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 ¹²⁹Xe_I/¹³⁶Xe_Pu 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.