Using volcanological models to understand the volatile drivers of eruptions on the Moon

  • Marissa Lo

Student thesis: Phd

Abstract

Volcanism is a key process linking the interior of a planetary body to its surface. The behaviour and speciation of different volatiles in magma is very sensitive to changes in chemistry, temperature, and pressure, across a wide parameter space relevant to different planetary systems. Therefore, understanding the behaviour of volatiles in magmatic and volcanic systems allows the volatile inventory of a planetary body to be constrained, and provides a great deal of information on the composition, formation, and history of the body. This thesis focuses on studying the behaviour and effect of volatiles in basaltic volcanic systems on the Moon and analogues for basaltic magma. In the first part of the thesis, I adapted a numerical model for terrestrial magma ascent to the lunar environment, with the aim of quantifying the volatile content of the magmas that erupted to form deposits of pyroclastic glass beads on the lunar surface. By conducting a sensitivity analysis on the magma ascent model, I concluded that, for lunar magmatic systems, CO and H2 have a greater control on magma ascent dynamics compared with H2O. The outputs from the magma ascent model were then coupled with a lunar pyroclast dispersal model, which calculated the ejection distance of pyroclastic glass beads. By simulating different initial magmatic volatile contents and comparing the results with measurements of observed lunar pyroclastic deposits, I found that most lunar pyroclastic deposits (up to ~10 km in radius) could be formed by the eruption of magma containing up to 0.3 wt.% volatiles. For the largest lunar deposits (>50 km in radius), a threshold of 1.3-1.4 wt.% volatiles is needed. Less than 5% of observed pyroclastic deposits exceed 50 km in radius; therefore, such volatile-rich regions of the lunar mantle must be spatially limited, but could represent regions that gained volatiles delivered by comets and asteroids, or represent volatile-rich residual melts from the crystallisation of the lunar magma ocean. By developing and combining the magma ascent and pyroclast dispersal models, I have devised a new method for constraining the volatile content of lunar magmas. Since gas flow dynamics within magma exert a strong control on the eruptive behaviour of volcanic systems, in the second part of this thesis I focussed on investigating gas flow dynamics within low viscosity magmas, such as terrestrial basalts or lunar picritic magmas. Specifically, I investigated whether gas flow through connected bubbles could facilitate open-system degassing in low viscosity volcanic systems. Using experiments where gas was injected into a column of hydroxyethyl cellulose solution, a magma analogue, I observed different gas flow regimes, including gas flow as bubble chains. Bubble chain phenomena have previously only been reported in a fluid mechanics context, and describe bubbles connected at their head and base by a narrow neck, forming a continuous chain. The rheology of the hydroxyethyl cellulose solution was characterised and was found to resemble the viscoelastic rheology of basaltic magma, supporting the hypothesis that bubble chains could form in low viscosity volcanic systems and facilitate open-system degassing.
Date of Award31 Dec 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorKatherine Joy (Supervisor), Mike Burton (Supervisor) & Margherita Polacci (Supervisor)

Keywords

  • magma
  • volcano
  • Moon
  • lunar
  • modelling
  • bubble chain

Cite this

'