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Transformation, deformation, and formation of minerals in the Vredefort and Ries impact structure and implications for magnetic properties of impactites
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.
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Dellefant, Fabian
2024
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Dellefant, Fabian (2024): Transformation, deformation, and formation of minerals in the Vredefort and Ries impact structure and implications for magnetic properties of impactites. Dissertation, LMU München: Faculty of Geosciences
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Abstract

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.