Date of Award
Doctor of Philosophy
Brad Thomson, Nick Dygert, Alexei Sokolov
Impact cratering is the prevalent form of weathering in the solar system. Impacts can also eject samples off of planetary surfaces which can eventually fall to Earth. Lunar meteorite NWA 10986 is a consequence of impacts occurring on the surface of the Moon. Multiple impact events are recorded by the impact melt glass found in NWA 10986, however this meteorite still provides powerful petrogenetic information that can help us determine the origin and evolution of the Moon. The impact melt glass in NWA 10986, and other glasses found on the Moon, record the conditions during their formation including oxygen fugacity conditions. Oxygen fugacity is equally as important as temperature and pressure in terms of petrologic variables that control the crystallization of minerals and the formation of magmas from planetary interiors. Other glasses occur on the surface of the Moon, either as a result of volcanic eruptions generated from the lunar interior, or of impacts that produce impact melt glass and agglutinates. Investigating the oxygen fugacity conditions present during the formation of these glasses allows us to understand the equilibrium oxygen fugacity conditions of the lunar interior or during impact processes. Multivalent elements such as Fe and Cr are capable of recording oxygen fugacities through their oxidation states. On Earth, Fe can be used to determine the changes in oxidation state experienced during impacts using terrestrial analogs such as fulgurites and Trinitite. Beyond the Earth, Fe is not capable of measuring lower oxygen fugacities such as those found on the Moon. This work demonstrates the use of a new Fe and Cr oxybarometer to perform in situ measurements of oxygen fugacity using x-ray absorption spectroscopy. Iron oxidation states measured in fulgurites and Trinitite demonstrates how impacts are not always reducing and spatial variability in oxidation states can occur within microns. This chrome oxybarometer can measure the lower oxygen fugacities in lunar materials and again demonstrates the variable nature of impacts on primary oxygen fugacity signatures.
Roberts, Sarah, "Interpreting Lunar Geochemistry Through Impact Events and Terrestrial Analogues. " PhD diss., University of Tennessee, 2019.
Chapter 1, "New lunar meteorite NWA 10986: A mingled impact breccia from the highlands; a complete cross section of the lunar crust" is currently in print in Meteoritics and Planetary Sciences. Chapter 2, "Oxidation state of iron in fulgurites and Trinitite: Implications for redox changes during abrupt high-temperature and pressure events" was published in Geochimica et Cosmochimica Acta, vol. 266, p. 332-350