Abstract

Small but measurable variations in triple oxygen isotopic compositions result from purely mass dependent kinetic or equilibrium fractionation. In this talk I will outline how the underlying processes (kinetic vs. equilibrium vs. mixing) can be identified and how this additional information is used to solve geochemical problems. Presently, the main applications are related to the global water cycle were the 17Oexcess parameter is used just like the well-known d-excess parameter (both measures for the deviation from a given reference line). However, 17Oexcess can also be applied to silicates or carbonates. For instance, the δ18O of metamorphic gneisses from northwestern Russia is exceptionally low (-27‰). Strong water-rock interaction with meltwaters from snowball earth glaciers 2.4 Ga ago is hypothesized as cause. The triple oxygen isotope data support this hypothesis and imply that pristine meltwaters had δ18O of -40‰ or less. The principles outlined here for oxygen can be applied to all isotope systems with at least three stable isotopes.