Solubility and solution mechanisms of NOH volatiles in silicate melts at high pressure and temperature-amine groups and hydrogen fugacity

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (f(H2)), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1-2.5 GPa pressure and 1300-1500 degrees C temperature ranges. The f(H2) was controlled at values defined by the magnetite-hematite (MH), Mn3O4-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H2O. The nitrogen solubility ranges from about 1 to about 5 mol%, calculated as N, with partial derivative X-N/partial derivative P > 0 and partial derivative X-N/partial derivative f(H2) > 0. The partial derivative/partial derivative f(H2)(partial derivative X-N/partial derivative P) is also positive. Raman and FTIR spectroscopic data are consistent with solution mechanisms that involve reduction of nitrogen with increasing f(H2). At low f(H2) [f(H2)(MH) and f(H2)(MM)], nitrogen is dissolved in melts only as molecular N-2. At f(H2)(NNO) and f(H2)(MW), there is partial reduction of nitrogen to form N-2, NH2+ complexes and molecular NH3 in the melts, whereas at the highest f(H2)(IW), only molecular NH3 and NH2+ groups can be identified. OH groups are also formed whenever there is reduction of nitrogen from N-2. Solution in silicate melts of reduced, NH-bearing species results in silicate melt depolymerization. At f(H2)(NNO) and f(H2)(MW), depolymerization occurs via H+ interaction with oxygen and NH2+ groups serving as network-modifier. Under more reducing conditions, oxygen is replaced by NH2- groups. Solution of reduced nitrogen in silicate melts causes depolymerization of their structure. This implies that melt properties that depend on silicate structure depend on redox conditions.