Suevite Breccia Research Articles

Newly detected shock-induced high-pressure phases formed in amphibolite clasts of the suevite breccia (Ries impact crater, Germany): Liebermannite, kokchetavite, and other ultrahigh-pressure phases

Amphibolite clasts in the suevite of the Ries impact crater contain shock-induced melt veins (SMVs) with high-pressure phases such as majoritic garnet, jadeitic clinopyroxene and others. In addition, heat conduction from hot SMVs into adjacent rock portions locally produced further high P–T melt pools. These melts were preferentially generated in rock domains, where the SMVs cross older (‘pre-Ries’) veinlets with analcime or prehnite and larger grains of sericitized plagioclase. Melting of such chemically different local bulk systems (Na-, Ca-, Ca-Na- and K-Na-rich) was facilitated by low solidus temperatures of the original secondary OH-bearing phases. From the resulting shock-induced melts, liebermannite, kokchetavite, jadeite, nonstoichiometric and albitic jadeite, grossular, vuagnatite, lawsonite + coesite, and clinozoisite crystallized during pressure release. Vuagnatite is now proven to be a genuine high-pressure phase. Its ubiquitous distance of 20–35 μm from the hot shock veins suggests a temperature sensitivity typical for an OH-bearing phase. In local Na-rich melts albitic jadeite appears instead of the assemblage jadeite + SiO2. Liebermannite, a dense polymorph of K-feldspar was identified by Raman spectroscopy. After stishovite, liebermannite constitutes the second known high-pressure phase in the Ries that contains silicon exclusively in six-fold coordination. The KAlSi3O8-polymorph kokchetavite was formed in alkali-rich melt glasses. Pressure and temperature values in the range of about 8–11 GPa and ~ 800–1100 °C were estimated from the chemical compositions of locally occurring majoritic garnets (Si = 3.21–3.32 and 3.06–3.10 apfu), respectively, and the presence of fine-grained aggregates of lawsonite and coesite. Generally, the neighboring areas of the veins are characterized by a sequence of variable high-pressure phases documenting strongly falling P–T conditions with increasing distance from the vein. These novel features enlighten the dynamic event during passage of a shock wave.

Magnetic Properties of Impact UHPHT Glasses, Melt Rocks, Suevitic Breccia and Target Rocks of the Giant Kara Meteorite Crater (Pay-Khoy, Arctic Seashore, Russia)

The shock waves can strongly change the physical properties of the target rock minerals including their density and magnetism which determine petrochemical properties of impactites finely as a rule are resulted in astroblemes contours on geophysical maps. Following to the aero-magnetic mapping data the non-magnetic sedimentary rocks of the Kara target create a zero and negative magnetic field with an average intensity of -1 nT, against the background the southwestern region of the Kara astrobleme provides the positive magnetic anomalies with an intensity of 1 to 3 nT which are in a good correspondence with the Pay-Khoy ridge structure general orientation. The Kara dome is characterised with an isometric negative anomaly of intensity -5 nT. Here we present the magnetic properties of the different kinds of the Kara impactites including impact ultra-high pressure high temperature (UHPHT) melt glasses, melt rocks and suevitic breccia compare to sedimentary target rocks. The petrophysical measurements presented the specific magnetic susceptibility of the impactites in the range of 8 to 48×10-8 SI units, where the UHPHT glasses have the limits from 9 to 38×10-8 SI units (15×10-8 SI units, in average). The sedimentary target is characterised with essentially lower level of magnetic susceptibility – no higher than 15×10-8 SI units, where limestone has it about zero. Following to the similar level of the iron content within the impactites and target rocks the magnetism of the Kara impact melts is explained rather by changing of magnetic properties by the impact process. One of the possible source of magnetism can be partially an iron-containing matter of the asteroid component in the form of pyrrhotine accompanied with Ni and Co impurities. Also, we cannot exclude partial presence of magnetic iron component directly within the quenched impact glasses including UHPHT variety.

