The Fate of Nitrogen in Highly Reducing Magmatic Systems: Implications for Storage of Nitrogen during Meteorite Parent Body Processing and Magma Ocean Crystallization

Abstract:

A key step in the origin of major, life-essential volatiles such as nitrogen (N) in rocky planets is the processing of those volatiles during magma ocean (MO) crystallization and parent body partial melting. In addition, all terrestrial planets show evidence of having extremely reduced, enstatite chondrite-like precursors in some portion of their accretionary history (1). Plus, Mercury and Aubrite parent body present highly reducing interior. Therefore, knowing the fate of N in a partially molten system under highly reducing conditions is critical in assessing the origin of N in many inner Solar System bodies. To determine the fate of N during highly reduced parent body melting or MO crystallization, phase equilibria experiments were conducted using a Ti-free tholeiitic basalts + 20 wt.% Si₃N₄ at 1.5-3 GPa (P) and 1300-1600 °C (T) in graphite capsules at IW – 6.19 to IW – 7.55. All experiments yielded silicate melt + Si₃N₄ + metal alloy + vapor ± sinoite ± cpx, with sinoite and cpx restricted to temperatures above and below 1400-1500 °C, respectively. The lack of coexistence of sinoite and cpx is consistent with Fegley’s (3) suggestion that sinoite does not occur in equilibrium with silicate minerals. Nitrogen solubility in silicate melt and Nitrogen Concentration at Nitride Saturation (NCNS) both increase with increasing P and T and are 3.57-9.46 wt.%. Nitrogen solubility in cpx was also measurable using EPMA (1.51-2.05 wt%). The cpx/melt partition coefficient decreases from ~0.4 to ~0.2 from 2 to 3 GPa, but is much higher than previously estimated for less reduced systems (2). Because NCNS increases with temperature, Si-nitrides are more likely to form as magma cools, as sinoite is unstable with silicate minerals. The higher solubility of and lesser incompatibility of N in cpx along with stability of sinoite and Si₃N₄ suggest that nitrogen can be retained or enriched in solid assemblages more readily under very reducing conditions. Our experimental data have implications for understanding the origin of N in shallow reservoirs of highly reduced meteorite parent bodies and establishing N in planetary atmospheres.

1. A. E. Rubin, B.-G. Choi, Earth Moon Planet. 105, 41–53 (2009).
2. Y. Li, M. Wiedenbeck, S. Shcheka, H. Keppler, EPSL. 377–378, 311–323 (2013).
3. B. Fegley, JGR: Solid Earth. 88, A853–A868 (1983).