02 : Research

Analysis of lunar glass beads demonstrates that at least part of the lunar mantle retains significant volatile contents [Saal et al. (2008), Hauri et al. (2015)]. These beads were most likely formed in fire-fountaining volcanic eruptions, a process in which the majority of the volatile contents within their parent melts were depleted. Despite this loss, a fraction of the volatile elements de-sublimated out of the volcanic gas clouds, forming thin (20-100 nm) coatings around the beads.

Recent models demonstrate that there is a correlation between these condensed coatings and the chemical composition and physical conditions - namely pressure, temperature, and oxygen fugacity - of the volcanic gases from which they originate [Renggli et al. (2017), Zolotov and Liu, (2021), Liu & Ma, (2022)]. This relationship implies that the petrology, geochemistry, and stratigraphy of these coatings can provide insights into the composition and evolution of gases emitted by lunar volcanoes. Consequently, these findings hold the potential to increase our understanding of lunar volatiles.

To this end, I am using a comprehensive suite of nanoanalytical techniques including Nanoscale Secondary Ion Mass Spectrometry, Transmission Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Electron Energy Loss Spectroscopy, and Atom Probe Tomography to investigate the mineralogy and stratigraphy of sublimate coatings on pristine glass beads that have never been exposed to air. This information is coupled with thermodynamic modelling, with the primary goal of reconstructing the pressure, temperature, and compositional evolution of the lost lunar volcanic gas cloud.

High ³He/⁴He ratios in Ocean Island Basalts (OIBs) are viewed as evidence for sampling of an undegassed primitive mantle source by mantle plumes. However, this does not explain why helium concentrations and the elemental ratios He/Ar and He/Ne in OIBs are an order of magnitude lower than in Mid-Ocean Ridge Basalts (MORBs). This discrepancy, often referred to as the 'Helium Paradox', challenges our understanding of mantle degassing processes. Various studies invoke disequilibrium degassing as the solution to this problem [Gonnermann and Mukhopadhyay (2007), Weston (2015)]. However, there is significant disagreement between current models.

In response to these unresolved questions, we are developing a new model that more accurately reflects the behavior of gas bubbles during the degassing of noble gases. We employ a lattice Boltzmann method for free surface flows, based on the approach of Körner et al. (2005). This method is particularly suited for our purposes as it can simulate the solubility-pressure relationships of CO₂ and noble gases, account for hydrodynamic interactions between bubbles, and, importantly, model the nucleation of new bubbles driven by volatile supersaturation.

Such nucleation events are critical during the ascent of magma, as they can counteract kinetic fractionation by reducing the diffusional length scale. To the best of our knowledge, ours is the first degassing model to incorporate the nucleation of new bubbles with realistic physics, Through this work, we aim to offer a more comprehensive framework for interpreting noble gas signatures in MORBs and OIBs, thereby addressing a key aspect of the Helium Paradox.