A halogen budget of the bulk silicate Earth points to a history of early halogen degassing followed by net regassing

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