- © 2008 American Mineralogist
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 (fH2), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1–2.5 GPa pressure and 1300–1500 °C temperature ranges. The fH2 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 ∂XN/∂P > 0 and ∂XN/∂fH2 > 0. The ∂/∂fH2(∂XN/∂P) is also positive. Raman and FTIR spectroscopic data are consistent with solution mechanisms that involve reduction of nitrogen with increasing fH2. At low fH2 [fH2(MH) and fH2(MM)], nitrogen is dissolved in melts only as molecular N2. At fH2(NNO) and fH2(MW), there is partial reduction of nitrogen to form N2, NH2+ complexes and molecular NH3 in the melts, whereas at the highest fH2(IW), only molecular NH3 and NH2− groups can be identified. OH groups are also formed whenever there is reduction of nitrogen from N2. Solution in silicate melts of reduced, NH-bearing species results in silicate melt depolymerization. At fH2(NNO) and fH2(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.