Behaviour of zirconium and hafnium in natural systems: Fundamental and applied implications

Abstract

The Zr/Hf ratio is considered constant in meteorites, minerals and rocks while changes by several orders of magnitude in natural aqueous fluids. The difficulty of predicting the behavior in fluids results from lack of reliable thermodynamic constants in aqueous media where Zr and Hf concentrations are in the range of picomol/kg. For this reason, the behavior of these elements has been investigated in a variety of aqueous systems spanning from acidic waters of volcanoes, cold and hot brines to very alkaline natural waters. These 'natural laboratories' made possible the empirical evaluation on the influence of pH, ionic strength, redox conditions, complexing ions and size particles (less than 0.1 micron) of suspended matter. The field systematic-approach reveals that pH and high ionic forces (from 0.7 to 4 mol/kg) do not significantly influence the fluid Zr/Hf ratio while the colloidal fraction is responsible for significant fractionation. We found that the Zr/Hf ratio in dissolved cold waters is mainly related to both solid morphology and electrical properties of the mineral surfaces constituting the colloidal fraction. On the contrary, in thermal fluids fractionation is enhanced by the presence of complexing ions such as Cl, SO4 and HCO3. Systematic studies over more than 20 years allowed Professor Paolo Censi to propose the use of Zr/Hf ratio as tracer of rapid magma arrival. By monitoring the Zr/Hf ratio in volcanic fumaroles, he proposed a new geochemical tool to assess the volcanic risk.