Halogens in chondritic meteorites and terrestrial accretion

Volatile element delivery and retention played a fundamental role in Earth’s formation and subsequent chemical differentiation. The heavy halogens (Cl, Br and I) are key tracers of accretionary processes due to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. Noble gas proxy isotopes produced during neutron irradiation provide a high sensitivity tool for the determination of heavy halogen abundance. Here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites have concentrations ~6, ~9 and between 15-37 times lower, respectively, than previously reported and most commonly accepted estimates1. This is independent of the chondrites’ oxidation state or petrological type. Bromine/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth (BSE) estimates. Our results demonstrate that BSE depletion of halogens relative to primitive meteorites is now consistent with lithophile elements of similar volatility. The new results for carbonaceous chondrites demonstrate that late accretion, constrained to a maximum of 0.5 ± 0.2 % of Earth’s silicate mass2–5, cannot solely account for present-day terrestrial halogen inventories6,7. It is estimated that 80−90% of heavy halogens are concentrated in Earth’s surface reservoirs7,8 and have not undergone the extreme early loss observed in atmosphere-forming elements9. Therefore, in addition to late accretion of halogens and mantle degassing, which is <50% efficient over Earth history10, efficient extraction of halogen-rich fluids6 from the solid Earth during the earliest stages of Earth formation is also required. The hydrophilic nature of the halogens supports this requirement, and is consistent with a volatile/water rich late-stage terrestrial accretion</p