Compositional Characteristics and Spatial Distribution of Enriched Icelandic Mantle Components

Abstract

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 (δ18O), 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 Δ207Pb (–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 δ18Oolivine signature (+4·1‰ to +4·6‰), moderate 206Pb/204Pb values (18·4–18·6) and mid-ocean ridge basalt (MORB)-like 3He/4He (6·9–7·5 R/RA). Samples from Vestmannaeyjar and Snaefellsnes have mantle-like δ18Oolivine (+4·9‰ to +5·0‰), and radiogenic 206Pb/204Pb values (18·9–19·3) that fall on the Northern Hemisphere Reference Line for 208Pb/204Pb (Δ208Pb –5 to +5). Compared with the Vestmannaeyjar lavas, Snaefellsnes lavas have higher La/YbN (5–11 vs 3–5), lower eNd (5·5–6·5 vs 6·8–7·6) and lower 3He/4He (6·3–8·6 R/RA vs 11·4–13·5 R/RA). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3He/4He values (>15 R/RA) 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 Δ207Pb samples, high-precision Pb isotope data produce a kinked array on a 206Pb/204Pb vs 208Pb/204Pb 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.