Eclogites of Taconian age from the Eastern Blue Ridge in Bakersville, North Carolina record evidence of two episodes of metamorphism, one during the rock’s initial formation and another during its subsequent exhumation. The eclogite, metamorphosed from a basaltic protolith, consists of garnet + clinopyroxene + hornblende + plagioclase + quartz + rutile +/- orthopyroxene. A limitation of thermometry methods reliant upon major elements is their susceptibility to diffusion and resetting. In contrast, trace elements can be more resistant to diffusion and therefore may be able to more accurately record their host crystal’s formation temperatures. Zr-in-rutile thermometry is able to record temperatures up to 1190°C, and is therefore appropriate for reconstructing HT and UHT conditions.

As such, we apply zirconium-in-rutile thermometry to the Bakersville eclogite in order to test the efficacy of the method in cases of metamorphic overprinting, and to determine whether prograde rutile inclusions in garnet record different temperatures than matrix rutiles. Rutiles were categorized according to placement as inclusions or in the matrix, and trace element spot analyses were conducted with LA-ICP-MS. Analysis of one sample yields an average Zr concentration of 1175 ppm in the matrix rutiles, while the average Zr concentration in the inclusion rutiles is 1633 ppm. This corresponds to an average matrix rutile temperature of 805 ± 42°C and an average inclusion rutile temperature of 765 ± 42°C.  

The similarity between inclusion and matrix rutile temperatures suggests that both populations record peak or near-peak conditions of the rock’s metamorphism. These temperatures overlap with upper temperature estimates produced by cation exchange thermometry and equilibrium assemblage diagrams (EAD). The high temperatures obtained via Zr-in-rutile thermometry align with the post-peak stability of orthopyroxene indicated by EADs. The temperatures from the Bakersville eclogite approach those of the contemporaneous Winding Stair Granulite: the highest temperatures recorded in the Southern Appalachians (~850°C).

Ongoing work is using LA-ICP-MS to create trace element maps of garnet and inclusions, applying Zr-in-rutile thermometry to a wider range of samples, and comparing results with temperatures derived from additional trace element thermometers from different inclusions in garnets. Zr-in-rutile thermometry has refined estimates of the peak temperature conditions experienced by the Bakersville eclogite. Furthermore, these results suggest that Zr-in-rutile thermometry is well suited to reconstruct the temperature history of rocks that have undergone multiple pulses of metamorphism.