scholarly journals Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa

2021 ◽  
Author(s):  
Aaron J. Cavosie ◽  
Christopher L. Kirkland ◽  
Steven M. Reddy ◽  
Nicholas E. Timms ◽  
Cristina Talavera ◽  
...  
Keyword(s):  
South Africa ◽  
Secondary Ion Mass Spectrometry ◽  
Microstructural Characterization ◽  
Deformation Bands ◽  
Planar Deformation ◽  
Orientation Mapping ◽  
Age Variations ◽  
Band Contrast ◽  
Lattice Misorientation ◽  
Highly Strained

ABSTRACT Accessory mineral U-Pb geochronometers are crucial tools for constraining the timing of deformation in a wide range of geological settings. Despite the growing recognition that intragrain age variations within deformed minerals can spatially correlate to zones of microstructural damage, the causal mechanisms of Pb loss are not always evident. Here, we report the first U-Pb data for shock-deformed xenotime, from a detrital grain collected at the Vredefort impact structure in South Africa. Orientation mapping revealed multiple shock features, including pervasive planar deformation bands (PDBs) that accommodate up to 40° of lattice misorientation by <100>{010} slip, and also an ~50-µm-wide intragrain shear zone that contains {112} deformation twin lamellae in two orientations. Twenty-nine in situ secondary ion mass spectrometry (SIMS) U-Pb analyses from all microstructural domains yielded a well-defined discordia with upper-intercept age of 2953 ± 15 Ma (mean square of weighted deviates [MSWD] = 0.57, n = 29, 2σ), consistent with derivation from Kaapvaal craton bedrock. However, the 1754 ± 150 Ma lower concordia intercept age falls between the 2020 Ma Vredefort impact and ca. 1100 Ma Kibaran orogenesis and is not well explained by multiple Pb-loss episodes. The pattern and degree of Pb loss (discordance) correlate with increased [U] but do not correlate to microstructure (twin, PDB) or to crystallinity (band contrast) at the scale of SIMS analysis. Numerical modeling of the Pb-loss history using a concordia-discordia-comparison (CDC) test indicated that the lower concordia age is instead best explained by an alteration episode at ca. 1750 Ma, rather than a multiple Pb-loss history. In this example, the U-Pb system in deformed xenotime does not record a clear signature of impact age resetting; rather, the implied high dislocation density recorded by planar deformation bands and the presence of deformation twins facilitated subsequent Pb loss during a younger event that affected the Witwatersrand basin. Microstructural characterization of xenotime targeted for geochronology provides a new tool for recognizing evidence of deformation and can provide insight into complex age data from highly strained grains, and, as is the case in this study, elucidate previously unrecognized alteration events.

Towards disentangling climatic and tectonic changes of southernmost Africa using strontium isotope stratigraphy and clumped isotope thermometry

2021 ◽  
Author(s):  
B. Linol ◽  
I. Montañez ◽  
A. Lombardo ◽  
D. Kuta ◽  
D. Upadhyay ◽  
...  
Keyword(s):  
South Africa ◽  
Sea Level Rise ◽  
Sea Level ◽  
Secondary Ion Mass Spectrometry ◽  
Isotope Composition ◽  
Tectonic Activity ◽  
Elemental Ratios ◽  
Strontium Isotope Stratigraphy ◽  
Clumped Isotope ◽  
Global Sea Level

Abstract Upper Cretaceous-Cenozoic marine sequences preserved between 30 and 350 masl across southern South Africa record a complex history of climatic and tectonic changes. In this study, we measure the strontium (Sr) isotope composition of fossil shark teeth, echinoderms, corals and oyster shells to chronostratigraphically constrain the ages of these sequences. The method requires careful petrographic screening and micro-drilling of the samples to avoid possible alteration by diagenesis. To assess palaeoenvironmental effects in the shells we measured the Mg/Ca elemental ratios and O isotope values using electron microprobe analysis (EMPA) and secondary ion mass spectrometry (SIMS). In addition, we employed carbonate clumped isotope thermometry to test palaeotemperatures reconstruction. The analysis of recent to modern stromatolites by clumped isotopes yields an average temperature of 20.2°C, in agreement with present day observations. The fossil oyster shells suggest a warmer (23.0°C) seawater palaeotemperature, possibly due to major deglaciation and sea-level rise during the Neogene. We also find that transgressions occurred above 200 to 350 m elevation during: 1) the Campanian-Maastrichian (~75 Ma); 2) the mid-Oligocene (32 to 26 Ma); and 3) the Messinian-Zanclean (6 to 5 Ma). These three episodes are linked to well-known variations in global sea level and regional tectonic processes that could have affected the continental margin differently. The most recent transgression coincides with a maximum global sea-level rise of ~50 m at ca. 5.3 Ma, and a worldwide plate kinematic change around 6 Ma, which in Eurasia led to the closure of the Mediterranean Sea. In the Eastern Cape of South Africa, the new dates of analyzed oyster shells constrain a minimum uplift rate of ca. 150 m/Myr during this tectonic activity. The results have important implications for robust calibration of relative sea level in southern Africa.

