Grains In Thin Section Research Articles

Timing and duration of discrete tectono-metamorphic events of the polymetamorphic high-grade Central zone of the Limpopo Belt (South Africa): Insight from in situ geochronology of monazite and zircon

The Central Zone of the Limpopo Belt in southern Africa is a complex polymetamorphic terrane that has experienced three metamorphic events, at 3.22 Ga (M1), 2.71–2.56 Ga (M2) and 2.02 Ga (M3). On the basis of a previous detailed petrological and geothermobarometrical study of a newly discovered supracrustal enclave within the Bulai Pluton (ca. 2.61 Ga granitoid gneiss in the Central Zone), we conducted a geochronological in situ study of texturally and chemically different monazite and zircon grains, which occur respectively as inclusions in garnet, in decompressional cordierite coronas around garnet and as matrix grains in polymetamorphic granulite-facies metapelites. Secondary-ion mass spectrometry (SIMS) in situ analyses of the monazite and zircon grains in thin sections, complemented by analyses of separated zircon grains, allowed the texturally-controlled direct determination of U-Pb and 207Pb/206Pb ages at a scale of < 10 μm. A concordant age of ca. 2.71 Ga derived from zircon included in garnet is interpreted to reflect the age of initial garnet growth during the prograde M2 metamorphism. Zircons in the cordierite coronas around garnet revealed a slightly younger concordant age of ca. 2.56 Ga. Zircon growth in the coronas is related to the retrograde breakdown of garnet (Zr-bearing) through to the decompression reaction garnet + sillimanite + quartz → cordierite + zircon and hence the ca. 2.56 Ga zircon age is interpreted to reflect the timing of post-peak decompressional uplift. The zircon ages constrain the duration of M2 metamorphism to approximately 145 Myr. A concordant 2.71 Ga age of large monazite inclusions in garnet cores corroborates the 2.71 Ga zircon age of initial garnet growth during the prograde M2 metamorphism. Core domains of the largest monazite in cordierite corona revealed ages between 2.5 and 2.0 Ga that are interpreted to reflect (multiple) Pb-loss during the M3 granulite-facies metamorphism. Other monazites in cordierite coronas and matrix showed an age of ca. 2.0 Ga, reflecting complete re-equilibration or new growth during M3 overprinting.Owing to the preservation of the original micro-textures related to the growth of zircon and monazite we were able to link the age data with the P-T record, and refine the timing and duration of the M2 granulite-facies metamorphism (approximately 2.71–2.56 Ga) in the Limpopo Belt that was previously dated at ca. 2.62 Ga by zircon separation techniques. The ca. 145 Ma duration of granulite-facies metamorphism is consistent with a tectonic model of long-lasting asthenosphere upwelling and subsequent continent–continent collision in the late Neoarchean. Decompression triggered by post-collisional extension occurred at 2.56 Ga. Paleoproterozoic M3 granulite-facies metamorphism at ca. 2.0 Ga caused the widespread re-setting of the U-Pb and Pb-Pb data that also affected most zircon and monazite grains. This study reveals that precise dating of the timing and duration of distinct tectono-metamorphic events by texturally-controlled in situ geochronology allows a much better insight into the complex geodynamic evolution of high-grade polymetamorphic terranes.

ArcStereoNet: A New ArcGIS® Toolbox for Projection and Analysis of Meso- and Micro-Structural Data