Petrographic and Petrochemical Characteristics of Suevite Matrix, Western Part of the Kara Astrobleme (Russia)

The Kara Astrobleme is a unique geological object located in the northwestern part of the Arctic zone of Russia. Impactites from the Kara crater are diamondiferous to various degrees. At present the Kara suevites are relatively poorly studied. Until now the petrographic features of the matrix have been described in general terms, and petrochemically the cementing mass of the suevites has not yet been studied. In 2015 suevite breccias of the southern part of the Kara Astrobleme were subdivided into three different types. It was later suggested that the suevites of the western part of the impact crater, located at the Put’yu and Sayakha rivers, could be type II suevites, but at the same time they had a number of distinctive features which might be associated with different formation conditions of these rocks. Hence it is relevant to study the matrix of the suevites of the mentioned areas to clarify their facies and formation features. The paper presents results of a comprehensive study with a detailed description of the suevite matrix at Put’yu and Sayakha rivers conducted for the first time for the suevites of the Kara Astrobleme. Through optical and microprobe studies we described petrographic and petrochemical features of the cementing mass of the suevites. We discovered that the matrix of the studied rocks was characterized by the similarity of the mineralogical composition, but differing in structural and content features. We found that all the studied rocks belonged to the aerodynamic ejection facies, within which they differed in the temperature conditions of formation which might be associated with the different initial position of the suevite material in the impact cloud. The research results clarify the type and facies of suevites in other areas of the impact structure that will allow defining facies variability and detail characteristics of suevite breccias of the Kara Astrobleme. The obtained data demonstrate genetic regularities of impactites and allow predict the diamond content of the Astrobleme as a whole.

Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon

AbstractReidite, the high‐pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer‐sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4under extremeP–Tconditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite‐bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite‐bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon‐to‐reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4microstructural constraints, we decipher the grains evolution at specificP–Tconditions related to different impact stages, offering further understanding of the behavior of ZrSiO4during shock.

Shock deformation in zircon grains from the Mien impact structure, Sweden

AbstractRecognition of impact‐induced deformation of minerals is crucial for the identification and confirmation of impact structures as well as for the understanding of shock wave behavior and crater formation. Shock deformed mineral grains from impact structures can also serve as important geochronometers, precisely dating the impact event. We investigated zircon grains from the Mien impact structure in southern Sweden with the aim of characterizing shock deformation. The grains were found in two samples of impact melt rock with varying clast content, and in one sample of suevitic breccia. We report the first documentation of so‐called “FRIGN zircon” (former reidite in granular neoblastic zircon) from Mien (pre‐erosion diameter 9 km), which confirms that this is an important impact signature also in relatively small impact structures. Furthermore, the majority of investigated zircon grains contain other shock‐related microtextures, most notably granular and microporous textures, that occur more frequently in grains found in the impact melt than in the suevitic breccia. Our findings show that zircon grains that are prime candidates for establishing a new and improved age refinement of the Mien impact structure are present in the impact melt.

X-ray computed microtomography of diamondiferous impact suevitic breccia and clast-poor melt rock from the Kara astrobleme (Pay-Khoy, Russia)

Abstract X-ray computed microtomography (CT) of impact rock varieties from the Kara astrobleme is used to test the method’s ability to identify the morphology and distribution of the rock components. Three types of suevitic breccias, clast-poor melt rock, and a melt clast from a suevite were studied with a spatial resolution of 24 µm to assess CT data values of 3D structure and components of the impactites. The purpose is first to reconstruct pore space, morphology, and distribution of all distinguishable crystallized melt, clastic components, and carbon products of impact metamorphism, including the impact glasses, after-coal diamonds, and other carbon phases. Second, the data are applied to analyze the morphology and distribution of aluminosilicate and sulfide components in the melt and suevitic breccias. The technical limitations of the CT measurements applied to the Kara impactites are discussed. Because of the similar chemical composition of the aluminosilicate matrix, glasses, and some lithic and crystal clasts, these components are hard to distinguish in tomograms. The carbonaceous matter has absorption characteristics close to air, so the pores and carbonaceous inclusions appear similar. However, X-ray microtomography could be used to prove the differences between the studied types of suevites from the Kara astrobleme using structural-textural features of the whole rock, porosity, and the distributions of carbonates and sulfides.

