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Falko Langenhorst - One of the best experts on this subject based on the ideXlab platform.
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Time-resolved diffraction of Shock-released SiO 2 and diaplectic glass formation
Nature communications, 2017Co-Authors: Arianna Gleason, Falko Langenhorst, C. A. Bolme, H. J. Lee, B. Nagler, E. Galtier, W. Yang, Richard Kraus, Richard L. Sandberg, W. L. MaoAbstract:Understanding how rock-forming minerals transform under Shock loading is critical for modeling collisions between planetary bodies, interpreting the significance of Shock features in minerals and for using them as diagnostic indicators of impact conditions, such as Shock pressure. To date, our understanding of the formation processes experienced by Shocked materials is based exclusively on ex situ analyses of recovered samples. Formation mechanisms and origins of commonly observed mesoscale material features, such as diaplectic (i.e., Shocked) glass, remain therefore controversial and unresolvable. Here we show in situ pump-probe X-ray diffraction measurements on fused silica crystallizing to stishovite on Shock compression and then converting to an amorphous phase on Shock release in only 2.4 ns from 33.6 GPa. Recovered glass fragments suggest permanent densification. These observations of real-time diaplectic glass formation attest that it is a back-transformation product of stishovite with implications for revising traditional Shock Metamorphism stages. Our understanding of Shock Metamorphism and thus the collision of planetary bodies is limited by a dependence on ex situ analyses. Here, the authors perform in situ analysis on Shocked-produced densified glass and show that estimates of impactor size based on traditional techniques are likely inflated.
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mineralogy and defect microstructure of an olivine dominated itokawa dust particle evidence for Shock Metamorphism collisional fragmentation and ll chondrite origin
Earth Planets and Space, 2014Co-Authors: Falko Langenhorst, Kilian Pollok, Dennis Harries, Peter A Van AkenAbstract:We report here detailed analytical scanning and transmission electron microscopic investigations on an olivine-dominated dust particle (RB-QD04-0042) from the surface of asteroid 25143 Itokawa. The dust particle was returned to Earth by the Hayabusa spacecraft and was made available in the context of the first announcement of opportunity for Hayabusa sample investigation. Multiple thin slices were prepared from the precious particle by means of focused ion beam thinning, providing a unique three-dimensional access to its interior. The 40 × 50 μm sized olivine particle contains a spherical diopside inclusion and an intimate intergrowth of troilite and tetrataenite. The compositions of olivine (Fo69Fa31) and diopside (En48Wo42Fs10), as well as the high Ni content of the sulfide-metal alloy, indicate a LL ordinary chondrite origin in accord with previous classifications. Although no impact crater exists at the surface of RB-QD04-0042, transmission electron microscopy revealed the presence of various Shock defects in constituent minerals. These defects are planar fractures and [001] screw dislocations in olivine, multiple {101} deformation twins in tetrataenite and basal (0001) stacking faults in troilite. These diagnostic Shock indicators occur only in a small zone on one concave side of the dust particle characterized by a high fracture density. These observations can be explained by a collisional event that spalled off material from the particle's surface. Alternatively, the dust particle itself could be a spallation fragment of an impact into a larger regolith target. This suggests that Itokawa dust particles lacking visible microcraters on their surfaces might have still experienced Shock Metamorphism and were involved in collisional fragmentation that resulted in the formation of regolith.
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iron deficiency in pyrrhotite of suevites from the chesapeake bay impact crater usa a consequence of Shock Metamorphism
Meteoritics & Planetary Science, 2012Co-Authors: Christoph Mang, Falko Langenhorst, Dennis Harries, Agnes Kontny, L HechtAbstract:Abstract– Pyrrhotite from suevite of the 35 Ma Chesapeake Bay impact structure (CBIS) shows a Shock Metamorphism and we report on several mineralogical and magnetic features. Pyrrhotite shows strong brittle deformation with a high density of stacking faults, twinning parallel to the hexagonal (001) planes and average fault distances in the order of 10 nm. Although the determination of a superstructure was not possible due to the lattice defects, the reflections of the NiAs subcell, which is typical of all pyrrhotite modifications, were clearly detected. This phase is ferrimagnetic with a Curie temperature (TC) between 350 and 365 °C, and suevite with this phase does not show the 34 K transition. The most peculiar feature is the low metal/sulfur ratio of 0.81, which indicates a distinctly higher vacancy concentration than for 4C pyrrhotite and a composition close to smythite (Fe9S11). This phase carries a stable natural remanent magnetization and is relatively hard magnetic. Steep inclinations of the natural remanent magnetization vector, however, suggest that this phase has been remagnetized by the drilling process. A possible explanation is the magnetic domain size of faultless areas of about 10 nm in diameter, which is at the lower limit of the single domain size near the threshold, below which superparamagnetic behavior occurs. The low thermal stability of this phase excludes postShock heating above 300 °C for the suevite of the CBIS. Our results imply that the iron-deficient pyrrhotite is produced by Shock Metamorphism, although an iron loss due to Shock has never been reported before for pyrrhotite.
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Iron deficiency in pyrrhotite of suevites from the Chesapeake Bay impact crater, USA—A consequence of Shock Metamorphism?
