Date of Award
8-12-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
This dissertation is largely split into two major parts. The first part, Chapters 2 and 3, touches on stratigraphic analyses of Apollo 17 sample 73001/73002. The second part, Chapters 4 and 5, presents experimental studies on moderately volatile element behavior during high-temperature evaporation experiments. In Chapter 1, I provide some brief background and the motivations for the work that is presented in the ensuing Chapters. In Chapter 2, I present an overview of the stratigraphy of the “light mantle” deposit at the base of South Massif in the Moon’s Taurus-Littrow Valley, uncovered through the analysis of double drive tube 73001/73002. Here I combine coordinated analyses from multiple Apollo Next Generation Sample Analysis (ANGSA) Science Team groups to provide a comprehensive picture of the stratigraphy of this deposit. The possibility that the “light mantle” was a landslide triggered by ejecta from Tycho Crater is critical for establishing the age of Tycho and constraining recent lunar impact chronology; however, this mechanism has recently been questioned. The newly opened 73001/73002 double drive tube from Station 3 sampled 70.6 cm deep into the regolith and represents the first stratigraphic section of an extraterrestrial landslide deposit returned to Earth. Coordinated laboratory analyses of this deposit support a single event for the emplacement of the deposit, which is representative of materials from the South Massif. The core did not penetrate through the light mantle deposit and thus provides a minimum thickness at this location and contributes key information to assess the landslide origin. I present another study on 73001/73002 in Chapter 3, this time taking a closer look at the bulk chemistry trends with depth. Using a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) and fused-bead electron-probe microanalysis (FB-EPMA), I determined the chemical composition of every 0.5 cm dissection interval of the entire 56.9 cm final extruded length of the double drive tube. I used the chemical compositions to model the proportions of different lithologic components found at the Apollo 17 site. Elemental variations with depth were linked to different proportions of these components. Higher amounts of high-Ti mare basalt near the 73002 surface (uniformly dark-toned regolith from 0–1.5 cm) indicate mixing of local mare materials by small impact cratering. Decreasing proportions of high-Ti mare basalt below 1.5 cm result from mixing of dark and light regolith components during the dissection process on Earth. Below about 7.5 cm, compositions indicate consistent amounts of primarily highlands material (<5% high-Ti mare basalt), which can be described as a mixture of noritic impact-melt and anorthositic-norite components. In detail, the modeled anorthositic-norite component, which may represent the pre-basin upper crust in this part of the Moon, ranges from 50–60 wt.%. The modeled noritic impact-melt breccia component remains relatively uniform at 35–40 wt.% throughout the length of 73002 and increases to 45 wt.% at the bottom of 73001. For the second major part of this dissertation, I present the results of 1 atm gas-mixing furnace evaporation experiments in Chapter 4. The chemical and isotopic signatures of moderately volatile elements (MVE) are useful for understanding processes of volatile depletion in planetary formation and differentiation. However, the fractionation factors between gas and melt phases during evaporation that are required to model these planetary volatile depletion processes are still sparse. To address this, I did twenty heating experiments in to constrain the behavior of K, Cu, and Zn evaporation and isotopic fractionation from basaltic melts at high temperatures ranging from 1300 ºC to 1400 ºC, and durations from 2 to 8 days. Oxygen fugacities (fO2) range from one log unit below to ten log units above that of the iron-wüstite buffer (IW–1 to IW+10, corresponding to logfO2 of –10.7 to –0.68 at 1400 ºC). The conditions were selected to achieve an evaporation-dominated regime (where timescales of diffusion << evaporation for trace elements) to avoid diffusion-limited evaporation. My results show during evaporation Zn behaved as the most volatile, followed by Cu and then K, regardless of temperature and oxygen fugacity. Partitioning of Zn into spinel layers within experimental capsules, however, has been observed, which has substantial effects on the Zn isotope fractionation factor. Therefore, Zn results are presented but further discussion is excluded. Element loss depends on both temperature and oxygen fugacity, where higher temperatures and lower oxygen fugacities promote evaporation. However, with varying temperature and oxygen fugacity, the kinetic isotopic fractionation factors for K and Cu remain constant, thus these factors can be applied to a wider range of conditions than those in this study. The experimentally determined fractionation factors for K, and Cu during evaporation from basaltic melts are 0.9944, and 0.9961, respectively. The fractionation factors for these elements with varying volatilities are all significantly larger than the “apparent observed fractionation factors,” which approach one and are inferred from lunar basalts relative to the Bulk Silicate Earth. This observation suggests near-equilibrium conditions during volatile-element loss from the Moon as the “apparent observed fractionation factors” of lunar basalts are similar for all three elements. In Chapter 5, I present experimental results obtained with a novel technique that uses flowing gas to levitate samples and a laser to heat them. This approach offers the advantage of preventing undesired sample-container reactions at elevated temperatures and enabling ultrafast quenching. I analyzed the MVE depletion (e.g., Na, K, Cu, Zn, Ga and Rb) and potassium isotope fractionation associated with heating basalt and loess materials at temperatures up to 2000 °C under multiple oxygen fugacities. My results show that the staring composition has a strong effect on the observed MVE depletion and K isotope fractionation factor for evaporation and we attribute different diffusion coefficients in the melts as the reason for such difference during evaporation. I additionally computed the evaporation coefficients of K and Zn across various oxygen fugacities and melt compositions. Our new values for the evaporation coefficients of K and Zn are notably lower compared to previous estimates, underscoring the pivotal role of melt composition and oxygen fugacity in regulating evaporation. My experiments on loess materials offer valuable insights into the formation of Australasian tektites, shedding light on the significant Cu and Zn depletion relative to K. However, these observations reveal higher levels of K isotopic fractionation in loess materials compared to basaltic materials, alongside greater isotopic fractionation under reducing conditions. These findings are inconsistent with the minimal K isotopic fractionation observed in Australasian tektites, suggesting a need for a significantly quicker heating and quenching process during their formation.
Language
English (en)
Chair and Committee
Kun Wang
Committee Members
Astrid Holzheid; Bradley Jolliff; Bruce Fegley; Katharina Lodders
Recommended Citation
Neuman, Mason, "Elemental and Isotopic Studies on Lunar Apollo Samples and High-Temperature Evaporation Experiments" (2024). Arts & Sciences Electronic Theses and Dissertations. 3314.
https://openscholarship.wustl.edu/art_sci_etds/3314