The core of the COOLER project involves developing a world-leading new 4He/3He thermochronology in Europe. Setup of this facility started with delivery of a noble-gas mass spectrometer (Thermo-Fisher Helix SFT) and preparation line in December 2020. Two items that were ordered externally, the diode laser and cryogenic trap, suffered from delays due to supply-chain issues and were delivered in March and June 2021, respectively. Testing and calibration of the instrument started immediately after its completion under the responsibility of the lab manager (Julien Amalberti; JA), using both gas and mineral standards. Both gave very satisfactory results regarding accuracy and precision, demonstrating that our setup can deliver world-class 4He/3He data. JA spent significant time developing an automated sample processing scheme, which will lead to significant time gains when routine measurements on natural samples start. Automation has required making some modifications to the preparation line, which are being implemented at this time.
As soon as we received our first irradiated samples (see below), we have started analysing these to both test the irradiation protocols developed and further calibrate the instrument. Initial measurements focused on full degassing of samples, to quantify the amount of 3He generated by the irradiations. Subsequent step-heating experiments allowed verifying homogenous production of 3He during the irradiations. Calibrating the laser for step-heating experiments has proven challenging, in part because the pyrometer delivered with the laser is not completely suited for our needs. JA has developed an in-house sample holder that allows precise and accurate temperature readings using a thermocouple, which we will use to calibrate the laser and pyrometer. This is the last required step before the instrument will be ready for routine use.
To date, a total of 5 irradiation experiments have been conducted under the responsibility of post-doc 1 (Cody Colleps; CC): one at the Paul Scherrer Institute (PSI; July 21), and four at the Helmholtz Zentrum Berlin (HZB; Sept. & Dec. 21; March 22; Jan. 23). The irradiation conducted at PSI involved a conventional sample assembly exposed to a 1-cm-wide proton beam at 2 nA and 230 MeV. Due to regulations that limit beam intensities at PSI, samples only reached a total fluence of 2 x 1014 protons/cm2 after 28 hours of exposure—nearly an order of magnitude less than the required >1 x 1015 protons/cm2. The HZB experiments involved a new protocol of in-vacuum irradiations with a 3-mm-wide, 68 MeV proton beam at exceptionally high intensities. The first HZB irradiation successfully revealed that the maximum-permitted beam energy of 68 MeV was sufficient to activate apatite for 3He production, and some grains were exposed to a fluence >1 x 1015 protons/cm2 within only 15 minutes of exposure. The second HZB irradiation focused on improving the in-vacuum sample beam-alignment and was used to assess the distribution of 3He produced across the sample stack. The third HZB irradiation incorporated a beam-scattering foil that improved the uniformity of exposure across the sample stack. Uniform 3He concentrations were induced in most samples, but improvements were still necessary. The fourth test irradiation refined the sample-stack geometry (informed by computer simulations) to ensure that 3He is uniformly induced in all mounted grains. Samples from the fourth HZB irradiation will be analyzed as soon as activity levels are low enough to safely handle.
We have further developed a thermal-kinematic code and coupled it with a glacial erosion model. Post-doc 2 (Maxime Bernard; MB) has redesigned the popular Pecube code to enable sample-specific thermochronometer predictions using quantitative multi-kinetic models for He diffusion and fission-track annealing in apatite and zircon. MB also developed a Graphical User Interface for the code that largely facilitates its running. User testing of the code is currently in progress before its planned release as an Open-Source package coupled with a publication this year. MB has worked on coupling of Pecube with the glacial-erosion model iSOSIA and running coupled models to develop the conceptual framework that will guide field sample collection. These models focus on both the European Alps the Norwegian. In parallel, PI Peter van der Beek has developed a simpler model for rapid synoptic interpretation of large datasets, in collaboration with the PI of the ERC Consolidator project Gyroscope T. Schildgen. This code, Age2exhume, has been published recently as is freely available, together with an example dataset from the Himalaya.
A first sampling campaign in the western Alps (Switzerland) was performed in July 2021; collected samples were prepared by CC and have been included in the most recent irradiation. These will be the first natural samples to be analysed in the next few months. Coupled iSOSIA-Pecube models developed by MB (see above) have focused on this region and will serve as a quantitative guide to interpreting the data. A PhD student (Isabel Wapenhans; IW) started in the project in June 2022; she will be involved in further analysis of Alpine samples. A field campaign in Norway (Jostedalsbreen) in July 2022 allowed collecting a suite of strategic samples for quantifying the amount and timing of glacial fjord incision as well as the pre-glacial topography of the area. These samples will be analysed by MB. Sampling was undertaken in August 2022 in the Hohe Tauern area (Eastern Alps) to provide IW with initial samples for analysis and comparison/contrasting to the western Alps data.
