Nitrogen solubility in basaltic silicate melt-Implications for degassing processes

The distribution of nitrogen between the different terrestrial reservoirs (core-mantle-atmosphere) and how this may have changed since the earliest planetary stages is uncertain. In particular, the primordial degassing processes of the magma ocean and its role in the formation of the atmosphere remains to be quantified. Since no geological samples can capture this early degassing process, we need to go through the thermodynamic modeling of the nitrogen solubility in silicate melt. We hence performed experiments on basaltic samples at fluid saturation in the C-H-O-N system, using an Internally Heated Pressure Vessel (IHPV) and Piston Cylinder (PC) in the pressure range 0.8 kbar to 10 kbar, temperature between 1200 and 1300 ?C, and a wide range of fO2 conditions from IW + 4.9 to IW-4.7 (IW standing for the Iron-Wustite redox buffer). The nitrogen concentration in the quenched silicate melts at fluid saturation was analysed by secondary ion mass spectrometry (SIMS), and the speciation of the dissolved C-O-H species was determined by Fourier transform infrared spectroscopy (FTIR). We identified two nitrogen species in the silicate melt: N2 dominating at fO2 > IW and N3- at lower fO2. Using these data and a database constraining nitrogen concentration at fluid saturation from 1 bar to 10 kbar pressure, we calibrated a solubility law for nitrogen in basalts defining its P-T-fO2 dependences. This model expands the model of Libourel et al. (2003) to high pressure and higher C-O-H activities. It can be used to investigate the nitrogen degassing processes for different pressure, temperature and fO2 conditions relevant to planetary accretion and modern volcanism.