Oxygenisotopefractionationbetweencoexistingmineralsinslowlycooledrocksconveysinformationabouttheircoolinghistory.Byusingthefastgrainboundary(FGB)modeltosimulateclosed-systemdiffusiveexchangeofoxygenisotopesbetweencoexistingminerals,Ishowthattheapparentequilibriumtemperatures(Tae)bythemineralpairwiththelargestisotopicfractionation(PLIF)alwaysliesbetweentheclosuretemperatures(Tc)ofthosetwominerals.Therefore,whentherateofoxygendiffusionandhenceTcforthePLIFchancetobecomparable(suchasinthecaseofquartzandmagnetite),TaewillserveasagoodapproximationofTcregardlessofvariationinmineralproportions.ThespecialtyofthePLIFinconstrainingTaewithintheirTcrangecanbegeneralizedtootherstableisotopesystemsandelementpartitioning.ByapproximatingTcwithTaeandinvertingDodson'sequation,thecoolingrateofplutonicormetamorphicrockscanbeinferred.
