Clay Mineral Characterization and Production in Impact Settings: A Case Study on the Chicxulub Impact Structure, Mexico

Sarah L. Simpson, The University of Western Ontario

Abstract

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.