Geothermometry in the Kiglapait Aureole: Part II. Evaluation of Exchange Thermometry in a Well-Constrained Thermal Setting

Previous determination of a well-constrained thermal profilerepresenting peak conditions of metamorphism for the contactaureole of the Kiglapait Intrusion, Labrador, provides the basisfor critical comparative geothermometry of cation exchange thermometersapplied to mafic and ultramafic granulites. Knowledge of theshape of the profile and constraints on the temperature of theintruding magma allow calculation of cooling rates which rangefrom 150?C/m.y. at the contact to 30?C/m.y. at 2500 m from thecontact. Substitution of these rates in diffusion equationspermits discrimination between geothermometers which are eitheradequately or poorly calibrated, as well as thermometers whichare probably inappropriate for use in metamorphic terranes.Our results suggest that the two-pyroxene thermometer (Lindsley& Andersen, 1983) is accurate from 950 to 750?C. Below 750?C,both limbs of the miscibility gap require at least minor recalibration,or the projection scheme of Lindsley & Andersen (1983) requiresadjustment. Results from the cpx-ilm thermometer are erraticand imply that the solution models for pyroxene and ilmenite,as presently formulated in this thermometer, are inadequate.Results from the opx-ilm, ol-ilm, and opx-ol thermometers areinconsistent. Theoretical calculations at both high and lowtemperatures show that these three thermometers cannot giveinternally consistent results. Results from Fe-Ti oxide thermometrysuggest that these minerals are easily re-equilibrated due tocation interdiffusion between grains. However some grain pairsappear to retain compositions appropriate to peak thermal conditions.