Denis Horlait

Our understanding of atmosphere formation essentially relies on noble gases and their isotopes, with xenon (Xe) being a key tracer of the early planetary stages. A long-standing issue, however, is the origin of atmospheric depletion in Xe¹ and its light isotopes for the Earth² and Mars³. Here we report that feldspar and olivine samples confined at high pressures and high temperature with diluted Xe and krypton (Kr) in air or nitrogen are enriched in heavy Xe isotopes by +0.8 to +2.3‰ per amu, and strongly enriched in Xe over Kr. The upper +2.3‰ per amu value is a minimum because quantitative trapping of unreacted Xe, either in bubbles or adsorbed on the samples, is likely. In light of these results, we propose a scenario solving the missing Xe problem that involves multiple magma ocean stage events at the proto-planetary stage, combined with atmospheric loss. Each of these events results in trapping of Xe at depth and preferential retention of its heavy isotopes. In the case of the Earth, the heavy Xe fraction was later added to the secondary CI chondritic atmosphere through continental erosion and/or recycling of a Hadean felsic crust.

A new heating and gas treatment line for Thermo-Desorption Spectrometry (TDS) of noble gases (He, Ne, Ar, Kr, and Xe) is presented. It was built with the primary objective to offer advanced temperature controls and capabilities while working in a cold environment. By choosing a high-power continuous wave laser as the heating source and using a proportional–integral–derivative controller system, TDS of noble gases can now be performed with fast and highly steady heating ramps (e.g., less than 1 °C deviation from the set point for ≤1 °C s⁻¹ ramps). Sample temperature over 2000 °C can also routinely be reached, with limited heating of the sample support and the sample chamber, offering the possibility to have several samples awaiting in the ultra-high vacuum chamber. We also present the development efforts made to increase temperature homogeneity of the heated sample while limiting the contact with the sample holder. Recent results acquired with this TDS setup on krypton thermal diffusion in uranium dioxide (UO₂) as a function of O₂ additions are also presented as an application example.

The dependencies of the enhanced thermomechanical properties of zirconium carbide (ZrC_x) with sample purity and stoichiometry are still not understood due to discrepancies in the literature. Multiple researchers have recently reported a linear relation between the carbon to zirconium atomic ratio (C/Zr) and the lattice parameter, in contrast with a more established relationship that suggests that the lattice parameter value attains a maximum value at a C/Zr ~ 0.83. In this study, the relationship between C/Zr atomic ratio and the lattice parameter is critically assessed: it is found that recent studies reporting the thermophysical properties of ZrC_xhave unintentionally produced and characterised samples containing zirconium oxycarbide. To avoid such erroneous characterization of ZrC_xthermophysical properties in the future, we propose a method for the accurate measurement of the stoichiometry of ZrC_xusing three independent experimental techniques, namely: elemental analysis, thermogravimetric analysis and nuclear magnetic resonance spectroscopy. Although a large scatter in the results (ΔC/Zr = 0.07) from these different techniques was found when used independently, when combining the techniques together consistent values ofxin ZrC_xwere obtained.