Transformation, deformation, and formation of minerals in the Vredefort and Ries impact structure and implications for magnetic properties of impactites

Shock effects of rock-forming minerals with a focus on Fe-Ti-oxides from the Ries- and the Vredefort impact structures were studied in relation to the magnetic properties. Therefore, samples were investigated from locations characterized by enigmatic magnetic anomalies attributed to the respective impact events. The main aim is to gain insights into the host rocks' stress, temperature and oxygen fugacity evolution. Based on different shock effects in impact breccias, the emplacement conditions of the rocks are discussed. Archean basement gneisses within the pronounced magnetic anomaly northwest of the Vredefort impact structure center have quartz (SiO2) grains with shock-generated planar fractures, as documented by two drill cores with ≈10 m depth. Ilmenite (FeTiO3) revealed that shock loading and unloading at relatively low shock pressures (<16 GPa) can result in the formation of mechanical (0001) and {10-11} twins. At re-equilibration temperatures of 600-700°C, exsolution of magnetite (Fe3O4) within ilmenite occurred, forming a few µm-sized magnetite lamellae parallel to the {10-11} twin boundaries and spheroid magnetite along twin and grain boundaries. Furthermore, shearing fractured and locally melted Fe-bearing oxides, which resulted in their intrusion into adjacent shear fractures within neighboring quartz and feldspar. Dauphiné twins associated with shock-induced planar fractures within quartz suggest that the temperatures before the impact event (paleo-depth of 11-23 km resulting in 650-725°C) were higher than the Curie temperature of magnetite (580°C), which is the carrier of the paleomagnetic orientation. Therefore, uplift of the Archean gneiss upon shock-unloading and subsequent cooling in the magnetic field direction present during the Vredefort impact best explains the observed magnetic remanence. The study, furthermore, found no microstructural difference (i.e., phase assemblage, planar fracture abundance and frequency) between samples from the surface and depth of the two drill cores. Lightning strikes heavily influenced the magnetic record of the surficial rocks, however, microstructural products formed from lightning strikes are likely nm-sized and reside below the resolution of the scanning electron microscope. Ilmenite in the Ries impact breccias recorded that at moderate shock pressures (>16 GPa), transformation twin lamellae were generated that share a common {11-20} plane with the host and a 109° angle between the c-axes of host and twin. Moreover, new grains with foam structure formed, which are characterized as orientation domains that also share a common {11-20} plane and whose c�axes span 109° or 99° angles. This crystallographic orientation relationship of new grains and the inferred twins indicates the back-transformation from FeTiO3 high-pressure polymorphs (liuite and wangdaodeite). A variety of different high-temperature reactions generated rutile (TiO2; T=850-1050°C) and minerals of the ferropseudobrookite-armalcolite solid-solution [(Fe,Mg)Ti2O5; T>1140°C] from ilmenite. Furthermore, redox reactions recorded variations in oxygen fugacity. At high temperatures, an enrichment of iron, in terms of elevated Fe/Ti ratios at the rims of ilmenite aggregates, indicates the presence of a reducing agent during the impactite formation, which generated elemental iron. Cooling and subsequent oxidation of iron formed magnetite. Below 700°C at high oxygen fugacity conditions in combination with a leaching agent, pseudorutile (Fe2Ti3O9) was locally created around single ilmenite grains or completely replaced them. A new occurrence of polymict crystalline breccia in the Ries impact structure at the Aumühle quarry exhibits the direct lithological relationship to the underlying Bunte Breccia and overlying suevite. The polymict crystalline breccia consists of ≈50% shocked crystalline clasts from the Variscan basement and ≈50% components from the sedimentary cover sequence, which display no apparent shock effects. Its emplacement likely occurred during the excavation stage of impact cratering. The mathematical Maxwell Z-model describes flow fields during excavation, indicating that shocked material from the crystalline basement was ballistically ejected. A mixture with ballistically ejected sedimentary clasts was subsequently placed on top of Bunte Breccia and then covered by suevite. Reworking of Bunte Breccia and suevite to form polymict crystalline breccia can be excluded based on the absence of glass fragments, larger clast sizes, and random paleomagnetic directions of polymict crystalline breccia compared to suevite. The proposed emplacement is consistent with observations of polymict crystalline breccias from other impact structures. Ballen SiO2 with characteristically curved fractures within impact melt rocks from the Ries impact structure was investigated to elucidate its formation mechanisms and conditions. It likely originated from fluid-inclusion-rich quartz grains in the gneisses of the crystalline basement. Quartz transformed into diaplectic glass upon shock loading, which partly retained structural information about the precursor phase. As a result, the fluid inclusions dissolved into the amorphous phase. Upon shock unloading and subsequent cooling, dehydration caused fracturing of the glass resulting in curved interfaces as similarly observed from volcanic glasses, i.e., perlitic structures. Structural remnants within the diaplectic glass enabled topotactic crystallization, resulting in preferred crystallographic orientations within quartz. In cases without structural information within the amorphous phase, quartz as well as cristobalite (at elevated temperatures) formed with random crystallographic orientations. Dendritic cristobalite only occurs at the rim of the aggregates in correlation with adjacent vesicles and is interpreted to have formed from a fluid-rich melt

Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering

Shock-related calcite twins are characterized in calcite-bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre-impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (1/μm) of twins with widths <100 nm. Different types of twins (e-, f-, and r-twins) crosscutting each other can occur in one grain. Interaction of r- and f-twins results in a-type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a-axis. Such a-type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer

Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany

AbstractThe Ries impact structure (Germany) contains well‐preserved ejecta deposits consisting of melt‐free lithic breccia (Bunte Breccia) overlain by suevite. To test their emplacement conditions, we investigated the magnetic properties and microstructures of 26 polymict breccia clasts and a stratigraphic profile from the clasts into the suevite at the Aumühle quarry. Remanent magnetization directions of the Bunte Breccia clasts fall into two groups: those whose directions mostly lie parallel to the reversed field during impact carried mostly by magnetite, and those whose directions vary widely among each clast carried by titanohematite. Basement clasts containing titanohematite acquired a chemical remanent magnetization (CRM) during the ejection process and then rotated during turbulent deposition. Clasts of sedimentary rocks grew magnetite after turbulent deposition, with CRM directions lying parallel to the paleofield. Suevite holds a thermal remanent magnetization carried by magnetite, except for ∼12 cm from the contact with the Bunte Breccia, where hematite concentrations increase due to hydrothermal alteration. These observations lead us to propose a three‐stage model of (a) turbulent deposition of the melt‐free breccia with clast rotation &lt;580°C, (b) deposition of the overlying suevite, which acted as a semi‐permeable barrier that confined hot (&lt;300°C) oxidizing fluids to the permeable breccia zone, and (c) prolonged hydrothermal activity producing further alteration which ended before the next geomagnetic reversal. Basement outcrops have significantly different magnetic properties than the Bunte Breccia basement clasts with similar lithology. Two basement blocks situated near the inner ring may have been thermally overprinted up to 550°C.Plain Language Summary: The 26‐km‐diameter, ∼15‐million‐year‐old Ries meteorite impact structure in southern Germany is characterized by well‐preserved ejecta deposits expelled from the crater within seconds after the impact. These deposits consist of two main layers: melt‐free, lithic breccia (Bunte Breccia), overlain by melt‐bearing breccia (suevite). To understand the formation conditions of the ejecta deposits, we performed paleomagnetic and rock magnetic measurements and microstructural experiments on clasts within Bunte Breccia and on the overlying suevite at the Aumühle quarry. We found that clasts derived from crystalline basement materials experienced high pressures during the impact. These clasts had randomly oriented magnetization directions carried by titanohematite. In contrast, clasts derived from sedimentary rocks experienced only low pressures and had coherent magnetization directions oriented parallel to the reversed field during the impact that are carried by magnetite. Our findings can be interpreted by a three‐stage model that explains the thermal and structural formation of impact ejecta at the Ries impact structure.Key Points: Randomly oriented paleomagnetic directions in basement clasts in ejecta deposits suggest turbulent emplacement Bunte Breccia was chemically altered and locally heated by the overlying suevite, resulting in hydrothermal activity up to 300°C Basement rocks near the inner ring may have experienced temperatures up to 550°C from cratering Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659https://earthref.org/MagIC/1998

Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering

AbstractShock‐related calcite twins are characterized in calcite‐bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre‐impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (&lt;7 GPa). Calcite, however, shows a high density (&gt;1/μm) of twins with widths &lt;100 nm. Different types of twins (e‐, f‐, and r‐twins) crosscutting each other can occur in one grain. Interaction of r‐ and f‐twins results in a‐type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a‐axis. Such a‐type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer.Bavarian Natural History Collection

Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany

The Ries impact structure (Germany) contains well-preserved ejecta deposits consisting of melt-free lithic breccia (Bunte Breccia) overlain by suevite. To test their emplacement conditions, we investigated the magnetic properties and microstructures of 26 polymict breccia clasts and a stratigraphic profile from the clasts into the suevite at the Aumühle quarry. Remanent magnetization directions of the Bunte Breccia clasts fall into two groups: those whose directions mostly lie parallel to the reversed field during impact carried mostly by magnetite, and those whose directions vary widely among each clast carried by titanohematite. Basement clasts containing titanohematite acquired a chemical remanent magnetization (CRM) during the ejection process and then rotated during turbulent deposition. Clasts of sedimentary rocks grew magnetite after turbulent deposition, with CRM directions lying parallel to the paleofield. Suevite holds a thermal remanent magnetization carried by magnetite, except for ∼12 cm from the contact with the Bunte Breccia, where hematite concentrations increase due to hydrothermal alteration. These observations lead us to propose a three-stage model of (a) turbulent deposition of the melt-free breccia with clast rotation <580°C, (b) deposition of the overlying suevite, which acted as a semi-permeable barrier that confined hot (<300°C) oxidizing fluids to the permeable breccia zone, and (c) prolonged hydrothermal activity producing further alteration which ended before the next geomagnetic reversal. Basement outcrops have significantly different magnetic properties than the Bunte Breccia basement clasts with similar lithology. Two basement blocks situated near the inner ring may have been thermally overprinted up to 550°C

