PhD defense of Fabien Bernadou

Abstract

The degassing of nitrogen in planetary atmospheres is a phenomenon that is still poorly constrained despite the abundance of this element (i.e. 78% of our atmosphere) and its importance for the origins of life on Earth. To understand the abundance of nitrogen in our atmosphere, it is important to identify its source and the conditions responsible for its presence in the atmosphere. The work of this thesis focused on the study of the solubility of nitrogen in molten silicates under magmatic conditions; solubility being closely linked with magmatic degassing. Solubility corresponds to the maximum nitrogen concentration that a silicate liquid can assimilate for P-T-fO2 and fH2 conditions. Beyond this threshold concentration, the silicate liquid becomes saturated and produces a fluid phase through degassing. To answer the question on solubility, the thesis can be separated into three main phases which are: the acquisition of experimental samples at fluid saturation at high pressure (250 to 10,000 bar) and high temperature (1200 to 1400°C) , the analysis of these samples (IR, Raman, EPMA, SEM, IMS) and finally the development of a solubility model. During the experimental phase, in addition to pressure and temperature, we varied the parameters of fO2 as well as fH2. These experiments allowed us to better define the important effects for the pressure and fO2 parameters. The characterization of the control of nitrogen solubility by fH2 is a significant step forward which resolves a series of inconsistencies noted in the literature. An increase in fO2 reduces the solubility of nitrogen and therefore leads to degassing. On the other hand, a reduction in the pressure parameter, more particularly in the partial pressure of N2 (PN2) as well as the fH2 parameter also leads to a reduction in the nitrogen concentration and triggers the degassing phenomenon. Following all these observations and using a database augmented with our experimental points, we produced a thermodynamic model of the degassing phenomenon. This model takes into account three equilibria describing the solubilization of nitrogen in silicate liquids in the form of N2, N3- and NH3. Each of these species corresponds to a different solubilization mechanism in the silicate liquid: N2 is physically dissolved in the ionic porosity of the silicate liquid, N3- is chemically dissolved in the structure of the silicate liquid. This model allowed us to rationalize a bundle of elements from the literature: the nitrogen concentration in the form (1) N2 was linked only to PN2, (2) N3- to PN2 and fO2, and (3) NH3 at PN2 and fH2 becoming the majority species for high fH2 conditions. The created and calibrated model can predict the solubility of nitrogen in silicate liquids under a wide range of conditions and can also be included in models of planetary atmosphere outgassing.