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Influence of radiation damage on He diffusivity in apatite: implications and applications to low-temperature thermochronology

  • Author(s): Willett, Chelsea Diane
  • Advisor(s): Shuster, David L
  • et al.

The radiogenic production and subsequent temperature-dependent diffusion of 4He in natural apatites provides means of constraining the thermal history of samples in the upper few kilometers of Earth’s crust. This technique, known as apatite (U–Th)/He thermochronology, has come into wide use in the past twenty years to quantify both rates and patterns of erosion in terrestrial settings. Critically important to the interpretation of these data, however, is the understanding of the diffusion of He and the effects of radiation damage over geologic time. Chapter 1 of this dissertation provides the background and basic concepts underlying the apatite (U–Th)/He thermochronology technique and describes the fundamental challenge of accounting for radiation damage when applying this technique. Chapter 2 describes one application of apatite (U–Th)/He thermochronology to a novel detrital study of glacially- transported cobbles in central Patagonia. This chapter provides evidence of a transient fast pulse of glacial incision with the onset of periodic glaciation in the region after ∼6 million years ago. As with all geochemical techniques used to study Earth science, the necessary assumptions made when interpreting data can have a consequential effect on the conclusions. Chapter 3 of this dissertation revisits a critical assumption—one regarding the role of radiation damage—that is made in the most often-used data interpretation models and then develops and proposes an alternate model (the alpha damage annealing model, or ADAM). In certain cases, but not all, the comparison between the ADAM and other radiation damage models shows that the radiation damage assumption can greatly affect the conclusions drawn from a data set. Additional experimental work aimed at improving the ADAM is described in Chapter 4, which finds that sample-dependent diffusion kinetics may be the key to interpreting challenging apatite (U–Th)/He thermochronologic data sets.

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