Quartz and cristobalite ballen in impact melt rocks from the Ries impact structure, Germany, formed by dehydration of shock‐generated amorphous phases

Quartz and cristobalite ballen aggregates surrounded by dendritic cristobalite in gneiss clasts of impact melt rocks from the Ries impact structure are analyzed by Raman spectroscopy, microscopy, and electron backscattered diffraction to elucidate the development of the characteristic polycrystalline ballen that are defined by curved interfaces between each other. We suggest that the investigated ballen aggregates represent former fluid inclusion‐rich quartz grains from the granitic gneiss protolith. Upon shock loading, they transformed into an amorphous phase that partly retained information on the precursor structure. Volatiles from inclusions dissolved into the amorphous phase. During decompression and cooling, dehydration takes place and causes fracturing of the amorphous phase and disintegration into small globular ballen, with the fluid being expelled along the fractures. A similar formation of small globules due to dehydration of silica‐rich glass is known for perlitic structures of volcanic rocks. Remnants of the precursor structure are present in the amorphous phase and enabled topotactic crystallization of quartz, leading to a crystallographic preferred orientation. Crystallization of more distorted parts of the amorphous phase led to random orientations of the quartz crystals. Ballen comprised of cristobalite formed from a dehydrated amorphous phase with no structural memory of the precursor. Dendritic cristobalite exclusively occurring at the rim of quartz ballen aggregate is interpreted to have crystallized directly from a melt enriched in fluids that were expelled during dehydration of the amorphous phase.SNSB-Innovativ Projekt Ballenquar

Ilmenite and magnetite microfabrics in shocked gneisses from the Vredefort impact structure, South Africa

Abstract We investigated microfabrics of shocked Archean gneisses from two, 10 m-deep drill cores located near the center of the Vredefort impact structure in an area that is characterized by a prominent, long-wavelength negative magnetic anomaly ( 100 µm) ilmenite and magnetite host grains. These fine-scaled veins suggest mobilization of magnetite and ilmenite during shear deformation of host Fe-phases and adjacent silicates, probably associated with frictional heating. Coarse ilmenite has fine-lamellar mechanical twins parallel to {10 $$\overline{1}$$ 1 ¯ 1} and single (0001) twins, indicative of dislocation-glide-controlled deformation under non-isostatic stresses related to shock. A few µm-wide magnetite lamellae parallel to {10 $$\overline{1}$$ 1 ¯ 1} and spheroidal magnetite (diameter ≈10 µm) within coarse ilmenite document exsolution after shock. Dauphiné twins associated with planar features in quartz imply cooling from 650 to 725 °C after shock, which accords with estimates of pre-impact basement temperatures from petrographic studies. The Curie temperature of magnetite is 580 °C; therefore, the central negative magnetic anomaly was produced as a thermoremanent magnetization acquired during cooling of the initially hot crust. The long-wavelength anomaly was likely amplified by the newly created magnetite that also acquired a thermal remanence. Although the magnetic properties of surface samples are often influenced by lightning strikes, we found no microstructural evidence for lightning-related processes

Middle Neoproterozoic (Tonian) Polar Wander of South China: Paleomagnetism and ID‐TIMS U‐Pb Geochronology of the Laoshanya Formation

Abstract Paleomagnetic records of middle Neoproterozoic (820 to 780 Ma) rocks display high amplitude directional variations that lead to large discrepancies in paleogeographic reconstructions. Hypotheses to explain these data include rapid true polar wander (TPW), a geomagnetic field geometry that deviates from a predominantly axial dipole field, a hyper‐reversing field (&gt;10 reversals/Ma), and/or undiagnosed remagnetization. To test these hypotheses, we collected 1,057 oriented cores over a 85 m stratigraphic succession in the Laoshanya Formation (Yangjiaping, Hunan, China). High precision U‐Pb dating of two intercalated tuff layers constrain the age of the sediments between 809 and 804 Ma. Thermal demagnetization isolates three magnetization components residing in hematite which are not time‐progressive but conflated throughout the section. All samples possess a north and downward directed component in geographic coordinates at temperatures up to 660°C that is ascribed to a Cretaceous overprint. Two components isolated above 660°C reveal distinct directional clusters: one is interpreted as a depositional remanence, while the other appears to be the result of a mid‐Paleozoic (460 to 420 Ma) remagnetization, which is likely widespread throughout South China. The high‐temperature directions are subtly dependent on lithology; microscopic and rock magnetic analyses identify multiple generations of hematite that vary in concentration and distinguish the magnetization components. A comparison with other middle Neoproterozoic paleomagnetic studies in the region indicates that the sudden changes in paleomagnetic directions, used elsewhere to support the rapid TPW hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations.</p