Ammonium in aqueous fluids to 600 °C, 1.3 GPa: A spectroscopic study on the effects on fluid properties, silica solubility, and K-feldspar to muscovite reactions

Abstract

The behavior of ammonium, NH 4 +, in aqueous systems was studied based on Raman spectroscopic experiments to 600 °C and about 1.3 GPa. Spectra obtained at ambient conditions revealed a strong reduction of the dynamic three-dimensional network of water with addition of ammonium chloride, particularly at small solute concentrations. The differential scattering cross section of the ν 1-NH 4 + Raman band in these solutions was found to be similar to that of salammoniac. The Raman band of silica monomers at ∼780 cm −1 was present in all spectra of the fluid at high temperatures in hydrothermal diamond-anvil cell experiments with H 2O ± NH 4Cl and quartz or the assemblage quartz + kyanite + K-feldspar ± muscovite/tobelite. However, these spectra indicated that dissolved silica is less polymerized in ammonium chloride solutions than in comparable experiments with water. Quantification based on the normalized integrated intensity of the H 4SiO 4 0 band showed that the silica solubility in experiments with H 2O + NH 4Cl was significantly lower than that in equimolal NaCl solutions. This suggests that ammonium causes a stronger decrease in the activity of water in chloridic solutions than sodium. The Raman spectra of the fluid also showed that a significant fraction of ammonium was converted to ammonia, NH 3, in all experiments at temperatures above 300 °C. This indicates a shift towards acidic conditions for experiments without a buffering mineral assemblage. The estimated pH of the fluid was ∼2 at 600 °C, 0.26 GPa, 6.6 m initial NH 4Cl, based on the ratio of the integrated ν 1-NH 3 and ν 1-NH 4 + intensities and the HCl 0 dissociation constant. The NH 3/NH 4 + ratio increased with temperature and decreased with pressure. This implies that more ammonium should be retained in K-bearing minerals coexisting with chloridic fluids upon high- P low- T metamorphism. At 500 °C, 0.73 GPa, ammonium partitions preferentially into the fluid, as constrained from infrared spectroscopy on the muscovite and from mass balance. The conversion of K-feldspar to muscovite proceeded much faster in experiments with NH 4Cl solutions than in comparable experiments with water. This is interpreted as being caused by enhancement of the rate-limiting alumina solubility, suggesting complexation of Al with NH 4. Nucleation and growth of mica at the expense of K-feldspar and NH 4 +/K + exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH 4 + into K-feldspar was distinctly faster than K-feldspar consumption.