3D Modeling of Suevite Massifs with UHPHT Glasses at the Kara Astrobleme and the Ust`-Kara Area (Pay-Khoy, Kara seashore, Arctic, Russia)

The unusual natural ultrahigh pressure high temperature (UHPHT) impact glasses have been discovered at the Southern part of the Kara astrobleme (Pay-Khoy, Russia) in 2015. The glasses form a complex of stockwork-like system of thin vein bodies set within suevite breccia at the right bank of the Kara river. The Kara glasses have many specific features pointing to UHPHT origin. For better understanding of the geological position of the UHPHT impact glasses we have provided additional field observation with the use of copter facility to observe the outcrops from the «air-bird high» and collecting data for 3D modeling. Here we present natural topological details for the more complete analysis of the discovered UHPHT complex at the Kara River (Pay-Khoy, Russia) and “bottom flow” suevites with UHPHT impact ribbon-like glasses on the Baydarata Bay shore (Kara Sea, Arctic, Russia).

UHPHT Glasses in Bottom Suevite Facies (Ust`-Kara, Arctic Ocean, Russia)

Amorphous substances, including glasses, are very important kind of value materials for numerous applications. Among the glasses the impact matter has especially high fundamental interest and use potential, being formed under extremely high PT conditions - up to hundreds GPa and thousands degrees Celsius. In this direction the detail studies of new natural occurrences with UHPHT impact glasses and their features are very actual fundamentally and probably potentially useful for new ideas for technologies. Our analytical studies of the UHPHT ribbonlike impact glasses of the Ust`-Kara area at the Baydarata Bay (Kara Sea, Arctic Ocean) revealed the presence a number of special features such as multilevel differentiation of impact melt and coesite abundance. The specifics of the Ust`-Kara UHPHT glasses rather point to bottom facies of the suevitic breccia. The observed impactites characteristics allow to see new geological importance of the studied Ust`-Kara suevites, allow to correct geological model of the impact structure.

Hyperspectral imaging of drill core from the Steen River impact structure, Canada: Implications for hydrothermal activity and formation of suevite‐like breccias

AbstractHyperspectral imaging can be used to rapidly identify and map the spatial distributions of many minerals. Here, hyperspectral mapping in three wavelength regions (visible and near‐infrared, shortwave infrared, and thermal infrared) was applied to drill cores (ST001, ST002, and ST003) penetrating a continuous sequence of crater‐fill breccias from the Steen River impact structure in Alberta, Canada. The combined data sets reveal distinct mineralogical layering, with breccias derived predominantly from sedimentary rocks overlying those derived from granitic basement. This stratigraphy demonstrates that the breccias were not appreciably disturbed following deposition, which is inconsistent with formation models of similar breccias (suevites) by explosive impact melt–fluid interaction. At Steen River, volatiles from sedimentary target rocks were an inherent part of forming these enigmatic breccias. Approximately three quarters of terrestrial impact structures contain sedimentary target rocks; therefore, the role of volatiles in producing so‐called suevitic breccias may be more widespread than previously realized. The hyperspectral maps, specifically within the SWIR wavelength region, also delineate minerals associated with postimpact hydrothermal activity, including ammoniated clay and feldspar minerals not detectable using traditional techniques. These nitrogen‐bearing minerals may have originated from microbial processes, associated with oil‐ and gas‐producing units in the crater vicinity. Such minerals may have important implications for the production of habitable environments by impact‐induced hydrothermal activity on Earth and Mars.

Evidence for shock‐induced anhydrite recrystallization and decomposition at the UNAM‐7 drill core from the Chicxulub impact structure

AbstractDrill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SOx gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation.

Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon

Zircon (ZrSiO4) is used to study impact structures because it responds to shock loading and unloading in unique, crystallographically controlled manners. One such phenomenon is the transformation of zircon to the high-pressure polymorph, reidite. This study quantifies the geometric and crystallographic orientation relationships between these two phases using naturally shocked zircon grains. Reidite has been characterized in 32 shocked zircon grains (shocked to stages II and III) using a combination of electron backscatter diffraction (EBSD) and focused ion beam cross-sectional imaging techniques. The zircon-bearing clasts were obtained from within suevite breccia from the Nordlingen 1973 borehole, close to the center of the 14.4 Ma Ries impact crater, in Bavaria, Germany. We have determined that multiple sets (up to 4) of reidite lamellae can form in a variety of non-rational habit planes within the parent zircon. However, EBSD mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misorientation relationship with the host zircon that is characterized by an approximate alignment of a {100}zircon with a {112}reidite and alignment of a {112}zircon with a conjugate {112}reidite. Given the tetragonal symmetry of zircon and reidite, we predict that there are eight possible variants of this interphase relationship for reidite transformation within a single zircon grain. Furthermore, laser Raman mapping of one reidite-bearing grain shows that moderate metamictization can inhibit reidite formation, thereby highlighting that the transformation is controlled by zircon crystallinity. In addition to lamellar reidite, submicrometer-scale granules of reidite were observed in one zircon. The majority of reidite granules have a topotaxial alignment that is similar to the lamellar reidite, with some additional orientation dispersion. We confirm that lamellar reidite likely forms via a deviatoric transformation mechanism in highly crystalline zircon, whereas granular reidite forms via a reconstructive transformation from low-crystallinity ZrSiO4 within the reidite stability field. The results of this study further refine the formation mechanisms and conditions of reidite transformation in naturally shocked zircon.

CHARAKTERYSTYKA PETROGRAFICZNA SKAŁ POCHODZĄCYCH ZE STRUKTURY IMPAKTOWEJ GARDNOS, NORWEGIA

PETROGRAPHIC CHARCTERSITICS OF THE ROCKS FROM THE IMPACT STRUCTURE GARDNOS, NORWAY Abstract. The crater of the Gardnos meteorite is located in Norway, about 125 km north-west of Oslo. The structure is well seen due to the outcrops of impact breccias and post-impact deposits that form a circle with a radius of about 5 km. Main types of the impact rocks in the Gardnos area are the autochthonous Gardnos breccia and its cover – allochthonous suevite breccia. The petrological characteristics of these rocks are presented. The Gardnos breccia is a lithic breccia, composed of fissured rock fragments, without melt components. The suevite is a polymictic breccia, built of small fragments composed of recrystallized melt and clasts of rocks and minerals.

10Be content in clasts from fallout suevitic breccia in drill cores from the Bosumtwi impact crater, Ghana: Clues to preimpact target distribution

AbstractRocks from drill cores LB‐07A (crater fill) and LB‐08A (central uplift) into the Bosumtwi impact crater, Ghana, were analyzed for the presence of the cosmogenic radionuclide 10Be. The aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater‐filling breccia, and also to detect meteoric water infiltration within the impactite layer. 10Be abundances above background were found in two (out of 24) samples from the LB‐07A core, and in none of five samples from the LB‐08A core. After excluding other possible explanations for an elevated 10Be signal, we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface. Our results suggest that in‐crater breccias were well mixed during the impact cratering process. In addition, the lack of a 10Be signal within the rocks located very close to the lake sediment–impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited. This may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore‐space, but rather through a localized system of fractures.

Petrography, geochemistry, and Hf‐Nd isotope evolution of drill core samples and target rocks from the El'gygytgyn impact crater, NE Chukotka, Arctic Russia

AbstractEl'gygytgyn in northeast Chukotka (Russia) is a 3.6 Ma, 18‐km‐diameter impact structure. The impact crater was recently drilled in the framework of a project sponsored by the International Continental Scientific Drilling Program (ICDP). Target rocks at the El'gygytgyn area are dominated by the felsic members of the Late Cretaceous Okhotsk‐Chukotka Volcanic Belt (OCVB). Such a target lithology is unique among terrestrial impact craters, thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks. Here, we present a petrographic, geochemical, and isotopic study of the section of the drill core underneath the lacustrine sediments, extending from ∼316 m to 517 m below the lake bottom (blb). The drill core stratigraphy includes ∼80 m of suevite and a cross section through a volcanic suite, which consists of (1) a middle section (∼390–423 mblb) with dominant felsic tuffs and a few mafic members, and (2) a welded rhyolitic‐dacitic ignimbrite (∼423–517 mblb). The melt fragments embedded in the suevite are interpreted as being impact‐related by comparison with impact glasses from the crater and in opposition to the target rock, which does not include similar melts. A suevitic dyke crosscuts the lower section of the core at the depth 471.40 mblb. Evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia. The geochemical and isotope (Nd and Hf) composition of samples from the target and the drill core reveals relationships to the “Berlozhya magmatic assemblage” (BMA) arguing for similar source magmas. The primitive upper mantle (PUM)‐normalized trace element plot of rocks investigated here confirms a subduction‐related signature, as previously proposed for rocks from both OCVB and BMA.

