The almost lithophile character of nitrogen during core formation

Abstract

Nitrogen is a key constituent of our atmosphere and forms the basis of life, but its early distribution between Earth reservoirs is not well constrained. We investigate nitrogen partitioning between metal and silicate melts over a wide range of conditions relevant for core segregation during Earth accretion, i.e. 1250–2000 °C, 1.5–5.5 GPa and oxygen fugacities of ΔIW-5.9 to ΔIW-1.4 (in log units relative to the iron–wüstite buffer). At 1250 °C, 1.5 GPa, DN metal melt/silicate melt ranges from 14 ± 0.1 at ΔIW-1.4 to 2.0 ± 0.2 at ΔIW-5, N partitioning into the core forming metal. Increasing pressure has no effect on DN metal melt/silicate melt, while increasing temperature dramatically lowers DN metal melt/silicate melt to 0.5 ± 0.15 at ΔIW-4. During early core formation N was hence mildly incompatible in the metal. The partitioning data are then parameterised as a function of temperature and oxygen fugacity and used to model the evolution of N within the two early prevailing reservoirs: the silicate magma ocean and the core. Depending on the oxidation state during accretion, N either behaves lithophile or siderophile. For the most widely favoured initially reduced Earth accretion scenario, N behaves lithophile with a bulk partition coefficient of 0.17 to 1.4, leading to 500–700 ppm N in closed-system core formation models. However, core formation from a magma ocean is very likely accompanied by magma ocean degassing, the core would thus contain ≤100 ppm of N, and hence, does not constitute the missing N reservoir. Bulk Earth N would thus be 34–180 ppm in the absence of other suitable reservoirs, >98% N of the chondritic N have hence been lost during accretion.