HR: 11:55h
AN: V22A-07    [Abstracts]
TI: Melt Extraction Versus Melt Stagnation Along the Ultraslow Spreading Lena Trough, Arctic Ocean
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: * Snow, J E
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: von der Handt, A
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Feig, S
AF: Inst. fuer Mineralogie, Callinstr 3, Hannover, 30167 Germany
AU: Gao, Y
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Jovanovic, Z
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Nauret, F
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AB: During cruise ARK XX-2 of RV Polarstern in summer 2004, 2400 kg of basement rocks were recovered by 23 dredge hauls along the oblique spreading Lena Trough. This 350 km long linear deep links the orthogonal spreading Gakkel Ridge in the north to the Knipovich Ridge in the south, and was until now barely explored. Lena Trough can be divided into three clearly defined segments: (1) a northern basalt-dominated segment; (2) a central 140 km long basalt-free, peridotitic segment; and (3) a southern basalt-bearing segment. Exclusive basalt recovery in the four northernmost axial Lena Trough dredges suggest that the change in spreading geometry from magmatically robust orthogonal spreading at the western Gakkel Ridge to oblique near-continental spreading at Lena Trough did not lead to an abrupt cessation in partial melting underneath northernmost Lena Trough. However, eruptive magmatism in northern Lena Trough may not have been active for a long time, since an off-axis dredge haul on the western flank at 82.8$\deg$ N contained only peridotites. Central Lena Trough is completely devoid of basalts, and is therefore the longest amagmatic spreading segment globally. Conventionally, the absence of basalt and exclusive exposure of mantle rocks directly translates to amagmatic extension. However, some partial melting may have occurred underneath central Lena Trough. Instead of being extracted from the mantle, melt stagnated and is now present as disseminated plagioclase and crosscutting gabbroic veins, as observed in 20-30% of all abyssal peridotites at elsewhere along the Gakkel Ridge, SWIR and CIR. The textures and mineral compositions of plag-peridotites can only be explained by melt entrapment. In addition, plag-peridotites are virtually absent in the southern- and northernmost dredge hauls and their abundance gradually increases to 100% in central Lena Trough. Melt generation occurred along the entire length of Lena Trough. In northermost Lena Trough, these melts were extracted from the mantle and formed a (thin) basaltic layer. In central Lena Trough, melts were formed in the mantle, but never managed to focus, pool and erupt. The reason for this may be an increase in lithospheric thickness from the Gakkel Ridge to Lena Trough intersection towards the south. In central Lena Trough, the lithosphere may be exceptionally thick resulting from the conductively cooling influences of the extreme obliquity, the ultraslow spreading and the vicinity to continental margin of Greenland and Svalbard.
DE: 9315 Arctic region
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting