Geothermobarometry using minerals at non-equilibrium: a corona example

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.