IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_2G-P4

Date/Time: July 23 Poster

Quantitative analysis of water concentration in melt inclusions by reflectance micro-FTIR
spectroscopy
Atsushi Yasuda
Earthquake Research Institute, Univ. Tokyo, Japan
E-mail: yasuda@eri.u-tokyo.ac.jp

A technique based on micro reflectance spectroscopy using a vacuum Fourier Transform infrared (FTIR)
spectrometer has been developed for the quantitative measurement of water content in melt inclusions inphenocrysts. Water in magma strongly affects how a volcano erupts. Thus it is important to determine the amountof water dissolved in the magma in order to predict how the eruption develops. Previously, a FTIR micro reflectancemeasurement was proposed to be a promising method to determine the concentration of water in volcanic glasses
rapidly because of its great advantage in sample preparation. It requires neither doubly polished wafers nor
measurement of sample thickness. However, application of the method to natural samples has been limited sofar because the resonance due to the O-H stretching vibration of dissolved water in the glass observed around3650cm-1 is weak. Moreover, requirement of smaller apertures for measurement of melt inclusions with several tens
micron in diameter significantly decreases the signal-to-noise ratio. Consequently quantitative analysis of water insuch a small melt inclusion was considered to be difficult. We overcome the difficulty by two improvements.
Firstly a vacuum FTIR apparatus was introduced. Evacuation the whole light path by a vacuum pump (<100Pa) can reduce more efficiently noisy absorption around 3650cm-1 due to water vapor than purging dry N2 gas.
Thirty-one glasses with various water content ranging from basaltic to rhyolitic composition were synthesized byan internal heated pressure vessel. Reflectance IR spectra were taken between 650-7000cm-1 on singly polishedglass samples. The maximum amplitude of the signal at around 3650 cm-1 resonance normalized to a baseline ofreflectivity was plotted against water contents of the samples determined by Karl-Fischer titration (ranging from 0.1
to 5.1 wt%), and a linear correlation was obtained. It appears that the slope of the correlation line slightly depends
on the bulk composition. When a reflectance spectrum is taken with a microscope aperture of 20x20 micron and1024 scans, the intensity of noise around 3650cm-1 is 0.02 (1σ), corresponding to approximately 0.2 wt% H2 O.
As the second improvement, we devised a correction method for spectrum interference by host crystal. When asmall melt inclusion is analyzed, some signal from host crystal may overlap the spectra of melt inclusion, resultingin underestimation of water content in the melt inclusion. However, fortunately enough, most silicate crystals havedistinct reflection peaks in the region 800-1200cm-1 . Using those peaks, the contribution of host crystal to themeasured reflective spectra can be estimated within a precision of 10% relative.
Olivine-hosted melt inclusions collected from 2011 Shinmoe-dake eruption were studied as a practical applicationof the new technique.

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