The core-mantle partitioning of carbon and nitrogen in carbon-undersaturated ultramafic systems

In addition to their highly volatile character, segregation into the metallic core could have played an important role in explaining the depletion of carbon (C) and nitrogen (N) in the bulk silicate reservoirs of rocky bodies [e.g., 1,2]. As the core-mantle partitioning character of C and N strongly depends on fO₂, they can act as a powerful tracers to understand the fO₂-dependent volatile accretion history of terrestrial bodies. Previous high P-T experimental studies have shown that the highly siderophile character of C increases with decrease in fO₂, but can get suppressed in systems with high fH₂, while N acts as a siderophile element at >IW-2.5 and is lithophile at <IW-2.5 [3]. High P-T experiments on D_C^{alloy/silicate} and D_N^{alloy/silicate} published till date were conducted under graphite saturated conditions with basaltic to andesitic melt compositions primarily to quench glasses so that equilibrium C and N content in the silicate melts could be determined. However, terrestrial Magma Oceans (MOs) are presumed to be C-undersaturated with ultramafic silicate melt compositions, but such systems have not been simulated yet.

Here we present 28 experiments in MgO capsules that yielded large silicate glass pools coexisting with quenched alloy melt. D_C^{alloy/silicate} and D_N^{alloy/silicate} were determined as function of fO₂ (IW-7 to -1.5) at T=1600-1800 °C with nominally anhydrous (as low as 0.06 wt.% H₂O) mafic-ultramafic silicate melts (NBO/T=0.6-3.2) at a fixed P (3 GPa). C and N in quenched metal and silicate products were measured using EPMA, while C and H₂O in silicate by SIMS. In contrast to previous studies our experiments show that the siderophile character of C can be much lower, while N remains siderophile at even more reducing conditions (≥IW-4.5) primarily due to lower C content in the alloy relative to graphite saturated conditions. Our calculations predict that during core formation, the siderophile character of C depends upon its activity in the metal, while N can act as a siderophile element across the entire fO₂ range relevant for terrestiral accretion; therefore, bulk C content in the MOs, in addition to fO₂, can strongly influence partitioning of C and N into the core forming alloys.

[1] Dalou et al. (2017) EPSL [2] Grewal et al (2019) Sci. Adv. [3] Grewal et al (2019) GCA