A halogen balance of the earth’s bulk silicate indicates a history of early halogen degassing followed by net regassing


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Importance

Halogens play an essential role in biochemistry and are useful in understanding how planets formed and evolved. As we have found that the traditional way of limiting the halogen balance within the Earth is unreliable, we have developed a method that better utilizes relevant geochemical data and estimated halogen abundances in various silicate reservoirs. of the earth. Our halogen balance indicates that the majority of halogens are more concentrated in the mantle than on the surface and suggests that the halogens probably underwent early outgassing and subsequent net regassing. This study also provides an important key to deciphering the geological history of ocean chemistry.

Summary

Halogens are important tracers of various planetary formation and evolutionary processes, and a precise understanding of their abundance in Earth’s silicate reservoirs can help us piece together the history of mantle-atmosphere interactions. and the oceans. Previous studies of halogen abundances in Earth’s silicate mass (BSE) are based on the assumption of constant element abundance ratios, resulting in a gross underestimation of the halogen balance of BSE. Here we present a more robust approach using a linear log-log model. Using this method, we provide an internally consistent estimate of halogen abundances in the source mantle of depleted mid-ocean basalt (MORB), the source mantle of enriched ocean basalt (OIB), depleted mantle and BSE. Unlike previous studies, our results suggest that the halogens in BSE are not more depleted relative to elements with similar volatility, thus indicating sufficient retention of halogens during planetary accretion. Based on the abundances of halogens in the depleted mantle and BSE, we estimate that approximately 87% of all stable halogens reside in the current mantle. Given our understanding of the history of mantle outgassing and the evolution of crustal recycling, the revised halogen balance suggests that the deep halogen cycle is characterized by efficient early earth outgassing and net regasing. later in the rest of Earth’s history. Such an evolution of the deep halogen cycle represents a major step towards a more complete understanding of the ancient alkalinity of the oceans, which affects the distribution of carbon in the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life. .

Footnotes

    • Accepted November 9, 2021.
  • Author contributions: research designed by JK; MG carried out research; MG and JK analyzed the data; MG wrote the paper; and JK discussed the results and commented on the manuscript.

  • The authors declare no competing interests.

  • This article is a direct PNAS submission.

  • This article contains additional information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2116083118/-/DCSupplemental.

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About Lucille Thompson

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