The fate of major volatiles during the evolution of protoplanetary bodies and its subsequent effect on the volatile accretion history of rocky planets is poorly understood. Evidence for the widespread differentiation of the earliest formed planetesimals and planetary embryos makes the volatile inventories of these bodies susceptible to fractionation between core, magma ocean (MO), and atmosphere (and subsequent loss to space). This is especially important for carbon (C) and nitrogen (N), which are prone to fractionation into all three reservoirs, albeit in different proportions depending upon the chemistry of the MOs. Recent findings have shown that even the parent bodies of primitive chondrites did not escape differentiation such that their interiors, overlain by unmelted chondritic crusts, also underwent large-scale melting. Therefore, to track the evolution of C and N from primitive dust to present-day planets, it is important to constrain their fate during end-member protoplanetary differentiation regimes, i.e., internal, closed system MOs (MO-core fractionation) and external, open system MOs (atmosphere-MO-core fractionation).
Here we present a thermodynamic modelling framework to track C and N fractionation between atmosphere, MO and core reservoirs as a function of the composition of their accreting materials and sizes of the parent bodies. For external MOs, C and N in the MOs were calculated based on their vapor pressure-induced solubility in the silicate melts while the exchange between MOs and core forming alloy melts for external as well as internal MOs were calculated using alloy-silicate melt partition coefficients. For bodies with external MOs, 89-99% of the accreted C and N inventories reside in the atmosphere rsulting from MO degassing, 1-11% in their cores, and less than 1% C-N in their silicate reservoirs. Whereas for bodies with internal MOs, the cores are the major C and N bearing reservoir (90-99%). The relative prevalence of external versus internal magma ocean regimes can be used to explain the C-N inventories of different groups of iron meteroites. Consequently, C-N inventories of larger planets were likely affected by the relative prevalence of feedstock rocky bodies in the Solar System that underwent end-member protoplanetary differentiation regimes.