OBSERVATIONS OF ACCRETIONARY LAPILLI FROM THE SUDBURY IMPACT EVENT

Accretionary lapilli are cm-scale rounded accretionary clasts, typically containing a core and concentric zones of accreted ash particles that are found in ejecta deposits associated with either volcanic eruptions or hypervelocity impact events. Although there have been abundant studies of accretionary lapilli associated with volcanoes, the features associated with impact events are not well understood. Petrographic investigations of accretionary lapilli from impact events have shown that they are remarkably similar, if not identical, to the volcanic equivalent. In this study, we have examined accretionary lapilli from the Sudbury impact event from five sites, the McLure site, the Connors Creek site, the Pine River site, the Hwy 588 site, and the Grand Trunk Pacific site, ranging from 480 to 700 km from the impact event. The lapilli were primarily investigated using petrographic and backscatter electron images for determining the large-scale and microscopic structures, and neutron activation analysis of powders drilled directly from accretionary lapilli to determine the geochemistry of the clasts. The lapilli are consistently found near the upper portions of the ejecta deposits, suggesting they arrived late in the depositional sequence. The sites vary somewhat in the expression of the lapilli, but the lapilli are largely found to have well-defined cores and concentric rims. The Pine River site is the major exception, where only cores of lapilli are found. The lapilli appear to be consistent with similar structures observed from the Chicxulub impact structure and also from experimental accretionary lapilli, particularly showing similarity to those lapilli formed in experiments where water aids the binding of ash particles, including some lapilli that appear to be oversaturated and have thus lost some cohesion. The geochemical results show that the lapilli in the ejecta deposits are similar to the Onaping Formation, and the Ni and Cr ratios trend towards chondritic ratios. This suggests that the ash that formed the accretionary lapilli must have been similar in composition between the site of the crater and the deposits that are 500-700 km from the rim of the structure. Additionally, there must have been some component of the impactor that was still present in the ash that was forming accretionary lapilli.