Analysis of hydrogen in olivine by SIMS: evaluation of standards and protocol.

    We measured hydrogen concentrations in 12 olivines using secondary ion mass spectrometry (SIMS and NanoSIMS), cross-calibrated against Fourier transform infrared (FTIR) spectroscopy and nuclear reaction analysis (NRA). Five of these samples are routinely used for calibration in other laboratories. We assess the suitability of these olivines as standards based on over 300 SIMS analyses, comprising 22 separate calibrations. Seven olivines with 0-125 ppm H₂O give highly reproducible results; in contrast to previous studies, the data are fit to well-constrained calibration lines with high correlation coefficients (r² = 0.98-1). However, four kimberlitic megacrysts with 140-245 ppm H₂O sometimes yield ¹⁶O¹H/³⁰Si ratios that have very high uncertainties and can  vary by up to a factor of two even in sequential analyses. An obvious cause of such behavior is the presence of sub-microscopic inclusions of hydrous minerals, such as serpentine. In most cases, however, we link the anomalies to the presence of sub-micron to micron-scale pores (as small as 100 nm), imaged using SEM and NanoSIMS. These pores are interpreted to be fluid inclusions containing liquid H₂O, other volatiles (including fluorine) and/or condensed hydrous phase precipitates. Ionization of the pores contributes variably to the measured ¹⁶O¹H, resulting in analyses with erratic depth profiles and corresponding high uncertainties (up to 16%, 2σ_mean). After filtering of these analyses using a simple criterion based on the error predicted by Poisson counting statistics, all the data fit well together. In contrast to previous studies, our results imply that the Bell et al. (2003) calibration can be applied accurately to all olivines with IR  bands from ~3400-3700 cm^-1, without the need for band-specific IR absorption coefficients.