Hot carbonates deep within the Chicxulub impact structure

Abstract

Constraining the thermodynamic conditions within an impact structure during and after hypervelocity impacts is extremely challenging due to the transient thermal regimes. This work uses carbonate clumped-isotope thermometry to reconstruct absolute temperatures of impact lithologies within and close to the ∼66 Myr old Chicxulub crater (Yucatán, México). We present stable oxygen (δ¹⁸O), carbon (δ¹³C), and clumped-isotope (Δ₄₇) data for carbonate-bearing impact breccias, impact melt rock, and target lithologies from four drill cores on a transect through the Chicxulub structure from the northern peak ring to the southern proximal ejecta blanket. Clumped isotope-derived temperatures (T(Δ₄₇)) are consistently higher than maximum Late Cretaceous sea surface temperatures (35.5°C), except in the case of Paleogene limestones and melt-poor impact breccias outside of the crater, confirming the influence of burial diagenesis and a widespread and long-lived hydrothermal system. The melt-poor breccia unit outside the crater is overlain by melt-rich impact breccia yielding a much higher T(Δ₄₇) of 111 ± 10°C (1 standard error [SE]), which likely traces the thermal processing of carbonate material during ejection. Finally, T(Δ₄₇) up to 327 ± 33°C (1 SE) is determined for the lower suevite and impact melt rock intervals within the crater. The highest temperatures are related to distinct petrological features associated with decarbonation and rapid back-reaction, in which highly reactive CaO recombines with impact-released CO₂ to form secondary CaCO₃ phases. These observations have important climatic implications for the Cretaceous-Paleogene mass extinction event, as current numerical models likely overestimate the release of CO₂ from the Chicxulub impact event.

Keywords: Chicxulub; back-reaction; clumped isotopes; decarbonation; impactites.

Fig. 1.

Chicxulub impact structure and sample locations. A) Simplified surface geological map of the northern part of the Yucatán Peninsula in México showing the location of the buried Chicxulub peak ring crater and the boreholes selected in this study (modified from Refs. [3, 4, 18–20]). The blue arrow indicates a gap in the inner ring of the Chicxulub structure and shows a potential pathway of water re-entering the crater after formation (4). B) Schematic geological cross-section through the Chicxulub impact structure displaying the boreholes and the interpreted sequence of crustal rock, Cretaceous carbonate basement (in gray), impact melt rock (in black), suevite (in green), breccia (Bunte breccia type in brown), polymict lithic impact breccia (in blue), and postimpact sediments (in orange, modified from Refs. [17, 20, 21]). C) Model of the Chicxulub postimpact hydrothermal system in cross-section, with isotherms representing the initial temperature distribution immediately after impact (modified from Ref. [22]).

Fig. 2.

Stratigraphic overview showing T(Δ₄₇) data vs. depth, lithology, and sample type of the four Chicxulub drill cores. A) IODP–ICDP Exp. 364 drill core from Site M0077, with in the inset between 616.2 and 617.8 mbsf also compiled T(Δ₄₇) data from Bralower et al. (30); B) PEMEX Yucatán-6 (Y6); C) ICDP Yaxcopoil-1 (Yax-1); D) UNAM-7. The red dashed line represents the maximum Late Cretaceous SSTs, as compiled by Upchurch et al. (40). Error bars correspond to 2 SE.

Fig. 3.

Isotopic cross-relationships of the Chicxulub impactites. A) Relationship between δ¹⁸O data and average T(Δ₄₇) data with the symbols referring to the lithological units from the four different drill cores. Dashed lines are constant δ¹⁸O_water (VSMOW) values of possible diagenetic fluids based on the carbonate–water equilibrium relationships of O’Neil et al. (41). Data plotting along these curves would indicate alteration via secondary processes, which can explain a large part of the dataset but not all data points (see explanation in the main text). B) Isotopic cross plot showing δ¹³C vs. δ¹⁸O data, with colors indicative of the corresponding average T(Δ₄₇) values. Dashed lines from right to left correspond to O-C isotope depletion patterns observed in contact metamorphic case studies 1 (Mount Royal pluton, Quebec), 2 (Alta Stock, Utah), and 3 (Pine Creek, California) as summarized in Baumgartner and Valley (42), reflecting a trend from unaltered carbonates in the upper right toward marbles and eventually igneous compositions in the lower left. Most of the isotopic signatures of the Chicxulub impactites from this study can be explained by hydrothermal processes similar to contact metamorphism. As a comparison, literature δ¹³C and δ¹⁸O values of Y6 (cores N15–N19) (43) and Yax-1 (between depths 800 and 883 mbs) (19) are shown (lacking clumped-isotope data) that largely follow the same trend. However, an IODP–ICDP Exp. 364 impact melt rock sample (721.45 mbsf) and suevites from UNAM-7, Y6, and IODP–ICDP Exp. 364 yield different isotopic compositions and higher T(Δ₄₇), which likely reflects mixing with (molten) silicates, and the preservation of a signal primarily linked to thermal impact processes superimposed by the hydrothermal overprint. C) Relationship between δ¹⁸O and CaCO₃ content (in wt%, based on µXRF analysis) of the different Chicxulub crater lithologies, with colors indicating the T(Δ₄₇) data. The dashed lines represent the evolution of the δ¹⁸O composition of a marine carbonate (with typical unaltered calcite values around 25‰ VSMOW [44]) or an anhydrite-dominated lithology (exemplified by the UNAM-7 lithic breccia with ∼50 wt% CaCO₃ and an δ¹⁸O value of ∼28‰ VSMOW) toward a quartz composition (with 0 wt% CaCO₃ and an δ¹⁸O value of ∼10‰ VSMOW [44]). Large parts of the impactite dataset follow one of these lines, which illustrates the importance of mixing with silicate components. Exp. 364 Pg lst—Paleogene limestone of the IODP–ICDP Exp. 364 core (616.24–616.56 mbsf). TU—Transitional unit of the IODP–ICDP Exp. 364 core (616.56–617.34 mbsf).

Fig. 4.

Petrographic similarities between Chicxulub samples and experimental analogs. The IODP–ICDP Exp. 364 impact melt rock interval, yielding high T(Δ₄₇) (327 ± 33°C [1 SE]), shows similar macro- and micro-textures compared to results from laser-irradiation experiments on a limestone target (8). A) Halfcore photograph of IODP–ICDP Exp. 364 core section 87_2 (69–81 cm; 721.35–721.47 mbsf) displaying smooth green textures, which were analyzed in this isotopic study (see yellow area and Fig. S1B sample #6). B) Reflected-light photomicrograph and C) zoomed-in scanning electron microscopy-backscatter electron (SEM-BSE) image of an ooid-limestone plate after laser irradiation, showing successive thermal decomposition of CaCO₃ resulting in white decomposition products and eventually in lustrous green CaO slag (modified from Hamann et al. [8]). D) Equigranular-like calcite matrix of the smooth textures in sample 721.45 mbsf, displaying subhedral calcite (Cal) grains of ∼5–15 μm in size, interspersed with minor smectite (Smc). E) Equigranular matrix of the zone with the decomposition products from the laser experiment showing euhedral calcite crystallites of ∼1–2 μm in size (modified from Hamann et al. [8]).