synthesis of the different approaches where isotopes have been applied to constrain the rates and timescales of water-rock interaction. We have adopted a very broad definition of the word “kinetics” for this review and have not limited the studies to reaction rates of particular reactions. Because most of the reactions occurring at the Earth’s surface are occurring at low temperatures, they do not proceed rapidly enough to reach thermodynamic equilibrium; therefore, we face the problem of determining the rates at which they proceed. Physical processes, such as climate variability, glacier dynamics, landsliding and tectonic processes often proceed more rapidly than chemical weathering reactions. For example, mineral weathering and the subsequent formation of secondary minerals in soils occurs over timescales on the order of tens to hundreds of thousands of years, while the characteristic timescales of climate variability are much shorter. Isotopic approaches have proven to be very powerful in constraining rates of water-rock interaction in natural systems, where physical and chemical processes are inevitably coupled.