HR: 1330h
AN: V32A-1000    [PDF]
TI: Age and Geochemical Data From the Madeira-Tore Rise and Surrounding Seamounts: New Insights Into East Atlantic Volcanism
AU: * Geldmacher, J
EM: jgeldmacher@geomar.de
AF: Geomar Forschungszentrum, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: Hoernle, K A
EM: khoernle@geomar.de
AF: Geomar Forschungszentrum, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: Kluegel, A
EM: akluegel@uni-bremen.de
AF: Dept. of Geosciences, Universitaet Bremen, Bremen, 28334 Germany
AU: van den Bogaard, P
EM: pbogaard@geomar.de
AF: Geomar Forschungszentrum, Wischhofstrasse 1-3, Kiel, 24148 Germany
AB: Located off the NW African continental margin is a $>$3000 km long belt of volcanic archipelagoes (e.g. Canary, Madeira Islands) and large seamounts including the 900 km long Madeira-Tore Rise (MTR). The cause of the East Atlantic volcanism, and in particular the origin and age of the MTR, which is composed of a widespread plateau with several seamount groups, is controversial. Proposed models include an origin of the MTR at the Mid Atlantic Ridge, formation over a hotspot, or as a product of diffuse small-scale mantle convection. All recently dredged volcanic samples (RV Meteor cruise M51/1) from the MTR and nearby off-rise seamounts exhibit enriched Ocean Island Basalt-like incompatible trace element signatures similar to HIMU (high time integrated $^{238}$U/$^{204}$Pb) ocean islands. Their isotope compositions are similar to those from Madeira with $^{87}$Sr/$^{86}$Sr and $^{143}$Nd/$^{144}$Nd overlapping Atlantic N-MORB to a large extent, but Pb isotope ratios plotting well below the Northern Hemisphere Reference Line and extending to $^{206}$Pb/$^{204}$Pb values of 19.90. Our preliminary data confirm the existence of at least two distinct isotopic domains in the eastern North Atlantic: 1) a Madeira-like domain (as characterized above) stretching from Madeira Island along the MTR to the NE as far as the Azores Gibraltar fracture zone, and 2) a Canary-like domain with Sr, Nd, Pb isotope ratios intermediate between N-MORB and HIMU ($^{206}$Pb/$^{204}$Pb = 19.0-20.2; $^{207}$Pb/$^{204}$Pb = 15.54-15.66) but with lower $^{143}$Nd/$^{144}$Nd ratios ($<$0.5130) than the Madeira domain. We interpret the distinct compositions of these domains to reflect spatial zonation in the deeper sources of the mantle upwellings. The domains include zones influenced by Enriched Mantle (EM) I and II, which appear to be orientated along the extension of the Oceanographer (at $\sim$ 34$\deg$N) and the Azores-Gibraltar fracture zones (at $\sim$ 37$\deg$N), respectively. The first zone includes a hitherto unknown seamount that exhibits the most extreme EM I composition yet discovered in the North Atlantic ($^{206}$Pb/$^{204}$Pb = 17.72, $^{143}$Nd/$^{144}$Nd = 0.5124). If the association with the fracture zones can be confirmed, a shallow source for the EM I and II signatures can be assumed. One explanation would be that splinters of subcontinental lithosphere or continental crust have been incorporated into the oceanic upper mantle during the break up of Pangaea and subsequently trapped along the fracture zones. $^{40}$Ar/$^{39}$Ar age determinations on MTR lavas and off-rise seamounts (currently underway) will help in constructing a geodynamic model for the East Atlantic volcanism.
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting