Water concentrations and hydrogen isotope compositions of alkaline basalt-hosted clinopyroxene megacrysts and amphibole clinopyroxenites: the role of structural hydroxyl groups and molecular water
Résumé
The aim of this study was to determine both `water' contents (as OH- and H2O) and 8D values of several clinopyroxene samples from alkaline basalts. These parameters were first obtained from five clinopyroxene samples using both the classical `off-line' vacuum extraction technique and the `on-line' high-temperature pyrolysis technique. Blanks measured with the `on-line' gas extraction techniques were low enough to prevent any contamination by atmospheric water vapour. The comparison of data has revealed that our `on-line' procedure is more effective for the extraction of `water' from clinopyroxenes and, consequently, this `on-line' technique was applied to ten additional clinopyroxene samples. Sample 8D values cover a similar range from 95 to 45 %c (VSMOW) regardless of the studied locations, whereas the total `water' content varies from -115 to -2570 ppm. The structural hydroxyl content of clinopyroxene samples measured by microFTIR spectrometry varies from -0 to 476 ppm expressed in molecular water equivalent. The total `water' concentrations determined by mass spectrometry differ considerably from structural hydroxyl contents constrained by microFTIR, thus indicating that considerable proportion of the `water' may be present in (nano) -inclusions. The structural hydroxyl concentration apart from clinopyroxenes separated from amphibole clinopyroxenite xenoliths correlates positively with the 8D values of clinopyroxene megacrysts for each locality, indicating that structurally bond hydrogen in clinopyroxenes may have 8D values higher than molecular water in inclusions. This implies that there may be a significant hydrogen isotope fractionation for structural hydroxyl during crystallization of clinopyroxene, qwhile for molecular water there may be no or only negligible isotope fractionation.