ArcStereoNet is a new ArcGIS® based toolbox for stereographic projections that we implement here using Python 2.7 programming language. The reason to develop another stereographic projection package arises from the recent use of Python as an exclusive programming language within the ArcGIS® environment. This permits a more flexible approach for the development of tools with very intuitive GUIs, and also allows the user to take full advantage of all potential GIS mapping processes. The core of this new projections toolbox is based on the capability to easily apply and compare most of the commonly used statistical methods for cluster and girdle analysis of structural data. In addition to the well-known Fisher, K-means, and Bingham data elaborations, a completely new algorithm for cluster analysis and mean vector extraction (Mean Extractor from Azimuthal Data), was developed, thereby allowing a more reliable interpretation of any possible structural data distribution. Furthermore, as in any other GIS platform, users can always precisely correlate each single projected data point with the corresponding geographical/locality position, thereby merging or subdividing groups of structural stations with a simple selection procedure. ArcStereoNet also creates rose diagrams, which may be applied not only to fault/joint planes orientation data, but also for the analysis of 2D microstructural fabric parameters. These include geometrical datasets derived from the minimum bounding approach as applied to vectorized grains in thin sections. Finally, several customization settings ensure high-quality graphic outputs of plots, that also allow easy vector graphics post-processing.

Full crystallographic orientation (&amp;lt;i&amp;gt;c&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T&gt;-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Red sandstone as raw material of Baden culture (Late Copper Age) grinding stones (Balatonőszöd - Temetői dűlő site, Hungary), with a review of the red sandstone formations of SW Hungary

Balatonőszöd - Temetői dűlő is one of the largest excavated and longest-lived sites of the Late Copper Age Baden Culture in Hungary, where 500 lithic finds were registered. In the site finds belonging to the late Middle Copper Age Balaton-Lasinja Culture and the Late Copper Age Boleraz Culture were found too.&#x0D; This paper presents petrographic and geochemical analyses of stone utensils, mostly of grinding stones, made of red, or discoloured white sandstones.&#x0D; Almost all sandstone artefacts are upper and lower stones of grinding equipment and polishers, as well as objects whose function is not known, worked and non-worked fragments; boulders of raw material are also in the studied set.&#x0D; The detailed petrographic and geochemical methods applied here are polarized light microscopy and a distribution study of the framework grains in thin section, and ICP-OES and ICP-MS as bulk rock chemical methods. The results were compared to published petrographic and geochemical data.&#x0D; Most of the studied artefacts were made of the rocks of the Red Sandstone and Siltstone Member of the Balaton Highland Sandstone Formation, especially from the mature type sandstone in which quartz is predominant, and which is almost free of feldspar. This type is characteristic of the confines of the Southern Balaton Highland and the lower part of the formation in the Northern Balaton Highland.&#x0D; A minor part of the studied artefacts - red or purple, purplish grey sandstones - originates from the sandstones of the Jakabhegy Sandstone Formation (Western Mecsek mountains).

A New High-Paleolatitude Late Permian Permineralized Peat Flora from the Sydney Basin, Australia

Premise of research. Permineralized peats are prized for hosting three-dimensionally preserved plant remains that provide insights into fossil plant anatomy and the composition of coal-forming ecosystems. A new record of siliceous permineralized peat is documented from a Lopingian-aged (upper Permian) strata from the southern Sydney Basin. It represents the fifth Permian permineralized peat identified from eastern Australia.Methodology. The single permineralized peat block was cut into smaller blocks, and both cellulose acetate peels and standard thin sections were prepared for study using transmitted light microscopy. Quantitative analysis of the peat was carried out using point counts perpendicular to bedding. One block examined using synchrotron X-ray computed tomography (CT) revealed the three-dimensional anatomy of abundant fossil seeds.Pivotal results. The peat contains a plant assemblage dominated by glossopterid leaves, seeds, and axes; although degraded, probable pteridophyte remains represent a significant subsidiary component of the assemblage. A new leaf form (Glossopteris thirroulensis McLoughlin et Mays sp. nov.) and a new type of seed (Illawarraspermum ovatum McLoughlin et Mays gen. et sp. nov.) are described. Leaf-, wood/seed-, and fine detritus-rich organic microfacies with gradational boundaries are evident within the peat.Conclusions. Regular growth rings in the small permineralized axes, together with the occurrence of autumnal mats of glossopterid leaves, signify a strongly seasonal climate. The presence of abundant charcoal in the peat indicates that fire was a significant influence on the high-paleolatitude mire ecosystem. Differentiation of organic microfacies within the peat profile indicates subtle variation in the contribution of plant components to the peat through time. The absence of mineral grains in thin section and CT, together with the presence of authigenic sulfides, indicates accumulation of organic matter in a stagnant mire away from the influence of clastic input.

