Abstract

To better constrain the formation and evolution of the lunar crust, this dissertation addresses two key aspects of lunar crustal formation: the primary feldspathic crust, formed during solidification of an ancient lunar magma ocean (LMO), and non-mare silicic volcanic constructs, which represent either secondary or tertiary crustal formation processes. Specifically, we performed photometry on Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images to determine the single-scattering albedo (SSA) of surface features that expose these two non-mare crustal rock types. Using digital terrain models (DTMs) derived from LROC NAC images of each analysis region, we accounted for potential topographic effects by calculating the local incidence and emission at the scale of the DTM (2-5 meters per pixel), producing high spatial resolution SSA values that correlate approximately linearly with the mafic mineral content of mature regolith. Chapter 2 uses photometric analyses of LROC NAC images and the relationship between SSA and mafic mineral content to assess the extent and distribution of plagioclase within the Moon’s primary crust. Previous spectral analyses have detected the presence of highly pure plagioclase in numerous locations in uplift features, leading workers to infer that a thick, global layer of highly pure anorthosite (“PAN”; > 98% plagioclase) exists in the Moon’s crust. The production of this layer would have required highly efficient separation of plagioclase crystals from mafic cumulates and melt during the formation of the primary crust. We determined the composition of eight uplift features across the lunar surface where spectral analyses have determined the presence of highly pure anorthosite. Of the sites analyzed, only the inner Rook Mountains of Orientale Basin possess SSA values that indicate the presence of high percentages of PAN over extensive portions of the uplifted units. Other analysis locations contain only small exposures of highly pure anorthositic material, with the majority of material indicating a feldspathic crust with anorthosite noritic (~75-80% plagioclase) compositions. Our results suggest that the primary flotation crust within the LMO contained moderate proportions of trapped melt, with high separation efficiencies occurring in some cases on local scales. Chapters 3 and 4 focus on examples of lunar nonmare volcanism, which morphologic and spectral analyses have determined formed from highly viscous, silicic magmas. These features could be instances of the Moon’s secondary crust, formed from the silica-enriched residual melt derived from silicate liquid immiscibility or extended fractional crystallization of a basaltic melt reservoir, or tertiary crust, created from partial remelting of fertile crust by basaltic underplating. Evaluating the degree and distribution of silicic material within and between non-mare silicic volcanic features can help elucidate likely formation mechanism(s). In chapter 3, we performed photometric analyses using LROC NAC images to evaluate the compositions of two regions of nonmare silicic volcanism: the Gruithuisen Domes (GD) and the Compton-Belkovich Volcanic Complex (CBVC). Analyses of the SSA values within separate morphologic units within the CBVC revealed high values of SSA, indicating the presence of a range of rhyolitic (SiO2 ~72-77 wt%) compositions mixed with pyroclastic ejecta. On the basis of this compositional variation, we suggest that the CBVC was formed from highly silicic melt produced from extended fractional crystallization. Analysis of the distribution of SSA values of the Gruithuisen Domes indicates extensive mixing between components with distinct compositions: a silica-rich dome component of dacitic or rhyolitic composition (SiO2 = 67-70 wt%) material and at least one or more FeO-rich component. The smaller range of silicic compositions in this region supports previous hypotheses that the Gruithuisen Domes formed from melt generated from crustal partial melting; however, the unknown origin of the more FeO-rich component complicates interpretations. Likely sources of the more FeO-rich materials are ejecta deposits from nearby large impact craters such as Iridum and Mairan, and mare basalt ejected onto the domes by nearby impact craters into the embaying mare basalts. In Chapter 4, we test the hypothesis of multiple sources of material on the Gruithuisen Domes and implications for the mode of their emplacement. Geologic and morphologic studies indicate the presence of three major compositional types within and surrounding the Gruithuisen Domes: (1) silicic dome material, (2) non-mare KREEP-rich material exemplified by deposits to the north of the domes, and (3) basaltic mare material such as embays the domes. Using published compositions of Apollo 12 and Apollo 14 samples to represent component compositions, we calculate the concentrations of FeO (wt%) and Th (ppm) when various proportions of the components are linearly mixed. Based on the observed FeO and calculated Th concentrations across Gamma and Delta Dome, we suggest that the regolith across the domes is composed of mostly nonmare KREEP material (50-80%) mixed with smaller amounts of dome material (15-40%) and a variable amount of mare material that is usually < 10% but could be as large as 20%. These mixing proportions differ in and around impact craters where dome material can compose up to 100% of the surface regolith. We infer that the Domes formed by a mechanism that facilitated this scale of mixing. We hypothesize that buoyant, silicic melt formed by crustal partial melting in the shallow upper crust rose into the megaregolith, where it attained neutral buoyancy before breaching the surface. As silicic melt continued to rise, buoyancy forces from the melt exerted pressure on the material above it, inflating and doming the surface from below, forming the Gruithuisen Domes. After emplacement, impacts exposed and excavated the silicic dome material, where it mixed with preexisting KREEP-rich surface regolith. Finally, local impacts into the embaying mare basalt added small amounts of basaltic material onto the domes. The analyses completed in this dissertation leverage the high spatial resolutions of LROC NAC images, enabling assessment of local-scale compositional variation. Photometric analyses of LROC NAC images facilitate the determination and comparison of compositions at the outcrop scale at multiple locations across the lunar surface, allowing us to determine both the composition of individual crustal features as well as the presence of broad compositional trends. These variations are useful for assessing both local crustal rock types and evaluating the implications of rock type variability on mechanisms of emplacement. Analyzing photometric parameters of the surface, such as SSA, derived from LROC NAC images at high resolution allows us to perform detailed and comprehensive compositional analyses that can only be rivaled by in situ data collection or sample return.

Committee Chair

Bradley Jolliff

Committee Members

Kerri Donnaldson Hanna; Rita Parai; Roger Michaelides; William McKinnon

Degree

Doctor of Philosophy (PhD)

Author's Department

Earth & Planetary Sciences

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-14-2025

Language

English (en)

Author's ORCID

https://orcid.org/0009-0002-1729-8125

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Available for download on Thursday, August 13, 2026

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