Cyril Aubaud

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

Abstract 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) × (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.

Petrologic structure of a hydrous 410 km discontinuity

We examine a simple thermodynamic model relating the effect of pressure and H 2 O content on the relative stabilities of olivine, wadsleyite, and hydrous melt through the upper mantle/transition zone boundary. As noted previously, the strong preference of H 2 O for wadsleyite relative to olivine means that H 2 O increases the depth interval over which the two polymorphs coexist and displaces the boundary to shallower depths. Olivine and wadsleyite become increasingly hydrous with decreasing depth through the transformation interval, and if the available H 2 O exceeds the storage capacity of olivine, then partial melting will occur at the shallowest portions of the interval. Increases in H 2 O beyond that necessary to generate incipient melt result in thinning of the transformation interval, but additional shoaling. In the extreme case where melting occurs throughout the melting interval, the transformation interval will be as thin as for the dry case (i.e., ∼7 km). These calculations may serve as a starting point for dynamical calculations of possible melting at the 410 km discontinuity and as a guide for interpretation of observational geophysical characteristics of putative hydrous features at the transition zone/upper mantle boundary.