Hydrothermal alteration in the core of the Yaxcopoil‐1 borehole, Chicxulub impact structure, Mexico

Abstract

Abstract-Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil-1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside-hedenbergite (Aeg 3 Fs 18-33 En 32-11 Wo 47-53 ) after primary augite and widespread Na-K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite ± (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre-existing mafic minerals, as well as very abundant calcite veins and open-space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine-rich (Cl/F >10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logƒO 2 units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt. The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil-1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre-existing saline brine and heat that traveled to the Yaxcopoil-1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiary marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post-hydrothermal ambient-temperature diagenesis, or ocean and/or meteoric water circulation.

First petrographic results on impactites from the Yaxcopoil‐1 borehole, Chicxulub structure, Mexico

“…7c). Zurcher et al (2003) suggested that this phase could have been chlorite that is now altered to other phyllosilicate phase(s); our XRD study failed to detect any chlorite. Microprobe analyses and SEM observations indicate that the material in such veins comprises acicular clay minerals such as saponite.…”

Volume 364: Chicxulub: Drilling the K-Pg Impact Crater

“…Models of hydrothermal systems in impact craters imply enhanced flow in peak rings (e.g., Figure F7) (Ames et al, 2004;Zürcher and Kring, 2004;Schwenzer and Kring, 2009), and thus Site M0077 is an excellent location to investigate the hydrothermal system at Chicxulub. This line of inquiry will include petrological assessment and thermochronology, as well as X-ray diffraction (XRD), X-ray fluorescence (XRF), fluid inclusion, and stable isotope analyses to examine the alteration assemblage and characterize the composition of the hydrothermal fluids (Ames et al, 2004;Lüders and Rickers, 2004;Zürcher and Kring, 2004;Osinski et al, 2005Osinski et al, , 2013. Was hydrothermal circulation focused in specific zones, as it was in Hole Yax-1 (along faults and lithologic contacts), or was it more pervasive?…”

“…Drilling recovered core from the ~800 m thick Cenozoic sequence, suevite, and underlying parautochthonous Cretaceous rocks to 1511 m below the surface (Urrutia-Fucugauchi et al, 2004b). The earliest Cenozoic sediments indicate gravity flows and resurge deposits formed part of the initial crater fill (Goto et al, 2004;Whalen et al, 2008Whalen et al, , 2013 and contain geochemical evidence for long-lasting hydrothermal venting into the ocean (Rowe et al, 2004;Zürcher and Kring, 2004). Unfortunately, its location on a steep slope meant studies of the post-impact section were plagued by coarse-grained redeposited carbonates and lithification, and the very basal Paleocene appears to be missing (Arz et al, 2004;Smit et al, 2004;Rebolledo-Vieyra and Urrutia-Fucugauchi, 2004;Whalen et al, 2013).…”

“…Of interest is the ocean chemistry and temperature immediately following the impact and any indicators of climatic recovery. Are there signs of local hydrothermal venting (Zürcher and Kring, 2004), short-term global cooling (Vellekoop et al, 2014), and/or indicators of ocean acidification? How long did it take to return to normal conditions?…”

“…Immediately after impact, the peak ring was submerged under water and located adjacent to a thick pool of hot impact melt. Hence, we would expect intense hydrothermal activity within the peak ring (Ames et al, 2004;Zürcher and Kring, 2004). This activity may have provided a niche for exotic life forms possibly similar to that of hydrothermal vent systems in the oceans.…”