Experimental determination of N2 solubility in silicate melts and implications for N2–Ar–CO2 fractionation in magmas

Abstract

The solubility of nitrogen in magmatic melts is key for understanding the degassing of Earth’s interior and formation of Earth’s N-rich atmosphere. We performed high-pressure experiments to determine the solubility of nitrogen at N2-saturation in basaltic to granitic melts at pressures of 0.3–8 GPa, temperatures of 1200–1600 °C, and oxygen fugacities between the Fe–FeO and Ni–NiO oxygen buffers. The solubility and speciation of nitrogen in the quench silicate melts were quantified by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), electron probe microanalyzer (EPMA), and Nano-scale secondary ion mass spectrometry (NanoSIMS). The results show that the predominant N-species in our silicate melts is N2, which dissolves physically by filling the interstitial sites of the silicate melt network. The measured N-solubilities in silicate melts (SN2melt) by using different approaches agree with each other, and they vary from 200 to 17,000 ppm by weight. The SN2melt increases with increasing pressure and/or decreasing silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations), but it is temperature-independent. Using our newly obtained SN2melt and previous SN2melt and SArmelt data, we develop an empirical model that describes the physical dissolution of N2 and Ar in silicate melts as a function of gas partial pressure and silicate melt composition, and this model can be used to predict SN2melt and SArmelt in magmatic melts of Earth’s mantle and crust. The comparison of our SN2melt data with previous SArmelt and SCO2melt data shows that the solubility in granitic melts is in the order of SArmelt >SCO2melt >SN2melt, but in basaltic melts it is in the order of SCO2melt >SArmelt > SN2melt. The lower SN2melt than SArmelt at Earth’s mantle and crustal conditions suggests N2–Ar fractionation during degassing of mid-ocean ridge basalts and arc magmas. Therefore, the N2/Ar ratios in degassed magmas may not fully represent those in their mantle source regions. The solubility-controlled, preferential degassing of N2 relative to CO2 in Earth’s oxidized shallow magma ocean, in conjunction with collision-induced atmospheric loss, can explain the superchondritic C/N ratio in the bulk silicate Earth. However, the two orders of magnitude higher N2/36Ar ratio in Earth’s mantle than in atmosphere could have resulted from preferential retention of reduced N-species in Earth’s reduced silicate magma ocean or preferential deep subduction of nitrogen.