02 : Research

Lunar pyroclastic glass beads preserve a record of physical and chemical conditions within volcanic gas clouds in the form of nanoscale minerals vapour-deposited onto their surfaces. However, the scale of these mineral deposits - less than 100 nm - has presented challenges for detailed analysis. Using SEM, TEM, APT, and NanoSIMS, we analysed pristine black glass beads from Apollo drive tube 74001 and found a sequence of sulfide depositionthat directly evidences lunar gas cloud evolution. The deposits are predominantly micromound structures ofnanopolycrystalline sphalerite ((Zn,Fe)S), with iron enrichment at the bead-micromound interface. Thermo-chemical modelling indicates that hydrogen and sulfur were major elements within the volcanic plume and ties the iron gradient to decreasing gas pressure during deposition. This pressure drop may also be consistent with our observed trend of δ34 S depletion. Finally, Apollo 17 74220 orange beads, deposited higher in the Shorty Crater sequence, appear to lack abundant ZnS nanocrystals (Liu and Ma, 2024a), suggesting a change in vapour deposition between orange- and black-glass bead deposition. Together, our results suggest a change in eruption style over the course of a pyroclastic volcanic eruption in the Taurus-Littrow Valley.

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Noble gas concentrations in ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are affected by degassing processes, obscuring their pre-eruptive characteristics. Although it is common to correct for degassing by assuming near-equilibrium partitioning between melt and vapour, kinetic disequilibrium may significantly impact measured noble gas ratios. For example, kinetic effects may allow for OIBs to obtain lower ³He contents than MORBS, even if OIB plumes sample a low ⁴He/³He "undegassed" primordial lower mantle - the origins of the so-called "Helium Paradox" (Gonnermann and Mukhopadhyay, 2007). Additionally, disequilibrium effects may be consistent with OIB and MORB sources sharing a homogenous pre-degassing ³He/²²Ne ratio. However, inferred mantle compositions may not require the presence of an "undegassed" reservoir, and there is evidence that OIB and MORB sources exhibit heterogenous ³He/²²Ne ratios.

We have developed a Lattice Boltzmann method for free surface flows to model the kinetics of CO₂ and noble gas exchange between bubbles and melt during magmatic ascent to evaluate the magnitude of non-equilibrium fractionation. Across plausible ranges of ascent rate, bubble populations, and noble gas diffusivity, our results indicate that disequilibrium cannot induce variations in ³He contents and ³He/²²Ne ratios sufficient to reproduce the range observed in OIB and MORB. Therefore, OIB and MORB sources cannot share a homogenous ³He/²²Ne ratio, and disequilibrium does not resolve the "Helium Paradox".

This manuscript has beent submitted to G-Cubed.