The application of quartz grain morphology measurements to studying iron-rich duricrusts

Iron-rich duricrust profiles are commonly located on old flat surfaces. They promote landscape stability by the protective action of ferricrete, providing important records of multiple soil and geomorphic processes, as well as paleoclimatic events. In the Southern Serra do Espinhaço (SSE) in Brazil, iron-rich duricrust profiles occur on flat to slightly convex hills of quartzite at elevations of mostly 1200–1400 m a.s.l. Some of these profiles may be remnants of old paleosurfaces. Here we describe an image analysis technique to measure morphological attributes of quartz grains in thin sections of iron-rich duricrusts; we present a way to investigate the genesis of these materials. Size and shape variations of quartz grains were measured for iron-rich duricrust profiles from four different locations (GS2, PF1, PF2 and PF3). Thin sections of 5 × 7.5 cm from surface horizons and iron-rich duricrusts were studied. Distinct morphological regions (iron concretion and matrix) of iron-rich duricrusts were separately analyzed to identify variations in regolith morphology. A TESCAN VEGA3 scanning electron microscope (SEM) was used to produce backscattered electron (BSE) images for regions of sufficient area to enable the use of image analysis software (ImageJ 1.50i). BSE images provide a clear contrast between quartz grains and iron-rich duricrust matrix allowing resolution of quartz grains for specific investigation. The size, circularity and roundness of many quartz grains were easily determined. This procedure enables the quantitative analysis of soil materials allowing comparisons of soils and the testing of hypotheses for soil genesis. Quartz grains within iron concretions of PF3 had different morphological properties from quartz in the surrounding ferricrete indicating that different processes had occurred during the genesis of the two materials. In contrast, PF1 which showed no evidence of parent material diversity, contained quartz grains that are indicative of a polycyclic character. We observed fragments of iron concretions with pores previously occupied by quartz grains indicating that extensive quartz dissolution had occurred during duricrust evolution.

Electron-microprobe dating of monazite: The story

This paper is a testimony of how electron microprobe dating of monazite was developed in the early days of this technique, in the 90's. Five contributions have been collected from Nagoya, Clermont-Ferrand, Orléans, Salzburg, and Amherst. The technique was designed in Nagoya by Professor Suzuki, using an old JXA-5A JEOL electron microprobe. The first results were published in 1991, with great difficulty. Professor Suzuki also created the isochron-like Pb-Th* diagram to present the data. Progressive improvement of the method finally allowed the Nagoya team to date Cenozoic monazite. The technique was re-discovered in Clermont-Ferrand using a Cameca Camebax microprobe. This team demonstrated the accuracy of the method, and proposed statistical tools to improve the precision and to handle multimodal populations. The first results were published in 1993. In Orléans, the French Geological Survey (BRGM) developed the method with a Cameca SX50 to support mapping projects. They developed a new method for calculating the age, and showed that this technique was very efficient for dating monazite in migmatites. In Salzburg, the first results were obtained in 1996, using a Jeol JX 8600. It proved to be very efficient in the Alps to distinguish Cadomian, Ordovician, Variscan and Alpine events. It allowed this team to directly date a monazite-allanite reaction they described for the first time. Finally, the Amherst team investigated in detail the technical issues of electron microprobe monazite dating. They showed that the best analytical procedures must include at least: detailed chemical mapping, several interference corrections and special care for background measurements. With the help of Cameca they designed a specific microprobe for dating, named “Ultrachron”. With this machine, high precision ages can be obtained on well characterized individual grains in thin section, giving birth to a new era in geochronology of complex metamorphic rocks.

