Experimental determination of the ammonium partitioning among muscovite, K-feldspar, and aqueous chloride solutions

The cation exchange equilibriaMuscovite+NH4Cl⇔Tobelite+KClKAl2AlSi3O10(OH)2+NH4Claq⇔NH4Al2AlSi3O10(OH)2+KClaqK-feldspar+NH4Cl⇔Buddingtonite+KClKAlSi3O8+NH4Claq⇔NH4AlSi3O8+KClaqare determined by hydrothermal experiments at 400, 500, 600 °C at 400 MPa, and piston cylinder experiments at 500, 600 °C at 1500 MPa along the entire K–NH4 compositional range. The composition of solid phases are determined by XRD, EMP and FTIR-methods, NH4–K ratios in coexisting 2–3 molal chloride solutions by ion chromatography. Muscovite–tobelite and K-feldspar–buddingtonite form complete solid solutions series at all conditions. Consistent phase relations and mass balances indicate equilibrium between solid solutions and fluids. In both the (K,NH4)-muscovite and (K,NH4)-feldspar-fluid systems, NH4 strongly fractionates into the fluid. In the range from 400 to 600 °C, 400 to 1500 MPa, fractionations are slightly temperature and pressure-dependent. The distribution coefficientKDsolid-fluid=XNH4solidXKsolidXKfluidXNH4fluidvaries within the experimental range from 0.10 at XKbulk=0.81 to 0.46 at XKbulk=0.20 for muscovite-fluid, and from 0.05 at XKbulk=0.91 to 0.70 at XKbulk=0.15 for feldspar-fluid. NH4 is preferentially incorporated into feldspar relative to mica with KDfsp-ms ranging between 1.36 and 2.0 at 400 MPa, and between 1.13 and 1.5 at 1500 MPa as XKbulk varies from 0.15 to 0.89. Results are evaluated thermodynamically assuming equal mixing of NH4Cl and KCl in the fluid. Mixing energies of solids were estimated using multidimensional regression and a regular solution model. For the muscovite–tobelite equilibrium, values of Δs°=−11.7 J K−1 mol−1, Δv°=−2.1 J MPa−1 mol−1, and W=4.6 kJ mol−1, and for the K-feldspar–buddingtonite equilibrium, Δs°=−8.8 J K−1 mol−1, Δv°=−1.1 J MPa−1 mol−1, and W=5.4 kJ mol−1 were derived. Results are used to calculate partition coefficients D among phases at very low NH4 bulk concentrations. Between 400 and 600 °C, DNH4fluid-ms ranges from 7 to 8 at 400 MPa and is 5 at 1500 MPa. DNH4fluid-ms is 6 to 7 at 400 MPa, and about 5 at 1500 MPa. DNH4fsp-ms is between 1 and 1.2 at all conditions. Partition coefficients are valid for NH4 contents of up to several hundred ppm in mica and feldspar, well within the concentration range observed in many rocks. Combining the data with that of K–NH4-partitioning between phlogopite and fluid results in DNH4phl-ms≈3.5 and DNH4phl-ms≈3 at 550 °C, 200 to 400 MPa. NH4 concentrations in coexisting muscovite, biotite, and K-feldspar from a variety of rocks show near-equilibrium distributions. NH4–K-partitioning between major K-bearing minerals and saline fluids allows for assessment of the dehydration history of metamorphic rocks. During prograde metamorphism, water is progressively produced by dehydration reactions and expelled along rockʹs P–T path. Nitrogen is subsequently removed due to preferred fractionation of NH4 into fluids. The remaining NH4 is continuously redistributed among muscovite and biotite, and at higher grades, K-feldspar with biotite as the main solid phase carrier of ammonium. The large fractionation effect of NH4 among most phases highlights its potential as tracer of devolatilization processes and fluid–rock interactions.