Compositional Features of Enriched Icelandic Mantle Components

We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank-zone regions in Iceland (Vestmannaeyjar in the south, Snaefell in the east, and Snaefellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr-Nd-Pb-Hf), stable isotope data (delta O-18), and trace element data to characterize the compositional features of the trace element enriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones (including Oraefajokull in the SE) and the Northern and Eastern rift zones. Most Icelandic lavas have negative delta Pb-207 (-4 to -1), with higher values (-1 to +4) found only in samples from Oraefajokull, Snaefell, and parts of the Reykjanes Peninsula. At Snaefell, this EM1-type component is characterized by a low delta O-18(olivine) signature (+4 center dot 1 parts per thousand to +4 center dot 6 parts per thousand), moderate Pb-206/Pb-204 values (18 center dot 4-18 center dot 6) and mid-ocean ridge basalt (MORB)-like He-3/He-4 (6 center dot 9-7 center dot 5 R/R-A). Samples from Vestmannaeyjar and Snaefellsnes have mantle-like delta O-18(olivine) (+4 center dot 9 parts per thousand to +5 center dot 0 parts per thousand), and radiogenic Pb-206/Pb-204 values (18 center dot 9-19 center dot 3) that fall on the Northern Hemisphere Reference Line for Pb-208/Pb-204 (delta Pb-208 -5 to +5). Compared with the Vestmannaeyjar lavas, Snaefellsnes lavas have higher La/Yb-N (5-11 vs 3-5), lower epsilon(Nd) (5 center dot 5-6 center dot 5 vs 6 center dot 8-7 center dot 6) and lower He-3/He-4 (6 center dot 3-8 center dot 6 R/R-A vs 11 center dot 4-13 center dot 5 R/R-A). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high He-3/He-4 values (> 15 R/R-A) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high delta Pb-207 samples, high-precision Pb isotope data produce a kinked array on a Pb-206/Pb-204 vs Pb-208/Pb-204 plot, which is not consistent with simple binary mixing between two end-members. This requires significant lateral heterogeneity within the Icelandic mantle and the presence of more than just two compositionally distinct local mixing end-member components. Samples from each of the main axial rift zones define different trends. Despite the tectonic continuity between the Northern Volcanic Zone and the Eastern Volcanic Zone, lavas from these two rift zones define separate sub-parallel linear arrays. Lavas from the adjacent Western Volcanic Zone and the Eastern Volcanic Zone define oblique linear arrays that converge on a common local end-member that is not involved in the magmatism of the Northern Volcanic Zone. Therefore, there is a distinct NE-SW compositional heterogeneity within the Icelandic mantle.