Geochemical and Pb-Sr-Nd isotopic evidence for separate hot depleted and Iceland plume mantle sources for the Paleogene basalts of the Faroe Islands

New trace element and Sr, Nd and Pb isotopic analyses on 43 tholeiites from the younger part of the early Paleogene Faroe Islands basalt plateau are p resented. The samples are mainly dykes and were formed by the proto-Iceland ic mantle plume during the final phase of continental rifting of the NE Atl antic. They may be grouped into four main types: (1) Low-Ti picritic and ol ivine basaltic dykes (MgO > 10%) with N-type MORE incompatible element patt erns and isotopic compositions with La/Sm-cho = 0,4-0.8, unfractionated HRE Es, epsilon (Sr) = -31 to -28, epsilon (Nd) = +11.8 to +9.8 and Pb-206/Pb-2 04 = 17.9-18.4. (2) Low-Ti basaltic dykes (MgO < 10%) also with unfractiona ted HREEs but trending towards more enriched compositions with e.g. higher La/Sm and epsilon (Sr) and lower Pb-206/Pb-204. (3) High-Ti magnesian basal t lavas with isotopic compositions like some Atlantic MORBs but enriched in incompatible elements, La/Sm-cho approximate to 1.2, and fractionated HREE s (Sm/Yb-cho = 2.4). (4) High-Ti basalt dykes are relatively enriched in th e highly incompatible trace elements with La/Sm-cho = 1.3-1.8, and relative ly high Sm/Yb-cho = 2.0-2.7; the main group clusters around epsilon (Sr) = - 19, epsilon (Nd) = +7.4, Pb-206/Pb-204 = 18.1, (207)pb/(204)pb = 15.37 an d Pb-208/Pb-204 = 37.9, which is lower for Pb-207/Pb-204 for any Pb-206/Pb- 204 or Pb-208/Pb-204 than any composition on Iceland. Only a few dykes have isotopic compositions intermediate between the high-Ti and low-Ti groups. High-degree melting (ca. 20%) is indicated for the low-Ti melts, whereas th e high-Ti melts may be modelled by low-degree melting (2-4%) at the transit ion between garnet-bearing and garnet-free mantle beneath around 85 km of l ithosphere. The eruption of the high-degree low-Ti melts with MgO = ca. 17 wt.% through continental lithosphere indicates that also the depleted mantl e was considerably hotter than the ambient asthenosphere. Contamination is important in the evolved low-Ti basalts, which can be mode lled to have assimilated Precambrian amphibolite facies gneisses. The other samples are inferred to closely represent mantle-derived compositions, alt hough minor contamination cannot be totally ruled out. The low-Ti magmas we re derived from an isotopically well defined depleted source, which we term the Faroe depleted component. The high-Ti magnesian lavas are modelled as derived from mixtures of this source and a source like that of the Icelandi c samples with the most radiogenic Ph. The cluster of basaltic high-Ti dyke s, representing the major part of Faroe Island magmas, are believed to be d erived from the main mantle plume component beneath the Faroes in the Paleo gene. This is either not available or has not yielded pure melts in Iceland , but indicates a significant change in the Iceland plume with time, as als o corroborated by the common occurrence of the High Pb-206 component in Ice land magmas. We envisage a rising plume mainly consisting of Paleogene Icelandic plume m antle and hot depleted mantle. While the former produced melts throughout P aleogene igneous activity of the Faroe Islands, the latter only yielded mel ts after the lithosphere was significantly and rapidly thinned at the onset of break-up. The Faroe magmas demonstrate that composite mantle plumes may rise virtually without the individual components being mixed. (C) 2001 Els evier Science B.V. All rights reserved.