scholarly journals Effect of Mineral Processes and Deformation on the Petrophysical Properties of Soft Rocks during Active Faulting

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 444
Author(s):  
Anita Torabi ◽  
Juan Jiménez-Millán ◽  
Rosario Jiménez-Espinosa ◽  
Francisco Juan García-Tortosa ◽  
Isabel Abad ◽  
...  
Keyword(s):  
Young’S Modulus ◽  
Young's Modulus ◽  
Damage Zone ◽  
Active Faults ◽  
Soft Rock ◽  
Microstructural Characterization ◽  
Rock Properties ◽  
Deformation Bands ◽  
Carbonate Cement ◽  
Granada Basin

We have studied damage zones of two active faults, Baza and Padul faults in Guadix-Baza and Granada basins, respectively, in South Spain. Mineral and microstructural characterization by X-ray diffraction and field emission electron microscopy studies have been combined with structural fieldwork and in situ measurements of rock properties (permeability and Young’s modulus) to find out the relation between deformation behavior, mineral processes, and changes in the soft rock and sediment properties produced by fluid flow during seismic cycles. Our results show that microsealing produced by precipitation of dolomite and aragonite along fractures in the damage zone of Baza Fault reduces the permeability and increases the Young’s modulus. In addition, deformation bands formed in sediments richer in detrital silicates involved cataclasis as deformation mechanism, which hamper permeability of the sediments. In the Granada Basin, the calcarenitic rocks rich in calcite and clays in the damage zone of faults associated to the Padul Fault are characterized by the presence of stylolites without any carbonate cement. On the other hand, marly lithofacies affected by faults are characterized by the presence of disaggregation bands that involve cracking and granular flow, as well as clay smear. The presence of stylolites and deformation bands in these rocks reduces permeability.

SIMS of Biomineralized Tissues: Present Trends and Potentials

1997 ◽  
Vol 11 (4) ◽  
pp. 364-379 ◽  
Author(s):  
A. Lodding
Keyword(s):  
Secondary Ion Mass Spectrometry ◽  
Microstructural Characterization ◽  
Surface Distribution ◽  
Distribution Maps ◽  
Lack Of Information ◽  
Low Concentrations ◽  
Dental Hard Tissues ◽  
Bone Research ◽  
Secondary Ion

The technique of dynamic secondary ion mass spectrometry (SIMS) has, during the 1980s, become a firmly established tool in the microanalytical and microstructural characterization of dental hard tissues. SIMS has proved to be outstandingly suited for charting the distributions of most elements, even at extremely low concentrations, in tooth and bone materials. In-depth concentration profiles as well as surface distribution maps of elements have been recorded with excellent (sub-micron) morphologic resolution. In spite of documented success, only relatively few teams, in a handful of countries, are presently engaged, to any significant extent, in conducting tooth or bone research by the application of SIMS. For dental-medical-surgical laboratories, a partial reason for non-communication is a lack of information about SIMS and its particular assets. Another reason may be connected with an essentially groundless reputation, among non-specialists, of SIMS being an exclusive and expensive technique. Among SIMS laboratories, on the other hand, the inertia in tackling biomineralization is partly due to some particular artifacts of analysis, hitherto not generally known and controlled. The present paper briefly sketches the chief principles of modem SIMS, emphasizing factors of special relevance in the characterization of biomineralized tissues. Examples of recent applications are provided. Present procedures and their limitations are discussed, especially with regard to elemental quantification and imaging. Suggestions for relatively simple modifications to existing routines are offered with the aim of enhancing the ease and availability of SIMS in odontological and surgical research.

