Measuring Noble Gases for Thermochronology

To extract gases from rocks and minerals in the laboratory, we often take advantage of the same properties that make them useful for thermochronology: they diffuse with an exponential dependence on temperature, and they are highly incompatible elements. Heating rocks and minerals to high temperatures in an ultrahigh vacuum (<10−9 torr) liberates noble gases, either by diffusion in stable phases or though melting. In the early days, this heating was usually by radiofrequency induction, but labs turned to double-vacuum resistance furnaces that isolate the sample vacuum from the vacuum surrounding the heating element, reducing exposure of the sample to gases sourced from heated metal. Today, heating and extraction with a laser is also very common. Laser heating typically adds a lower background signal than furnaces because lasers heat a smaller volume, and they have become cheaper and easier to work with. Lasers can either couple directly with rocks and minerals to achieve heating or can heat samples indirectly by coupling with metal packets holding sample material (e.g., House et al. 2000; see title image in Gautheron and Zeitler 2020 this issue). An advantage of both furnace and indirect laser heating is that they can be used to control sample temperatures accurately and precisely, which is necessary for quantifying the kinetics of noble gas diffusion. However, conducting such measurements can be technically challenging and does require care in experimental design. A less frequently used way to extract noble gases is laser ablation, whereby the surface of a sample is removed (ablated) with a high-energy, short-wavelength laser pulse (e.g., Boyce et al. 2006). This approach can be used to extract noble gases from a targeted subregion of a mineral or rock, but is often challenging because, in most minerals, small ablated volumes release proportionately small amounts of noble gases.