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

Abstract

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 presented. The samples are mainly dykes and were formed by the proto-Icelandic mantle plume during the final phase of continental rifting of the NE Atlantic. They may be grouped into four main types: (1) Low-Ti picritic and olivine basaltic dykes (MgO>10%) with N-type MORB incompatible element patterns and isotopic compositions with La/Sm cho=0.4–0.8, unfractionated HREEs, ϵ Sr=−31 to −28, ϵ Nd=+11.8 to +9.8 and 206Pb/ 204Pb=17.9–18.4. (2) Low-Ti basaltic dykes (MgO<10%) also with unfractionated HREEs but trending towards more enriched compositions with e.g. higher La/Sm and ϵ Sr and lower 206Pb/ 204Pb. (3) High-Ti magnesian basalt lavas with isotopic compositions like some Atlantic MORBs but enriched in incompatible elements, La/Sm cho≈1.2, and fractionated HREEs (Sm/Yb cho≈2.4). (4) High-Ti basalt dykes are relatively enriched in the highly incompatible trace elements with La/Sm cho=1.3–1.8, and relatively high Sm/Yb cho=2.0–2.7; the main group clusters around ϵ Sr=−19, ϵ Nd=+7.4, 206Pb/ 204Pb=18.1, 207Pb/ 204Pb=15.37 and 208Pb/ 204Pb=37.9, which is lower for 207Pb/ 204Pb for any 206Pb/ 204Pb or 208Pb/ 204Pb 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 the high-Ti melts may be modelled by low-degree melting (2–4%) at the transition between garnet-bearing and garnet-free mantle beneath around 85 km of lithosphere. The eruption of the high-degree low-Ti melts with MgO=ca. 17 wt.% through continental lithosphere indicates that also the depleted mantle was considerably hotter than the ambient asthenosphere. Contamination is important in the evolved low-Ti basalts, which can be modelled to have assimilated Precambrian amphibolite facies gneisses. The other samples are inferred to closely represent mantle-derived compositions, although minor contamination cannot be totally ruled out. The low-Ti magmas were 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 Icelandic samples with the most radiogenic Pb. The cluster of basaltic high-Ti dykes, representing the major part of Faroe Island magmas, are believed to be derived from the main mantle plume component beneath the Faroes in the Paleogene. 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 also corroborated by the common occurrence of the High 206Pb component in Iceland magmas. We envisage a rising plume mainly consisting of Paleogene Icelandic plume mantle and hot depleted mantle. While the former produced melts throughout Paleogene igneous activity of the Faroe Islands, the latter only yielded melts 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.