Postdoctoral Fellowship: EAR-PF: The effects of grain-scale deformation on helium diffusion and thermochronometric ages of accessory minerals

Ross, Catherine Haggard, Austin TX

Investigators

Abstract

The Earth’s thermal structure is altered by many processes which show up in rocks as distinct geochemical and isotopic fingerprints. Thermochronology uses these isotopic records in minerals to infer thermal histories and the (U–Th)/He system is one of these tools. This technique’s uncertainties are relatively low and the method has yielded reproducible, geologically reasonable ages in many studies. Sometimes, however, the data are “overdispersed”, meaning that the spread in dates is greater than the quantified uncertainty, which can make datasets challenging to interpret. There could be several reasons for this, but one understudied reason is crystal defects. PI Ross aims to investigate this grain-scale process as a potential contributor to overdispersion by carrying out a cross-disciplinary study of how these defects within minerals affect helium diffusion. It has become increasingly important in the geology community to carefully characterize thermochronology samples to study a wider set of geological problems including better estimating erosion, constraining plate tectonics, exploring other planets, and developing energy and mineral resources for our future. PI Ross will mentor an undergraduate student by working with two well-established University of Colorado Boulder research training programs aimed at recruiting and retaining underrepresented students. PI Ross will also choreograph a dance that helps non-geologists understand how complicated deformation is accommodated on the atomic scale within a single crystal, bolstering the connection between the arts and sciences. Although the thermochronology community has studied many of the factors that affect He diffusion such as grain size, radiation damage, and anisotropy, there are still aspects of diffusion kinetics that are not well understood, as manifested by the overdispersion of many (U-Th)/He datasets. Defects due to deformation may substantially affect helium diffusion kinetics in minerals and therefore contribute to (U-Th)/He age dispersion in an underappreciated way. This project integrates detailed microscopic characterization via scanning and transmission electron microscopy of intragrain defects, He diffusion experiments (Continuous Ramped Heating and 4He/3He), and dating via conventional (U-Th)/He analyses and Laser-Ablation He mapping and spot analyses. By completing a variety of analyses yielding different types of insights on the same grains, a direct comparison and quantification can be made between single-grain deformation, diffusion kinetics, and age. The role that deformation plays in these processes at the sub-grain scale is understudied, despite frequent applications of these chronometers in deformed rocks from tectonically complex regions. The broader impacts of this project are two-fold: PI Ross will mentor underrepresented undergraduate students and create a “Dance of Dislocations”. This EAR-PF project has been fully supported by the EAR Tectonics program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.