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Halogens in chondritic meteorites and terrestrial accretion
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Halogens in chondritic meteorites and terrestrial accretion
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Journal Article
Halogens in chondritic meteorites and terrestrial accretion
2017
Overview
Halogen abundances in chondrites are 6 to 37 times lower than previously reported, which is consistent with the low abundances of these elements found in Earth.
An Earthly halogen history
The heavy halogens chlorine (Cl), bromine (Br) and iodine (I) are key tracers of accretion during the formation of Earth owing to their high volatility and incompatibility. Patricia Clay and co-authors show that the abundances of these heavy halogens in carbonaceous, enstatite, Rumuruti and ordinary chondrites are much lower than reported previously. The authors also find that the Br/Cl and I/Cl ratios in all the chondrites studied show a limited range, indistinguishable from bulk silicate Earth estimates. This indicates that the depletion of halogens relative to primitive meteorites is consistent with lithophile elements of similar volatility. They conclude that the observed terrestrial halogen inventories cannot be explained by late accretion alone, but also require the efficient extraction of halogen-rich fluids from the solid Earth during the earliest stages of its formation.
Volatile element delivery and retention played a fundamental part in Earth’s formation and subsequent chemical differentiation. The heavy halogens—chlorine (Cl), bromine (Br) and iodine (I)—are key tracers of accretionary processes owing to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. However, noble gas proxy isotopes produced during neutron irradiation provide a high-sensitivity tool for the determination of heavy halogen abundances. Using such isotopes, here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites are about 6 times, 9 times and 15–37 times lower, respectively, than previously reported and usually accepted estimates
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. This is independent of the oxidation state or petrological type of the chondrites. The ratios Br/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth estimates. Our results demonstrate that the halogen depletion of bulk silicate Earth relative to primitive meteorites is consistent with the depletion of lithophile elements of similar volatility. These results for carbonaceous chondrites reveal that late accretion, constrained to a maximum of 0.5 ± 0.2 per cent of Earth’s silicate mass
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, cannot solely account for present-day terrestrial halogen inventories
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. It is estimated that 80–90 per cent of heavy halogens are concentrated in Earth’s surface reservoirs
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and have not undergone the extreme early loss observed in atmosphere-forming elements
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. Therefore, in addition to late-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth’s dissolved mantle gases
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, the efficient extraction of halogen-rich fluids
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from the solid Earth during the earliest stages of terrestrial differentiation is also required to explain the presence of these heavy halogens at the surface. The hydropilic nature of halogens, whereby they track with water, supports this requirement, and is consistent with volatile-rich or water-rich late-stage terrestrial accretion
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