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 (δ 18 O), and trace element data to characterise the compositional features of the trace-elementenriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones (including Öraefajökull in the southeast) and the Northern and Eastern rift zones. Most Icelandic lavas have negative ∆ 207 Pb (-4 to-1), with higher values (-1 to +4) found only in samples from Öraefajökull, Snaefell, and parts of the Reykjanes Peninsula. At Snaefell, this EM1-type component is characterised by a low δ 18 O olivine signature (+4.1‰ to +4.6‰), moderate 206 Pb/ 204 Pb values (18.4-18.6) and MORB-like 3 He/ 4 He (6.9-7.5 R/R A). Samples from Vestmannaeyjar and Snaefellsnes have mantle-like δ 18 O olivine (+4.9‰ to +5.0‰), and radiogenic 206 Pb/ 204 Pb values (18.9-19.3) that fall on the NHRL for 208 Pb/ 204 Pb (∆ 208 Pb-5 to +5). Compared to the Vestmannaeyjar lavas, Snaefellsnes lavas have higher La/Yb N (5-11 vs. 3-5), lower ε Nd (5.5-6.5 vs. 6.8-7.6) and lower 3 He/ 4 He (6.3-8.6 R/R A vs. 11.4-13.5 R/R A). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3 He/ 4 He 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 ∆ 207 Pb samples, high-precision Pb isotope data produce a kinked array on an 206 Pb/ 204 Pb vs. 208 Pb/ 204 Pb 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.

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Rare high-3He/4He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-3He/4He OIB exhibit anomalous 182W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high 3He/4He. However, it is not understood why some OIB host anomalous 182W while others do not. We provide geochemical data for the highest-3He/4He lavas from Iceland (up to 42.9 times atmospheric) with anomalous 182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high 3He/4He and anomalous 182W. These data, together with data on global OIB, show that the highest-3He/4He and the largest-magnitude 182W anomalies are found only in geochemically depleted mantle domains—with high 143Nd/144Nd and low 206Pb/204Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low 3He/4He and lack anomalous 182W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-3He/4He mantle domains with anomalous 182W have low W and 4He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low 3He/4He and normal (not anomalous) 182W characteristic of subducted crust. Thus, high 3He/4He and anomalous 182W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high 3He/4He and anomalous 182W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.