CHROMIUM ISOTOPES OF CHROMITE GRAINS FROM THE KT BOUNDARY AT THE WESTERN CRETACEOUS SEA PALEO-MARGIN: EVIDENCE FOR EXCAVATION AND EJECTION OF OCEANIC CRUST AND/OR UPPER MANTLE BY THE IMPACT

Location of Chicxulub Crater [1] on ~35 km thick continental crust may be inconsistent with geochemical and mineralogical evidence for a mantle and/or basaltic component in KT boundary distal ejecta [2-5] published after the Alvarez KT boundary impact hypothesis [6]. Evidence for oceanic target rocks was reinforced by discovery of chromites, some with shock planar deformation features (PDF), in impact layer samples from sites in southern Colorado [7] and eastern Wyoming (collected by Bruce Bohor in 1986, separated and found by us in 2012). However, until now it was unclear whether the chromites originated with an impactor or with terrestrial target rocks. To this end, high-precision ⁵⁴Cr/⁵²Cr isotope ratios were measured on KT boundary chromites. The ⁵⁴Cr/⁵²Cr ratio varies systematically with meteorite class [8] and has been used to identify possible impactor types [9, 10]. Digestion of chromite grains, Cr purification from the matrix, and mass spectrometry methods have been previously described in [11]. We find a terrestrial ⁵⁴Cr/⁵²Cr ratio in KT boundary chromites from impact layer samples collected at the above sites. Ejected terrestrial chromites imply the impact sampled terrestrial ophiolitic and/or oceanic crust and upper mantle target rocks. Since ophiolites are not known to exist in the Chicxulub target area, we suggest oceanic crust and upper mantle rock, exposed as ophiolite in the Greater Antilles island chain, marks the rim of an additional ~700 km diameter impact basin deformed and dismembered from an originally circular form by at least 50 million years of left lateral shear along the North American-Caribbean plate boundary.

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