Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N plus H)-bearing fluids at high pressure and temperature: Effects of melt structure

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+H-2+N-2 and H2O+H-2 fluid and H- and N-isotope concentrations ill these melts were determined experimentally at 1.5 GPa and 1400 degrees C as a function of hydrogen fugacity,fa, 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 similar to 0.4-1). The f(H2) was controlled between that of the iron-wftstite + H2O [logf(H2)(IW) similar to 3.42 (bar)] and that of the magnetite-hematite + H2O [logf(H2)(MH) similar to-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 f(H2)(IW) and decreases by about 50% between f(H2)(IW) and f(H2)(MH). The H solubility at f(H2)(IW) is insensitive to N BO/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 f(H2)(IW) and f(H2)(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 H-2, N-2, 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-/H-2 abundance ratios vary by similar to 55 and similar to 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 delta N-15 of (N+H)-saturated melts, relative to that of melts with NBO/Si = 0.4, varies by similar to 2 parts per thousand, whereas the delta D varies by similar to 87 parts per thousand. In N-free melts, the SD varies by similar to 12 parts per thousand. 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.