A New Halogen Budget of the Bulk Silicate Earth Points to a History of Catastrophic Halogen Degassing Followed by Net Regassing

Halogens are important tracers of various planetary formation and evolution processes, and an accurate understanding of their abundances in the Earths silicate reservoirs can help us reconstruct the history of interactions among mantle, atmosphere, and oceans. The previous studies of halogen abundances in the bulk silicate Earth (BSE) are based on the assumption of constant ratios of element abundances, which is shown to result in a gross underestimation of the BSE halogen budget. Here, we present a new, more robust approach using a log-log linear model. Using the new method, we provide an internally consistent estimate of halogen abundances in the depleted MORB-source mantle (DMM), the enriched OIB-source mantle (EM), the depleted mantle, and the BSE. Unlike previous studies, our results suggest that BSE is not severely depleted in halogens compared to CI chondrites, which indicates sufficient halogen retention during planetary accretion. According to halogen abundances in DM and BSE, we estimate that ~70% of halogens reside in the present-day mantle. Given our understanding of the history of argon degassing and the evolution of crustal recycling, the revised halogen budget suggests that the catastrophic degassing of halogens in the early Earth was followed by continuous net regassing in the rest of Earth history. The degassing history of halogens presents an important step towards a more comprehensive understanding of ancient ocean alkalinity, which greatly affects carbon partitioning within the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life.