Synthesizing uniform 3He concentrations in accessory minerals for 4He/3He thermochronology: Current status, complications, and prospects

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.