Unraveling Zr/Hf fractionation: Hydrothermal zirconium and hafnium fluoride complexation up to 400°C

High field strength elements (HFSE) such as Zr and Hf are relatively insoluble in most natural hydrothermal solutions and consequently immobile in most geological systems. However, fluoride forms stable aqueous complexes with many HFSE ions, including Zr⁴⁺ and Hf⁴⁺, and is thus a potent mobilizer of these elements. Due to their identical charge and similar ionic radius (590 pm and 580 pm, respectively), Zr and Hf behave almost identically in geological system and are therefore referred to as geochemical twins. Fluoride complexation in hydrothermal environments is one of few processes in the Earth's crust that can effectively fractionate them from one another. This fact can be used to trace past fluoride activity in fossil hydrothermal systems by investigating Zr/Hf ratios, if fluoride complexation of Zr and Hf is sufficiently well understood. Mobility of metals as complexes is controlled by two distinct but related mechanisms: Formation of the complex itself and solvation of that complex in the solvent. Poly(hydrogen-fluoride) bridging of fluoride complexes to the surrounding aqueous solvent is crucial to the understanding of the solvation and therefore the mobility of fluoride complexes.

We report geometries of Zr and Hf fluoride complexes up to 400°C, determined by extended X-Ray absorption fine structure (EXAFS) in a hydrothermal autoclave. Existing data sets on the stability of those complexes at lower temperatures are extended to 400°C. Our data show strong temperature dependence of the complex stability for both metals. However, the effect of temperature is not equally strong for Zr and Hf. Fractionation of the twin pair is thus a function of temperature as well as fluoride activity.