Water concentrations and hydrogen isotope compositions of alkaline basalt-hosted clinopyroxene megacrysts and amphibole clinopyroxenites: the role of structural hydroxyl groups and molecular water

István Kovács, Attila Demény, György Czuppon, Christophe Lécuyer, Francois Fourel, Qun Ke Xia, Jia Liu, Zsanett Pintér, Edit Király, Kálmán Török, Ábel Szabó, Etienne Deloule, György Falus, Tamás Fancsik, Zoltán Zajacz, Judit Sándorné Kovács, Beatrix Udvardi

Research output: Article

6 Citations (Scopus)

Abstract

The aim of this study was to determine both ‘water’ contents (as OH and H2O) and δD 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 δD values cover a similar range from −95 to −45 ‰ (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 micro-FTIR 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 micro-FTIR, 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 δD values of clinopyroxene megacrysts for each locality, indicating that structurally bond hydrogen in clinopyroxenes may have δD 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, while for molecular water there may be no or only negligible isotope fractionation.

Original languageEnglish
Article number38
JournalContributions to Mineralogy and Petrology
Volume171
Issue number5
DOIs
Publication statusPublished - máj. 1 2016

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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    Kovács, I., Demény, A., Czuppon, G., Lécuyer, C., Fourel, F., Xia, Q. K., Liu, J., Pintér, Z., Király, E., Török, K., Szabó, Á., Deloule, E., Falus, G., Fancsik, T., Zajacz, Z., Sándorné Kovács, J., & Udvardi, B. (2016). Water concentrations and hydrogen isotope compositions of alkaline basalt-hosted clinopyroxene megacrysts and amphibole clinopyroxenites: the role of structural hydroxyl groups and molecular water. Contributions to Mineralogy and Petrology, 171(5), [38]. https://doi.org/10.1007/s00410-016-1241-0