A comparative study of nitrogen incorporation in silicate, metallic, and bulk earth melts at high pressure

Abstract

Nitrogen as the dominant volatile element of the atmosphere is also expected to exist underneath the Earth’s surface. Its interior budget and distribution may have largely been set early on by the core formation and other processes that led to apparent volatile depletion in the terrestrial planet. To better understand how nitrogen may have behaved during the accretion stages when proto-Earth experienced multiple episodes of magma ocean environments, we report a first-principles computational study of nitrogen incorporation in silicate melts over wide pressure range 0 to 125 GPa (3000 to 5000 K), as well as in iron-rich metallic melt and bulk Earth-like melt at selected conditions. The results show that the speciation of nitrogen in silicate melts at low pressures consists of almost entirely N₂ molecules with interstitial occupancy. As pressure increases, nitrogen interacts increasingly with the silicate network and bonds with iron more strongly than with any other cations (Mg, Si, Ca, Al, and Ni) present in the melt. Both pressure and reducing conditions help nitrogen chemically dissolve as nitride species thus promoting nitrogen retention of possible deep-seated dense silicate melts in the mantle. Metallic liquid incorporates nitrogen by bonding with iron with weak or no interactions with itself or with other impurities. The simulated bulk Earth melt system shows a phase segregation to form an iron-rich cluster which is surrounded by a silicate region. The metal-silicate partition coefficient of nitrogen evaluated using the relevant local coordination statistics from the two-phase system is ∼31 at 30.5 GPa (3000 K, IW-3.1), ∼18 at 37.1 GPa (4000 K, IW-2.2), and ∼24 at 131 GPa (5000 K, IW-2.1) which are generally consistent with the measured trends. Based on the predicted strong preferential partitioning to the metallic liquid, we argue that while nitrogen may be depleted from the silicate mantle, it may be sequestered in the core.