Nitrogen-carbon-argon features of the silicate Earth established by deep core-mantle differentiation

Abstract

The processes and periods during which volatile elements were accreted to terrestrial planets provide crucial insights into their evolution and habitability. The bulk silicate Earth (BSE) is extremely depleted in nitrogen and features super-chondritic C/N and ³⁶Ar/N ratios, but their origins are elusive. Here using ab initio molecular dynamics combined with the thermodynamic integration method, we demonstrate that nitrogen remains siderophile under high-pressure and high-temperature, and predict a positive but nonlinear effect of pressure on nitrogen partitioning, which is caused by structural modifications in molten silicate. The nitrogen-carbon-argon characteristics of the BSE could have been established by deep core-mantle differentiation accompanied by simultaneous degassing from the surface of a deep magma ocean. These results underline the significant role of the deep core-mantle differentiation in shaping volatile ratios of the BSE and suggest that a substantial proportion of the Earth's nitrogen-carbon-argon may have been delivered to the proto-Earth by carbonaceous chondrite-like materials during the late stage of the Earth's main accretion. The Earth's distinct volatile ratios from those of carbonaceous chondrites may indicate different accretion times of the Earth's volatiles instead of different volatile sources.