Significance of Secondary Fe-Oxide and Fe-Sulfide Minerals in Upper Peak Ring Suevite from the Chicxulub Impact Structure

Abstract

The suevite (polymict melt rock-bearing breccia) composing the upper peak ring of the Chicxulub impact crater is extremely heterogeneous, containing a combination of relict clasts and secondary minerals. Using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) and electron probe microanalysis (EPMA), we investigated the nature and occurrence of primary and secondary Fe-oxide and Fe-sulfide minerals to better understand hydrothermal trends such as mineral precipitation and dissolution, and to document the remobilization of Fe and associated siderophile elements within suevites. Large primary Fe-oxides (~20–100 µm) reveal decomposition and dissolution patterns, forming sub-micrometer to micrometer Fe-oxide phases. Secondary sub-micrometer Fe-oxide crystals are also visibly concentrated within clay. The occurrence of Fe-oxide crystals within clay suggests that these likely formed at temperatures ≤100 °C, near the formation temperature of smectite. The formation of Fe-oxide minerals on clay surfaces is of interest as it may form a micro-setting, where free electrons (from the oxidation of Fe2+) and the adsorption of simple organic molecules on the surface of clay could generate reactive conditions favorable to microbial communities. Primary and secondary Fe-sulfide minerals exhibiting a variety of morphologies are present within samples, representing different formation mechanisms. Secondary Fe-sulfide minerals occur within rims of clasts and vesicles and in fractures and voids. Some secondary Fe-sulfide grains are associated with Ni- and Co-rich phases, potentially reflecting the post-impact migration of siderophile elements within the suevite of the Chicxulub crater.