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 estimates1. 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 mass2,3,4,5, cannot solely account for present-day terrestrial halogen inventories6,7. It is estimated that 80–90 per cent 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-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth’s dissolved mantle gases10, the efficient extraction of halogen-rich fluids6 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 accretion5,11,12,13,14.
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We acknowledge the following organisations and individuals for the provision of samples: NASA Antarctic Meteorite Working Group (MIL 07139, MIL 07028 and ALH 77295); P. Heck, Chicago Field Museum (Indarch); Izikio Museum of South Africa (St Marks); M. Boyet (SAH 97096); M. Schönbächler (GRA 06100, EET 92159, Murray, Orgueil); and A. Ruzicka (NWA 753 and NWA 755). We thank J. Cowpe for assistance with noble gas measurements, K. J. Theis for laboratory assistance and W. Akram for help with sample preparation. H.B. acknowledges support from the PlanetS National Center of Competence in Research (NCCR) of the Swiss National Science Foundation. This project was funded by the European Research Council FP7 ‘NOBLE’ grant number 267692.
The authors declare no competing financial interests.
Reviewer Information Nature thanks H. Becker and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Histograms of chlorine (a), bromine (b) and iodine (c), showing the distribution of halogens in CI chondrites (masses as shown), as reported in Supplementary Table 1. Data for Orgueil from this study are shown in each panel by the black line, with uncertainty shown in grey (for iodine, this is within the thickness of the line). NAA, neutron activation analysis, INAA, instrumental neutron activation analysis, CHEM, chemical methods, GSIRMS, gas source isotope ratio mass spectrometry.
Extended Data Figure 2 A comparison of meteorite find environments can be used as a diagnostic tool to assess terrestrial contamination.
Chlorine concentrations in hot desert, cold desert and non-desert meteorites analysed in this study.
Extended Data Figure 3 Comparison of chlorine in meteorite finds and falls can be used to assess terrestrial contamination.
Chlorine is expected to be higher in finds than in falls, where contamination has occurred, owing to high relative Cl in the terrestrial environment. However, not much difference is observed between falls and finds, illustrating that the samples are not strongly contaminated, and some of the highest concentrations are present in the falls. We consider this to reflect variations in the amounts of halogen carrier phases present, rather than resulting from terrestrial input.
Extended Data Figure 4 Br/Cl and I/Cl ratios in some known terrestrial contaminants as indicators of terrestrial contamination.
Sample halogen ratios are shown in the context of some known terrestrial contaminants, marine aerosol, ice and atmospheric particles from McMurdo and the South Pole. The dashed lines encompass the region of contamination. Samples are generally below these values, apart from SAH 97096 (EH3), which is affected by a contaminant with a high Br concentration. The composition of the contaminants is given in ref. 108.
Extended Data Figure 5 Backscattered electron image of SAH 97096, showing that some halogen-carrier phases are susceptible to thermal metamorphism.
Shown is the sulfide breakdown reaction90 due to thermal metamorphism in enstatite chondrite SAH 97096 (EH3). Djerfisherite [(K, Na)6(Cu, Ni, Fe)25S26Cl] breaks down into porous troilite, with loss of Na, K, Cl and so on. Original djerfisherite is shown in the red boxes, while the reaction product is the large mass in the centre of the image. If djerfisherite is a host to bromine and iodine as well as chlorine, halogen loss may be synchronous with alteration, which could be an explanation for the consistent loss of all halogens in enstatite chondrites with increasing petrologic type.
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Clay, P., Burgess, R., Busemann, H. et al. Halogens in chondritic meteorites and terrestrial accretion. Nature 551, 614–618 (2017). https://doi.org/10.1038/nature24625
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