New Insights into Lunar Basaltic Magmatism: A Study of Volatiles, Vesicles, and Volcanics

Abstract

Within our Solar System, diverse planetary bodies show evidence of volcanic processes, ranging from ancient to on-going activity. The Earth’s Moon is an important witness to volcan-ism owing to its well-preserved rock record. Unlike Earth, whose plate tectonics erase geologi-cal history, the Moon’s single-plate world allows for billions of years of volcanic activity to be preserved on its surface. The products of lunar volcanism are lava flows that constitute the ‘maria’ and pyroclastic deposits. Apollo-era missions returned to Earth with material from both varieties of lunar volcanism, and these materials display remarkable chemical and isotopic di-versity. Without definitive mantle samples in the lunar sample collection, mantle differenti-ates, such as basalts, are key to understanding the thermochemical evolution of the Moon’s in-terior. Mare basalts are subdivided based upon their titanium concentrations (i.e., high-Ti composed of >6 wt.% TiO2, low-Ti composed of 1.5-6.0 wt.% TiO2, and very low-Ti composed of <1.5 wt.% TiO2). In this dissertation, I investigate the eruptive and crystallization histories of samples belonging to each TiO2 grouping through detailed mineralogic and petrologic study. I place emphasis on the analysis of apatite, as it is a ubiquitous mineral in lunar samples and can accommodate variable amounts of F, Cl, and OH. Apatite typically crystallizes late in mare basalts and is key to understanding the volatile histories of lunar volcanics. Vesicles and vugs within mare basalts represent small cavities formed from vapor bub-bles trapped within magma/lava, and their presence is an important testament to gas-rich vol-canic activity on the Moon. Gaining an understanding of the volume and morphology of vugs and vesicles can provide valuable information on the eruptive histories of these samples. Since mare basalts were largely collected from the lunar regolith, rather than their respective lava flows, I couple mineral chemistry and 3D analysis of mineral morphology and vesicularity to reconstruct the lava flow stratigraphy of mare basalts. The high-Ti Steno Crater basalts (71035, 71036, 71037, and 71055) were collected from the same boulder located on the rim of Steno Crater. Sample 71036 was stored frozen after its return to Earth and was released for study through the Apollo Next Generation Sample Analysis (ANSGA) Program, while the companion samples were curated using traditional methods. In Chapter 3, I investigate the mineralogy and 3D morphology of 71036 and its companion sam-ples and show that these basalts crystallized quickly in the upper crustal region of a mare lava flow. These basalts contain fluorapatite, and petrographic evidence suggests these basalts expe-rienced volatile exsolution (degassing) contemporaneous with apatite crystallization. The Apollo 17 mission returned double drive tubes that core vertically into the lunar regolith, sampling a diversity of lunar materials. Clasts from the double drive tube 73001 were released to the community through the ANGSA program. In Chapter 4, I investigate the petro-genesis of one high-Ti basalt clast (73001,531) and a basalt clast denoted low-Ti (73001,538). Detailed mineralogic analysis shows that 73001,538 is actually a very-low Ti basalt, adding important context for basalts that are undersampled in our collection, but likely represent a large portion of effusive flows on the Moon. In Chapter 5, I study compositionally and texturally diverse low-Ti Apollo 15 basalts (15495, 15499, 15555, 15556, and 15608). Combined 2D and 3D measurements help place these samples into lava flow stratigraphic context and show these basalts degassed prior to the onset of apatite crystallization. These studies ultimately serve to improve our understanding of how basalts crystallize in mare flows in preparation for proposed robotic investigations of lava tube stratigraphy. This work shows the wealth of data gleaned from the nondestructive 3D analysis of priceless Apollo basalts. 3D measurements combined with traditional 2D analysis helps us un-derstand how lunar basalts of varying textures and compositions crystallized on the surface of the Moon and allow for a prediction of the timing of volatile exsolution. As outlined in Chapter 6, measurements of the abundances and isotopic compositions of hydrogen within apatite in the studied basalts will further constrain the timing of degassing relative to apatite crystallization.