Characteristics of Hot Compressed Magnesium Alloy AZ31 with Initial Textures Analyzed by Orientation Mapping

2005 ◽  
Vol 495-497 ◽  
pp. 639-644 ◽  
Author(s):  
Ping Yang ◽  
Li Meng ◽  
Wei Min Mao ◽  
Leng Chen
Keyword(s):  
Dynamic Recrystallization ◽  
Texture Evolution ◽  
Strain Rates ◽  
Plastic Slip ◽  
Relative Contribution ◽  
Orientation Mapping ◽  
Subgrain Rotation ◽  
Ebsd Technique ◽  
Plain Strain ◽  
Band Contrast

Orientation mapping based on EBSD technique was applied to reveal the orientations of new grains and their relationships to the surrounding matrices, to analyze Kikuchi band contrast and the influence of strain rates on local orientation evolution. This information is used to understand the dynamic recrystallization mechanism and the relative contribution of plastic slip versus grain boundary glide or grain rotation related with super-plasticity. For this purpose samples with different initial textures were deformed by (quasi-)plain strain compression at two strain rates. It is suggested that the dynamic recrystallization in this alloy proceeds in continuous way by progressive subgrain rotation. No evident non-basal slip of <a+c> was observed by referring texture evolution in the sample with initial basal texture. A high strain rate promotes more contribution of plastic slip accompanied by fast orientation changes. The fact that groups of grains with very similar orientations in basal oriented samples is discussed in terms of viscous flow.

Studies of In0.53Ga0.47As/InP Superlattice Mixing and Conversion

1988 ◽  
Vol 144 ◽  
Author(s):  
S.A. Schwarz ◽  
P. Mei ◽  
D.M. Hwang ◽  
C.L. Schwartz ◽  
T. Venkatesan ◽  
...  
Keyword(s):  
Secondary Ion Mass Spectrometry ◽  
Threshold Concentration ◽  
High Dose ◽  
Cation Site ◽  
Transmission Electron Microscopy Micrographs ◽  
Strained Layers ◽  
Transmission Electron ◽  
The Difference ◽  
Si Films ◽  
Highly Strained

ABSTRACTSealed ampoule Zn diffusions into an unstrained In0.53Ga0.47As/InP superlattice are observed to preferentially induce cation (In, Ga) interdiffusion. The extent of interdiffusion is monitored in these studies by SIMS (secondary ion mass spectrometry). In some cases, Zn entirely displaces In in the InP layers forming an In1−xGaxAs/Zn3P2 superlattice. Under still more stringent conditions, all In and Ga is replaced, resulting in a Zn3As2/Zn3P2 superlattice. These structures are capable of supporting considerable strain due to the absence of grown-in defects. TEM (transmission electron microscopy) micrographs of several samples reveal defect free highly strained layers with thicknesses exceeding the predicted critical values. A high dose Zn implant was examined in which mixing proceeded until the Zn concentration dropped below a threshold concentration of approximately 1020 cm−3 during the anneal. These results, in total, strongly support a kickout mechanism for Zn induced mixing. Si diffusion was observed, in one case, to induce comparable cation and anion interdiffusion, thereby reducing the layer bandgap disparity, as opposed to Zn which increases the difference in layer bandgaps. Almost no interdiffusion was induced by Si diffusion from MBE (molecular beam epitaxy) polycrystalline Si films, due either to the very high resultant Si concentrations or to the capping effect of the film. The data suggest that both anion and cation site defects play a role in the Si induced mixing process.

scholarly journals Interrogating the Effects of Hydrogen on the Behavior of Planar Deformation Bands in Austenitic Stainless Steel

2021 ◽  
Vol 52 (4) ◽  
pp. 1516-1525
Author(s):  
J. E. C. Sabisch ◽  
J. D. Sugar ◽  
J. Ronevich ◽  
C. San Marchi ◽  
D. L. Medlin
Keyword(s):  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Tensile Deformation ◽  
True Strain ◽  
Deformation Bands ◽  
Electron Backscattered Diffraction ◽  
Planar Deformation ◽  
Internal Hydrogen ◽  
Transmission Electron ◽  
Scanning Transmission

AbstractThe effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε-martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α′-martensite was observed at the intersection of ε-martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α′-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α′-martensite is a secondary effect of increased ε-martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.

Time and Latitude in South Africa

1972 ◽  
Vol 1 ◽  
pp. 27-38
Author(s):  
J. Hers
Keyword(s):  
South Africa ◽  
Cape Town ◽  
Astronomical Observations ◽  
Royal Observatory ◽  
The Republic ◽  
Astronomical Determination ◽  
Limited Accuracy ◽  
Better Than

In South Africa the modern outlook towards time may be said to have started in 1948. Both the two major observatories, The Royal Observatory in Cape Town and the Union Observatory (now known as the Republic Observatory) in Johannesburg had, of course, been involved in the astronomical determination of time almost from their inception, and the Johannesburg Observatory has been responsible for the official time of South Africa since 1908. However the pendulum clocks then in use could not be relied on to provide an accuracy better than about 1/10 second, which was of the same order as that of the astronomical observations. It is doubtful if much use was made of even this limited accuracy outside the two observatories, and although there may – occasionally have been a demand for more accurate time, it was certainly not voiced.

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