A SHOCKING DEVELOPMENT: MICROANALYSIS OF SHOCKED IRON SPINELS FROM THE WEAUBLEAU AND DECATURVILLE IMPACT STRUCTURES IN SOUTHERN MISSOURI, USA

Spinel is a common mineral in some meteorites, and can be produced by a variety of igneous and metamorphic processes, including those related to extraterrestrial projectiles. Although this mineral is formed as a product of meteorite ablation, impact melt, and impact plume condensates, much is still unknown about when and how spinel forms during impact. This study documents the composition and structure of spinel crystals in polymict breccia in the Weaubleau and Decaturville impact structures, two of five tructures along the thirty-eighth parallel in southern Missouri, in order to infer the relationship between both structures. Transmission electron microscope (TEM) analyses reveal a Fe₃O₄ diffraction pattern for both structures, and defects related to impact in the arrangement of the mineral crystals. Although iron spinel crystals have been created in experimental studies as a result of oxygenated conditions, this study is their first TEM documentation in terrestrial impact structures. Due to the extreme environment under which impact spinel crystals are formed, their composition and structure may be reflective of the conditions of formation, and provide insights into temperature, oxygen fugacity, and shock pressure, as well as potential trace input from the projectile. Since impact volatilization typically results in a reducing environment, iron spinel formation is a rarity due to oxygen being a limiting factor in redox reactions and the potential to be depleted in many terrestrial environments. TEM data potentially provides evidence for marine impact and similarities between the Weableau and Decaturville structures.