Influence of composition and thermal history of volcanic glasses on water content as determined by micro-Raman spectrometry

• Published in: Applied geochemistry Vol. 21; no. 5; pp. 802 - 812
• Main Authors: Muro, A. Di, Giordano, D., Villemant, B., Montagnac, G., Scaillet, B., Romano, C.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 01.05.2006; Elsevier
• Subjects: Alkaline magmas; Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Sciences; Exact sciences and technology; Geochemistry; Glasses; Global Changes; Obsidian; Pollution, environment geology; Raman; Sciences of the Universe; Viscous dissipation; Volatiles; Volcanology; Water; Water; Alkaline magmas; Glasses; Obsidian; Raman; Viscous dissipation; Volatiles; thermal history; Raman spectra; degassing; substitution; phonolites; alkali basalts; cycles; low frequency; basaltic andesite; basalts; volcanic glass; water content; calibration; cations; obsidian; andesites; volcanic rocks; topology; igneous rocks; magmas; rhyolites; heterogeneity; standard samples; trachytes; thermal annealing; natural glasses; melts; volatiles
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2006.02.009

Development of Raman spectrometry for quantification of water content in natural glasses requires the assessment of the dependence of the technique on glass composition and thermal history. In the low frequency domain, Raman spectra topology varies due to glass depolymerization and substitution in the framework of (Si 4+) IV by alkali-balanced (Al 3+) IV and (Fe 3+) IV in calcalkaline (rhyolite to basaltic andesite) and alkaline (trachyte, phonolite to alkali basalt) glasses. These processes result in strong dependence of previous analytical procedure (internal calibration) on glass composition. Here, we show that an analytical procedure based on calibration to an external standard is only faintly composition-dependent for Si-rich alkaline glasses (trachytes–phonolites). For a given glass composition, thermal history also plays a fundamental role in the choice of Raman procedure for water analysis. Repeated cycles of thermal annealing induce microcrystallization of hydrous trachyte glasses and modify cation distribution in the glass structure. Application of these concepts to analysis of banded obsidians suggests that small-scale heterogeneities in glasses are not simply related to magma degassing, but could depend on thermal history and consequent relaxation paths in the melt.