ACCRETIONARY LAPILLI IN THE SUDBURY IMPACT LAYER, LAKE SUPERIOR REGION, U.S. AND CANADA
Chemical analyses of lapilli from three sites show variable major element compositions reflecting variable carbonate and silica content. All three samples have very low Na2O contents (≤ 0.12 wt.%) and high K20/Na2O ratios (9.4-19.4). The lapilli have variable trace element abundances, but show similar chondrite-normalized trace element patterns with enriched light REE (La/Sm = 5-8), relatively flat heavy REE (Gd/Yb = 1.7-2.5), and negative Ba, Nb, Ta, Sr, P, and Ti anomalies. These patterns are similar to those in melt spherules from the impact layer, suggesting that the lapilli composition includes a significant component of impact melt or materials from the same source as the melt.
Accretionary lapilli are believed to have formed in the atmosphere as wet accumulations of dust in the presence of liquid water vapor and thus are an indicator of a low temperature environment where they formed. However, lapilli in the impact layer are found in very high energy ground surge breccias and ballistic deposits, and have been reworked into tsunamites. The survival of most lapilli as undeformed, unbroken, and unabraded spheres suggests that they became cohesive and indurated during or very rapidly after formation and prior to deposition.