HR: 0800h
AN: V41C-1464    [Abstracts]
TI: Isotopic Composition of Lithium in Volcanic Rocks From the Northern Icelandic Rift Zone
AU: * Hansen, H
EM: hansen@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 101 Iceland
AU: Gronvold, K
EM: karlgr@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 101 Iceland
AU: Kosler, J
EM: jan.kosler@geo.uib.no
AF: University of Bergen, Department of Earth Science, Allegt. 41, Bergen, 5007 Norway
AU: Pedersen, R B
EM: rolf.pedersen@geo.uib.no
AF: University of Bergen, Department of Earth Science, Allegt. 41, Bergen, 5007 Norway
AB: Isotopic composition of Li (δ⁷Li) can potentially be used to trace different mantle sources, including recycled crustal material. To test the variability in Icelandic volcanic rocks, samples from Theistareykir and Krafla in the northern part of the Icelandic rift zone were used in this study. The samples cover a range from primitive olivine-tholeiites to highly evolved rhyolites. The Theistareykir rift segment includes a number of primitive (high MgO) early post glacial lavas erupted over less than three thousand years. Minimal crustal influence is expected from thin crust (< 20 km) and the primitive nature of the magmas. The Krafla central volcano has a significantly greater compositional range and displays a suite of rock types from olivine tholeiite through quartz tholeiite and intermediate compositions to rhyolite. The studied samples range in age from hundred thousand years to recent. Krafla has a thicker and more complicated crust. Li concentration and δ⁷Li were studied in 27 whole rock samples. Li concentration varies regularly with evolution from 3.3 ppm, in the most primitive volcanics, to 32.9 ppm in the rhyolites. Theistareykir primitive samples have δ⁷Li-values between +0.8 and +4.3 ‰, while the Krafla samples show a significantly larger range from -0.8 to +6.3 ‰. Rhyolites and dacites have δ⁷Li-values from 2.6 to 6.3 ‰, coinciding with the higher basaltic values. There is no correlation between δ⁷Li and Li concentration. MgO-content varies considerably between the two studied areas, with Krafla showing a wider range than Theistareykir (0.04 - 9.73 wt% and 7.31 - 12.40 wt%, respectively). Li concentration correlates with evolution (decreasing MgO-content), which suggests that Li behaves incompatibly during melting and crystallization processes. There is no obvious correlation between δ⁷Li and other isotopic ratios measured so far. The rocks from Theistareykir and Krafla are known to have variable Sr, Nd, Hf, Pb and δ¹⁸O isotopic ratios. The samples are all fresh, so these ratios and the new δ⁷Li-data represent magmatic values. The range of δ⁷Li observed in the basalts should therefore represent Icelandic mantle values. Since Li is unlikely to fractionate in high temperature processes, the variations observed most likely represents earlier incorporation of crustal material into the mantle.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005