Martin Schmieder

Dating impact craters: palaeogeographic versus isotopic and stratigraphic methods – a brief case study

Isotopic and stratigraphic ages of the ~ 80 km diameter Puchezh-Katunki (Russia; 220 ± 10 to 167 ± 3 Ma) and the ~ 20 km diameter Obolon (Ukraine; 215 ± 25 to 169 ± 7 Ma) impact structures are associated with significant age uncertainties. As a case study, reconstructions of the palaeogeography at the time of crater formation (Late Triassic to Middle Jurassic) based on recent palaeogeographic maps help further to constrain impact ages. Palaeogeographic studies suggest that Puchezh-Katunki is older than 170 Ma and that Obolon is younger than 185 Ma. This also rules out that Obolon formed during a ~ 214 Ma Late Triassic multiple impact event as recently discussed.

Meteorite traces on a shatter cone surface from the Agoudal impact site, Morocco

The recently discovered Agoudal impact site in Morocco is a small, eroded impact structure with well-developed shatter cones. A scanning electron microscopic study of a shatter cone surface has revealed the presence of schreibersite – a phosphide very rare on Earth but common in iron meteorites – and Fe–Ni oxides. This is the first reported evidence for primary meteoritic matter adherent to shatter cones and suggests that the Agoudal crater was formed by the impact of an iron meteorite, probably the Agoudal IIAB iron. Shatter cones from other terrestrial impact structures might also hold valuable information about the nature of the impacting projectiles.

U–Th–Pb systematics in zircon and apatite from the Chicxulub impact crater, Yucatán, Mexico

This work presents a systematic study of zircon and apatite in melt-bearing impactites from the annular trough of the ~180 km and ~66.04 Ma Chicxulub impact crater, Yucatan, Mexico, using in situ laser ablation – inductively coupled plasma mass spectrometry, in which the petrologic context of the analysed minerals was assessed. Geochronologic U–Pb results for variably shocked zircon from the Yaxcopoil-1 core, including monocrystalline grains and neocrystallised granular aggregates, yielded a discordant array of ages representing the Early Palaeozoic age of the crystalline–metamorphic Maya block in the crater basement and the timing of the Chicxulub impact, respectively, and provide evidence for impact-induced resetting of the U–Pb system. Zircon and fluor-chlorapatite from the Yaxcopoil-1 core, and fluorapatite in clasts of impact melt from the Yucatan-6 core have low 206 Pb/ 204 Pb, suggesting the presence of detectable common Pb. The Chicxulub impactites were altered in an initially hot hydrothermal system that lasted up to ~2 Myr; locally, Pb-rich sulphides precipitated. Hydrothermal conditions did not reset the U–Th–Pb systematics of relict zircon, however, due to elevated closure temperatures for Pb diffusion at the fast cooling rates associated with the crater locations of the Yucatan-6 and Yaxcopoil-1 boreholes. Thus, the zircon preserves pre-impact and impact-related ages, rather than those of the hydrothermal system. In contrast, no useful geochronologic information was obtained from relict apatite, because common Pb in these grains overwhelmed radiometrically derived isotope ratios.

Rare metals on shatter cone surfaces from the Steinheim Basin (SW Germany) – remnants of the impacting body?

The ~3.8 km Steinheim Basin in SW Germany is a well-preserved complex impact structure characterized by a prominent central uplift and well-developed shatter cones that occur in different shocked target lithologies. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron probe microanalysis have revealed, for the first time, the occurrence of rare metals on the Steinheim shatter cone surfaces. Shatter cones produced from the Middle Jurassic (Aalenian) Opalinus Claystone (‘Opalinuston’), temporarily exposed in the central uplift in spring 2010, and shatter cones in Upper Jurassic (Oxfordian) limestones from the southeastern crater rim domain are commonly covered by faint coatings. The Opalinus Claystone shatter cone surfaces carry coatings dominated by Fe, Ca, P, S and Al, and are covered by abundant small, finely dispersed microparticles and aggregates of native gold, as well as locally elevated concentrations of Pt. On several surfaces of the claystone shatter cones, additional Fe, Ni and Co was detected. The Ca–Mn-rich coatings on the limestone shatter cone surfaces locally include patches of Fe, Ni, Co, Cu and Au in variable amounts and proportions. The intriguing coatings on the Steinheim shatter cones could either stem from the impacted Lower Jurassic to Palaeogene sedimentary target rocks; from the crystalline-metamorphic Variscan crater basement; or, alternatively, these coatings might represent altered meteoritic matter from the Steinheim impactor, possibly an iron meteorite, which may have been remobilized during post-impact hydrothermal activity. We here discuss the most plausible source for the rare metals found adherent to the shatter cone surfaces.

An updated and refined Holocene uplift history of southern Tenerife (Canary Islands) and the possible consequences for future volcanic activity

Various uplift markers suggest asymmetrical uplift of Tenerife Island, with stable conditions in the north but significant uplift of up to 45 m in the south over the past ~42 ka. Fossil shells in beach deposits uplifted by 7.5–9 m were 14 C-dated at a Holocene age of 2460±35 bp (1σ). This confirms earlier results and documents very young, and probably still ongoing, uplift of southern Tenerife potentially caused by ascending magma. This underlines that southern Tenerife is probably undergoing a further cycle of volcanic activity that started ~95 ka ago.