HR: 1340h
AN: V43E-1462    [Abstracts]
TI: H$_{2}$O and Hydrogen Isotopic Study of Glasses from Subglacial Volcanoes in the Neovolcanic Zone of Iceland
AU: * Peterson, A H
EM: ahp@uwm.edu
AF: Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201-0413 United States
AU: Cameron, B I
EM: bcameron@uwm.edu
AF: Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201-0413 United States
AB: Worldwide interest in subglacial volcanism was rekindled by the spectacular 1996 Gjalp eruption in Iceland. In Iceland, the flat-topped volcanoes that form beneath glaciers are called table mountains or tuyas. Sections of the basaltic subglacial volcanoes in Iceland reveal a crude stratigraphic progression from pillow lavas to hyaloclastite deposits from the base upward. Locally the sections are capped by subaerial basaltic lava flows. Based on classic models, the glassy pillow basalts and overlying hyaloclastite deposits indicate eruption of basalt within melt water caverns of increasingly shallow depths. Capping subaerial flows would record the emergence of lava above the ice sheet. Large thicknesses of ice and melt water overlying the basaltic volcanoes significantly impede the release of volatiles from the ascending magma. As degassing and ascent rate are viewed as the critical variables controlling eruptive style, H$_{2}$O contents and hydrogen isotopic analyses of glasses were conducted in order to improve our understanding of degassing processes at subglacial volcanoes. These data may also help to refine current models of subglacial basaltic eruptions. Consequently, samples of glassy pillow basalts, pillow breccias, and hyaloclastites were collected at Hlodufell, Raudafell, Efstadalsfjall, and Geitafell. Major element compositions of the glasses measured by electron microprobe reveal uniform basaltic compositions with relatively primitive MgO values ranging only from 7.37-7.68 wt%. The glass compositions show both tholeiitic and alkalic affinities on an alkalis-silica classification diagram. In order to definitively track degassing at these subglacial volcanoes, pillow and hyaloclastic glasses were analyzed for their H$_{2}$O concentration and hydrogen isotopic composition. Glasses from the Raudafell section have H$_{2}$O contents ranging from 0.21-1.24 wt% and bulk $\delta$D values from -87.2 to -105.7$\permil$. Vapor saturation pressures calculated from these dissolved H$_{2}$O contents range from 5 to 151 bars assuming that associated CO$_{2}$ contents are zero. This range in pressure corresponds to an ice thickness of 57-1710 m based on an ice density of 0.9 g cm$^{-3}$. Step heating hydrogen isotopic analyses were undertaken to evaluate the nature of the H$_{2}$O in the basaltic glasses. A Raudafell sample released 43% of its water at temperature steps of 200 and 400$\deg$C indicating a meteoric origin, whereas the remaining 57% of the H$_{2}$O liberated during the fusion step is likely magmatic in origin. Accurate determination of the magmatic H$_{2}$O component would reduce the estimation of ice thickness above these subglacial volcanoes.
DE: 8414 Eruption mechanisms
DE: 8499 General or miscellaneous
DE: 3655 Major element composition
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting