New high-resolution step heating experiments using a coupled Diode laser and thermocouple for thermochronology applications

Abstract

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 4He provides crucial information on the thermal history of apatite crystals. Characterizing the diffusion and distribution of 4He 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.Within the ERC-funded COOLER project, we aim to further the development of high-resolution, ultra-low temperature 4He/3He 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™ 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.Our initial results show very short response times for the thermocouple (a few seconds) and excellent agreement with the melting point of Indium (Tmelt = 157°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.  This new approach, coupled with analytical automation, will lead to significant improvement in the accessibility and efficiency of routine 4He/3He analyses for geologic applications.