Doctor of Philosophy
Planetary Science and Exploration
Osinski, Gordon R
Longstaffe, Fred J
Impact cratering is the most ubiquitous geologic process shaping the surface of solid bodies in our Solar System. Despite their deleterious effects, impacts have the potential to initiate transient hydrothermal systems, making them attractive targets in the search for water and extraterrestrial life. The relatively low temperature nature of these environments and poor preservation state of craters on Earth leads to difficulties in determining the provenance of many alteration phases, particularly clay minerals. This becomes especially problematic on other clay-rich planetary bodies (e.g., Mars) where limited geologic information can lead to ambiguous and/or inaccurate interpretations. This thesis presents a case study on drill core from the peak-ring of the Chicxulub impact crater, Mexico, with the following goals in mind: (1) document the alteration assemblages preserved within the impact melt-bearing breccias; (2) examine the impactite clay mineralogy (i.e., µm) using X-ray diffraction, electron microprobe and spectroscopic analysis; (3) determine the clay δ2H and δ18O signatures, and (4) study the geochemistry and alteration patterns in glasses preserved throughout the melt-bearing breccias. The secondary assemblages are predominantly zeolites, clay minerals, and carbonates plus other phases in minor amounts. The clay minerals consist of hydroxy-interlayered saponite and montmorillonite. Changes in smectite mineralogy correlate with host rock physical properties and clay δ18O (+10.4 to +18.6‰), which indicates relatively low formation temperatures for the montmorillonite (~10-25°C), and only slightly higher temperatures for the saponite (~35-50°C). The clays’ δ2H (–105 to –87‰) remains relatively unchanged through the core. The isotopic data indicate clay mineral formation from a meteorically-derived Gulf Coast Brine, not seawater or it’s evolved equivalents. Impact glass is altered predominantly to a nanocrystalline clay-like material, palagonite. Despite being altered, the palagonite preserves textures indicating the initial presence of two chemically distinct melts, one more felsic and the other more mafic, reflective of the mixed sedimentary-crystalline target. Results from this thesis support the continued exploration of impact structures on other hydrous, rocky bodies. Craters should be considered a major source of di- and trioctahedral smectites and poorly-crystalline materials, although they are not necessarily indicators of hydrothermal formation temperatures.
Summary for Lay Audience
Impact cratering is the most common geologic process that shapes the surface of every planetary body in our Solar System. Despite their biologically devastating immediate consequences, the long term effects of cratering have the potential to create environments that are favorable for life to thrive, making impact craters attractive targets in the search for extraterrestrial life. The immense amount of heat generated during an impact event interacts with water (e.g., liquid water, subsurface ice) and can initiate ephemeral hydrothermal systems. There are no currently active impact-generated hydrothermal systems on Earth, however approximately one-third of ancient craters show evidence that they existed, usually in the form of mineral deposits and degassing structures. These processes are also believed to occur on the surface of Mars and any other water-bearing rocky planet or satellite; however, detecting fossilized impact-generated hydrothermal systems on distant worlds is a challenging endeavor. Discerning the geologic history of other celestial bodies is currently limited to what can be learned using remote observations (i.e., satellites and ground-based rovers), and interpreting scientific measurements made on other planets is based on ground-truth observations of geologic features on Earth. This concept, called Earth analogue studies, introduces the foundation of this thesis which explores the hydrothermal system preserved in the peak ring of the 66 million year old, 180 km-diameter Chicxulub impact structure buried beneath the Gulf of Mexico and northern Yucatán peninsula, using drill core collected in 2016. This work specifically focuses on the characterization and production of clay minerals within the context of impact-generated hydrothermal systems, as these are one of the most ubiquitous and complex mineral groups found in ancient impact structures on Earth and Mars. Results from this thesis showcase the complexity of clay minerals, their stable isotope signatures and more broadly their formation conditions through the peak ring, and also gives some insight into regional geologic processes affecting the impact structure today. This work, overall, provides some progress towards discerning the origin of clay minerals within the context of impact processes on other water bearing rocky planets and satellites in our Solar System.
Simpson, Sarah L., "Clay Mineral Characterization and Production in Impact Settings: A Case Study on the Chicxulub Impact Structure, Mexico" (2020). Electronic Thesis and Dissertation Repository. 7403.