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 |
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| Authors: | Christian Schmidt Anke Watenphul |
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| Institution: | GFZ German Research Centre for Geosciences, Section 3.3, Telegrafenberg, 14473 Potsdam, Germany |
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| Abstract: | The behavior of ammonium, NH4+, 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 ν1-NH4+ 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 H2O ± NH4Cl 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 H4SiO40 band showed that the silica solubility in experiments with H2O + NH4Cl 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, NH3, 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 NH4Cl, based on the ratio of the integrated ν1-NH3 and ν1-NH4+ intensities and the HCl0 dissociation constant. The NH3/NH4+ 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 NH4Cl 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 NH4. Nucleation and growth of mica at the expense of K-feldspar and NH4+/K+ exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH4+ into K-feldspar was distinctly faster than K-feldspar consumption. |
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