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Evidence for post-depositional diffusional loss of hydrogen in quartz phenocryst fragments within ignimbrites

Tamas Biro, Istvan Janos Kovacs, David Karatson, Roland Stalder, Edit Kiraly, Gyorgy Falus, Tamas Fancsik and Judit K. Sandorne
Evidence for post-depositional diffusional loss of hydrogen in quartz phenocryst fragments within ignimbrites
American Mineralogist (June 2017) 102 (6): 1187-1201


Ignimbrite-hosted quartz phenocryst fragments contain much lower hydroxyl defect concentration than quartz in igneous rocks. Pre-eruptive and post-depositional loss of hydrogen were hypothesized as the main processes for lowering the initial magmatic concentrations of hydroxyl defects. The aim of this study was to examine the hydroxyl defect concentration of quartz phenocryst fragments from various vertical positions above the base of pyroclastic density current (PDC) deposits. It aims to record the vertical variations of hydroxyl defect concentrations to have an insight into potential post-depositional hydrogen loss of PDC deposits. Ignimbrite-hosted quartz phenocryst fragments were examined from two different ignimbrites in the Bukk Foreland Volcanic Area (North Hungary). Unpolarized micro-FTIR measurements on 23-35 unoriented crystal fragments from each sample were performed representing four different vertical positions of each site. Present results imply that hydroxyl defect concentrations show a pronounced decrease upward from the base of the deposits. The initial approximately 12 ppm hydroxyl defect concentration decreases to <3 ppm within <10 m from the base. Ignimbrites with contrasting degree of welding are characterized by different hydroxyl defect concentrations of quartz phenocryst fragments at the same height above the base. Thus, post-depositional dehydration is supposed to be the main factor causing the observed vertical decreasing trend. The modeling of post-depositional dehydration by considering typical ignimbrite emplacement temperatures (300-700 degrees C) and thicknesses (20-50 m) revealed that neither different cooling rates or different crystal diameters could cause the observed decrease in hydroxyl defect concentrations in ignimbrites. Other factors, such as contrasting pre-depositional thermal history, presence of melt- and fluid inclusion, and crack density of crystals could also play an important role in affecting the final hydroxyl defect concentrations.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 6
Title: Evidence for post-depositional diffusional loss of hydrogen in quartz phenocryst fragments within ignimbrites
Affiliation: Eotvos University, Institute of Geography and Earth Sciences, Budapest, Hungary
Pages: 1187-1201
Published: 201706
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 77
Accession Number: 2017-052112
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
N45°45'00" - N48°30'00", E16°10'00" - E23°00'00"
Secondary Affiliation: Geological and Geophysical Institute of Hungary, HUN, HungaryUniversity of Innsbruck, AUT, AustriaHungarian Institute for Forensic Sciences, HUN, Hungary
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201728
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