Zircon Exsolution from Rutile: Constraints on Ultra-High Temperature Metamorphism of Lower Crustal Granulites
College of Arts and Sciences
Zircon exsolution from rutile in ca 2.7Ga ultra-high temperature (UHT; >900°C) lower-crustal granulite xenoliths from South Africa provides a record of the evolving conditions of extreme metamorphism of continental crust. In back-scatter electron images, rutile grains show a range of textures from a homogeneous single-phase to crystals with zircon exsolution as either lamellae oriented along crystallographic planes or as larger crystallites. By measuring the Zr content of rutile and reintegrating zircon into the rutile, we can calculate the peak temperatures the rocks reached during metamorphism using the Zr-in-rutile thermometer. LA-ICP-MS was used to sample the age and Zr content of rutile grains, targeting discrete-phase rutile with a small beam diameter, and also attempting to reintegrate multi-phase rutile and fine zircon lamellae using a large beam diameter. Temperature-time plots show that rutile crystals with evenly distributed exsolution of zircon have analytically reintegrated temperatures ranging from 900-1000°C, consistent with growth during UHT metamorphism. By contrast single-phase rutile temperatures, both in grains without zircon or in grains with coarser zircon lamellae, cluster round 550-650°C. 206Pb/238U ages of single-phase rutile range from 500-1000Ma suggesting that these rocks experienced a younger episode of upper-amphibolite to granulite facies metamorphism that modified the lower-crust subsequent to 2.7Ga UHT metamorphism.
Leach, Travis, "Zircon Exsolution from Rutile: Constraints on Ultra-High Temperature Metamorphism of Lower Crustal Granulites" (2018). 2018 Undergraduate Research and Scholarship Conference. 82.