Stable isotope record of post-impact fluid activity in the core of the Yaxcopoil-1 borehole, Chicxulub impact structure, Mexico
Lukas Zurcher, David A. Kring, Mark. D. Barton, David Dettman, Mark Rollog, 2005. "Stable isotope record of post-impact fluid activity in the core of the Yaxcopoil-1 borehole, Chicxulub impact structure, Mexico", Large Meteorite Impacts III, Thomas Kenkmann, Friedrich Hörz, Alex Deutsch
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Carbon, oxygen, and hydrogen isotope results from carbonate and silicate fractions of altered core samples from the Yaxcopoil-1 borehole drilled into the 65 Ma Chicxulub impact crater provide constraints on the physico-chemical parameters of the hydrothermal solutions, and their likely origin.
Yaxcopoil-1 impactites were initially permeated with calcite and halite at ambient temperature. This was followed by thermal metamorphism (diopside after igneous augite) and widespread Na-K metasomatism (feldspar after igneous plagioclase), which were overprinted by abundant lower-temperature clay and calcite.
Silicate fraction isotopic values have δ18OSMOW values between 10 and 23‰ indicating important isotopic exchange between impact melt (∼8‰) and Cretaceous limestone (∼26‰). Heavier δ18O values occur over depth intervals with intense feldspar alteration (813–833 m and 864–872 m). The δDSMOW values (−34 to −54‰) are chiefly influenced by smectite abundance and roughly mirror δ18O values. Carbonate fraction δ18OSMOW values (22–30‰) are controlled by calcite contents, and several exceed the limestone signature. Most δ13CPDB (−1 to +2‰) values also cluster around that of local limestone, but a number are significantly lighter (down to −7‰).
Isotopic and fluid inclusion results indicate hydrothermal fluid temperatures between 270 and 100 °C, high salinities (∼20%), and minor kerogen contents. These data are compatible with mineralogical constraints, which further support an increase in oxidation state with decreasing temperature. Isotopic data point to a saline CO2-bearing fluid mixed with small amounts of reduced carbon, and decarbonation and infiltration processes. Combined results are most consistent with a basinal oilfield saline brine that was driven by impact-induced heat.