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<p>Step-heating experiments constitute a key technique to study the release of volatile elements from geological materials as a function of temperature. In the case of noble gases (He, Ne, Ar, Kr, and Xe), step-heating is particularly useful to determine diffusion kinetics, structural defects, or spatial homogeneity within the material. These parameters are critical in the application of diffusion-based thermochronology such as the apatite (U-Th)/He system, where mapping out the spatial distribution of natural <sup>4</sup>He provides crucial information on the thermal history of apatite crystals. Characterizing the diffusion and distribution of <sup>4</sup>He via step-heating additionally has the potential to detect anomalously behaved grains and to directly constrain grain-to-grain variability in diffusivities within samples with significant radiation damage-induced age dispersion.</p> <p>Within the ERC-funded COOLER project, we aim to further the development of high-resolution, ultra-low temperature <sup>4</sup>He/<sup>3</sup>He thermochronology. To this end, we developed a new technique for precise step-heating experiments coupled with a diode laser including an inline single-wavelength pyrometer. The new protocol uses an all-alumina ceramic crucible fitted with a K-thermocouple ~0.1 mm below the center of the crucible pit. The head of the thermocouple is located directly below the sample within the ceramic matrix, allowing precise temperature measurements of the sample. The crucible is mounted on an alumina rod connected to a noble-gas preparation line. Gas released from the sample is purified and analyzed by a Thermo Scientific Helix SFT&#8482; multi-collector mass spectrometer. The sample is wrapped in Pt foil and indirectly illuminated with a diode laser. Laser and PID temperature controls are carried out by a custom LabVIEW program. Temperature calibration is performed by comparing measured and theoretical melting points of well-known materials loaded in the alumina crucible pit.</p> <p>Our initial results show very short response times for the thermocouple (a few seconds) and excellent agreement with the melting point of Indium (T<sub>melt</sub> = 157&#176;C). Although the current design is limited to hold only a single sample, it enables precise calibration of the emissivity value for a specific capsule assembly, which is a key parameter for pyrometer control of the temperature. Consequently, by calibrating the Pt capsule emissivity prior to the step-heating experiment, they can then be mounted in a multiple laser sample holder (up to 36 samples per chamber). The single-wavelength pyrometer of our system enables temperature measurements for large sample batches. Temperature is also cross-calibrated between the pyrometer and the thermocouple to ensure its correct reading. &#160;This new approach, coupled with analytical automation, will lead to significant improvement in the accessibility and efficiency of routine <sup>4</sup>He/<sup>3</sup>He analyses for geologic applications.</p>
<p>High-resolution <sup>4</sup>He/<sup>3</sup>He thermochronometry involves stepped-heat degassing of U and Th-bearing accessory minerals with simultaneous measurement of natural <sup>4</sup>He (non-uniform bulk distribution) and synthetically produced <sup>3</sup>He (uniform bulk distribution) at each step. The ratio evolution of <sup>4</sup>He/<sup>3</sup>He measured across all heating steps reflects the spatial distribution of <sup>4</sup>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 <sup>4</sup>He/<sup>3</sup>He method is commonly hindered by the necessity to uniformly synthesize <sup>3</sup>He within crystals at concentrations >1x10<sup>9</sup> atoms/mg for single grain analysis. This high concentration is required to ensure that the <sup>3</sup>He released at initial heating steps&#8212;where the most important geological information is preserved&#8212;is sufficiently above blank-detection limits of modern, highly-sensitive noble gas mass spectrometers. Synthesis of high <sup>3</sup>He concentrations is conventionally achieved via the spallation of targeted nuclei during high-energy proton irradiations to fluences >1x10<sup>15</sup> protons/cm<sup>2</sup>; 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 <sup>3</sup>He in geologic materials, and present preliminary <sup>4</sup>He/<sup>3</sup>He measurements on gem-quality Durango apatite using conventional and alternative approaches to induce <sup>3</sup>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 <sup>3</sup>He implantation via sample exposure to an energy-modulated <sup>3</sup>He beam. We discuss the advantages and disadvantages of both conventional and alternative methods in regards to <sup>3</sup>He uniformity, concentration limitations, crystal lattice damage, efficiency, post-experiment &#8216;cool-down&#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 <sup>4</sup>He/<sup>3</sup>He analyses for geologic applications.</p>
