A new technique using FT-IR micro-reflectance spectroscopy for measurement of water concentrations in melt inclusions

This paper presents a new technique to measure water concentrations in small volcanic glasses such as melt inclusions. The technique uses Fourier transform infrared (FT-IR) micro-reflectance spectroscopy, and thus does not require a doubly polished sample. To enhance the signal-to-noise (S/N) ratio, a vacuum pump and a narrow-band mercury-cadmium-telluride (MCT) detector were introduced into the FT-IR spectrometer. The pump reduced noise caused by atmospheric water vapor and carbon dioxide, and the detector enhanced the signal, which resulted in significant improvement of the S/N ratio. For standards, we synthesized 32 glasses with different water concentrations ranging from basaltic to rhyolitic composition. An empirical relationship was established between total water content and the negative peak height normalized by the baseline extrapolated from the neighboring region (ΔR/Rbaseline); for example, H2O wt.% = 49.76 × ΔR/Rbaseline − 0.08 for basaltic composition. The regression error corresponded to approximately 0.29 wt.% water (2σ) with an aperture of 20 μm× 20 μm and 2,048 scans. To apply the technique to natural samples of various shapes, we developed a method to correct the spectrum of melt inclusion contaminated with host minerals. This method calculates the overlapping volume of the host crystal using the reflectance spectra at 800 to 1,300 cm and is applicable to melt inclusions hosted by olivine, orthopyroxene, and plagioclase.