EXPANDING THE TEMPERATURE SENSITIVITY RANGE AND APPLICABILITY OF THE (U-TH)/HE SYSTEM: SOME EXAMPLES

The field of (U-Th)/He thermochronology is rapidly changing with the continuing development of new He thermochronometers and their novel application to an increasing array of Earth science problems. The emergence of new thermochronometers has increased the wealth of thermal information accessible through the He system, and extended the variety of lithologies amenable to thermochronologic analysis. Moreover, improved understanding of how radiation damage affects the He diffusion kinetics of the two minerals most widely used in (U-Th)/He studies – apatite and zircon – has expanded the temperature range that we can decipher using these minerals alone. The radiation damage effect can cause the closure temperatures of these minerals to vary by 10’s of °C and thus lead to complex datasets, but if treated carefully this complexity can allow for extraction of detailed thermal history information.

Here we give several examples of our work to both develop new He thermochronometers and refine our understanding of existing ones. Our typical approach is to use data from better characterized thermochronometers as a natural calibration experiment and/or conduct incremental outgassing experiments in the lab to explore the mineral diffusivity characteristics, with the goal of applying the tool to a geologic problem that capitalizes on the new development. Our studies include: 1) work on zircon from Proterozoic basement in the Colorado Front Range that sheds insight into the temperature sensitivity of high-damage zircon, 2) investigation of cratonic titanites, from which we find a strong influence of radiation damage on titanite He retentivity, 3) analysis of perovskite from kimberlites to date pipe emplacement, 4) a multichronometer He study of the McClure Mountain syenite to evaluate relative mineral temperature sensitivities, and 5) preliminary study of the conodont (U-Th)/He system.