Hf isotope analysis and U-Pb geochronology of rutile : technique development and application to a lower crustal section (Ivrea-Verbano Zone, Italy)

Abstract

This thesis presents technical developments and geological applications for in situ uranium-lead (U-Pb) and hafnium (Hf) isotope analysis of rutile. A new protocol for in situ Hf isotope analysis of rutile is presented, and its accuracy and reproducibility demonstrated. Precision of ± 1-4 𝛆Hf units (95% confidence) is achieved for populations of 10-15 rutiles with ≥ 40 ppm Hf. A new rutile standard for in situ U-Pb dating, Wodgina, has been characterised by SHRIMP, TIMS and limited LA-ICPMS analysis. Wodgina allows SHRIMP Pb-U calibration with a precision of 3.5-5.5% (2σ).

A case study in the Permian lower crustal section of the Ivrea-Verbano Zone (IVZ) incorporated U-Pb dating, Zr thermometry and Hf isotope analysis of rutile. The results demonstrate the robustness of the Hf isotope system in rutile even under conditions of high temperature (>1000° C) metamorphism and crustal melting. The Zr-in-rutile thermometer is shown to be similarly robust, and in the IVZ records peak temperatures more faithfully than the Ti-in-zircon thermometer. Rutile is found to control bulk rock zirconium (Zr) and Hf in metapelites at high temperatures, due to the breakdown of zircon. Zircon dissolution controls the Hf isotope composition of subsequently-formed phases, which has implications for the interpretation of Hf isotope signatures in both metamorphic and magmatic settings. U-Pb ages of zircon from the IVZ record Permian growth and resetting events related to high temperature metamorphism and fluid influx, while U-Pb ages of rutile record cooling in the Jurassic and constrain the thermal evolution of the lower crustal section during rifting and exhumation. The IVZ case study demonstrates that trace element and isotopic analysis of rutile is a powerful tool for investigations in metamorphic petrology. Show more