The Granulite Uncertainty Principle: Limitations on Thermobarometry in Granulites

Abstract

There are three geothermometers based on reversed experimental data and applicable to granulites: the two-pyroxene, two-oxide, and garnet-clinopyroxene thermometers. All have apparent closure temperatures below those of the granulite facies. This casts significant doubt on the concept that "peak" temperatures are routinely obtained from ion-exchange thermometers, particularly those that are empirically calibrated. The best way to recover high temperatures in granulite terrains is through the reintegration of exsolution lamellae or the use of relict mineral assemblages; we call these features fossil thermometers. Because fossil thermometers can be destroyed by deformation, in many terrains it may be impossible to tell whether the temperature they record is a true maximum or whether it is a temperature locked in during cooling. Low closure temperatures also affect the oxygen fugacites obtained from two-oxide equilibria. Compilation of the available data indicate that a number of granulite terrains either crystallized on the graphite saturation surface or underwent re-equilibration until they intersected this surface. Subsequent oxyexsolution appears to have taken place on the graphite saturation surface by the reaction: $$6 Fe_{2}TiO_{4} + CO_{2} = 2 Fe_{3}O_{4} + 6 FeTiO_{3} + C$$, suggesting that a $$CO_{2}$$ vapor phase was present during the cooling history of these granulites. Recognition of the low closure temperature of ion-exchange thermometers also calls into question the validity of P-T paths calculated from retrograde garnet zoning profiles. Because geobarometers are likely to lock in at higher temperature than do geothermometers, the P-T paths obtained from the zoned rims of granulite-grade garnets may be artifacts of the differential closure temperatures for both the barometer and the thermometer. Consideration of the garnet-plagioclase-aluminosilicate-quartz, garnet-rutile-aluminosilicate-ilmenite-quartz, and garnet-plagioclase-orthopyroxene-quartz barometers indicates that the apparent P-T path obtained due to differential closure temperatures will be indicative of isobaric cooling, regardless of the actual P-T path followed by the rock. This problem is illustrated with an example from the Kerala Khondalite Belt of southern India in which the garnet zoning profile records an apparent isobaric cooling path, although fossil thermobarometry strongly suggests that the rock underwent considerable decompression during cooling.