ALBERT

Description: Pseudotachylytes resulted from frictional melts associated with ultramylonites in high-grade metapelitic rocks from the Ivrea-Verbano zone in the Southern Alps (Northern Italy) were studied with focus on the deformation microstructures in zircon. The aims were to investigate the characteristics of zircon deformation in seismic zones, and to recognize specific microstructures generated in zircon during earthquakes, which could be useful for mineral dating of paleo-seismic events; helps to understand how seismic energy is released at depth and interacts with metamorphic processes. The interior of polished zircon grains ranging from 30 to 150 μm in length were investigated with optical microscope and scanning electron microscope (SEM) techniques, including secondary electron (SE), backscattered electron (BSE), forward scattered electron (FSE), cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction analysis (EBSD). Grains were studied in situ and as separated fractions embedded in epoxy disks. Among different cataclastic and crystal-plastic deformation microstructures in zircon we identified characteristic planar deformation bands (PDBs), planar fractures (PFs), and curviplanar fractures (CFs). Planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain, which varies from 0.4° to 2.7°. PDBs are usually parallel to {100} crystallographic planes, have width from 0.3 to 1 μm and average spacing of 5 μm in 2D sections. Planar deformation bands appear as contrast lamellae in orientation contrast images and in EBSD maps, and in rare cases can be observed with the optical microscope. PDBs form in specifically oriented grains due to high differential stresses, high temperatures, and high strain rates generated in seismically active environment and/or due to shearing in the vicinity of frictional melts. Discovered structures represent a result of crystal-plastic deformation of zircon grains with operating dislocations having 〈100〉{010} glide system and 〈001〉 misorientation axis, therefore, they can be classified as a new type 4 lattice distortion pattern, according to the existing classification for zircon ( Piazolo et al. 2012 ; Kovaleva et al. 2014 ). We have demonstrated that formation of planar fractures in zircon takes place not only during impacts, but also in seismically active zones. We observe at least two cases of formation of PFs with {100} orientation: (1) as a result of evolution of PDBs and (2) as micro-cleavage. This study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events, but also as a result of seismic events at depth due to unusually high differential stress, strain rate, and temperature. According to the new findings, PDBs in zircon from the deep-crust are supposed to represent newly recognized evidence of seismicity.

Description: 〈span〉The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO〈sub〉4〈/sub〉) and reversion to zircon resulted in zircon grains composed of fine neoblasts (~0.5–3 μm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.〈/span〉

Description: The emerging method of femtosecond crystallography (FX) may extend the diffraction resolution accessible from small radiation-sensitive crystals and provides a means to determine catalytically accurate structures of acutely radiation-sensitive metalloenzymes. Automated goniometer-based instrumentation developed for use at the Linac Coherent Light Source enabled efficient and flexible FX experiments to be...

