Origin of the superchondritic carbon/nitrogen ratio of the bulk silicate Earth − an outlook from iron meteorites
Damanveer S. Grewal, Paul D. Asimow
Geochimica et Cosmochimica Acta
Available online 20 January 2023
“Disagreement regarding the origin of the bulk silicate Earth’s (BSE) superchondritic carbon/nitrogen (C/N) ratio is due, in part, to the unknown C/N ratios of differentiated planetesimals − the building blocks of Earth-like rocky planets. In this study we report solid-liquid metal partitioning experiments for C and N that allow us to reconstruct, from the C and N contents of iron meteorites, the C/N ratios of the cores of the earliest formed planetesimals. Due to their siderophile character, most of the C and N retained in these bodies after differentiation resides in their cores. Therefore, estimates of the bulk C and N contents and C/N ratios of the cores yield confident estimates of these quantities in the complete parent bodies of iron meteorites. Our experimental data, at 1 GPa and 1200-1400 °C, show that C and N are incompatible in solid metal relative to S-poor liquids but compatible in solid metal relative to S-rich liquids. Crucially, N is approximately an order of magnitude more compatible than C in S-rich systems. S itself is incompatible in solid metal and so the late-crystallizing liquids persisting at the end of core freezing were S-rich for most cores. Although these late-crystallizing liquids are unsampled by iron meteorites, we infer that their N contents and C/N ratios were generally lower and higher, respectively, than those in iron meteorites. Depending upon the fraction of unsampled late-crystallizing liquids as well as their S contents, the C/N ratios of the bulk cores and complete parent bodies are either similar to or higher than those measured in iron meteorites. The reconstructed C/N ratios of most of the parent bodies of iron meteorites are chondritic, except that the volatile-rich IC and IIC groups have superchondritic C/N ratios. Importantly, the C/N ratio of the parent body of the IC iron meteorite group lies within the estimated range of the BSE, whereas the C/N ratios of all other groups are distinctly lower. Correlated depletion of moderately volatile elements like Ge and Ga with C and N, variations in metallographic cooling rates, and Pd-Ag isotope systematics suggest that the parent cores of the volatile-depleted iron meteorite groups were likely affected by volatile degassing. If volatile-rich iron meteorites like the IC group better capture the C and N inventories of the parent cores of the earliest formed planetesimals, then delivery of C and N via such planetesimals makes the superchondritic C/N ratio of the BSE a natural consequence of the Earth’s accretion history. Otherwise, poorly constrained processes like atmospheric erosion or C and N delivery by exotic materials are required to explain the superchondritic C/N ratio of the BSE.”