<p>Constraining the impact of Quaternary glaciations on landscape dynamics is required to better understand the interaction between tectonics, climate, and erosion. Over the years, low-temperature thermochronology such as apatite (U-Th)/He (AHe) has been used to quantify glacial erosion in different climatic and tectonic settings. However, in some contexts, AHe records lack temporal resolution because of limited exhumation due to glacial incision and/or low geothermal gradients. In addition, significant spatial variability in erosion can affect the quality of thermal-kinematic inversions when combining spatially distributed AHe data. This effect may be significant in glacial settings where a switch from a fluvial to a glacial landscape induced a significant change in the spatial distribution of erosion.</p> <p>However, the <sup>4</sup>He/<sup>3</sup>He thermochronology can extract lower-temperature and higher-resolution thermal histories from an AHe dataset. The method uses the spatial distribution of natural <sup>4</sup>He in an apatite crystal, which reflects the rate of cooling through the AHe partial retention zone. It has been successfully applied to track glacial incision and relief-development histories that would have been untraceable with conventional AHe thermochronology. Consequently, thermochronology data can now provide more detailed and localized thermal history. While <sup>4</sup>He/<sup>3</sup>He thermochronology has been successfully used in settings where background exhumation rates are moderate, the sensitivity of the technique remains untested in settings with notably low exhumation-rates, such as at passive margins.</p> <p>Here, we couple a glacial landscape-evolution model (iSOSIA) with a new version of a thermo-kinematic model (PecubeGUI), incorporating radiation-damage effects on helium diffusion, to explore the ability of apatite (U-Th)/He and <sup>4</sup>He/<sup>3</sup>He thermochronometers to record glacial incision. To do so, we model a range of synthetic glacial scenarios in different tectonic, climatic, and thermal settings. &#160;Our landscape-evolution models include glacial, fluvial and hillslope erosion, as well as sediment transport. We assess model predictions of thermochronologic parameters, including age-elevation relationships and <sup>4</sup>He/<sup>3</sup>He spectra, and their evolution when switching from a steady-state fluvial to a glacial topography. This modelling exercise aims to provide a guide for sampling strategies and interpretations for both conventional apatite (U-Th)/He and <sup>4</sup>He/<sup>3</sup>He thermochronology when working in glacial settings, considering their particular tectonic and climatic context.</p>
<p>Next generation, high-resolution datasets to assess the dynamics of geological systems are becoming increasingly important to answer scientific questions that require higher spatial and temporal resolution than the current state-of-the-art. Such questions involve the couplings and feedbacks between tectonic, climatic, and surficial processes that constitute a heavily debated topic in Earth-Systems research. Over the last decades, the insufficient temporal resolution of conventionally derived (U-Th)/He thermochronometric datasets has limited the necessary quantification to track recent changes in erosion rates and relief&#8212;two metrics essential to reconstruct the past dynamics of landscapes and evaluate the relative contribution of surface and tectonic processes on erosion.</p><p>To overcome this limitation, the ERC-funded COOLER project aims to further the development of high-resolution, ultra-low temperature thermochronology by setting up a world-leading <sup>4</sup>He/<sup>3</sup>He laboratory at the University of Potsdam. The centerpiece of the newly established laboratory is a split-flight-tube multi-collector gas-source sector mass spectrometer from Thermo Scientific&#8482; connected to a sample-gas preparation bench, which includes He gas purification equipment along with a diode laser for stepped-heat sample degassing. Important topics of research the instrument will be utilized for include 1) investigation of the glacial imprint on topography, 2) characterization of the couplings between tectonic activity and topographic relief development in response to glaciation, and 3) quantification of glacial erosion relative to fluvial erosion in mountain belts. In addition to serving researchers and students at the University of Potsdam and collaborating institutions, the facility will provide analytical, research, and educational opportunities within the frame of the COOLER project to researchers from across the globe through external workshops.</p><p>To illustrate the capabilities of the new laboratory, we present our analytical and experimental methodologies used to obtain reliable high-resolution <sup>4</sup>He/<sup>3</sup>He datasets. We focus on accuracy and cross-calibration to ensure minimal analytical bias in our measurements. Growing efforts in the (geo)science community are aimed at establishing best standardization practices and ensuring consistencies between laboratories and/or communities. Accordingly, we focus on ensuring that our methodologies are leading toward a noble-gas standardized method to compare mass spectrometry capabilities over various laboratories, and analytical techniques among the noble-gas communities. Accordingly, our standardized approach, coupled with analytical automation will lead to significant improvement in the accessibility and efficiency of routine <sup>4</sup>He/<sup>3</sup>He analyses for geologic applications.</p>
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