Complex Planetary Processes During Earth’s Magma Ocean Phase Could Account for the Superchondritic C/N Ratio of Bulk Silicate Earth

Cartoon illustrating potential processes responsible for bulk silicate Earth’s superchondritic C/N ratio. Proto-Earth could a) accrete differentiated planetesimals with superchondritic C/N ratios during equilibrium accretion, b) accrete C-saturated embryos with superchondritic C/N ratios in the silicate mantles during disequilibrium accretion, and c) accrete differentiated planetesimals with superchondritic C/N ratios while the magma ocean is solidifying and oxidizing metal. Credit: Li et al., 2023.

Unraveling the origin of Earth’s volatile elements from meteorites, the building blocks of the solar system, may be complicated by planetary-scale processes such as differentiation, crystallization, and degassing. Previous models have argued that Earth accreted dry and acquired its volatiles from a “late veneer” of carbonaceous chondrite-like material. However, the bulk silicate Earth (BSE) — the bulk Earth minus its metallic core — has a superchondritic C/N ratio. This implies a different source for most of Earth’s volatiles because core formation after adding this veneer would have resulted in a subchondritic C/N ratio in the BSE. The C/N ratio of BSE could be a powerful tool for understanding the types of planetary bodies that contributed volatiles to Earth if it were better understood how C/N changes during large-scale planetary processes such as magma ocean crystallization and degassing.

A new study by Yuan Li from the Guangzhou Institute of Geochemistry and co-authors report the results of new experiments on the solubility and partitioning of C and N between Fe-rich metallic melts (representing planetary cores) and silicate melts (representing planetary mantles) under high-pressure (0.3-3 GPa) and high-temperature (1400-1600°C) conditions relevant to the magma ocean environment on Earth. Results showed that C and N partition coefficients (e.g., DCmetal/silicate, the ratio between the C content of metal and the C content of silicate) are controlled by several variables, including pressure, temperature, silicate melt composition, and oxygen fugacity (abundance of oxygen).

Results of these experiments showed that at low oxygen fugacity, C/N ratios in silicates in equilibrium with metal (e.g., BSE) would be subchondritic unless significant degassing occurred. Degassing of the magma ocean could increase its C/N ratio. These results suggest that the superchondritic C/N ratios of BSE require a complex combination of accretion of metallic cores of differentiated planetesimals, degassing of the magma ocean during crystallization, and degassing due to impacts. READ MORE