ON THE FATE OF SULFATES DURING THE CHICXULUB EVENT - MICROSCOPIC EVIDENCE

In the context of the Chicxulub event at the K/T boundary, impact-released CO2 and SOx likely changed composition and radiative balance of the atmosphere, and hence, Earth's climate. This assumption prompted experimental research and modeling attempts. So far we lack direct mineralogical proof for dissociation of carbonates and sulfates, which are major target lithologies at the Chicxulub impact site.

PEMEX and UNAM drill cores reveal that the Chicxulub crater fill consists of impact melt rocks (tagamites - well C-1 "bottom"), overlain by melt breccias (well Y-6, N 19 to N 15) and suevites (Y-6, N 14). Not only textural features of these impactites, yet also amount and type of clasts vary strongly: The C-1 bottom tagamite is virtually free of sulfate and carbonate fragments. The absence of clasts either reflect a spectrum of precursor materials different to that for the impact melt breccias, or an extremely high formation temperature, possibly associated with some thermal dissociation of CaSO4 and CaCO3. The impact melt breccias from deeper levels of well Y-6 frequently contain quartz and anhydrite (anhy) clasts, embedded in a matrix with qtz + clinopyroxene + feldspar. We interpret this mineral assemblage to represent a mixture of all target lithologies The qtz - anhy clasts are composed of equant-sized, defect-free crystals with 120° triple junctions. This feature indicates solid-state recrystallization of the rock fragments in the hot melt matrix. The melt breccias from upper levels in Y-6 contain anhy-clasts displaying the original fine-grained sedimentary texture including still preserved lath-like crystals. Occasionally, we found carbonate clasts whose texture reflects crystallization from melt. The Y-6 N14 suevites mostly lack melted carbonate clasts and annealed anhy-clasts are absent, fragments mostly display sedimentary features. In summary, our observations indicate that large-scale melting of platform sediments has occurred in the impact melt layer, possibly associated with some degree of degassing.