Hydroxide in olivine: A quantitative determination of the absolute amount and calibration of the IR spectrum

Abstract

[1] Olivine is an important host of hydrogen in the Earth's upper mantle, and the OH abundance in this mineral determines many important physical properties of the planet's interior. To date, natural and experimentally hydrated olivines have been analyzed by uncalibrated spectroscopic methods with large (±100%) uncertainties in accuracy. We determined the hydrogen contents of three natural olivines by 15 N nuclear reaction analysis and used the results to calibrate the common infrared (IR) spectroscopic method for quantitative hydrogen analysis of olivine. OH content (expressed as parts per million H 2 O by weight) is 0.188 times the total integrated absorbance of the fundamental OH stretching bands in the 3750-3100 cm À1 region. The results indicate that an upward revision of some previous determinations by factors of between 2 and 4 is necessary. The most hydrous naturally occurring mantle-derived olivine analyzed to date contains 240 ppm wt. H 2 O. Retrospective application of this calibration to experimentally hydrated olivines may be limited by spectral differences in some cases and by the previous use of nonpolarized IR spectra.

Experimental deformation of olivine single crystals at mantle pressures and temperatures

“…We use the calibration of Bell et al (2003) to reduce the data in terms of water concentrations. Since this calibration is intended for polarized measurements, A c c e p t e d M a n u s c r i p t 5 water concentrations measured with an unpolarized beam may be underestimated by a factor of 1.5 or more (see Bell et al, 2003). We thus multiplied by 1.5 the measured water contents before reporting them in Table 1.…”

The Role of Solar Wind in the Formation of Hydroxyl on Airless Silicate Bodies in Space

“…Our results below show that they cannot be more than 10% lower in our case since the OH produced by implantation must be one or less per incident proton. The average OH peak position for the silica glass was 3673 cm -1 , identical to that of Davis et al The absorbance in San Carlos olivine peaked near ~3570 cm -1 , similar to previous studies [Bell et al 2003 ;Koch-Muller, 2006], but exhibited a broadband structure without sharp peaks as seen in silicate glass spectra [Stolper, 1982].…”

“…Cooling the sample allowed molecular water to be distinguished from structurally bound OH as noted in Section 3.2.4 and discussed further below. The absorbance band of the olivine was broad and structureless, very different than previously published olivine spectra [Bell et al, 2003] though quite similar to absorptions for glassy silicate samples [Stolper, 1982;Paterson, 1982]. Paterson (1982) determined that the strength of the absorption coefficient depends on the peak wavenumber of the OH absorption band and developed a calibration relating the two.…”

“…Paterson (1982) determined that the strength of the absorption coefficient depends on the peak wavenumber of the OH absorption band and developed a calibration relating the two. Libowitzky and Rossman (1996) pointed out that unpolarized measurements for crystalline minerals yielded unreliable estimates of the water contents, and this was further discussed for olivine by Bell et al (2003) with direct comparisons being made to the Paterson calibration. However, the OH absorbance peak height for the olivine spectra of Bell et al (2003) was several orders of magnitude higher than in the current experiments, and because the shape of the band found in the irradiated olivine was most similar to the absorption bands discussed for quartz by Paterson (1982), the calibration given there was used to determine the OH column density increase due to H + implantation:…”

“…The integrated molar absorption coefficients used to calculate column density were taken from published studies where OH concentrations were measured using more direct techniques such as 15 N nuclear reaction and dehydration weight loss [Bell et al, 2003;Davis et al, 1996].…”