Combining shock barometry with numerical modeling: insights into complex crater formation – The example of the Siljan impact structure (Sweden)

Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz-bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best-fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best-fit model results in a final crater (rim-to-rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak-ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure

Combined neutron and x-ray tomography—a versatile and non-destructive tool in planetary geosciences

With several upcoming sample return missions, such as the Mars Sample Return Campaign, non-destructive methods will be key to maximizing their scientific output. In this study, we demonstrate that the combination of neutron and X-ray tomography provides an important tool for the characterization of such valuable samples. These methods allow quantitative analyses of internal sample features and also provide a guide for further destructive analyses with little to no sample treatment, which maintains sample integrity, including minimizing the risk of potential contamination. Here, we present and review the results from four case studies of terrestrial impactites and meteorites along with their analytical setup. Using combined X-ray and neutron tomography, a Ni-Fe silicide spherule, that is, projectile material, was located within a Libyan Desert Glass sample and the distribution of hydrous phases was pinpointed in selected impactite samples from the Chicxulub IODP-ICDP Expedition 364 drill core and the Luizi impact structure, as well as in the Miller Range 03346 Martian meteorite

Rochechouart 2017-Drilling Campaign: First Results

No abstract available

CIRIR programs: drilling and research opportunities at the Rochechouart Impact Structure

International audienc

Exceptionally Preserved Cambrian Trilobite Digestive System Revealed in 3D by Synchrotron-Radiation X-Ray Tomographic Microscopy

The Cambrian ‘Orsten’ fauna comprises exceptionally preserved and phosphatised microscopic arthropods. The external morphology of these fossils is well known, but their internal soft-tissue anatomy has remained virtually unknown. Here, we report the first non-biomineralised tissues from a juvenile polymerid trilobite, represented by digestive structures, glands, and connective strands harboured in a hypostome from the Swedish ‘Orsten’ fauna. Synchrotron-radiation X-ray tomographic microscopy enabled three-dimensional internal recordings at sub-micrometre resolution. The specimen provides the first unambiguous evidence for a J-shaped anterior gut and the presence of a crop with a constricted alimentary tract in the Trilobita. Moreover, the gut is Y-shaped in cross section, probably due to a collapsed lumen of that shape, another feature which has not previously been observed in trilobites. The combination of anatomical features suggests that the trilobite hypostome is functionally analogous to the labrum of euarthropods and that it was a sophisticated element closely integrated with the digestive system. This study also briefly addresses the preservational bias of the ‘Orsten’ fauna, particularly the near-absence of polymerid trilobites, and the taphonomy of the soft-tissue-harbouring hypostome

The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)

The Brunflo fossil meteorite was found in the 1950s in mid-Ordovician marine limestone in the Grde quarry in Jmtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L-chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule-size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent-body breakup. Chromite maximum diameters and well-defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Grde quarry is relatively rich (about 0.45 grain kg^(-1)) in sediment-dispersed extraterrestrial chromite grains (63 m) with chemical composition similar to those in L chondrites and the limestone (1-3 grains kg^(-1)) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

A Consortium status report: the characterisation of the asteroid Itokawa regolith - a correlated study by X-ray tomography, micro-raman spectroscopy, and high-sensitivity noble gas analysis

Precious samples from S-type as-teroid 25143 Itokawa have been sampled by the JAXA (Japanese Space Agency) Hayabusa mission in 2005 and returned to Earth in 2010. Itokawa is, succeeding the Moon and comet Wild 2, the third planetary body successfully probed by a sample return mission. The initial studies revealed that Itokawa consists mostly of type LL5-6 material. It experienced severe surface alteration due to space weathering, as docu-mented by surficial, nanosize S-and Fe-bearing phases in some grains. Noble gas studies indicate that Itokawa experiences a surprisingly intense surface loss at a rate of tens of cm/Ma, implying that Itokawa (largest dimension ~540 m) will be destroyed quickly. We received material through JAXA in Sept. 2012 and aim to analyze noble gases in Itokawa samples with high sensitivity, including Kr and Xe, which could not be studied previously, because of the low concentrations. We will combine the noble gas studies with scanning microRaman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and synchro-tron radiation X-ray tomographic microscopy (SRXTM). These provide non-destructive characteri-zations of grain density, mineralogy, structure, and potential space weathering, which are essential to as-sess gas concentrations of potentially present cosmogenic, solar, trapped and radiogenic compo-nents. Here, we summarize the work of our consortium performed so far. Further studies will be presented at the meeting

The Vakkejokk Breccia : An Early Cambrian proximal impact ejecta layer in the North-Swedish Caledonides

The ≤27 m thick Vakkejokk Breccia is intercalated in autochthon Lower Cambrian along the Caledonian front north of Lake Torneträsk, Lapland, Sweden. The spectacular breccia is here interpreted as a proximal ejecta layer associated with an impact crater, probably ~2-3 km in size, located below Caledonian overthrusts immediately north of the main breccia section. The impact would have taken place in a shallow-marine environment ~520 Ma ago. The breccia comprises i) a strongly disturbed lower polymict subunit with occasional, in themselves brecciated, crystalline mega-clasts locally exceeding 50 m surrounded by contorted sediments; ii) a middle, commonly normally graded, crystalline-rich, polymict subunit, in turn locally overlain by iii) a thin fine-grained quartz sandstone, <30 cm thick. The upper sandstone is sporadically either overlain, or replaced, by a conglomerate. In progressively more distal parts of the ejecta layer, the lower subunit is better described as only slightly disturbed strata. The lower subunit is suggested to have formed by ejecta bombardment of the strata surrounding the impact crater, even causing some net outwards mobilization of the sediments. The middle subunit and the uppermost quartz sandstone are considered resurge deposits. The top conglomerate may be caused by subsequent wave reworking and slumping of material from the elevated rim. Quartz grains showing planar deformation features are present in the graded polymict subunit and the upper sandstone, that is, the inferred resurge deposits