The Chicxulub impact crater (~180 km) is one of the largest impact craters on Earth and currently the only one that features an intact peak ring. In 2016, the IODP and ICDP jointly drilled this peak ring at site M0077A to achieve a better understanding of the mechanics that form large impact structures. Melt bodies consisting of impact melt and breccia have been discovered inside and below crystalline basement rocks at the base of the core, from 748 to 1334.7 meters below sea floor. The dynamic collapse model of crater formation hypothesizes that the crystalline basement was uplifted from 8 to 10 km deep and emplaced in the peak ring during the excavation of the crater (Morgan, et al., 2016). These “impact melt dikes” and the uplifted crystalline basement rocks have been studied by means of petrography and geochemical analyses to understand the emplacement mechanics and characterize their relationship to the overlying melt sheet and suevite sequence.
Whole-rock major and trace element analyses have shown that impact melt dikes consist for an estimated 60 to 95 % of granitic basement rocks. Other major components that have been identified include gneissic rocks (10 to 30 %), which possibly originate from the middle to lower Yucatán crust, and dolerite rocks occurring as dikes in the crystalline basement. Analysis of 87Sr/86Sr ratios confirm granite and gneiss as the most abundant components in impact melt dikes. Syenite dikes and fine-grained monzodiorite dikes were also encountered in the basement and possibly represent minor components of the impact melt dikes.
A comparison with the Chicxulub melt sheet and suevite rocks has shown that CaO concentrations are very low in impact melt dikes (< 2.6 wt%) but are high to very high in suevite rocks, mostly in the range of 12 to 30 wt%. Carbonate clasts from the targeted carbonate platform have been encountered in these suevites (Op de Beeck, 2018). The melt sheet shows a continuous increase in CaO from base to top, from approximately 4 wt% to over 15 wt% and seem to form a gradual transgression from the carbonate-poor impact melts at the base of the core to the carbonate-rich suevites higher in the core. Whole-rock trace element data indicates a depletion in Pb and enrichment in Sr in a suevite sample at approximately 666 meters below sea floor compared to impact melt dikes, but otherwise, impact melt dikes and suevites are very similar in trace element composition. The impact melts may thus not form as one homogeneous and continuous melt sheet in the Chicxulub crater, but rather as discrete melt bodies that were isolated from the main melt sheet during the outward migration of the peak ring.