Chromium isotope anomaly in an impactite sample from the El'gygytgyn structure, Russia: Evidence for a ureilite projectile?

AbstractThe 3.6 Ma, 18‐km‐diameter El'gygytgyn impact structure (Arctic Russia) is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks. Previous analyses of impactites from El'gygytgyn showed minor enrichments of the siderophile elements, including Ir, which, together with distinct Cr enrichments, gave rise to speculation that an achondritic projectile was involved. We studied the major and trace element composition in samples from the new ICDP drill core obtained near the center of the structure, as well as the chromium isotopic composition of an impact glass sample collected on the surface. Several suevitic breccias from the upper part of the suevite sequence in the drill core show higher Cr and Ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples. However, it is difficult to unambiguously establish a meteoritic component from trace element data, as input from (rare) mafic target rocks is a possibility. In contrast, the Cr isotopic composition of the impact glass sample yielded a nonterrestrial ε54Cr value of −0.72 ± 0.31 (2 std. err.). This negative ε54Cr is different from known carbonaceous chondrite values (ε54Cr of +0.95 to +1.65), but is nearly identical to reported values for ureilites (approximately −0.77). The value is, however, also within analytical error of eucrites (approximately −0.38) and ordinary chondrites (approximately −0.42). Given the chemical signatures found in previous analyses of El'gytgytgyn impactites and the similarity of our Cr isotopic data to ureilites, we suggest that the impacting asteroid could have been an F‐type asteroid of mixed composition, similar to the recent Almahata Sitta fall in Sudan.

Evidence of Shock Metamorphism Effects in Allochthonous Breccia Deposits from the Colônia Crater, São Paulo, Brazil

The 3.6 km-diameter Colonia impact crater, centred at 2352'03S and 4642'27W, lies 40 km to the south-west of the S?o Paulo city. The structure was formed on the crystalline basement rocks and displays a bowl-shaped with steeper slope near the top that decreases gently toward the centre of the crater. Over recent years were drilled two boreholes inside the crater, which reached a maximum depth of 142 m and 197 m. Geological profile suggests four different lithological associations: 1) unshocked crystalline basement rocks (197 - 140 m); 2) fractured/brecciated basement rocks (140 - 110 m); 3) polymictic allochthonous breccia deposits (110 - 40 m); and 4) post-impact deposits (40 - 0 m). Petrographic characterisation of the polymictic allochthonous breccia reveals a series of distinctive shock-metamorphic features, including, among others, planar deformation features in quartz, feldspar and mica, ballen silica, granular texture in zircon and melt-bearing impact rocks. The occurrence of melt particles and very high-pressure phase transformation in suevite breccia suggest a shock pressure regime higher than 60 GPa.

Ca-rich majorite derived from high-temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the Ries impact crater (Germany)

Shock veins up to 1.1 mm thick were found within non-porous lithic clasts from suevite breccia of the Nordlinger Ries impact structure. These veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy. In shocked amphibolites, two types of Ca-rich majorite occur within and adjacent to the veins. The first type crystallized from shock-induced melts within the veins. Si contents of these majorites suggest dynamic pressure of ~15–17 GPa, implying minimum temperatures in the range of ~2,150–2,230°C. The second type of majorite was formed adjacent to the shock veins within pargasitic hornblende. This majorite contains significant amounts of H2O (0.7–0.9 wt%). Based on the textural setting, the shrinkage cracks and the chemical compositions of both phases, a solid-state mechanism is deduced for the hornblende to majorite phase transition. Both genetic types of Ca-rich majorite are described for the first time from a terrestrial impact crater. Along with stishovite, majorite constitutes the second silicate mineral displaying sixfold coordination of Si at Ries. Using micro-Raman spectroscopy, jadeite + coesite and jadeite + grossular were identified within local melt glasses of alkali feldspar and plagioclase composition, respectively. Stishovite aggregates, produced by solid-state reaction, along with shock-induced high-pressure melt glasses of almandine composition were also detected in shock veins of a garnet-cordierite-sillimanite restite. The quenched, homogeneous almandine glasses point to melting temperatures of more than ~2,500°C for the veins. Our findings demonstrate that terrestrial shock veins can give valuable information on shock-induced mineral transformations and transient high pressures of host rocks during a natural impact event.

