Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N+H)-bearing fluids at high pressure and temperature; effects of melt structure
Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N+H)-bearing fluids at high pressure and temperature; effects of melt structure
American Mineralogist (July 2010) 95 (7): 987-999
Solubility and solution mechanism(s) of reduced (N+H)- and H-containing N-O-H volatile components in Na (sub 2) O-SiO (sub 2) composition melts in equilibrium with NH (sub 3) +H (sub 2) +N (sub 2) and H (sub 2) O+H (sub 2) fluid and H- and N-isotope concentrations in these melts were determined experimentally at 1.5 GPa and 1400 degrees C as a function of hydrogen fugacity, f (sub H2) , 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 approximately 0.4-1). The f (sub H2) was controlled between that of the iron-wustite+H (sub 2) O [log f (sub H2) (IW) approximately 3.42 (bar)] and that of the magnetite-hematite+H (sub 2) O [log f (sub H2) (MH) approximately -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 (sub H2) (IW) and decreases by about 50% between f (sub H2) (IW) and f (sub H2) (MH). The H solubility at f (sub H2) (IW) is insensitive to NBO/Si and averages 0.76+ or -0.28 wt% and 0.48+ or -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 approximately 70% between f (sub H2) (IW) and f (sub H2) (MH). Their N and H isotope ratios are systematic functions of the abundance ratio of structurally bound N and H (as NH (sub 2) (super -) - and OH (super -) -groups bonded to Si (super 4+) ) relative to molecular H (sub 2) , N (sub 2) , and NH (sub 3) in the melts. Molecular H (sub 2) O plays a subordinate role in these melts, the bulk H (sub 2) O content of which is <5 wt%. The NH (sub 2) (super -) /NH (sub 3) and OH (super -) /H (sub 2) abundance ratios vary by approximately 55 and approximately 500% between NBO/Si=1.18 and 0.4 relative to the values at NBO/Si=0.4. In this same NH (sub 2) (super -) /NH (sub 3) abundance ratio range, the delta (super 15) N of (N+H)-saturated melts, relative to that of melts with NBO/Si=0.4, varies by approximately 2 per mil, whereas the delta D varies by approximately 87 per mil. In N-free melts, the delta D varies by approximately 12 per mil. 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.