MULTIPHASE MODEL OF EJECTA TRANSPORT AWAY FROM EARTH IMPACT CRATERS

Ejecta from impact craters has long been interpreted to represent primary ballistic movement of particles, large and small, that fall to ground at a rate related to their size. However, study of ejecta from the Chicxulub impact crater - the best documented impact ejecta blanket on Earth - has shown prevalence and widespread distribution (to 2500 km) of accretionary particles to several mm size that are deposited in intimate association with similar sized glassy ejecta particles previously assumed to be products of ballistic transport. Study of volcanic eruptions shows that accretionary particles are produced within eruptive plumes and carried in suspension in turbulent eruptive clouds before deposition. Furthermore, Chicxulub accretionary lapilli are dominantly carbonate (mixed with some silicic glass), a material produced in particulate form within the impact plume by condensation from the vapor phase: carbonate lapilli were not part of the ballistic ejecta of impact. These factors suggest that the primary mechanism of transport for Chicxulub impact spherules of all types, including bubbly glass spherules, is high altitude turbulent eruptive plumes. This demonstrates the need for using a more complex multiphase model of impact for the Chicxulub event, one that includes continuing eruptive activity in the impact crater after the initial blast dispersal of ejecta materials. These conditions are probably the norm for impacts on Earth and multiphase impact models should be used for all large impact events.