Whole-rock Fe isotope compositions as mantle source and process tracers across the Hawaiian archipelago

We present a geographically extensive Hawaiian Fe isotope dataset, with the primary goal of testing the use of stable Fe isotopes to identify mantle heterogeneities in the source of Hawaiian volcanoes, including whether the isotopically distinct Hawaiian geochemical groups (Loa, Kea, and Enriched Loa) exhibit distinct Fe isotope compositions. Hawai‘i is an excellent natural laboratory for assessing newer isotopic systems as it is well-studied and thoroughly geochemically characterized. We analyzed 43 carefully selected shield-stage tholeiitic basalt samples from 14 subaerial and submarine volcanoes that represent the past ∼ 5 Ma of volcanic activity along the Hawaiian chain. These samples have previously been thoroughly characterized for their major and trace element concentrations, and Pb, Sr, Nd, Hf, and Tl isotope compositions. Their compositions span the entire range of Kea, Loa and Enriched Loa Hawaiian geochemical groups. Iron isotope results reported to IRMM-14 vary from a δ 56 Fe (±2SD) of 0.00 (±0.06 ‰) to 0.21 (±0.02 ‰). The data adjusted for olivine fractionation show a similar range, and the mean δ 56 Fe of the new data (0.10 ± 0.10 ‰) overlaps with the average MORB value (∼0.10 ‰). This work provides the first comprehensive, large-scale overview of Fe isotope compositions in shield-stage tholeiitic basalt across the Hawaiian archipelago. Important observations include: 1) there is no statistical difference in δ 56 Fe values between Loa, Kea, and Enriched Loa geochemical groups; 2) δ 56 Fe and lithophile radiogenic isotope ratios (Pb, Sr, Nd, Hf) show no correlations; and 3) δ 56 Fe and ε 205 Tl (used to track the presence of recycled pelagic sediments in Hawaiian basalts) are not correlated. Hawaiian δ 56 Fe values are slightly lower than those from other enriched oceanic islands (EM-I, EM-II, HIMU), including Pitcairn, Azores, and Samoa, which is a feature also observed in Iceland. On a broader scale, these observations suggest that the mantle heterogeneities contributing to the unique radiogenic isotopic signatures in Hawaiian lavas, which are interpreted as being linked to the Pacific Large Low Shear Velocity Province, do not exhibit distinct or heavy Fe isotope compositions. Alternatively, if the deep mantle reservoirs sampled by the Hawaiian plume have distinct Fe isotope compositions, these signatures are not preserved during the ascent through the mantle, melting, and eruption at the surface.