¹A. Losiak et al. (>10)*
¹Planetary Geology Lab, Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
*Find the extensive, full author and affiliation list on the publishers website

The estimates of the age of the Kaali impact structure (Saaremaa Island, Estonia) provided by different authors vary by as much as 6000 years, ranging from ~6400 to ~400 before current era (BCE). In this study, a new age is obtained based on 14C dating charred plant material within the proximal ejecta blanket, which makes it directly related to the impact structure, and not susceptible to potential reservoir effects. Our results show that the Kaali crater was most probably formed shortly after 1530–1450 BCE (3237 ± 10 14C yr BP). Saaremaa was already inhabited when the bolide hit the Earth, thus, the crater-forming event was probably witnessed by humans. There is, however, no evidence that this event caused significant change in the material culture (e.g., known archeological artifacts) or patterns of human habitation on Saaremaa.

Reference
Losiak A et al. (2016) Dating a small impact crater: An age of Kaali crater (Estonia) based on charcoal emplaced within proximal ejecta. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12616]
Published by arrangement with John Wiley & Sons

¹Ronny Schoenberg, ¹Alexandra Merdian, ²Chris Holmden, ¹Ilka C. Kleinhanns, ¹Kathrin Haßler, ¹Martin Wille, ¹Elmar Reitter
¹Department of Geosciences, University of Tuebingen, Germany
²Department of Geological Sciences, University of Saskatchewan, Canada

The depletion of chromium in Earth’s mantle (∼2,700 ppm) in comparison to chondrites (∼4,400 ppm) indicates significant incorporation of chromium into the core during our planet’s metal-silicate differentiation, assuming that there was no significant escape of the moderately volatile element chromium during the accretionary phase of Earth. Stable Cr isotope compositions – expressed as the ‰-difference in 53Cr/52Cr from the terrestrial reference material SRM979 (δ53/52CrSRM979 values) – of planetary silicate reservoirs might thus yield information about the conditions of planetary metal segregation processes when compared to chondrites. The stable Cr isotopic compositions of 7 carbonaceous chondrites, 11 ordinary chondrites, 5 HED achondrites and 2 martian meteorites determined by a double spike MC-ICP-MS method are within uncertainties indistinguishable from each other and from the previously determined δ53/52CrSRM979 value of –0.124 ± 0.101 ‰ for the igneous silicate Earth. Extensive quality tests support the accuracy of the stable Cr isotope determinations of various meteorites and terrestrial silicates reported here. The uniformity in stable Cr isotope compositions of samples from planetary silicate mantles and undifferentiated meteorites indicates that metal-silicate differentiation of Earth, Mars and the HED parent body did not cause measurable stable Cr isotope fractionation between these two reservoirs. Our results also imply that the accretionary disc, at least in the inner solar system, was homogeneous in its stable Cr isotopic composition and that potential volatility loss of chromium during accretion of the terrestrial planets was not accompanied by measurable stable isotopic fractionation. Small but reproducible variations in δ53/52CrSRM979 values of terrestrial magmatic rocks point to natural stable Cr isotope variations within Earth’s silicate reservoirs. Further and more detailed studies are required to investigate whether silicate differentiation processes, such partial mantle melting and crystal fractionation, can cause stable Cr isotopic fractionation on Earth and other planetary bodies.

Reference
Schoenberg R, Merdian A, Holmden C, Kleinhanns IC, Haßler K, Wille M, Reitter E (2016) The stable Cr isotopic compositions of chondrites and silicate planetary reservoirs. Geochmica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.03.013]
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