Carbon/Nitrogen ratio of Bulk Silicate Earth as a probe to understand planetary accretion and differentiation

In addition to the presence of liquid water on its surface, Earth's ability to host life is dependent on the unique budget of bio-essential volatile elements like carbon (C) and nitrogen (N) in the surficial reservoirs. To use Earth as an analog to search for the potential of life elsewhere in the Solar System and beyond, it is important to understand the role played by planetary accretion and differentiation in setting up the initial inventory of C and N in Bulk Silicate Earth (BSE), which communicates with the surface reservoirs to regulate the composition of ocean-atmosphere. Initial abundances of C and N in accreting bodies, alloy-silicate equilibration as well as volatility related losses during accretion have the potential to fractionate C relative to N. To understand the relative importance of these processes in establishing the BSE C/N, we performed high P-T experiments to constrain the ratio of partition coefficient of C and N between alloy and silicate (DC/DN).

The experiments were conducted at 1-7 GPa, 1500-2200 °C, logfO₂ (ΔIW -4.5-0), silicate melt composition (NBO/T-0.5-2.5), and S (0-30 wt.%) and Si (0-3 wt.%) contents of the alloy. The experiments were performed using piston cylinder and multi-anvil apparatus under graphite saturated conditions. C and N in quenched metal and silicate were measured using EPMA or SIMS and C-O-H-N speciation in silicate glasses was studied using IR and Raman spectroscopy. Our experiments show that DC/DN decreases with increase in S content in the alloy, fO₂ and NBO/T while it increases with increase in T and Si content in the alloy. Additionally, under relatively oxidized conditions (> ΔIW-2), C becomes less siderophile in the presence of N while N has no effect on the alloy-silicate partitioning character of C in increasingly reduced conditions. Our calculations predict that accretion of Earth from either oxidized or reduced chondrite-like material should have left behind sub-chondritic C/N ratios in bulk silicate reservoirs even if loss of early atmosphere(s) is taken into account. Therefore, superchondritic C/N ratio in BSE can only be explained if silicate portion of Earth accreted from material that already had elevated C/N ratios.