Synthesizing uniform 3He concentrations in accessory minerals for 4He/3He thermochronology: Current status, complications, and prospects
Authors: Colleps Cody , van der Beek Peter , Amalberti Julien , Bernard Maxime 
Published: March 2022

&lt;p&gt;High-resolution &lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He thermochronometry involves stepped-heat degassing of U and Th-bearing accessory minerals with simultaneous measurement of natural &lt;sup&gt;4&lt;/sup&gt;He (non-uniform bulk distribution) and synthetically produced &lt;sup&gt;3&lt;/sup&gt;He (uniform bulk distribution) at each step. The ratio evolution of &lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He measured across all heating steps reflects the spatial distribution of &lt;sup&gt;4&lt;/sup&gt;He 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 &lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He method is commonly hindered by the necessity to uniformly synthesize &lt;sup&gt;3&lt;/sup&gt;He within crystals at concentrations &gt;1x10&lt;sup&gt;9&lt;/sup&gt; atoms/mg for single grain analysis. This high concentration is required to ensure that the &lt;sup&gt;3&lt;/sup&gt;He released at initial heating steps&amp;#8212;where the most important geological information is preserved&amp;#8212;is sufficiently above blank-detection limits of modern, highly-sensitive noble gas mass spectrometers. Synthesis of high &lt;sup&gt;3&lt;/sup&gt;He concentrations is conventionally achieved via the spallation of targeted nuclei during high-energy proton irradiations to fluences &gt;1x10&lt;sup&gt;15&lt;/sup&gt; protons/cm&lt;sup&gt;2&lt;/sup&gt;; 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 &lt;sup&gt;3&lt;/sup&gt;He in geologic materials, and present preliminary &lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He measurements on gem-quality Durango apatite using conventional and alternative approaches to induce &lt;sup&gt;3&lt;/sup&gt;He 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 &lt;sup&gt;3&lt;/sup&gt;He implantation via sample exposure to an energy-modulated &lt;sup&gt;3&lt;/sup&gt;He beam. We discuss the advantages and disadvantages of both conventional and alternative methods in regards to &lt;sup&gt;3&lt;/sup&gt;He uniformity, concentration limitations, crystal lattice damage, efficiency, post-experiment &amp;#8216;cool-down&amp;#8217; 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 &lt;sup&gt;4&lt;/sup&gt;He/&lt;sup&gt;3&lt;/sup&gt;He analyses for geologic applications.&lt;/p&gt;