Synthesizing uniform 3He concentrations in accessory minerals for 4He/3He thermochronology: Current status, complications, and prospects
- University of Potsdam, Institute of Geosciences, Potsdam-Golm, Germany (email@example.com)
High-resolution 4He/3He thermochronometry involves stepped-heat degassing of U and Th-bearing accessory minerals with simultaneous measurement of natural 4He (non-uniform bulk distribution) and synthetically produced 3He (uniform bulk distribution) at each step. The ratio evolution of 4He/3He measured across all heating steps reflects the spatial distribution of 4He within a single crystal, which can be coupled with its (U-Th)/He date to model high-resolution low-temperature thermal histories. Although an exceptionally powerful tool to elucidate disputed drivers of crustal exhumation in various geologic settings (e.g., climatic vs. tectonic mechanisms), the 4He/3He method is commonly hindered by the necessity to uniformly synthesize 3He within crystals at concentrations >1x109 atoms/mg for single grain analysis. This high concentration is required to ensure that the 3He released at initial heating steps—where the most important geological information is preserved—is sufficiently above blank-detection limits of modern, highly-sensitive noble gas mass spectrometers. Synthesis of high 3He concentrations is conventionally achieved via the spallation of targeted nuclei during high-energy proton irradiations to fluences >1x1015 protons/cm2; however, facilities capable of, or willing to, efficiently carry out such anomalously high-fluence irradiations using previously defined methods remain few and far between. Here, we summarize the current state-of-the-art of synthesizing uniform distributions of 3He in geologic materials, and present preliminary 4He/3He measurements on gem-quality Durango apatite using conventional and alternative approaches to induce 3He to sufficient concentrations. Alternative approaches include (1) in-vacuum proton-irradiation with a narrowly focused proton beam to maximize intensities for short-duration experiments, and (2) direct uniform 3He implantation via sample exposure to an energy-modulated 3He beam. We discuss the advantages and disadvantages of both conventional and alternative methods in regards to 3He uniformity, concentration limitations, crystal lattice damage, efficiency, post-experiment ‘cool-down’ times, and accessibility. Both alternative approaches are considerably less demanding on particle accelerator facilities, and can significantly reduce the post-experiment waiting time required to safely handle activated samples. Accordingly, these approaches, if proven successful, yield great promise to improve the accessibility and efficiency of routine 4He/3He analyses for geologic applications.
How to cite: Colleps, C., van der Beek, P., Amalberti, J., and Bernard, M.: Synthesizing uniform 3He concentrations in accessory minerals for 4He/3He thermochronology: Current status, complications, and prospects, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3544, https://doi.org/10.5194/egusphere-egu22-3544, 2022.