A plate tectonic origin for Earth’s hydrogen isotope dichotomy

The Earth’s hydrogen isotope signature is a primary tool for investigating the origins of terrestrial water and other volatiles. However, this signature is not uniform within the Earth. Whereas seawater D/H resembles that of carbonaceous chondrites and falls outside of the enstatite chondrite range, mantle water is consistent with the enstatite chondrite signature. A recently popularized interpretation is that the Earth’s surface and interior had distinct cosmochemical sources for volatiles during planetary accretion: mantle volatiles were initially sourced by enstatite chondrite-like material, followed by a relatively late shift to a carbonaceous chondrite-like source for surface volatiles. This scenario requires that initially disparate isotopic signatures of the early mantle and surface have been preserved until present day, but the likelihood of such preservation is uncertain. I will present coupled geochemical and geodynamic models that constrain the hydrogen isotopic evolution of the mantle and surface throughout Earth’s history. Models indicate that the present-day isotopic dichotomy is a natural consequence of the fractionation associated with plate tectonic processes, and is decoupled from the initial (post-accretion) isotopic distribution between mantle and surface. We may still use the bulk silicate Earth D/H to infer the integrated origin of Earth’s water, and I will discuss a refined estimate of BSE D/H based on model constraints.