Description: A clear case study of local-scale, fluid-induced dehydration of the Paleoarchean Sand River biotite–hornblende gneiss from the Central Zone of the Limpopo Complex is presented here. Field and petrographic examination of three adjacent zones—darker Sand River orthogneiss with local occurrence of orthopyroxene and clinopyroxene, a lighter intermediate gneissic zone with more orthopyroxene than the Sand River orthogneiss, and tonalitic veins containing large orthopyroxene-bearing patches—indicates the local transformation of a light grey, fine- to medium-grained, hornblende–biotite gneiss into a greenish brown, medium- to coarse-grained orthopyroxene-bearing dehydration zone. Field evidence indicates that the tonalitic veins were emplaced in discrete ductile shear zones, with development of large orthopyroxene-bearing patches in a sigmoidally transposed foliation bounded by shear planes. Orthopyroxene-forming reaction textures after biotite and amphibole together with the occurrence of microveins of K-feldspar along quartz–plagioclase grain boundaries in the three adjacent zones, and the higher modal abundance of orthopyroxene and K-feldspar with lesser biotite and amphibole from the Sand River orthogneiss to the intermediate gneissic zone to the orthopyroxene-bearing patches, indicate that the three adjacent zones represent progressive stages of the dehydration process. Such K-feldspar microveins along quartz–plagioclase grain boundaries have been proposed as evidence for the presence and passage of a low H 2 O activity fluid. Further, the occurrence of monazite inclusions in fluorapatite in orthopyroxene-bearing zones suggests dissolution and reprecipitation involving a free fluid phase. Fluid inclusion studies indicate the presence of a fluid with CO 2 , NaCl and H 2 O components, with higher salinity of the fluid (up to 29% NaCl) in the orthopyroxene-bearing patches relative to the intermediate gneissic zone. The increase in Cl content in amphibole, biotite and fluorapatite from the Sand River orthogneiss to the orthopyroxene-bearing patches supports the presence of a Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of plagioclase at the contact with the K-feldspar rims on quartz reflects an increase in potassium activity in the fluid. The whole-rock major, trace and rare-earth element enrichment or depletion patterns of the orthopyroxene-bearing zones relative to the precursor support the dehydration process. The diffuse contact relationship of a granite pegmatite occurring in the vicinity of the dehydration zones, together with fluid inclusion and whole-rock major, trace and rare element characteristics of samples collected along a traverse from the granite pegmatite to the Sand River orthogneiss, suggests a scenario in which the dehydrating fluids derived from an external source utilized lithological contrasts, such as the gneiss–pegmatite boundaries, as fluid conduits. Dehydration of the gneissic wall-rock occurred where permeability was sufficient for fluid penetration. The occurrence of orthopyroxene-bearing tonalitic veins along deformation-transposed foliation planes further attests to a structural control to the channeling of the dehydrating fluids.

Description: 〈span〉〈div〉Abstract〈/div〉The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO〈sub〉4〈/sub〉) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.〈/span〉

Description: Lattice defects in zircon can cause trace elements redistribution and disturbance of isotopic systems. This study investigates in detail how seismically induced deformation microstructures in zircon correlate with trace element and isotope re-equilibration. Felsic mylonites with pseudotachylyte veins from the Ivrea-Verbano Zone in Northern Italy were studied with special focus on deformed zircon. We have revealed the following post-growth deformation microstructures: planar deformation bands (PDBs), planar fractures (PFs), non-planar fractures (both healed and open), and finite strain patterns. PDBs are planar portions of crystal lattice that are strictly parallel to {100} crystallographic planes, and are rotated to up to 3° with respect to the host grain. They are from 0.5 to 1 μm wide and have average spacing of 5 μm. PDBs originate in seismically active environment at elevated differential stress, strain rate and temperature. Several grains, in which PDBs are observed, were analyzed with ion microprobe. Ion maps indicate redistribution of radiogenic Pb isotopes associated with PDB formation. Isotopic redistribution preferably occurs in PDBs with larger crystallographic misorientation. Profiling demonstrated clear spatial correlation of PDBs with variations of REE abundances (both gain and loss), and possible correlations with increased and decreased Hf, Ti, and P abundances. Trace elements can be depleted or enriched (compared to the abundance in surrounding matrix) in deformed domains, depending on the spacing of PDBs and the proximity of the analyzed volume to grain boundary or to detrital core. 207 Pb/ 206 Pb ratio demonstrates systematic Pb-loss in the PDB-bearing lattice domains with respect to PDB-free domains; in some cases Pb-gain is observed, where the PDBs source radiogenic Pb from older detrital cores. Our study has important implications for geochronology and microchemistry of zircon from seismically deformed sections of Ivrea-Verbano Zone and from other paleo-seismic zones of the world. Zircon found in seismically deformed rocks near pseudotachylyte veins may demonstrate distorted and even reset isotopic ages, and altered trace element abundances. Enhanced trace element exchange between deformed zircon and host mylonite can influence mass-balance calculations for the bulk rock.