Probing the hydrothermal system of the Chicxulub impact crater

Abstract

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth's crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 10⁵ km³ of Earth's crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 10⁶ years.

Fig. 1. Spatial context of hydrothermal system.

Chicxulub hydrothermal evolution model (12) that was tested by Expedition 364 with a borehole into the peak ring ~40 km from the crater center. (A) Thermal contours of 25°, 50°, 100°, 200°, 300°, 600°, 900°, and 1200°C illustrate the location of the central melt pool (left side of diagram) and the thermal effect beneath the peak ring (middle of diagram). Temperature decreases with distance but is still ~300°C at the point corresponding to the base of the Expedition 364 borehole. (B) Water and steam flux vectors in the same region. Hydrothermal flow is particularly vigorous adjacent to the melt pool, in the vicinity of the peak ring. Water and steam do not penetrate the central melt pool until it has crystallized. Both panels represent the system 4000 years after impact. Full model details are available in (13). The ICDP Yaxcopoil-1 borehole, located on land, was drilled at a radial distance of ~65 km.

Fig. 2. Hydrothermally altered rock.

Sawn surfaces through hydrothermally altered core samples, 83 mm wide. (A) Dark green porous, permeable channel that cuts through an impact melt-bearing breccia (suevite). The matrix of the breccia was dissolved before being partially replaced by secondary carbonate that grew from the fluid. Sample 007A-48R-3, 40 to 70 cm (643 mbsf). (B and C) Red-orange swath of Na-dachiardite and analcime cutting through impact melt-bearing breccia. The carbonate matrix of the breccia was dissolved in the hydrothermal channel, before Na-dachiardite, analcime, clay, and sparry calcite partially filled the channel. Samples 0077A-53R-3, 0 to 30 cm (658 mbsf), and 0077A-54R-1, 50 to 80 cm (660 mbsf). (D) Highly deformed, porous, and permeable peak-ring granitoid rock cut by a fault adjacent to altered feldspar and veins partly filled with quartz and epidote. Sample 0077A-129R-2, 0 to 30 cm (832 mbsf). (E) Granitoid rock with centimeter-size quartz dissolution cavities. Sample 0077A-275R-2, 0 to 30 cm (1248 mbsf).

Fig. 3. Hydrothermal alteration with depth.

Range chart with postimpact hydrothermal minerals correlated with core lithologies (15) and physical properties in the Chicxulub peak ring as measured in Expedition 364 Site 0077A core. The uppermost units of impact melt-bearing breccia (suevite) and impact melt rock between 617 and 747 mbsf were subdivided by the expedition (15) into units 2A, 2B, 2C, 3A, and 3B, which are shown in an inset beneath the ranges of low-temperature minerals. Garnet is andradite-grossular in the upper core section and mainly andradite in the lower core section. Natural remanent magnetization (NRM) of core samples shows both reverse and normal polarity.

Fig. 4. Hydrothermal mineralization.

(A to K) Hand specimen (C and J), optical microscopic (B, E, I, and K), and backscattered electron images (A, D, F, G, and H) of hydrothermal alteration minerals in the Chicxulub peak ring, presented in order of increasing depth. (A) Domain of euhedral FeS₂, either pyrite (Py), and/or the alternative dimorphic structure marcasite (Mrc), in suevite sample 0077A-40R-2, 105 to 107 cm (619 mbsf). (B) Impact glass fragment altered to montmorillonite-like sheet silicate in suevite sample 0077A-41R-1, 58 to 60 cm (638 mbsf). (C) Red Na-dachiardite (Dac) and transparent analcime (Anl) in suevite sample 0077A-60R-1, 90 to 92 cm (678 mbsf). (D) Framboidal pyrite (Py) associated with zeolite (analcime and Na-dachiardite) in suevite sample 0077A-63R-2, 69.5 to 72 cm (685 mbsf). (E) Sheaf-like barite (Brt) associated with zeolite (heulandite; Heu), smectite, and calcite (Cal) in suevite sample 0077A-71R-1, 55.5 to 57 cm (697 mbsf). (F) Secondary magnetite (Mag) surrounding a clay-lined vesicle filled with calcite in impact melt rock sample 0077A-85R-1, 26 to 28 cm (717 mbsf). (G) Zoned euhedral andradite-grossular garnet (Grt) crystals in mafic sheet silicate alteration domain, impact melt breccia sample 0077A-89R-3, 39 to 43 cm (729 mbsf). (H) Vein of albite (Ab) and K-feldspar (Kfs) crosscutting shocked basement granitoid sample 0077A-150R-3, 25.5 to 27 cm (887 mbsf). (I) Green epidote (Ep) vein crosscutting shocked granitoid sample 0077A-155R-1, 12 to 14 cm (897 mbsf). (J) Euhedral analcime crystal along open fracture in impact melt breccia sample 007A-293R-1, 10 to 12 cm (1301 mbsf). (K) Euhedral andradite garnet crystals in postimpact calcite (Cal) vein, impact melt breccia sample 0077A-299R-2, 10 to 12.5 cm (1321 mbsf). Photo credit for (C) and (J): M.S., LPI.

Fig. 5. Manganese anomaly.

Manganese is strongly enriched in the transitional unit that is interpreted as sediment deposited in the immediate wake of the impact (15). Elevated concentrations of Mn were measured in samples beyond 2.1 Ma after the impact, based on correlated micropaleontologic data (26). Aluminum concentrations in these samples are shown as proxies for suevite components, suggesting the Mn anomalies in the transitional unit and the lowermost postimpact sediments are not correlated with reworked impactite debris.