Abstract
Core from Hole M0077 from IODP/ICDP Expedition 364 provides unprecedented evidence for the physical processes in effect during the interaction of impact melt with rock-debris-laden seawater, following a large meteorite impact into waters of the Yucatán shelf. Evidence for this interaction is based on petrographic, microstructural and chemical examination of the 46.37-m-thick impact melt rock sequence, which overlies shocked granitoid target rock of the peak ring of the Chicxulub impact structure. The melt rock sequence consists of two visually distinct phases, one is black and the other is green in colour. The black phase is aphanitic and trachyandesitic in composition and similar to melt rock from other sites within the impact structure. The green phase consists chiefly of clay minerals and sparitic calcite, which likely formed from a solidified water–rock debris mixture under hydrothermal conditions. We suggest that the layering and internal structure of the melt rock sequence resulted from a single process, i.e., violent contact of initially superheated silicate impact melt with the ocean resurge-induced water–rock mixture overriding the impact melt. Differences in density, temperature, viscosity, and velocity of this mixture and impact melt triggered Kelvin–Helmholtz and Rayleigh–Taylor instabilities at their phase boundary. As a consequence, shearing at the boundary perturbed and, thus, mingled both immiscible phases, and was accompanied by phreatomagmatic processes. These processes led to the brecciation at the top of the impact melt rock sequence. Quenching of this breccia by the seawater prevented reworking of the solidified breccia layers upon subsequent deposition of suevite. Solid-state deformation, notably in the uppermost brecciated impact melt rock layers, attests to long-term gravitational settling of the peak ring.
Highlights
IntroductionThe melt is thought to be chemically homogeneous prior to cooling to the liquidus temperature and may maintain this homogeneity, if solidification is rapid enough to prevent the melt from differentiating (Floran et al 1976, 1978; Grieve et al 1976; Phinney and Simonds 1977; Zieg and Marsh 2005)
Deposits of impact melt, at times continuous sheets, covering crater floors are known from a number of impact structures on Earth and are paramount for elucidating hypervelocity impact processes (Grieve 1975; Grieve et al 1977; Ontario, London, ON N6A 5B7, Canada 9 Institute for Earth and Space Exploration, University of Western Ontario, London, ON N6A 5B7, Canada1 3 Vol.:(0123456789)International Journal of Earth Sciences (2021) 110:2619–2636Floran et al 1978; Onorato et al 1978; Kring and Boynton 1992; Therriault et al 2002)
Impact melt rock recovered at Site M0077 of IODPICDP Expedition 364 was divided into two units (Morgan et al 2017), which are from bottom to top: units 3B and 3A, and the overlying unit 2, with sub-units 2C, 2B, and 2A interpreted as suevite (Fig. 2b)
Summary
The melt is thought to be chemically homogeneous prior to cooling to the liquidus temperature and may maintain this homogeneity, if solidification is rapid enough to prevent the melt from differentiating (Floran et al 1976, 1978; Grieve et al 1976; Phinney and Simonds 1977; Zieg and Marsh 2005). In this case, impact melt rock will lack any petrographic layering
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