A simple extension of geothermobarometric technique is developed, from the
usual case of assumed equilibrium among all minerals, to layered coronas wi
th gradients of chemical potential. For reactions among end-members in the
minerals, instead of the assumption of zero affinity (-DeltaG) = 0, (-Delta
G) ratios are used. The values of these ratios are defined by diffusion mod
elling of corona growth. The variations in composition of solid-solution mi
nerals across the corona must be considered. The method is demonstrated usi
ng a corona, for which diffusion modelling has been published, in a metabas
ic granulite from the Yenisey Ridge, Siberia. Provided that an appropriate
set of affinity ratios is selected, the result is fairly well constrained,
varying only slightly with the choice of activity model for garnet, and ind
icating T = 740 +/- 20 degreesC with P = 9.5 +/- 0.7 kbar. The method indic
ates the extent of departure from equilibrium PT conditions during such a r
eaction.