AbstractContinuing element exchange between minerals during cooling from the metamorphic peak modifies mineral compositions from those existing at the peak. After closure, when the minerals stop changing composition, the compositional relationships hold important information regarding, for example, cooling rates. Whereas the classic result of Dodson (1973) regarding closure provides an important limit, the situation involving element exchange with a slow diffusing mineral equilibrating with afinitefaster diffusing mineral is more valuable in considering metamorphic systems. An analytical extension to Dodson's equation for this situation in one dimension is presented here, and some of the consequences of departing from Dodson's limiting case are illustrated.