Calibration of Rutile (U-Th)/He Thermochronology: assessing the thermal evolution of the KTB drill hole, Germany and adjacent Bohemian Massif

Wolfe, Melissa Renee

Publication

Calibration of Rutile (U-Th)/He Thermochronology: assessing the thermal evolution of the KTB drill hole, Germany and adjacent Bohemian Massif

Wolfe, Melissa Renee

Abstract

This study empirically calibrates zircon and rutile (U-Th)/He dating by evaluating the experimentally-derived He diffusion kinetics and their extrapolation over geological time by (U-Th)/He dating a suite of borehole samples with a welldefined thermal history. Age dating is coupled with cycled step-heating diffusion experiments and thermal modeling of zircon and rutile. The German Continental Deep Drilling Project (KTB) is ideally suited for this approach given its depth and zircon- and rutile-bearing metamorphic rocks. Results from zircon (U-Th)/He analyses display a helium partial retention zone between ~140-210°C which agrees well with laboratory-derived He diffusion kinetics. While regional rutile (U-Th)/He age data confirm the feasibility of rutile (U-Th)/He thermochronometry (Tc ≈ 220°C) in eclogites and granulites, anomalous rutile (U-Th)/He ages from the KTB drill hole illustrate the implications of retrograde rutile breakdown to titanite and the importance of petrographic characterization of rutile for obtaining reliable and meaningful thermochronometric data.