HR: 1330h
AN: V42B-0367    [PDF]
TI: Degassing at Basaltic Tuyas in northern British Columbia: Insights from the Hydrogen Isotopic Composition of Glasses and Whole Rocks
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
AU: * Boscov-Parfitt, S
EM: Boscovp2@uwm.edu
AF: Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201-0413 United States
AB: Interest in subglacial volcanism was rekindled by the spectacular 1996 Gjalp eruption in Iceland. Northern British Columbia, Canada, hosts numerous flat-topped subglacial volcanoes called tuyas. Ash Mountain, South Tuya, and Three Caribou Tuya are three examples of basaltic volcanoes that began eruptive activity under glacial ice during the Fraser glaciation (25 to 10 ka). Sections of the three superbly exposed tuyas reveal a consistent 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 eruptive models, the glassy pillow basalts and overlying hyaloclastite deposits indicate eruption of basalt within melt water caverns of increasingly shallow depths. Capping subaerial flows record the emergence of lava above the ice sheet. Large thicknesses of ice and melt water overlying the basaltic volcanoes would significantly impede the release of volatiles from the ascending magma. Degassing and ascent rates of magma are thought to exert control on eruptive style. Despite hydrogen isotopes being sensitive measures of degassing, to our knowledge no hydrogen isotope data has been collected on glassy subglacial basaltic rocks. H$_{2}$O contents and hydrogen isotopic analyses of glasses from the pillow basalts and hyaloclastite deposits should improve our understanding of degassing processes at these classic tuyas and help refine current models of subglacial basaltic eruptions. Consequently, samples of glassy pillow basalts and hyaloclastites along with crystalline basalt flows were collected at Ash Mountain, South Tuya, and Three Caribou Tuya in northern British Columbia. XRF major and trace element analyses of glassy and crystalline basalts reveal relatively primitive compositions with MgO values ranging from 5.52-10.07 wt%. Whereas Ash Mountain samples are largely tholeiitic, South Tuya and Three Caribou Tuya basalts show more alkalic affinities. In order to definitively track degassing at these subglacial volcanoes, glasses and whole rock basalts were analyzed for their H$_{2}$O concentration and hydrogen isotopic composition. At Ash Mountain, glasses from pillow basalts have higher water contents (0.54-0.66 wt%) and less depleted $\delta$D values (-87.2 to -102.0$\permil$) than associated hyaloclastite deposits (0.43-0.48 wt% H$_{2}$O and $\delta$D from -112.5 to -122.6$\permil$). Hyaloclastite glasses from South Tuya have H$_{2}$O contents similar to the Ash Mountain pillow glasses, but $\delta$D values resembling Ash Mountain hyaloclastites. Bulk rock basalts have even lower H$_{2}$O contents than either the pillow glasses or hyaloclastite deposits and exhibit extreme depletions in $\delta$D (i.e., less than -140$\permil$). These preliminary data support eruptive models that propose pillow basalts erupted in deeper water than hyaloclastite deposits. Moreover, the H$_{2}$O data can be used to constrain the thickness of ice overlying the basalts at the time of the eruption.
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting