Abstract
Solubility and solution mechanism(s) of reduced (N+H)- and H-containing N-O-H volatile components in Na2O-SiO2 composition melts in equilibrium with NH3+H2+N2 and H2O+H2 fluid and H- and N-isotope concentrations in these melts were determined experimentally at 1.5 GPa and 1400 °C as a function of hydrogen fugacity, fH2, and melt polymerization (composition), NBO/Si (NBO/Si = 0.4–1.18). This NBO/Si-range is similar to that between dacite and olivine tholeiite melt (NBO/Si ~0.4–1). The fH2 was controlled between that of the iron-wüstite + H2O [log fH2(IW) ~3.42 (bar)] and that of the magnetite-hematite + H2O [log fH2(MH) ~ −0.91 (bar)] buffer.
The N solubility decreases from 0.98 to 0.28 wt% in the melt NBO/Si-range from 0.4 to 1.18 at fH2(IW) and decreases by about 50% between fH2(IW) and fH2(MH). The H solubility at fH2(IW) is insensitive to NBO/Si and averages 0.76±0.28 wt% and 0.48±0.07 wt% H in (N+H)-saturated and in N-free and H-saturated melts, respectively. The H solubility in the melts decreases by at least ~70% between fH2(IW) and fH2(MH). Their N and H isotope ratios are systematic functions of the abundance ratio of structurally bound N and H (as NH2−- and OH−-groups bonded to Si4+) relative to molecular H2, N2, and NH3 in the melts. Molecular H2O plays a subordinate role in these melts, the bulk H2O content of which is <5 wt%. The NH2−/NH3 and OH−/H2 abundance ratios vary by ~55 and ~500% between NBO/Si = 1.18 and 0.4 relative to the values at NBO/Si = 0.4. In this same NH2−/NH3 abundance ratio range, the δ15N of (N+H)-saturated melts, relative to that of melts with NBO/Si = 0.4, varies by ~2‰, whereas the δD varies by ~87‰. In N-free melts, the δD varies by ~12‰. Changing abundance of volatiles dissolved in silicate melts in molecular form and as structural complexes that form bonds with the silicate melt structure is an important factor that can affect stable isotope fractionation during melting and crystallization at high pressure and temperature.
