The Effects of Radiation Damage and Crystallography on Helium Diffusion in Minerals: An Integrated Bulk and Laser Ablation Depth Profiling Study

Arizona State University, Scottsdale AZ

Investigators

Abstract

Thermochronology - the study of temperature histories for geologic samples using the tools of isotope and nuclear geochemistry - has evolved into a fundamental part of earth system research. Just as the geochemistry community in the 1970's and 1980's led an explosion in the development of new isotopic tracers as probes of mantle and crustal evolution, the thermochronology community is now driving a rapid expansion in the temperature range and precision of temperature history reconstructions. Such reconstructions have a variety of applications to understanding aspects of earth system evolution relevant to society. Reconstructing the thermal history of sedimentary basins is essential for our determination of oil and gas potential. More generally, we know that thermal histories are of rocks in the upper crust are strongly influenced by erosional processes. Those processes are, in turn, influenced by climatic variations such that low-temperature thermochronometry - the focus of this research project - has become our primary means of reconstructing past climate fluctuations on timescales of millions of years. Such information is essential for us to establish long-term climate variations that can help us to better compare current climate trends to those further back in earth history, and to more fully evaluate the societal implications of extreme changes in climate. The practical utility of mineral-isotopic systems for thermochronometry, is limited by the quality of our knowledge of helium diffusion kinetics. Recent experimental studies of apatite and zircon suggest that radiation damage substantially affects the diffusivity of helium. Recent studies of zircon suggest a strong helium diffusion anisotropy (orders of magnitude greater parallel to the c crystallographic direction). The investigators propose a study aimed at systematically quantifying both of these influences on helium diffusion on a variety of minerals with proven or anticipated value for (U-Th)/He thermochronometry: monazite, rutile, titanite, xenotime, and zircon. They will prepare samples with various levels of radiation damage through thermal annealing and proton irradiation for two kinds of diffusion experiments: 1) the more familiar incremental heating, bulk diffusion approach through which most of the currently available helium diffusion data were obtained; and 2) a newly developed laser ablation depth profiling approach that enables the interrogation of laboratory induced diffusion profiles in predetermined crystallographic orientations. The ultimate goal of the project will be to contribute to the development of a schema for assigning appropriately unique diffusion parameters to individual crystals so that themochronologists can more confidently use measured dates for thermal history reconstructions.