Distribution of chrome-spinel grains across the 3He anomaly of the Tortonian Stage at the Monte dei Corvi section, Italy

• The present-day ocean-climate system configuration took shape during the Miocene Epoch. Toward the end of the epoch, in the late Tortonian at ca. 8.5 Ma, there was an exceptional event: collisional disruption of an >150-km-diameter asteroid, which created the Veritas family of asteroids in the asteroid belt. This event increased the flux of interplanetary dust particles rich in 3He to Earth and probably caused a period of increased dust in the atmosphere, with consequent alteration of global and local environmental conditions. A late Miocene 3He anomaly likely related to the Veritas event has been registered in deep-sea sediments from Ocean Drilling Program (ODP) Site 926 (Atlantic Ocean), ODP Site 757 (Indian Ocean), and in the late Tortonian–early Messinian Monte dei Corvi section near Ancona, Italy. Here, we report the results of a study in the Monte dei Corvi section aimed to recover extraterrestrial chrome-spinel grains across the 3He anomaly interval, as has been done for the similar late Eocene 3He anomaly in the nearby Massignano section. In this study, three ~100 kg samples were collected from the Monte dei Corvi section: two within the 3He peak interval and one outside the anomaly interval as a background reference sample. In total, 1151 chrome-spinel grains (>63 µm) were recovered, but based on chemical composition, none of the grains has a clear extraterrestrial origin. This supports the inference that the 3He anomaly is indeed related to the Veritas event and not to an approximately coeval breakup of a smaller H-chondritic body in the asteroid belt, an event registered in meteoritic cosmic-ray exposure ages. Spectral studies of the Veritas asteroids indicate that they are made up of carbonaceous chondritic material. Such meteorites generally have very low chrome-spinel concentrations in the grain-size range considered here, contrary to the very chromite-rich ordinary chondrites. The terrestrial grains recovered were classified, and their composition showed that all the grains have an ophiolitic origin with no substantial compositional and distributional change through the section. The source area of the terrestrial grains was probably the Dinarides orogen.

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NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

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