Extraterrestrial chromite in latest Maastrichtian and Paleocene pelagic limestone at Gubbio, Italy: The flux of unmelted ordinary chondrites

Abstract

The distribution of sediment-dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (63 m) has been studied across the latest Maastrichtian and Paleocene in the Bottaccione Gorge section at Gubbio, Italy. This section is ideal for determining the accumulation rate of EC because of its condensed nature and well-constrained sedimentation rates. In a total of 210 kg of limestone representing eight samples of 14-28 kg distributed across 24 m of the Bottaccione section, only 6 EC grains were found (an average of 0.03 EC grains kg^(-1)). In addition, one probable pallasitic chromite grain was found. No EC grains could be found in two samples at the Cretaceous-Tertiary (K-T) boundary, which is consistent with the K-T boundary impactor being a carbonaceous chondrite or comet low in chromite. The average influx of EC to Earth is calculated to ~0.26 grain m^(-2) kyr^(-1). This corresponds to a total flux of ~200 tons of extraterrestrial matter per year, compared to ~30,000 tons per year, as estimated from Os isotopes in deep-sea sediments. The difference is explained by the EC grains representing only unmelted ordinary chondritic matter, predominantly in the size range from ~0.1 mm to a few centimeters in diameter. Sedimentary EC grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere. The average of 0.03 EC grain kg^(-1) in the Gubbio limestone contrasts with the up to ~3 EC grains kg^(-1) in mid-Ordovician limestone that formed after the disruption of the L chondrite parent body in the asteroid belt at ~470 Ma. The two types of limestone were deposited at about the same rate, and the difference in EC abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup.