Experimentally determined NH4+ – K+ exchange coefficient between phengite and fluids at 700°C/4.0GPa

Phengite is the main nitrogen (N) carrier in high-P/T metamorphosed rocks in subduction zones (e.g. Halama et al., 2017; Abdel-Hak et al., 2020). N is incorporated as ammonium (NH4+) substituting for K+. The significant amounts of NH4+ observed in natural phengite from the high-P metasediments of the Dora Maira Massif (Busigny et al., 2003) suggest that N can be retained during subduction and recycled into the deep mantle. Understanding the pathways of N during subduction, however, requires an understanding of the partitioning behavior of N within the hosting phases (Mikhail et al., 2017). Accordingly, we studied the exchange of NH4+ between phengite and fluid experimentally.

Piston-cylinder experiments using the ‘’synthesis exchange technique’’ (Zimmermann et al., 1997) were performed at 700°C, 4.0 GPa for the synthesis of K-NH4-phengites solid solutions. Starting materials were solid oxide mixtures and chloridic solutions.

After the experiments, solid run products were examined by X-ray powder diffraction with Rietveld analysis, and infrared spectroscopy. Cations in the product fluids were analyzed by ion chromatography.

Resulting coefficients (KD) for the K- NH4 exchange range from 0.77±0.13 to 1.42±0.18. Results show that at these P-T conditions, NH4+ partitions equally between phengite and coexisting fluid. Pöter et al. (2004) performed partitioning experiments between tobelite (an NH4+-muscovite mineral) and fluid at lower pressures (0.4 & 1.5 GPa). Here, contrary to our results, NH4+ shows preferential partitioning into the fluid phase. This pressure-induced change in the partitioning behavior highlights the significance of high pressures in increasing the extent to which N can be recycled during subduction.