Phase diagram modified from Schmidt et al. 46, and the phengite to K-hollandite transition is based on Fig. 2a. Circle symbols indicate the phengite to K-hollandite transition where a large fraction of nitrogen is lost along the IBM slab (cyan dashed line) and the CA slab (orange dashed line); the numbers beside the circle symbols are the calculated fractions of nitrogen preserved. The P-T conditions for the slab surfaces (D80 model 52) are extrapolated to 12 GPa. The red dashed line represents the geothermal gradient of the hottest slab (Mexico 52), where no phengite exists below the sub-arc depth. The numbers beside the square symbols are the phase proportions (wt%) of phengite at the corresponding pressure and temperature 46; the square symbols with blue color indicate sub-solidus runs, while the square symbols with yellow color mean that partial melting occurs, and the square symbols with green color indicate runs with supercritical fluids.
