Effects of chemical composition on the environments of D+ and H+ in alkali silicate glasses: with implications for D/H fractionation in magmatic processes

Abstract:

The δD is an important probe for studying the cycle of water within the Earth and between planetary bodies. D/H fractionation between silicate melts, minerals, aqueous fluids and gases governed the δD-evolution of the various geochemical reservoirs. It is usually assumed that D⁺ and H⁺ have the same chemical properties and structural environments in silicate melts and aqueous fluids, so that the only mass-dependent fractionation takes place with values approaching 1 at magmatic temperatures. However, recent in situ studies reveal important D/H fractionation between silicate melts and aqueous fluids even at high temperature. H and D MAS NMR data from sodium silicate glasses also shown that D⁺ and H⁺ occupy different structural positions in the structure of silicate glasses. This suggests that mass-dependent fractionation is not the only factor governing D/H fractionation in magmatic systems.

To assess how the chemical composition and the structure of alkali silicate glasses affect the environments of H⁺ and D⁺, the H and D MAS NMR spectra of M₂Si₄O₉ glasses (M = Li, Na or K) with different concentrations of pure H₂O or D₂O (from 3.3 up to 17.6 mol%) were recorded. Other spectra were acquired from M₂Si₄O₉ glasses with 17.6 mol%(1H,1D)₂O. Signals at ~1, ~3.5, ~5, ~12 and ~16 ppm in ¹H MAS NMR spectra are assigned to H⁺ in H₂O molecules and Si-OH groups in the glasses. These five signals indicates protons distribution between at least five environments with O^…O distances ranging from ~305 to ~240 pm. The ionic radius of alkali affects the distribution of H⁺ between those environments.

D MAS NMR spectra reveal that by exchanging H⁺ with D⁺, the intensity of the 16 ppm NMR line increases, whereas the intensity of the 5 ppm line decreases. Consequently, D⁺ seems to be more concentrated than H⁺ in environments with small O^...O distances. In other words, the structural environments of H⁺ and D⁺ in the silicate glasses, and hence in melts at their glass transition temperature, differ. These structural differences may promote fractionation of H⁺ and D⁺ during melting, crystallization and exsolution of aqueous fluids from magma in the Earth's interior. These effects, in turn, may cause variations of δD between the different Earth reservoirs.