LATE EOCENE IMPACT EJECTA IN ITALY: ATTEMPTS TO CONSTRAIN THE IMPACTOR COMPOSITION FROM ISOTOPIC ANALYSES OF SPINEL-RICH SAMPLES

The late Eocene is marked by multiple bolide impact events, possibly related to a comet or asteroid shower over a duration of about 2-3 million years, which may have played an important role related to the deterioration of the global climate at the end of the Eocene. Marine sediments worldwide contain evidence for at least two closely spaced impactoclastic layers. The upper layer correlates with the North American tektite strewn field (with the 85-km-diameter Chesapeake Bay crater (USA) as its source crater), whereas the lower, microkrystite layer (with clinopyroxene-bearing spherules) was most likely derived from the 100-km-diameter Popigai impact crater (Russia). At least five impact structures with late Eocene ages are known. The Eocene/Oligocene GSSP is located at Massignano, Italy, and below the boundary in the late Eocene 5.61-meter-level shocked quartz and pancake-shaped smectite spherules that contain Ni- and Cr-rich spinel crystals are found. These are associated with a positive Ir anomaly in upper Eocene deposits with the same age as the Popogai-derived cpx spherule layer. This layer is overlain by another Ir-rich layer, probably representing the NA-tektite event. The pancake spherules appear to be diagenetically-altered and flattened cpx spherules About 50 kg of “pancake-bearing” rock from was collected, largely dissolved, and then the pancakes extracted mostly by hand-picking, leaving a few hundred mg of this spinel-rich material. About 120 mg of the unspiked sample was analyzed for tungsten isotopic composition, resulting in a barely resolvable negative anomaly of about -0.2 ε182W, which is in line with a meteoritic admixture to the pancake sample. Chromium isotopic measurements were done on two separate batches of the “pancake-concentrate”, giving values of around -0.4 to -0.5 ε54Cr, which distinctly point to an ordinary chondritic impactor. This result supports the asteroid impact interpretation but not the comet impact hypothesis.