Geochemistry of meteorite-rich marine limestone strata and fossil meteorites from the lower Ordovician at Kinnekulle, Sweden

Abstract

Thirteen fossil meteorites (⊘ = 1.5–9 cm) have been found in lower Ordovician Orthoceratite Limestone at Kinnekulle, Sweden. The meteorites were recovered from a 3.5 m thick quarried interval in the middle of a 30 m thick section of Orthoceratite Limestone. The section formed in an epicontinental sea at very slow sedimentation rates, ≈ 0.2 cm/ka. The limestones of the lower two-thirds of the section are reddish brown, except in the quarried interval, where a conspicuous 1 m thick grey limestone bed occurs. Ten of the meteorites were found in a thin bed below the grey limestone, one was found in the grey limestone and two in the red limestone above. The meteorites are almost completely pseudomorphosed, primarily by calcite and barite. They were identified by their content of chromite grains and chondrule pseudomorphs. The central part of one of the meteorites shows iridium (800 ppb), gold (550 ppb) and chromium (4400 ppm) concentrations in typical chondritic abundances. Centimeter-resolution profiles for iridium and eight other elements have been established through the quarried interval (at a site at 4 km distant from the meteorite-producing quarry). In the reddish brown parts, Ir concentrations (carbonate-free basis) lie typically in the range 300–400 ppt, compared with ≈ 50 ppt Ir in average shale. In the grey limestone the Ir concentrations are lower, around 100–150 ppt. Iridium correlates with iron, and appears to be related to sedimentation rate and/or iron redox state rather than to any variations in the composition of the siliciclastic fraction. With estimated sedimentation rates as above, the excess Ir flux to the sediment was 30 ng/cm 2 Ma, which is a factor 2–3 higher than the present total Ir flux and one order of magnitude higher than the extraterrestrial Ir flux to the Pacific Sea floor. Our data cannot discriminate to what extent the Ordovician Ir enrichments are related to extraterrestrial matter or to precipitation of Ir from seawater. In recent deep-sea sediments, seawater-derived Ir enrichments are associated with Mn-oxides, but no similar association occurs in the Orthoceratite Limestone. The ten meteorites in the bed below the grey limestone reflect minimum meteorite-accumulation rates of one meteorite per 150 m 2 and 175 ka. Probably the ten meteorites are related to a local strewn field rather than to background influx. Minimum accumulation rates calculated on three meteorite finds > 20 g in the entire quarried interval give one meteorite per 570 m 2 and 1.17 Ma, which is one order of magnitude higher than present-day large-area averaged meteorite influx rates. The probability is low that the small area (< 1700 m 2) of the sea-floor studied would happen to be one where an unusually high number of meteorites struck. Our results may reflect that meteorite, and possibly also extraterrestrial Ir, influx rates were substantially higher than at the present during a period in the early Ordovician. Enhanced influx rates could be related to an asteroid collision in space.