Lunar Stories Unfolded by Volatile Elements: From Water in the Lunar Mantle to Volcanic Eruptions on the Lunar Surface

Abstract

The Moon, as the Earth's sole natural satellite, offers unique insights into the formation and evolution of both the Moon-Earth system and the broader solar system. Despite the wealth of knowledge garnered from the lunar missions, many aspects of the Moon remain enigmatic. In this dissertation, I use lunar samples collected from the Apollo missions and computational models to answer key science questions about the Moon. My research specifically targets enhancing our understanding of 1) the lunar mantle's composition, with a particular focus on its water content, as a proxy for the Moon’s formation and evolution history and its relation to the Earth, and 2) the processes that occurred during ancient lunar volcanic eruptions that shaped the current Moon.

In Chapter 2, mineral-hosted melt inclusions in various Apollo samples were studied to investigate the abundance of volatiles in the primitive lunar mantle like water. The elemental compositions of the prepared melt inclusions were measured and compared to the data of other lunar melt inclusions with different post-eruptive cooling rates. Our results provide key new data on lunar melt inclusions and further establish the positive relationship between H2O/Ce ratios of melt inclusions and their cooling rates. It indicates that the lunar sample with the highest cooling rate was least affected by diffusive water loss during eruptions and best represents the pre-eruptive H2O/Ce ratio in the lunar basalts and further the lunar mantle. Based on the melt inclusion data, we estimate concentrations of volatiles in the primitive lunar mantle.

Chapter 3 is focused on lunar volcanic orange glass beads to understand the processes occurred during ancient fire-fountain eruptions on the lunar surface. The distributions of moderately volatile element Na, K and Cu in the beads were measured and results show that these elements have inward diffusion from bead surfaces to interiors forming a “U” shaped profile, which has not been discovered before. We propose that the observed trend was formed due to a new and undiscovered process named in-gassing. A quantitative model was developed to simulate the concentration evolution of these moderately volatile elements in the orange beads, which successfully reproduced the observed trends and constrained the individual cooling history of the orange beads.

In Chapter 4, we expanded the study from Chapter 3 and examined sulfur in the orange glass beads. Our analyses reveal that sulfur exhibits a non-uniform distribution across the beads, displaying "U" or "W" shaped profiles typical of in-gassing. Sulfur contributions from different sources (magmatic versus atmospheric) in the orange beads were evaluated which indicates significant atmospheric contribution. In-gassed sulfur of atmospheric origin could undergo photochemical reactions induced by UV light, leading to mass independent fractionation and a distinct sulfur isotope signature. These findings provide new insights into the complex dynamics of volatile elements in lunar volcanic processes, highlighting the role of in-gassing in shaping sulfur isotope signatures in volcanic glass beads. Our work can be applied to explaining sulfur isotope anomaly observed in orange beads and reconstructing the lunar atmospheric composition.