Heat-induced alteration of glauconitic minerals in the Middle Stone Age levels of Blombos Cave, South Africa: Implications for evaluating site structure and burning events

In this paper we conduct geochemical and colourimetric measurements of glauconite grains in micromorphological thin sections from the Middle Stone Age site of Blombos Cave, South Africa, to investigate the formation, internal structure and reworking of heat-exposed cave deposits that are related to prehistoric burning events. Controlled heating experiments were first carried out on glauconite-rich loose sediments and block samples, both of which were collected from the Blombos Cave bedrock. The control samples were then subjected to Fourier transform infrared spectrometry (FTIR), microscopic Fourier transform infrared spectrometry (micro-FTIR) and petrographic-colourimetric analyses. The control experiment shows that glauconitic minerals undergo a gradual and systematic colour change when temperatures reach higher than c. 300–400 °C, primarily due to dehydration and iron oxidation. They also undergo clear structural changes when temperatures reach higher than c. 550 °C due to dehydroxylation and mineral transformation. By assessing the nature and degree of heat-induced optical and molecular alteration in glauconitic minerals, we demonstrate how glauconite grains in thin sections can be classified by the temperature to which they were exposed (20–400 °C, >400 °C, >600 °C and >800 °C). To assess the archaeological relevance of our controlled heating experiment, we applied this glauconite classification scheme to >200 grains found in three micromorphological thin sections of a Middle Stone Age (MSA) combustion feature. These grains were individually geo-referenced within the local coordinate system of Blombos Cave, through a thin-section-based GIS mapping procedure. With improved spatial control, we were able to study both the general distribution of non-altered and heat-altered glauconite grains in their original sedimentary context, as well as to calculate heat distribution models that cover the entire sampled section. This combined geo-chemical, optical and spatio-contextual approach provides insights into more elusive aspects of MSA site structure and burning events, such as heat intensity, burning frequency, temperature distribution, internal hearth structure and post-depositional reworking. The workflow we propose may easily be implemented and adapted to other archaeological contexts and to analogous sedimentary materials that show comparable heat-induced alteration patterns.

Use of multivariate analysis for synchrotron micro-XANES analysis of iron valence state in amphiboles

Microanalysis of Fe 3+ /∑Fe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe 3+ were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R 2 correlation with Fe 3+ /∑Fe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /∑Fe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe 3+ /∑Fe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /∑Fe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /∑Fe.

Mollusk maxima and marine events in the Middle Ordovician of Baltoscandia

Except for the scattered cephalopod conchs that have given the rock type its name, mollusk fossils are relatively rare in the Middle Ordovician ‘orthoceratite limestone’ of Sweden. However, an interval in the Darriwilian stands out as being unusually rich in various types of mollusks. Throughout southern Sweden, gastropods became relatively abundant close to the Volkhov–Kunda Baltoscandian Stage boundary, and their numbers increase significantly in the Kundan. This increase is most apparent in the microscopic realm. A distinct peak in abundance is seen toward the middle Kundan, across the boundary between the regionally recognized Asaphus expansus and Asaphus raniceps trilobite zones, close to the Lenodus variabilis–Yangtzeplacognathus crassus conodont Zone boundary (Dw1–Dw2 transition). Hyoliths, which are otherwise exceptionally rare, are also more common in these beds, as are putative bivalves. Moreover, macroscopic cephalopod conchs occur in unusually high numbers. Gastropods and other small mollusks become relatively rare again in the upper half of the Kundan, but cephalopods are concentrated in some beds. The mollusk-rich intervals coincide with diverse assemblages and multiple morphotypes were distinguished. Point counting of skeletal grains in thin sections showed that the relative abundance of gastropods fluctuates in a pulse-like (cyclic?) manner throughout the studied succession. The concurrent paleontologic and sedimentologic development suggests a correlation to sea level, and thus that gastropod abundance can be used as a paleobathymetric proxy. Abundance peaks are associated with inferred lowstand intervals. Closely similar abundance patterns throughout southern Sweden suggest that the increase in mollusks was a geographically widespread phenomenon, perhaps even global in extent.

