IMPACT-RELATED HYDROTHERMAL FLUID AND HYDROCARBON MIGRATION IN CREATACEOUS SEDIMENTS OF THE ICDP-CHICXULUB DRILL CORE YAX-1

Fluid inclusions studies in quartz and calcite in samples from the ICDP-Chicxulub drill core Yax-1 have revealed compelling evidence for impact induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200°C) highly saline, metal-rich, hydrocarbon-bearing brines. The source of the hot quartz-forming brines is assumed in the center of the crater structure. Trace hydrocarbon being present in some quartz inclusions may be derived from cracking of pre-impact organic matter. Remobilization of bitumen and oil occurred after the emplacement of the dikes and may have been accelerated by an impact-induced thermal overprint. Brittle shear zones and dikes were used as pathways and hosts. Fluid inclusions in abundant newly-formed calcite indicate low cyrstallization temperatures (75°-100°C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite-forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to convection of local pore fluids possibly triggered by impact-induced conductive heating of the sediments.