NEW INSIGHTS INTO THE FORMATION AND EMPLACEMENT OF IMPACT MELT ROCKS WITHIN THE CHICXULUB IMPACT STRUCTURE, FOLLOWING THE 2016 IODP-ICDP EXPEDITION 364

In 2016, IODP-ICDP Expedition 364 drilled the peak ring of the Chicxulub impact structure, offshore of the Yucatán Peninsula, México, recovering a near-continuous core containing, among other lithologies, largely uninterrupted granitoid crystalline basement and impact melt rocks. This drilling presents a unique opportunity to better understand Chicxulub impact melt rock formation and emplacement, something that has previously been largely impossible due to the lack of exposure of impact melt rocks.

We subdi­vide the Chicxulub impact melt rocks into two groups: the lower im­pact melt rock–bearing unit, which intrudes the granitoid peak ring at different intervals, and the upper impact melt rock unit, which overlies the peak ring basement. The geochemical characteriza­tion in major and trace element, and isotopic composition of the impact melt rock units and five identified pre-impact lithologies (i.e., granitoid, dolerite, dacite, felsite, and limestone) reveals that the composition of impact melt rock can be explained by the mixing of the primarily felsic and mafic lithologies, but with various degrees of carbonate dilution. The bulk geo­chemical composition of the pre-impact lithologies constrains the composition of both impact melt rock units. As such, these pre-impact lithologies likely rep­resent the main constituents that were melted to form the impact melt rock and also suggests that these lithologies are likely pervasive throughout the Yucatán subsurface.

Following these observations, we suggest that the two impact melt rock units were initially part of the same impact-produced melt, but discrete processes separated them during crater formation. The lower impact melt rock–bearing unit is interpreted to represent impact melt rock injected into the crystalline basement during the compression/excavation stage of cratering. These impact melt rock layers likely acted as delamination surfaces within the crystalline basement, accommodating its displacement during peak ring formation. The composition of the upper impact melt rock unit was contingent on the entrainment of carbonate components and is interpreted to have lined the transient crater surface during crater development. The upper melt formation was not finalized until the modification stage, when carbonate material would have re-entered the crater.