Revisiting26Al-26Mg systematics of plagioclase in H4 chondrites

Zinner and Göpel (1992, 2002) found clear evidence for the former presence of 26Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the 26Al-26Mg systematics of these chondrites date “metamorphic cooling of the H4 parent body.” Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al. 2011). Zinner and Göpel (2002) used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial 26Al/27Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel (2002), indicating that ratio bias does not significantly affect 26Al-26Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial 26Al/27Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAIs. The isochrons for Forest Vale and Beaver Creek also show clear evidence that 26Al was alive when they cooled, but the initial 26Al/27Al ratios are not well constrained. Sena does not show evidence that 26Al was alive when it cooled to below the Al-Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10–50 °C/Myr at 500 °C (Scott et al. 2014), we conclude that the Al-Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the parent body, instead of undisturbed cooling at depth in the H chondrite parent body, like many have assumed.

Crystal orientation measurements using transmission and backscattering

AbstractA method has been devised and tested for measuring the c-axis orientation of crystal grains in thin sections of glacier ice. The crystal orientation and grain size of ice are of great interest to glaciologists since these parameters contain information on the prior thermal and flow history of the ice. The traditional method of determining c-axis orientation involves a transmission measurement through an ice sample, a process that is time-consuming and therefore impractical for obtaining a continuous record. A reflection- or backscatter-based method could potentially be used inside boreholes, with bubbles as reflectors to avoid such drawbacks. The concept demonstration of this paper is performed on ice slices, enabling a direct comparison of accuracy with traditional methods. Measurements of the crystal orientations (θ, ϕ) in 11 grains showed an average error of ±0.8° in ϕ, with no grain error &gt;1.4°. Measurements of θ showed an average error of ±8.2° on ten grains, with unexplained disagreement on the remaining grain. Although the technique is applied specifically to glacier ice, it should be generally applicable to any transparent birefringent polycrystalline material.

Sedimentological Signature of A Deformable Bed Preserved Beneath An Ice Stream In A Late Pleistocene Glacial Sequence, Ross Sea, Antarctica

Abstract Subglacial processes are described using a nested set of samples from AND-1B drill core collected from beneath the Ross Ice Shelf as part of the ANtarctic geologic DRILLing program (ANDRILL). An interval from the late Pleistocene was chosen for study because it is preserved in the sedimentary record, yet the depositional processes are applicable to the widely studied glacial conditions at the Last Glacial Maximum. A complete glacial–interglacial sequence, between two glacial surfaces of erosion (GSEs) at 41.9 and 47.7 meters below sea floor, was studied by centimeter-scale core logging based on X-radiographs of the archive core halves in addition to core description, bulk samples and 45 mm × 65 mm thin sections of diamictites for micromorphology analysis. Above the lower GSE, 4.6 m of subglacial till occurs including a sequence of thin muddy conglomerate with diverse pebble lithologies, massive clast-rich muddy diamictite, and stratified diamictite with clast-rich and clast-free beds. The sand-size fraction of these deposits is dominated by aggregate grains, termed till pellets following terminology used for Ross Sea deposits. We present the first detailed description and two different conceptual models of tillpellet formation beneath an ice stream: one is related to mechanical processes, and the other is related to thermodynamics. Till pellets are rounded, spherical to prolate in form, and are associated with turbate structures and aligned grains in thin sections. The core of the pellet is either a lithic grain or stiff till with additional clay plastered on the outside, forming rounded grains from originally angular ones. Galaxy structures, skelsepic fabric, and pressure shadows around grains are indicative of rotational deformation within the till. Till pellets form in situ within a highly porous deformable bed under an ice stream either by mechanical shearing or by alternating thermal conditions in the till. In the first model, a slow glacial advance under dry subglacial conditions fractures the till, increasing the porosity as shear induces dilation. Basal melting and high pore-water pressure develop as the ice stream thickens, decreasing the intergranular effective stress and inducing rotation of the angular till aggregates. An alternative model calls for basal freezing with preferential ice growth in larger pore spaces causing rotation of fine-grained till aggregates due to concentration of shear stresses in a thin zone beneath the glacier sole. Survival of till pellets is enhanced in subglacial diamictite as the thin zone of deformation moves upward toward the base of the ice stream. A thin granular bed enriched in till pellets is melted out in the grounding zone, and this deposit marks the transition from subglacial till to stratified silty claystone representing sub–ice shelf deposits.

In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples

An in situ U–Pb SIMS (IN-SIMS) method to date micro-baddeleyite crystals as small as 3 μm is presented with results from three samples that span a variety of ages and geologic settings. The method complements ID-TIMS geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation. X-ray mapping and BSE imaging are used to locate target grains in thin section, followed by SIMS analysis on a CAMECA ims 1270, using the field aperture in the transfer column to screen out ions from host phases. Internal age precisions for the method are anticipated to range from 0.1% for Precambrian rocks to 3–7% for Phanerozoic rocks. Results establish a 2689 ± 5 Ma age for mafic dikes in the Wyoming craton, USA, a 1540 ± 30 Ma age for a subaerial lava flow from the Thelon Basin of northern Canada, and a 457 ± 34 Ma age for mafic dikes in the platform sequence of southeastern Siberia. The method is ideal for relatively non-destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples.

Origin of modern quartzarenite beach sands in a temperate climate, Florida and Alabama, USA

Quartzarenite coastal sands extending from eastern Louisiana eastward to Apalachee Bay, Florida, are anomalous: their position 7° north of the Tropic of Cancer contrasts with most other known modern quartzarenites, most of which are in a tropical setting. To determine the origin of these quartzarenite beach sands, we compared the mineralogy of samples taken from Alabama and Florida beaches, rivers that supply sand to the coast, and well cuttings representative of sandstone bedrock exposed in the Alabama coastal plain. To help assess the abundance of recycled quartz, and accepting the conventional wisdom that rounded sand-size quartz grains are recycled, we quantified the roundness of quartz grains in thin sections of river, beach, and well samples. We also determined the abundance of recycled grains with authigenic quartz using cathodoluminescence. River sands on Precambrian and Paleozoic bedrock in the study area have subarkose and sublitharenite compositions. However, as far as 200 km inland from the coast, river sands have attained quartzarenite composition and all rivers are presently delivering sand with at least 97% quartz to the coast. Rivers develop quartzarenite sand composition where they traverse poorly consolidated Tertiary sandstones, all of which we sampled are composed of > 95% quartz. Published experimental work indicates that abrasional rounding of sand-size quartz by rivers is insignificant and rounding in beaches is extremely slow. Hence, the abundance of quartz grains with some degree of rounding (96% for beaches; > 75% for rivers) further attests to the abundance of recycled quartz.

ESTRUTURAÇÃO TECTÔNICA E ANÁLISE DEFORMACIONAL DO SEGMENTO CENTRAL DO CINTURÃO RIBEIRA NA SERRA DA BOCAINA, NOS ESTADOS DE SÃO PAULO E RIO DE JANEIRO

The central segment of the Ribeira belt in southeastern Brazil comprises orthogneisses and paragneisses of medium to high grade metamorphic conditions, beeing dominated by regional NE-SW trending ductile shear zones. This paper presents the results of structural and strain analysis carried out in the Serra da Bocaina range, bordering region between the states of São Paulo and Rio de Janeiro, with the aim to better constrain the tectonic regime and the nature of the deformational process associated to the orogenic processes. In that area a pervasive strain is well recorded in the microstructures associated to gneissic (Sn) and mylonitic (Sm) foliations. Samples from 29 gneiss outcrops between the Rio Preto (ZCRP) and the Arcádia-Areal (ZCAA) shear zones were analysed using the Rf/φ method on quartz and feldspar grains in thin sections. On the main kinematic plane, the samples showed typically low to medium strain ratios (2&lt;R xz &lt;3) and some increasing values reaching 5.2 along the shear zones. On the YZ plane, R yz ranges between 1.0 and 3.3. The average ellipsoid estimated from all samples shows X:Y:Z semi-axes dimensions of 1.45:1.1:0.65, which corresponds to a stretching of 45% in X (sub-horizontal NE-SW), 10% in Y (vertical) and a shortening of 35% in Z (NW-SE). The overall finite strain ellipsoid (regional bulk strain) shows an oblate shape (medium K flinn of 0.38) that corresponds to the general flattening field. The highest vorticity values (Wk&lt;0.81) are found along strike-slip shear zones and in the Paraíba do Sul domain, while coaxial deformations (0.15&lt;Wk&lt;0.75) predominate inside the Embu and the Juiz de Fora domains. The integration of shear strain values γ (1.3&lt;γ&lt;8.9) obtained between the ZCRP and ZCAA allows to estimate an overall strike-slip displacement of almost 200 Km. The general tectonic model is represented by two types of deformations: dextral strike-slip and shortening perpendicular to the shear zones, which is visible by symmetrical and asymmetrical folds, as well as mineral aggregates flattening. The partitioning between NW-SE contraction and NE-SW transcurrent structures confirms the transpressional tectonic regime in the central segment of the Ribeira Belt, which is compatible to E-W collision.

Estruturação tectônica e análise deformacional do segmento central do Cinturão Ribeira na Serra da Bocaina, nos estados de São Paulo e Rio de Janeiro

The central segment of the Ribeira belt in southeastern Brazil comprises orthogneisses and paragneisses of medium to high grade metamorphic conditions, beeing dominated by regional NE-SW trending ductile shear zones. This paper presents the results of structural and strain analysis carried out in the Serra da Bocaina range, bordering region between the states of Sao Paulo and Rio de Janeiro, with the aim to better constrain the tectonic regime and the nature of the deformational process associated to the orogenic processes. In that area a pervasive strain is well recorded in the microstructures associated to gneissic (Sn) and mylonitic (Sm) foliations. Samples from 29 gneiss outcrops between the Rio Preto (ZCRP) and the Arcadia-Areal (ZCAA) shear zones were analysed using the Rf/φ method on quartz and feldspar grains in thin sections. On the main kinematic plane, the samples showed typically low to medium strain ratios (2<R xz <3) and some increasing values reaching 5.2 along the shear zones. On the YZ plane, R yz ranges between 1.0 and 3.3. The average ellipsoid estimated from all samples shows X:Y:Z semi-axes dimensions of 1.45:1.1:0.65, which corresponds to a stretching of 45% in X (sub-horizontal NE-SW), 10% in Y (vertical) and a shortening of 35% in Z (NW-SE). The overall finite strain ellipsoid (regional bulk strain) shows an oblate shape (medium K flinn of 0.38) that corresponds to the general flattening field. The highest vorticity values (Wk<0.81) are found along strike-slip shear zones and in the Paraiba do Sul domain, while coaxial deformations (0.15<Wk<0.75) predominate inside the Embu and the Juiz de Fora domains. The integration of shear strain values γ (1.3<γ<8.9) obtained between the ZCRP and ZCAA allows to estimate an overall strike-slip displacement of almost 200 Km. The general tectonic model is represented by two types of deformations: dextral strike-slip and shortening perpendicular to the shear zones, which is visible by symmetrical and asymmetrical folds, as well as mineral aggregates flattening. The partitioning between NW-SE contraction and NE-SW transcurrent structures confirms the transpressional tectonic regime in the central segment of the Ribeira Belt, which is compatible to E-W collision.

