Stable Isotope Fronts and Crustal Buffering - 1-Dimensional Mass Balance and Kinetics

Zero dimensional and 1-dimensional models of isotope exchange in fluid-rock interaction are discussed from a mass balance and kinetic point of view. A rate-controlled 1-dimensional model provides unique delta vs distance profiles for any given reaction rate constant and interstitial fluid velocity. In principle, the model allows to quantify even that fraction of fluid which, due to slow exchange rates, fails to leave an effect in the rock of the aquifer it has passed. All higher temperature aquifers or fluid trajectories have an isotopic inversion point given by the porosity. This point locates a more or less developed ‘isotope front’. The fluid/rock ratio over a whole aquifer is dictated by the porosity and fluid/rock ratios »1 are possible only when an aquifer discharges to the surface. In the 1-dimensional view, most geothermal systems (e.g. Wairakei) have reached a stage behind the inversion point, whereas few geothermal systems (Ngawha) but most metamorphic terranes so far as is known lie ahead of it and appear to be still rock-controlled in their isotopic properties.