A search for extraterrestrial chromite in the late Eocene Massignano section, central Italy

Abstract

The late Eocene may have been a period with an enhanced flux of extraterrestrial matter to Earth related either to a comet or an asteroid shower. The evidence comes from two very large and several medium-sized impact craters, at least two microtektite-microkrystite layers, and a stratigraphic interval with enhanced extraterrestrial He-3, all within the period ca. 36.3-34.3 Ma. Here, we show that the distribution of sediment-dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains in the Massignano section, central Italy, can be used to test whether the flux of ordinary chondritic matter to Earth was enhanced in the late Eocene. In twelve limestone samples, each weighing similar to 12-15 kg, from 1.25 m to 10.25 m above the base of the section, only 1 EC grain was found. Based on the total amount of limestone analyzed, 167 kg, this corresponds to 0.006 EC grain kg(-1) limestone. This is a factor of five lower than the 0.029 EC grain kg(-1) recovered in 210 kg of latest Cretaceous-Paleocene limestone from the Bottaccione Gorge section at Gubbio, central Italy. The difference can readily be explained by an approximately threefold higher sedimentation rate in the late Eocene at Massignano. In essence, our results speak against a late Eocene asteroid shower. Apparently, there was no significant increase in the flux of extraterrestrial chromite at this time, such as that after the disruption of the L-chondrite parent body in the mid-Ordovician, when the EC flux was enhanced by two orders of magnitude. We also discuss the potential to search for lunar minerals in the Massignano section in order to test the recent hypothesis that late Eocene He-3 enrichments originated from impact-ejected lunar regolith. (Less)