Comparative quantitative petrographical analysis of Cenozoic aquifer sands in Flanders (N Belgium): overall trends and quality assessment

The bulk mineralogy and grain size distribution of 30 sands of the Belgian Cenozoic were studied. From each sample, the grain size distribution was defined both by classical sieving and image analysis (IA) of thin sections. The grain size distribution by means of automated IA produces an overestimation of the smaller grain sizes; this is related to the artificial breaking up of heterogeneous grains and to the random sectioning of the grains in thin section. However, the results of the quantitative mineralogical analysis of the sands obtained through point counting indicate overall depositional or diagenetical influences. Continentally influenced sands are characterized by a high quartz content, whereas marine-influenced sands reveal a more diverse mineralogy. The potentially reactive chert content generally lies under 2%. Sedimentological and diagenetical influences on the mineralogy and physical properties of the sands thus control the applicability of such materials as building sands.

New finding of micro-diamonds in eclogites from Dabie-Sulu region in central-eastern China

Micro-diamonds were only found ten years ago in eclogite associated with marble at Xindian in the Dabie Mountains. This paper reports our new finding of micro-diamonds not only in eclogites at Maobei in the Sulu region and at Xindian and Laoyoufang in the south part of the Dabie Mountains (South Dabie), but also in eclogites at Baizhangya and Huangweihe in the northern part of the Dabie Mountains (North Dabie) that has usually been considered not to experience ultrahigh pressure metamorphism. Except the micro-diamond at Huangweihe that was found from the artificial heavy sands of zircons used for isotopic dating, the micro-diamonds from other localities were identified in thin sections of the eclogites. Besides a few interstitial grains, most of the micro-diamond grains in thin sections occur as inclusion in garnet. Three crystals of microdiamond at Maobei in the Sulu region are sized in 120, 60 and 30 μm, respectively. Crystal forms look like octahedron and the composite of octahedron and hexahedron. The largest micro-diamond crystal comes from Xindian, which is measured to be 180 μm in diameter with distinct zonal structure and inclusions. The zonal structure occurs as an inclined octahedron inside rounded by an incomplete hexagonal girdle. A smaller micro-diamond inclusion occurs inside the central octahedron, and a larger graphite inclusion is within the outer zone. The Laoyoufang micro-diamond is partially retrograded to graphite. Micro-diamond from the Baizhangya eclogite in the ultramafic rock belt of North Dabie is an aggregate of 70 μm×90 μm in size. All the micro-diamonds are confirmed by the Raman spectrum analysis. The occurrence of the micro-diamonds from the eclogites in the ultramafic rock belt of North Dabie demonstrates that this region was also subjected to ultrahigh pressure metamorphism as well as the South Dabie did.

Cloud Creek structure, central Wyoming, USA: Impact origin confirmed

Abstract— The circular Cloud Creek structure in central Wyoming, USA is buried beneath ˜1200 m of Mesozoic sedimentary rocks and has a current diameter of ˜7 km. The morphology/morphometry of the structure, as defined by borehole, seismic, and gravity data, is similar to that of other buried terrestrial complex impact structures in sedimentary target rocks, e.g., Red Wing Creek in North Dakota, USA. The structure has a fault‐bordered central peak with minimum diameter of ˜1.4 km, composed predominantly of Paleozoic carbonates thickened by thrust faulting and brecciation, and is elevated some 520 m above equivalent strata beyond the outer rim of the structure. There is a ˜1.6 km wide annular trough sloping away from the central peak (maximum structural relief, 300 m) and terminated by a detached, fault‐bounded, rim anticline. The youngest rocks within the structure are Late Triassic (Norian?) clastics and these are overlain unconformably by post‐impact Middle Jurassic (Bathonian?) sandstones and shales. Thus, the formation of the Cloud Creek structure is dated chronostratigraphicly as ˜190 ± 20 Ma.Reported here for the first time are measurements of planar deformation features (PDFs) in shocked quartz grains in thin sections made from drill cuttings recovered in a borehole drilled at the southern perimeter of the central peak. Other, less definitive microstructures consistent with impact occur in samples collected from boreholes drilled into the central peak and rim anticline. The shock‐metamorphic evidence confirms an impact origin for the Cloud Creek structure.