ABSTRACT Tabun Khara Obo is the only currently known impact crater in Mongolia. The crater is centered at 44°07′50″N and 109°39′20″E in southeastern Mongolia. Tabun Khara Obo is a 1.3-km-diameter, simple bowl-shaped structure that is well visible in topography and clearly visible on remote-sensing images. The crater is located on a flat, elevated plateau composed of Carboniferous arc-related volcanic and volcanosedimentary rocks metamorphosed to upper amphibolite to greenschist facies (volcaniclastic sandstones, metagraywacke, quartz-feldspar–mica schist, and other schistose sedimentary rocks). Some geophysical data exist for the Tabun Khara Obo structure. The gravity data correlate well with topography. The −2.5–3 mGal anomaly is similar to that of other, similarly sized impact craters. A weak magnetic low over the crater area may be attributed to impact disruption of the regional trend. The Tabun Khara Obo crater is slightly oval in shape and is elongated perpendicular to the regional lithological and foliation trend in a northeasterly direction. This may be a result of crater modification, when rocks of the crater rim preferentially slumped along fracture planes parallel to the regional structural trend. Radial and tangential faults and fractures occur abundantly along the periphery of the crater. Breccias occur along the crater periphery as well, mostly in the E-NE parts of the structure. Monomict breccias form narrow (<1 m) lenses, and polymict breccias cover the outer flank of the eastern crater rim. While geophysical and morphological data are consistent with expectations for an impact crater, no diagnostic evidence for shock metamorphism, such as planar deformation features or shatter cones, was demonstrated by earlier authors. As it is commonly difficult to find convincing impact evidence at small craters, we carried out further geological and geophysical work in 2005–2007 and drilling in 2007–2008. Surface mapping and sampling did not reveal structural, mineralogical, or geochemical evidence for an impact origin. In 2008, we drilled into the center of the crater to a maximum depth of 206 m, with 135 m of core recovery. From the top, the core consists of 3 m of eolian sand, 137 m of lake deposits (mud, evaporites), 34 m of lake deposits (gypsum with carbonate and mud), 11 m of polymict breccia (with greenschist and gneiss clasts), and 19 m of monomict breccia (brecciated quartz-feldspar–mica schist). The breccias start at 174 m depth as polymict breccias with angular clasts of different lithologies and gradually change downward to breccias constituting the dominant lithology, until finally grading into monomict breccia. At the bottom of the borehole, we noted strongly brecciated quartz-feldspar schist. The breccia cement also changes over this interval from gypsum and carbonate cement to fine-grained clastic matrix. Some quartz grains from breccia samples from 192, 194.2, 196.4, 199.3, 201.6, and 204 m depth showed planar deformation features with impact-characteristic orientations. This discovery of unambiguous shock features in drill core samples confirms the impact origin of the Tabun Khara Obo crater. The age of the structure is not yet known. Currently, it is only poorly constrained to post-Cretaceous on stratigraphic grounds.

ABSTRACT Early Archean spherule layers, widely accepted to represent distal ejecta deposits from large-scale impact events onto the early Earth, have been described from several stratigraphic levels of the Barberton greenstone belt in South Africa. Recently, exploration drilling at the Fairview Gold Mine (25°43′53″S, 31°5′59″E) in the northern domain of the belt resulted in the discovery of a new set of spherule layer intersections. The Fairview spherule layers in drill cores BH5901, BH5907, BH5911, and BH5949 were intersected just a few meters apart, at about the same stratigraphic position within the transition from the Onverwacht Group to the Fig Tree Group. The Fairview spherule layers have petrographic and chemical similarities to at least three other well-known Barberton spherule layers (S2–S4), and multiple spherule layer bed intersections in drill cores BARB5 and CT3, all from about the same stratigraphic position. They are not uniform in composition, in particular with respect to abundances of highly siderophile elements. The highest concentrations of moderately (Cr, Co, Ni) and highly siderophile (Ir) elements are within the range of concentrations for chondrites and, thus, reinforce the impact hypothesis for the generation of the Fairview spherule layers. Iridium peak concentrations and Cr/Ir interelement ratios for spherule layer samples from drill cores BH5907, BH5911, and BH5949 suggest admixtures of 50%–60% chondritic material, whereas for the BH5901 spherule layer, only an admixture of 1% chondritic material is indicated. We discuss whether these four Fairview spherule layers represent the same impact event, and whether they can be correlated to any of the S2–S4, CT3, and BARB5 intersections.

