Durgalakshmi1, Ian S. Williams1, K. Sajeev2
1 Research School of Earth Sciences, Australian National University, Canberra, Australia, 2 Centre for Earth Sciences, Indian Institute of Science, Bengaluru, India
Rutile (TiO2) is a common accessory mineral in hydrothermal and metamorphic rocks that is stable across a wide range of P-T conditions. It can incorporate up to 200 ppm of U, and has a lower closure temperature than zircon, making it a reliable mineral with which to date retrograde metamorphism and low- to medium-grade metamorphic events by the U-Pb technique. Further, Pb and Th are incompatible in rutile, so corrections for its small initial Pb can be made accurately using its 208Pb content.
During prograde metamorphism, rutile forms following the breakdown of biotite or ilmenite as part of a continuous reaction. During retrogression, rutile can be replaced by ilmenite and titanite. Secondary rutile can be formed by hydrothermal alteration, oxidation or exsolution. In high-grade metamorphic rocks, rutile occurs as single crystals in the matrix and/or as inclusions in other minerals such as garnet, pyroxene and amphibole. In low- to medium-grade rocks it usually occurs as needles or polycrystalline aggregates. Using an ion microprobe (SIMS), rutile can be dated in its textural context in thin section, providing age information directly linked to metamorphic reactions. Dating and trace element analysis of separated rutile grains have a wide range of applications from sedimentary provenance studies to dating vein mineralisation and granitic pegmatites that host mineral deposits.
In the Neoarchæan Southern Granulite Terrane, India, rutile preserves a record of the late thermal history that is not provided by other datable minerals such as zircon or monazite. One studied sample of granulite grade felsic gneiss contains at least three distinct generations of rutile that preserve a range of Neoproterozoic to early Palæozoic U-Pb ages. The zircon from the same sample is Neoarchæan, with no evidence of a younger component. The rutile occurs as single ~ 0.5–0.8 mm crystals in the matrix, some of which are rimmed by titanite. Rutile-forming reactions, which can be linked to the metamorphic conditions, have been dated, contributing to unravelling the polymetamorphic and tectonic history of this complex terrane.
Miss Durgalakshmi is a PhD student at Research School of Earth Sciences, ANU. She works on the Archaean rocks of Southern Granulite Terrane, India.