Meteoritics & Planetary Science, 2012Co-Authors: Christoph Mang, Falko Langenhorst, Dennis Harries, Agnes Kontny, L HechtAbstract:Abstract– Pyrrhotite from suevite of the 35 Ma Chesapeake Bay impact structure (CBIS) shows a Shock Metamorphism and we report on several mineralogical and magnetic features. Pyrrhotite shows strong brittle deformation with a high density of stacking faults, twinning parallel to the hexagonal (001) planes and average fault distances in the order of 10 nm. Although the determination of a superstructure was not possible due to the lattice defects, the reflections of the NiAs subcell, which is typical of all pyrrhotite modifications, were clearly detected. This phase is ferrimagnetic with a Curie temperature (TC) between 350 and 365 °C, and suevite with this phase does not show the 34 K transition. The most peculiar feature is the low metal/sulfur ratio of 0.81, which indicates a distinctly higher vacancy concentration than for 4C pyrrhotite and a composition close to smythite (Fe9S11). This phase carries a stable natural remanent magnetization and is relatively hard magnetic. Steep inclinations of the natural remanent magnetization vector, however, suggest that this phase has been remagnetized by the drilling process. A possible explanation is the magnetic domain size of faultless areas of about 10 nm in diameter, which is at the lower limit of the single domain size near the threshold, below which superparamagnetic behavior occurs. The low thermal stability of this phase excludes postShock heating above 300 °C for the suevite of the CBIS. Our results imply that the iron-deficient pyrrhotite is produced by Shock Metamorphism, although an iron loss due to Shock has never been reported before for pyrrhotite.
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Shock Metamorphism of some minerals: Basic introduction and microstructural observations
2002Co-Authors: Falko LangenhorstAbstract:Minerals show a unique behaviour when subjected to Shock waves. The ultradynamic loading to high pressures and temperatures causes deformation, transformation and decomposition phenomena in minerals that are unequivocal indicators of impact events. This paper introduces into the basics of Shock compression, required to understand the formation and experimental calibration of these Shock effects in minerals, and par- ticularly focuses on the recent advances in the field of Shock Metamorphism achieved by the application of transmission electron microscopy (TEM). TEM studies underline that the way minerals respond to Shock compression largely depends on their crystal structures and chemical compositions, as is illustrated here on the basis of four minerals: quartz, olivine, graphite and calcite. The crystal structure of a mineral exerts an important control on the Shock-induced deformation phenomena, comprising one- to two-dimension- al lattice defects, such as dislocations, mechanical twins, planar fractures, and amorphous planar deformation lamellae. For example, dislocations can- not be activated in quartz due to the strong covalent bonding, whereas the island silicate olivine easily deforms by dislocation glide. Transformation phenomena include phase transitions to (diaplectic) glass and/or high-pressure polymorphs. TEM studies reveal that high-pressure polymorphs such as coesite, stishovite and ringwoodite are liquidus phases, which form upon decompression by crystallization from high-pressure melts. The graphite-to-diamond transition is however a rare example for a solid-state transformation, taking place by a martensitic shear mechanism. Shock-induced decomposition reactions are typical of volatile-bearing minerals and liberate toxic gases that, in case of large impacts, may affect Earth's climate. Shock experiments show that degassing of calcite does not take place under high pressure but can massively occur after decompres- sion if the post-Shock temperature is sufficiently high. A recombination reaction happens however if CaO and CO2 are not physically separated.
B. C. Hyde - One of the best experts on this subject based on the ideXlab platform.
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variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
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Variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
Axel K. Schmitt - One of the best experts on this subject based on the ideXlab platform.
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variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
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Variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
James Darling - One of the best experts on this subject based on the ideXlab platform.
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variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
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Variable microstructural response of baddeleyite to Shock Metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events
Earth and Planetary Science Letters, 2016Co-Authors: James Darling, Desmond E. Moser, I. R. Barker, K. T. Tait, Kevin R. Chamberlain, Axel K. Schmitt, B. C. HydeAbstract:Abstract The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how Shock Metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U–Pb isotopic systematics, but there is little understanding of Shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-Shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate Shock-effects and their linkage with U–Pb isotopic disturbance that has previously been documented by in-situ U–Pb isotopic analyses. The Shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite Shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U–Pb isotope compositions. Primary igneous features in Group 1 baddeleyites ( n = 5 ) are retained in high Shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U–Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-Shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just Shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of Shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U–Pb isotope system. Furthermore, by linking in-situ U–Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-Shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.
Peter W. Haines - One of the best experts on this subject based on the ideXlab platform.
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zircons from the acraman impact melt rock south australia Shock Metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events
Geochimica et Cosmochimica Acta, 2015Co-Authors: Martin Schmieder, Eric Tohver, Fred Jourdan, Steven W. Denyszyn, Peter W. HainesAbstract:Abstract This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90 km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive Shock Metamorphism, from virtually unShocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO 2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of Shock Metamorphism and impact-induced recrystallization. ZrO 2 -bearing granular zircons indicate Shock pressures in excess of ∼65–70 GPa, which are considerably higher than previous Shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly Shocked crystalline grains yield ages on concordia at ∼1.59–1.60 Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and Shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40 Ar/ 39 Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from Shocked Yardea Dacite, resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500 Ma and ∼450 Ma that selectively affected the impactites exposed in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541 Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.
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Zircons from the Acraman impact melt rock (South Australia): Shock Metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact events
Geochimica et Cosmochimica Acta, 2015Co-Authors: Martin Schmieder, Eric Tohver, Fred Jourdan, Steven W. Denyszyn, Peter W. HainesAbstract:Abstract This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90 km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive Shock Metamorphism, from virtually unShocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO 2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of Shock Metamorphism and impact-induced recrystallization. ZrO 2 -bearing granular zircons indicate Shock pressures in excess of ∼65–70 GPa, which are considerably higher than previous Shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly Shocked crystalline grains yield ages on concordia at ∼1.59–1.60 Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and Shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40 Ar/ 39 Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from Shocked Yardea Dacite, resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500 Ma and ∼450 Ma that selectively affected the impactites exposed in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541 Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.
