Earth’s early differentiation recorded by halogen abundance ratios in Hawaiian lavas

Abstract

Halogens (F, Cl, Br, I) and radiogenic isotopes were investigated in a suite of submarine basalt glasses from across the Hawaiian archipelago in order to better understand the origin of xenon isotope anomalies related to extinct 129I and the nature of metasomatic agents involved in peripheral Arch magmatism. We found that Lō’ihi tholeiites with high 3He/4He of up to 26 Ra (where Ra denotes the atmospheric 3He/4He) are characterised by unusually low I/Cl of (27 ± 3) × 10−6, whereas peripheral alkaline lavas from the South Arch that have 3He/4He of up to 18 Ra are characterised by unusually high I/Cl of (120 ± 20) × 10−6. In comparison, rejuvenated lavas with 3He/4He of ∼ 8 Ra have I/Cl in the range of (60 ± 30) × 10−6 that is typical of mid-ocean ridge and ocean island basalt lavas elsewhere. The variations in I/Cl, 3He/4He and radiogenic isotopes are consistent with mixing between three mantle end-members. The most primitive end-member with high 3He/4He and low 129XeI/136XePu has low I/Cl but normal Br/Cl, H2O/Cl and F/Cl ratios. Compared to the primitive mantle it is strongly depleted in I and weakly depleted in the other volatiles F, Cl, Br and H2O. The depletion of I (and 129Xe derived from extinct 129I) probably results from a combination of Earth’s heterogenous accretion and preferential partitioning of I into the Earth’s core. The high I/Cl of the South Arch lavas can be explained by input of low degree carbonatitic melts derived from carbonated eclogite in the plume source. This is implied because carbonated ocean crust is uniquely enriched in I. The involvement of carbonatitic melts is consistent with previously reported H2O, Ba and light rare earth element enrichments of South Arch lavas. Taken together the halogen abundance ratios in lavas sourced from around Hawaii record Earth’s early accretion and differentiation, the subsequent subduction of carbonated ocean crust into the mantle and its mobilisation in minor carbonatitic components enriching the periphery of the plume.