The Fate of Hydrogen in the Early Earth

Abstract:

The source of Earth's volatiles, particularly hydrogen, provides crucial information about the planet's formation and evolving oxidation state. Estimates for the H concentrations of the bulk Earth vary by several orders of magnitude. Hydrogen may have been incorporated directly from the solar nebula, during heterogeneous accretion of hydrous planetesimals, or during late accretion of volatile-rich bodies. The D/H ratio of terrestrial water matches the volatile rich CI chondrites, suggesting late accretion of chondritic-like material material as am importance source for volatiles. However, these correlations ignore the H isotope fractionation that likely occurred during early volatile loss to space. Such loss would raise both the f(O₂) and the D/H ratio of the upper mantle.

Any H present during the accretionary stages of Earth should have undergone extreme isotope fractionation. During planetary accretion and following the Giant Impact, the mantle must have been at an f(O₂) of IW or below. Under such reducing conditions, the f(H₂)/f(H₂O) ratio at 1 bar is 3-10, depending on temperature. Virtually all early post-nebular processing would have raised the dD value of the bulk silicate Earth (BSE). Examples of these processes include: H₂ degassing from an hydrous melt; incorporation of iron hydride into the core; oxidation of atmospheric H₂ gas; loss of H₂ to space during early hydrodynamic escape via EUV radiation. In addition to raising the D/H ratio, loss of H₂ gas to space will have the effect of oxidizing the crust, and presumably upper mantle. A loss of 1/4 ocean-equivalent water as H₂ is sufficient to raise the f(O₂) to the present day FMQ buffer. In contrast, addition of water from late accretion will have no appreciable effect on f(O₂) unless it is followed by significant H₂ loss to space, which would, in turn, raise the dD value significantly above that of carbonaceous chondrites. Given these strong fractionation effects, it is possible that the initial D/H ratio of Earth was far lower than today and nebular water needs to be considered as an possible H component.