The Fate of Nitrogen during Parent Body Partial Melting and Accretion of the Inner Solar System Bodies at Reducing Conditions

Rajdeep Dasgupta, Emily Falksen, Aindrila Pal, Chenguang Sun

Geochimica et Cosmochimica Acta
Available online 23 September 2022


“Evolution of nitrogen (N), a life-essential volatile element, in highly reduced magmatic systems is a key for the origin of N on rocky planets formed via accretion of reduced chondritic parent body materials, planetesimals, and embryos that underwent partial or complete differentiation. However, the storage capacity of N in phases relevant for reduced silicate systems undergoing thermal processing is poorly known. To investigate the stability of N-bearing phases in partially molten silicate-rich systems as well as solubility of nitrogen in silicate melts and minerals, we performed laboratory experiments on a 80:20 synthetic basalt-Si3N4 mixture at 1.5-3.0 GPa and 1300-1600 °C in graphite capsules, yielding oxygen fugacity ranging from ∼IW– 3.0 to ∼IW – 4.0. All experiments produced silicate melt + nierite + Fe-rich alloy melt + N-rich vapor ± sinoite ± cpx. Sinoite was restricted to above while cpx was restricted below 1400-1500 °C. Nitrogen solubility and Nitrogen Concentration at Silicon-Nitride Saturation (NCNS) in silicate melts increases with increasing pressure and temperature and ranges between 3.6 and 9.5 wt %. Using our high pressure N solubility data and similar data at ambient and lower pressures, we derived a new N solubility model in silicate melts. Solubility of nitrogen in cpx was between 1.51 and 2.05 wt% and resulted in cpx/silicate melt partition coefficients for nitrogen, of D epx/silicatemelt/N ∼0.4 to ∼0.2. These D epx/silicatemelt/N are distinctly higher than those previously estimated at more oxidizing conditions, suggesting N maybe much less incompatible during thermal processing of rocky reservoirs at highly reducing conditions. Partition coefficient of N between Fe-rich alloy melt and cpx, D Fe/richalloymelt/epx/N was found to be between 1.6 and 2.1. The application of our N solubility data and model suggests that mobilization of N from the deeper, partially molten reservoirs to shallower reservoirs is possible in reduced planetesimals and internally differentiated meteorite parent bodies – leading to net loss of N via melt degassing or reprecipitation of N-bearing solid phases, depending on whether the surficial shell is oxidized or reduced, respectively. Similarly, comparison of the first measured D epx/silicatemelt/N values from our highly reducing experiments with those estimated at more oxidizing conditions suggest that N would be much less incompatible during internal and external magma ocean processing of rocky bodies under highly reducing conditions. Therefore, enrichment of N in the atmospheres of Earth and Venus is likely a result of more oxidizing penultimate phase of accretion, which would lead to N being more readily partitioned to residual liquid, which would also more readily degas at oxidizing conditions.”