Impact ejecta in upper Eocene deposits at Massignano, Italy

Abstract

Previous workers have shown that an impact ejecta layer at Massignano, Italy contains a positive Ir anomaly, flattened spheroids (pancake spherules), Ni-rich spinel crystals, and shocked quartz with multiple sets of planar deformation features. Because of sample sizes and work by different investigators, it was not clear if the shocked quartz is associated with the Ir anomaly and pancake spherules or if it belongs to a separate impact event. To address this problem, we carried out a high-resolution stratigraphic study of this ejecta layer. The ejecta layer was sampled continuously at 1 cm intervals in two adjacent columns. The carbonate was removed with dilute HCl, and the noncarbonate fraction was gently sieved. Pancake spherules were recovered from the 250-500 μm size fraction and counted. At the peak abundance, the number of pancake spherules in the 250-500 μm size fraction is about 67/g of sample. The pancake spherules removed from the 250-500 μm size fraction are mostly translucent to opaque pale green, but some have a grey color or dark opaque patches due to a coating of Ni- and Cr-rich spinel crystals. Energy-dispersive X-ray analysis and Xray diffraction data indicate that the green spherules are composed of iron-rich smectite, probably nontronite. Black opaque spinel stringers (dark spinel-rich pancake spherules), usually <200 μm across, can be seen in a polished section of a block that includes the ejecta layer. None of the dark spinel-rich pancake spherules were recovered from the sieved non-carbonate fraction due to their fragile nature, but we believe that they are from the same impact event as the green pancake spherules. The <250 μm size fractions from both columns were disaggregated using ultrasonics and re-sieved. The 63-125 μm size fractions were then searched for shocked quartz using a petrographic microscope. At the peak-abundance level, the number of shocked quartz grains in the 63-125 μm size fraction is about 7/g of sample. Some of the shocked quartz grains have a toasted appearance. These grains have a brownish color and contain a patchy distribution of faint, densely spaced planar deformation features (PDFs). Polymineralic fragments containing one or two shocked quartz grains with one or two sets of PDFs were observed. They appear to have an organic matrix and are probably fragments of agglutinated foraminiferal tests. We searched for, but did not find, coesite or shocked zircons. We found that the peak abundance of the shocked quartz is within a centimeter of the peak abundance of the green pancake spherules. We conclude that the pancake spherules are diagenetically altered clinopyroxene-bearing spherules and that the shocked quartz, green (and presumably the dark spinel-rich) pancake spherules, and Ir anomaly all belong to the same impact event. This conclusion is consistent with previous suggestions that the cpx spherule layer may be from the 100 km-diameter Popigai impact crater in northern Siberia.