Akaogiite: An ultra-dense polymorph of TiO2 with the baddeleyite-type structure, in shocked garnet gneiss from the Ries Crater, Germany

Akaogiite, an ultra-dense polymorph of TiO2 was encountered in heavily shocked garnet-cordieritesillimanite gneiss in the Suevite breccia of the Ries meteorite impact crater in Germany. The chemical formula of akaogiite is TiO2, containing (wt%) TiO2 97.6-97.7; Nb2O5 0.15-0.20, and FeO 0.11-0.14. The empirical formula is: Ti0.998Fe0.002Nb0.001O2. Akaogiite is optically distinguishable from rutile only in reflected light microscopy, through its slightly higher brightness and the intense royal blue color of its internal reflections in crossed nicols. The mineral grains consist of countless randomly oriented submicrometer particles. In situ micro-beam X-ray diffraction established akaogiite to be monoclinic with the baddeleyite-type structure. Cell parameters are a = 4.606(2) Å, b = 4.896(3) Å, c = 4.933(3) Å, and β = 99.17(6)°; space group P21/c; molar volume = 16.82(2) cm3/mol; density = 4.72 g/cm3. Akaogiite coexists in the shocked gneiss with graphite-diamond phase transition assemblage, a second dense TiO2 polymorph with the scrutinyite-type structure (TiO2-II), liquidus idiomorphic jadeite inclusions in melt pockets in garnet, and a new FeTiO3-polymorph with the Li-niobate structure. The high-pressure assemblage constrains the equilibrium peak-shock pressure to be ≤22 GPa and a post-shock temperature <500 °C, thus correcting previous estimates. The name was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2007-058). The name is for Masaki Akaogi, Professor at the Department of Chemistry, Gakushuin University in Tokyo, Japan.

Integrated interpretation of physical properties of rocks of the borehole Yaxcopoil‐1 (Chicxulub impact structure)

The borehole Yaxcopoil‐1, drilled within the Chicxulub meteoritic impact structure (Mexico), was completely cored from 404 to 1511 m through postimpact Tertiary limestones underlain by impactites. The impactites comprise impact melt‐rich, suevitic breccia followed by megablocks of Cretaceous limestones, calcarenites, dolomites, and anhydrites. Measurements of porosity, density, and thermal parameters on 450 samples (equidistant sampling, complete depth range) and of ultrasonic velocities and electric resistivity on 80 representative samples are used to investigate the physical properties of carbonate rocks and to study the influence of the impact. Experiments under elevated pressure, calculations using frequency‐dependent Biot‐Gassmann theory, and cross‐checking with borehole logs, where available, show that ultrasonic laboratory and sonic in situ data correspond. Sonic and electric quasi‐continuous logs are obtained from empirical correlations with thermal conductivity, density, and porosity and consideration of mineralogical composition and microstructure. These data give constraints on interpretation and geophysical modeling of, e.g., seismic and gravity data. In the Tertiary postimpact limestone section, the rock fabric (porosity) influences the physical properties. The upper boundary of the impactites is distinctly determined by the high inhomogeneity factor and anisotropy coefficient of thermal conductivity and by the temperature gradient from high‐resolution borehole temperature measurements. All physical properties indicate that the upper part of the suevitic breccia can be distinguished from the lower suevite unit. In the Cretaceous megablocks, a high variability of all properties (particularly, thermal conductivity, density of solid material, and temperature gradient) due to the high variability in the mineral composition (calcite, dolomite, anhydrite) is observed.

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

Abstract— The Bosumtwi impact crater in Ghana, arguably the best‐preserved complex young impact structure known on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1–2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near‐equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures.Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB‐07A and LB‐08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics &amp; Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB‐07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB‐08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke‐dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim.Deep drilling results confirmed the gross structure of the crater as imaged by the pre‐drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post‐impact sediments (less than 1800 m/s) and the impactites (2600–3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence.The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre‐drilling geophysics, and presents an interesting problem for comparative studies and requires re‐evaluation of existing data from other terrestrial impact craters, as well as modeling parameters.