During Earth's infancy, the planet was covered by a deep magma ocean. As the planet differentiated, heavy metals descended into the core, while lighter elements formed the silicate mantle. Volatile elements like nitrogen, carbon, and argon faced a different fate, their distribution depending on intense pressure and temperature conditions. Earth's atmosphere is 78% nitrogen today, yet nitrogen levels in the mantle remain remarkably low-just 1 to 5 parts per million. This imbalance puzzled scientists for decades.
Researchers from Ehime University's Geodynamics Research Center tackled this enigma using powerful simulations. They modeled conditions in Earth's early magma ocean at pressures up to 135 GPa and temperatures around 5000 K. Employing quantum mechanical techniques and thermodynamic models, the team assessed nitrogen's behavior in metal and silicate environments.
Their findings were conclusive: under high-pressure conditions, nitrogen preferentially bonded with metallic iron rather than silicates, becoming markedly "metal-loving." At 60 GPa, nitrogen was over 100 times more likely to migrate into the core than remain in the mantle. This preference intensified with pressure but followed a nonlinear trend, helping resolve prior experimental inconsistencies.
On an atomic level, nitrogen's interactions evolved with depth. Initially bonding with itself or hydrogen in silicate magma, nitrogen eventually bonded with silicon under pressure, forming nitride ions. In contrast, nitrogen remained neutral in the iron-rich core, slipping between iron atoms with ease.
The study also compared nitrogen's behavior to that of carbon and argon. While carbon showed some metal affinity, it was less than nitrogen's. Argon displayed no preference for metal. This ranking-nitrogen > carbon > argon-helps explain the high C/N and 36Ar/N ratios seen in Earth's mantle and atmosphere.
Using a planetary accretion model, the researchers proposed that if 5-10% of Earth's mass came from carbonaceous chondrites, the observed volatile element ratios could be replicated-but only if core formation occurred under deep magma ocean conditions. At such depths, over 80% of nitrogen would descend into the core, matching today's mantle nitrogen concentrations. Meanwhile, carbon would remain in the mantle, and argon would be confined to the atmosphere.
This discovery reframes the debate over Earth's volatile inventory. Rather than requiring exotic meteorite sources or massive nitrogen loss to space, the data suggest that deep planetary differentiation alone explains the anomalies. The findings also imply that Earth's capacity to retain life-essential elements may have been determined very early during core formation.
Far from being lost, Earth's nitrogen is simply hidden away in the planet's metallic heart-a revelation that underscores how the physics of planetary formation continues to shape Earth's chemical legacy.
Research Report:Nitrogen-carbon-argon features of the silicate Earth established by deep core-mantle differentiation
Related Links
Ehime University
Explore The Early Earth at TerraDaily.com
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