Extraterrestrisk och terrestrisk kromrik spinell i fanerozoiska kondenserade sediment

This study aims to assess the normal inflow of extraterrestrial chromite to Earth. The results from the investigationare compared to earlier studies of middle Ordovician Orthoceratite Limestone in which 1 - 3 extraterrestrial chromitegrains ( >63 ?Êm ) per kilogram limestone were found. These values have been interpreted as evidence of a twoorders of magnitude increase in the flux of extraterrestrial matter to Earth during a part of middle Ordovician.From an early Paleocene marine sedimentary sequence with known sedimentation rate in Zumaia, northernSpain, 90,5 kilogram of condensed limestone, dated to about 63 Ma, was sampled. The limestone was dissolved inhydrochloric- and hydrofluoric acid and the residue was sieved and searched for opaque minerals under the binocularmicroscope. In order to determine origin of the opaque minerals the chemical composition was analysed. Sixadditional samples from different time periods were treated in the same way. Two of these samples, weighing 15,1and 14,2 kilogram, respectively, were sampled in a part of the early Paleocene sequence in Zumaia where limestone-and marly layers form a cyclical pattern. This part of the sequence is dated to about 61 Ma. From Garde inJamtland 22,8 kilogram of middle Ordovician limestone was sampled. This limestone is about 4 million yearsyounger than the previously studied limestone showing an enhanced content of extraterrestrial matter. At StevnsKlint in Sjalland, Denmark, two samples of 29,8 and 14,3 kilogram of a late Maastrichtian limestone were collected.At Adnet, close to Salzburg, Austria, 14,4 kilogram of an extremely condensed limestone from the Sinemurianstage in the lower Jurassic was sampled. As a reference, a sample comprising 14,0 kilogram from the chromite-enriched part of the middle Ordovician Orthoceratite Limestone was examined.In this study, a total of 3 extraterrestrial chromite grains were found in 178 kilogram limestone from other timeperiods than the middle Ordovician. With a view to determine the number of chromite grains by unit area by unit oftime, the sedimentation rate has been estimated for localities where this has not been known. Most of our samplesfrom other time periods than the middle Ordovician have a much lower chromite content than the middle OrdovicianOrthoceratite Limestone. Our study indicates that the normal flux of extraterrestrial chromite is less than 1grain per square meter per 1000 years, whilst the flux when it was most enhanced during the middle Ordovician isestimated to 11,1 - 13,6 grains per square meter per 1000 years. For the younger of our middle Ordovician samples,our calculated value is 1,95 - 2,34 extraterrestrial grains per square meter per 1000 years. During this time periodthe flux of extraterrestrial matter to Earth had probably not yet returned to normal conditions.When interpreting the chromite content we have taken into consideration processes which could concentrateheavy minerals on specific levels in the geological record. Our middle Ordovician reference sample has a remarkablehigh content of terrestrial chromium-rich spinels, indicating influence of some kind of concentration process.Accumulation processes could be winnowing of light minerals by weak bottom currents and calcite dissolution. Ourstudy indicates that the provenance for the chromium-rich spinels could be mafic subvolcanic intrusions exposed onthe seafloor. The provenance could also be volcanic ash or subvolcanic layered intrusions from an ultramafic volcanismwith a composition similar to Alaskan Ultramafics. In the large sample from Zumaia and in both samples from Stevns Klint spherical particles, which could beof extraterrestrial origin, were found. Chemical analyses of the nickel content of the spherules were performed tointerpret the origin. The smaller sample from Stevns Klint, which contained one extraterrestrial chromite grain, alsocontained one spherule with a possible extraterrestrial origin. The sample from Zumaia contains several spherulesof which the provenance could not be established with certainty.The sequence in Zumaia has been studied and thoroughly interpreted since this sequence, because of knownmean sedimentation rate, is giving the most accurate indication of the background occurence of extraterrestrialchromite. Our study indicates that the amalgamated limestone, in which the background value of extraterrestrialchromite has been determined, is not the most condensed part of the sequence. Instead, the most condensed partsare the marly layers found in between the limestone bundles.

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