Crater-fill impact breccia and basement rock samples from the 1.07 Ma Bosumtwi impact structure (Ghana) were recovered for the first time in 2004 during an International Continental Scientific Drilling Program (ICDP)–sponsored drilling project. Here, we present detailed results of major- and trace-element analyses of 119 samples from drill core LB-08A, together with the chemical compositions of melt particles from suevite. The meta-graywacke and phyllite/slate crater basement rocks can be easily distinguished from each other on the basis of their bulk chemical compositions. A comparison of the chemical compositions of crater-fill and fallout suevites, as well as between proximal and distal impactites, reveals that LB-08A suevites have higher MgO, CaO, and Na 2 O contents than fallout suevites and, similarly, that the CaO and Na 2 O contents are higher by a factor of approximately two in LB-08A suevites than in Ivory Coast tektites. Noticeable differences occur in Cr, Co, and Ni contents between the different impactites; higher abundances are observed for these elements in distal impactites. The observed differences in composition in the various impactites mainly reflect mixing of different proportions of the original target lithologies, as can be seen in the differences in the clast populations between crater-fill and fallout suevites. However, the original impactite compositions may have also been modified by postimpact alteration, particularly in the proximal impactites. Melt particles in suevite show significant differences in major-element compositions between the different samples investigated, but also within a given sample, indicating that they represent melts derived from different lithologies.

The Chesapeake Bay impact structure, which is 85 km in diameter and 35.5 Ma old, was drilled and cored in a joint International Continental Scientific Drilling Program (ICDP) and U.S. Geological Survey (USGS) drilling project at Eyreville Farm, Virginia, U.S.A. In the Eyreville drill core, 154 m of impact breccia were recovered from the depth interval 1397–1551 m. Major- and trace-element concentrations were determined in 75 polymict impactite samples, 10 samples of cataclastic gneiss blocks, and 24 clasts from impactites. The chemical composition of the polymict impactites does not vary much in the upper part of the section (above ~1450 m), whereas in the lower part, larger differences occur. Polymict impactites show a decrease of SiO 2 content, and slight increases of TiO 2 , Al 2 O 3 , and Fe 2 O 3 abundances, with depth. This is in agreement with an increase of the schist/gneiss component with depth. Concentrations of siderophile elements (Co, Ni) are lower in the polymict impactites than in the basement-derived schists and do not indicate the presence of an extraterrestrial component. The five petrographically determined types of melt particles, i.e., clear glass, altered melt, recrystallized silica melt, melt with microlites, and dark-brown melt, have distinct chemical compositions. Mixing calculations of the proportions of rocks involved in the formation of various polymict impactites and melt particles were carried out using the Harmonic least-squares MiXing (HMX) calculation program. The calculations suggest that the metamorphic basement rocks (i.e., gneiss and schist) constitute the main component of the polymict impactites, together with significant sedimentary and possible minor pegmatite/granite and amphibolite components. The sedimentary component is derived mostly from a sediment characterized by a composition similar to that of the Cretaceous Potomac Formation. Compositions of the melt particles were modeled as mixtures of target rocks or major rock-forming minerals. However, the results of the mixing calculations for the melt particles are not satisfactory, and the composition of the particles could have been modified by hydrothermal alteration. Carbon isotope ratios were determined for 18 samples. The results imply a hydrothermal origin for the carbonate veins from the basement-derived core section; carbon-rich sedimentary clasts from the Exmore breccia and suevite have a δ 13 C range typical for organic matter in sediments.

We investigated whole-rock chemical compositions of 318 samples of Exmore breccia (diamicton), impactite (suevite, impact melt rock, polymict lithic impact breccia), and crystalline basement-derived rocks from 444 to 1766 m depth in the International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville A and B drill cores (Chesapeake Bay impact structure, Virginia, USA). Here, we compare the average chemical compositions for the Exmore breccia (diamicton), the impactites and their subunits, sandstone, granite, granitic gneiss, and amphibolite of the lithic block section (1095.7–1397.2 m depth), cataclastic gneiss of the impact breccia section, and schist and pegmatite/granite of the basal crystalline section (1551.2–1766.3 m depth). The granite of the megablock (1097.7–1371.1 m depth) is of I-type and is seemingly related to a syncollisional setting. The amphibolite (1377.4–1387.5 m depth) of the lithic block section is of igneous origin and has a tholeiitic character. Based on chemical composition, the Exmore breccia (diamicton) can be subdivided into five units (444.9–450.7, 450.7–468, 468–518, 518–528, and 528–~865 m depth). The units in the depth intervals of 450.7–468 and 518–528 m are enriched in TiO 2 , MgO, Sc, V, Cr, and Zn contents compared to the other Exmore breccia units. In some samples, especially at ~451–455 m depth, the Exmore breccia contains significant amounts of P 2 O 5 . The Exmore breccia is recognized as a mixture of all sedimentary and crystalline target components, and, when compared to the impactites, it contains a significant amount of a SiO 2 -rich target component of sedimentary origin. The chemical composition of the impactites overlaps the compositional range for the Exmore breccia. The impactites generally display a negative correlation of SiO 2 and CaO, and a positive correlation of TiO 2 , Al 2 O 3 , Fe 2 O 3 , and MgO with depth. This is the result of an increasing basement schist component, and a decreasing sedimentary and/or granitic component with depth. Suevite units S2 and S3 display distinct enrichment of Na 2 O by a factor of ~2 compared to all other impactite units, which is interpreted to reflect a higher granitic component in these units.