Hydrogen defects in feldspars: kinetics of D/H isotope exchange and diffusion of hydrogen species in alkali feldspars

Abstract

Diffusion of hydrogen in natural alkali feldspars (Eifel sanidine and adularia from unknown locality) containing strongly bonded OH defects was investigated by D/H isotope exchange in the T range 600–1050 °C at ambient pressure and at elevated pressures up to 8 kbar. Runs at 1 atm were performed in a fused silica tube connected to a liquid D2O reservoir at room temperature. In the high- pressure experiments samples were sealed with D2O in gold capsules and processed up to 4 kbar in externally heated pressure vessels using Ar/D2O as the pressure medium. Experiments at 6–8 kbar were performed in an internally heated gas pressure vessel using the double capsule technique to minimize isotopic contamination by the pressure medium. Diffusion coefficients were determined either by measuring concentration-distance profiles of OH and OD with an IR microscope or by measuring the total exchange of oriented plates after various run durations using a macroscopic IR technique. Both methods gave consistent data. D/H interdiffusion, DD/H, is almost identical in the adularia and in the sanidine implying that the chemical composition and the degree of Al/Si disorder have minor influence on the hydrogen isotope exchange in alkali feldspars. Furthermore, no effect of crystallographic orientation was found for DD/H in both feldspars. DD/H in sanidine, however, depends on the thermal pre-treatment. Heating for several days at 900 °C leads to a lowering of D by a factor of 2.3, indicating a corresponding decrease in mobile hydrogen species. Data for sanidine pre-annealed at 900 °C are well described in the T range 600–1050 °C by DD/H(m2/s)=6.9·10-6exp(-162kJ/molR·T)The diffusivity is strongly enhanced by water pressures (PH2O), i.e., in the range of 0–2 kbar. At PH2O = 2 kbar the following equation applies in the T-range of 645–800 °C: DD/H(m2/s)=1.2·10-6exp(-131kJ/molR·T)Experiments with D2O/CO2 mixture of ratio 1:1 gave smaller exchange rates compared to pure D2O fluids, confirming that that not the pressure but the water fugacity leads to the increase in the mobility of hydrogen species. At 720 °C and pressures of 4–8 kbar, chemical diffusivities of H2O, D~H2O, were determined by fitting the weighted sum of the absorbances of the OH and the OD band vs. distance. The D~H2O values are similar to those reported by Kronenberg et al. (Geochim Cosmochim Acta 60:4075–4094, 1996) for dehydration of Kristallina adularia at ambient pressure. It is concluded that in both cases high concentrations of H2O molecules on interstitial sites govern the transport of hydrogen. Comparison of D/H interdiffusion to O diffusion in sanidine (Freer et al. in Phil Mag A75:485–503, 1997) implies that not only interstitial H2O but also protons contribute to the transport of hydrogen under hydrothermal conditions. On the other hand, the high DD/H at ambient pressure is attributed to an interdiffusion of protons and Na+, which is supported by Na tracerdiffusion data for sanidine (Wilangowski et al. in Defect Diffus Forum 363:79–84, 2015). A basic conclusion of this research is that hydrogen storage capacity and hydrogen diffusion in feldspars are largely determined by extrinsic defects, such as substitutional defects (i.e., Al3+ + H+ for Si4+) and associates of water molecules with vacancies. The bonding of hydrogen species to the defects can vary greatly, depending on the genesis of the feldspars, so that quantitative predictions are difficult.