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'Water' content of quartz from various rhyolitic ignimbrite horizons: results from unpolarized micro-FTIR analyses on unoriented grains

Biro, Tamás and Kovács, István János and Király, Edit and Falus, György and Karátson, Dávid and Bendő, Zsolt and Fancsik, Tamás and Sándorné Kovács, Judit (2015) 'Water' content of quartz from various rhyolitic ignimbrite horizons: results from unpolarized micro-FTIR analyses on unoriented grains. European Journal of Mineralogy. ISSN 0935-1221 (Submitted)

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Abstract

Structural hydroxyl contents of quartz grains from various rhyolitic pyroclastic density current (PDC) deposits from the Bükk Foreland Volcanic Area, Hungary, were determined by using micro-FTIR spectrometry. Additional trace element analysis and SEM cathodoluminescence imaging were also performed. Structural hydroxyl content (expressed in water equivalent) of volcanic quartz ranges from ~2,0 to ~4,1 ppm, which is lower than those in quartz of plutonic (granitic), metamorphic and hydrothermal origin. The incorporation of structural hydroxyl is mainly due to coupled H+ + Al3+ substitutions into Si-tetrahedral vacancies of quartz. In addition, the presence of molecular water is also obvious in (nano-)inclusions. The post-eruptive out diffusion of H+ during cooling seems to be the main factor causing the very low concentration of structural hydroxyl. This may be also manifested in the almost homogeneous distribution of structural hydroxyl regardless the considerable zonation in Al- and Ticontent. Although complete disappearance of structural hydroxyl was possibly hindered by the cooling effect of phreatomagmatism (excess water-magma interaction during eruption) and deposition in ashallow submarine environment. Moreover, a reasonable linear correlation is observed between the integrated area of Si-O bonds (between 2110 and 1440 cm-1) and sample thickness up to ~300 μm by studying unoriented quartz crystals. This may enable the analysis of separated quartz crystals without the need for preparing oriented thin section providing a more practical tool to determine structural hydroxyl concentrations.

Item Type: Article
Subjects: Q Science / természettudomány > QD Chemistry / kémia > QD05 Crystallography / kristálytan
Q Science / természettudomány > QD Chemistry / kémia > QD06 Mineralogy / ásványtan
Q Science / természettudomány > QE Geology / földtudományok
Q Science / természettudomány > QE Geology / földtudományok > QE02 Geochemistry / geokémia
Q Science / természettudomány > QE Geology / földtudományok > QE06 Petrology. Petrography / kőzettan, petrográfia
Depositing User: Dr. István János Kovács
Date Deposited: 03 Oct 2016 14:25
Last Modified: 03 Oct 2016 14:25
URI: http://real.mtak.hu/id/eprint/41027

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