Japan Geoscience Union Meeting 2016

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International Session (Poster)

Symbol S (Solid Earth Sciences) » S-GC Geochemistry

[S-GC16] Volatile Cycles in the Deep Earth - from Subduction Zones to the Mantle and Core

Wed. May 25, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo), Colin Jackson(Geophysical Laboratory, Carnegie Institution of Washington)

5:15 PM - 6:30 PM

[SGC16-P01] Accurate measurement of H2O concentration and speciation in silicate glasses using FTIR spectroscopy

*Iona McIntosh1, Alexander Nichols1, Kenichiro Tani2, Ed Llewellin3, Ian Schipper4, Bob Stewart5 (1.Japan Agency for Marine-Earth Science and Technology, Japan, 2.National Museum of Nature and Science, Japan, 3.Durham University, UK, 4.Victoria University of Wellington, NZ, 5.Massey University, NZ)

Keywords:FTIR spectroscopy, H2O speciation, volcanic glass

Accurate measurement of H2O concentration and speciation in silicate glasses is important not only for studies of erupted glasses that use the volatile record to reconstruct eruption processes, but also for studies that use experimental glasses to find e.g. the partition coefficients of other volatiles that partition into H2O-rich vapour. Fourier Transform Infrared (FTIR) spectroscopy can be used to find not only the overall H2O concentration (H2Ot) of silicate glasses, but also the individual concentrations of the two H2O species: molecular H2O (H2Om), and dissociated hydroxyl groups (OH). Here we discuss key developments and refinements of FTIR methodology with respect to finding the water contents of silicate glasses. Firstly, we demonstrate the importance of using a species-dependent H2Ot molar absorptivity coefficient when using the 3500 cm-1 H2Ot absorbance peak to find H2Ot and OH (indirectly as [OH] = [H2Ot] – [H2Om]) concentrations, and in particular how this can be used to reconstruct the original H2O contents of glasses that have since undergone secondary hydration at low temperature. Secondly, we discuss the strong dependence on glass composition of ‘silicate peaks’ at ~1830 cm-1 and ~1600 cm-1, and demonstrate how the 1830 cm-1 peak can be used to correct FTIR imaging of H2O concentrations in samples of varying glass thickness, and how the superposition of the 1630 cm-1 H2Om and 1600 cm-1 silicate peaks may result in overestimation of H2Om concentration in thick and/or H2Om-poor samples.