Water in leucite, a nominally ahydrous volcanic mineral

The study of magmatic/volcanic volatiles is based on the determination of H2O and CO2 in glasses, presuming they represent the vast majority of the gas dissolved in the melt. However, recent work shows that several nominally anhydrous volcanic minerals (NAMs) do contain trace but significant water ( CO2). Leucite - KAlSi2O6 - is an anhydrous mineral typical of potassium-rich basic lavas. It is commonly altered into analcime due to percolating Na-rich fluids during cooling of the host rock. The process is not a solid-state reaction but proceeds by dissolution of leucite and reprecipitation of analcime (Putnis et al. 2007). We report here a single-crystal FTIR study of transparent, inclusion-free leucite phenocrystals occurring within lava flows, pyroclastic deposits and ejecta in the Roman Comagmatic Province. The IR spectra invariably show broad absorptions in the 4000-3000 cm-1 region consisting of overlapping components around 3604, 3500 and 3250 cm-1, thus showing that the examined samples contain structurally bound water molecules. Detailed FTIR mappings, obtained using an automated stage, show significant zoning of this water, consisting typically of an anhydrous leucite core mantled by an hydrous rim. Micro-chemical data show almost negligible Na in the samples (< 0.5 wt% ), thus excluding alteration into analcime. Using the molar absorption value for analcime (Libowitzky and Rossman, 1996) H2O contents up to 700 ppm are calculated. This water is believed to be primarily incorporated in leucite together with trace Na in the structural pores. Its patterns of zoning may allow to monitor the evolution of magmatic system (P, T, fO2) as a function of time. Libowitzky, E and Rossman, GR (1996) Am. Mineral., 82, 1111-1115. Putnis, CV, Geisler, T., Schmid-Beurmann, P, Stephan, T and Giampaolo, C. (2007) Am. Mineral., 92, 19-26.