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Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals

¹ Department of Geology and Geophysics, University of Minnesota, 108 Pillsbury Hall, 310 Pillsbury Drive, Minneapolis, Minnesota 55455, U.S.A.
² Department of Geology, Arizona State University, Box 871404, Tempe, Arizona 85287-1404, U.S.A.
³ Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
⁴ Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85282, U.S.A.

We present new Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe/secondary ion mass spectrometry (SIMS) analyses of ¹H in 61 natural and experimental geological samples. These samples include 8 basaltic glasses (0.17 to 7.65 wt% H₂O), 11 rhyolitic glasses (0.143 to 6.20 wt% H₂O), 17 olivines (~0 to 910 wt. ppm H₂O), 9 orthopyroxenes (~0 to 263 wt. ppm H₂O), 8 clinopyroxenes (~0 to 490 wt. ppm H₂O), and 8 garnets (~0 to 189 wt. ppm H₂O). By careful attention to vacuum quality, the use a Cs⁺ primary beam, and a resin-free mounting technique, we routinely achieve hydrogen backgrounds equivalent to less than 5 ppm by weight H₂O in olivine. Compared to previous efforts, the new calibration extends to a wider range of H₂O contents for the minerals and is more reliable owing to a larger number of standards and to characterization of anisotropic minerals by polarized FTIR on oriented crystals. When observed, discrepancies between FTIR and SIMS measurements are attributable to inclusions of hydrous minerals or fluid inclusions in the crystals. Inclusions more commonly interfere with FTIR analyses than with SIMS, owing to the much larger volume sampled by the former. Plots of H₂O determined by FTIR vs. (¹H/³⁰Si) x (SiO₂), determined by SIMS and electron microprobe (EMP) yield linear arrays and for each phase appear to be insensitive to bulk composition. For example, basalt and rhyolite calibration slopes cannot be distinguished. On the other hand, calibration slopes of different phases vary by up to a factor of 4. This reflects either phase-specific behavior of ¹H/³⁰Si secondary ion ratios excited by Cs⁺ ion beams or discrepancies between phase-specific FTIR absorption coefficient schemes.

Key Words: Spectroscopy • infrared • water • mantle • ion microprobe