Quantifying water in silicate glasses using FTIR: Extending calibrations to highly depolymerized compositions

Water is the most abundant volatile dissolved in magmas and exerts a major influence on the physical and chemical behavior of silicate melts and glasses. Even small amounts can strongly modify viscosity, phase relations, and crystal nucleation, and affect properties such as heat capacity and phase separation. Because of its broad impact on geological processes – from partial melting to magma ascent and crystallization – accurate quantification of water in silicate melts is essential.

Fourier-transform infrared spectroscopy (FTIR) is a widely used technique for determining water content in both natural and synthetic glasses. Its high sensitivity, non-destructive nature, and capability to analyze microscopic regions make it suitable for a broad range of samples. FTIR also allows distinguishing between different hydrogen-bearing species, such as hydroxyl groups and molecular water, through characteristic absorption bands. Silicate glasses exhibit multiple infrared bands associated with hydrogen, three of which are commonly used to quantify water: combination bands of H₂O and OH species at 5200 cm^-¹ and 4500 cm^-¹, respectively, and the fundamental O–H stretching band at 3530 cm^-¹. Quantifying water from these bands requires knowledge of molar absorption coefficients, which depend on glass composition. Previous calibrations focused on relatively polymerized compositions with non-bridging oxygen per tetrahedral cation ratios (NBO/T) of 0–0.8, leaving highly depolymerized melts poorly constrained.

To address this gap, we measured molar absorption coefficients in highly depolymerized hydrous peridotitic glasses (NBO/T ≈ 2.5). These glasses were synthesized using a rapid-quench multi-anvil technique, which preserve water in compositions previously inaccessible. Absolute hydrogen contents (0.3–4.7 wt.% H₂O) were determined independently using elastic recoil detection analysis, providing a robust basis for FTIR calibration. We determined both linear and integrated molar absorption coefficients of combination bands at 5200 cm^-¹ and 4500 cm^-¹, and the fundamental O–H stretching band.

We compiled and critically assessed over 350 published values of molar absorption coefficients across a wide range of glass compositions, creating the largest database of its kind. Combining these data with our new measurements for highly depolymerized peridotitic glasses, we identify systematic correlations between molar absorption coefficients and compositional parameters, such as SiO₂ wt.% and mol.%, excess modifiers, and the Al+Si/cations ratio, with coefficients generally decreasing as melts become more depolymerized. These trends enable reliable prediction of absorption coefficients for both combination and fundamental bands, extending FTIR calibrations to ultramafic melts and providing a consistent framework for accurate water quantification across diverse silicate glasses.