Provenance of Earth’s volatile building blocks inferred from the behaviour of nitrogen during core formation

Nitrogen (N) is the most abundant element in Earth's modern atmosphere, but is extremely depleted in the silicate crust and mantle. The volatile inventory of the bulk silicate Earth shows a well-established N deficit compared to CI chondrites, the primitive meteorites representative of the solar composition. However, it remains unclear whether the formation of the iron-rich core, early atmospheric loss, or a combination of both was responsible for this depletion, partly due to the large extrapolation from low-pressure experiments. Here, we study the effect of core formation on the inventory of nitrogen in a terrestrial magma ocean using laser-heated diamond anvil cells. Under core-forming conditions relevant to Earth-sized planets, we find that N is siderophile (iron-loving), making the core its largest reservoir, notwithstanding that the simultaneous dissolution of oxygen in the core lowers that of nitrogen. A combined core-mantle-atmosphere coevolution model, however, cannot account for the observed N anomaly in the silicate Earth via its core sequestration and/or atmospheric loss during accretion, unless Earth's building blocks had experienced vaporisation processes akin to those accountable for the volatile signatures found in CV-CO chondrites. The terrestrial volatile pattern requires severe N depletion (>99%) on precursor bodies but limited atmospheric loss (<5%), prior and posterior to their accretion to the proto-Earth. We argue that early vapour loss/depletion on Earth's building blocks is the key to establishing our planet's volatile budget.