Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2023 / Paper 1 / 7 / d / Solution

The fugacity coefficient $\varphi =f/P=0.7$ gives $f=3500\mathrm{bar}$. Keep the same solid thermodynamic activities and pressure correction to a mineral reaction as in part (c), changing only the water chemical fugacity:The lower water chemical fugacity makes the dehydrated assemblage more favorable, so the equilibrium temperature decreases by about $29.0\mathrm{K}$. This direction is a useful independent check on the logarithm's sign.