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

We present new Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe/secondary ion mass spectrometry (SIMS) analyses of [sup.1]H in 61 natural and experimental geological samples. These samples include 8 basaltic glasses (0.17 to 7.65 wt% [H.sub.2]O), 11 rhyolitic glasses (0.143 to 6.20 wt% [H.sub.2]O), 17 olivines (~0 to 910 wt. ppm [H.sub.2]O), 9 orthopyroxenes (~0 to 263 wt. ppm [H.sub.2]O), 8 clinopyroxenes (~0 to 490 wt. ppm [H.sub.2]0), and 8 garnets (~0 to 189 wt. ppm [H.sub.2]O). By careful attention to vacuum quality, the use a [Cs.sup.+] primary beam, and a resin-free mounting technique, we routinely achieve hydrogen backgrounds equivalent to less than 5 ppm by weight [H.sub.2]O in olivine. Compared to previous efforts, the new calibration extends to a wider range of [H.sub.2]O 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 [H.sub.2]O determined by FTIR vs. ([sup.1]H/[sup.30]Si) x (Si[O.sub.2]), 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 [sup.1]H/[sup.30]Si secondary ion ratios excited by [Cs.sup.+] ion beams or discrepancies between phase-specific FTIR absorption coefficient schemes. Keywords: Spectroscopy, infrared, water, mantle, ion microprobe