Population Of Grains Research Articles

Intrinsic Mesoscopic Spatial Variability in Macroscopically Uniform Soils

AbstractA soil judged to be uniform when viewed at a macroscopic scale may be a highly diversified population of grains and pores when viewed at a microscopic scale. If so, at intermediate scales, it will exhibit an intrinsic mesoscopic spatial variability (IMSV) that cannot be eliminated by thorough mixing and fastidious packing. For a given soil, data from a stepwise mechanical analysis or a stepwise drying curve can be conventionally construed as counts of grains or pores assigned to specific size classes. For any set of parcels of equal volume, it is postulated that counts for the jth size class will be normally distributed about the mean. The smaller the parcels, the greater the expected amplitude of normalized deviations. A description of IMSV uses the raw data and a table of probabilities of suggest replies to questions about how small parcels of macroscopically uniform soil must be if there is to be some predetermined probability that the data for the jth size‐class in any one parcel will deviate to a predetermined degree from the mean for that class. In effect, IMSV suggests lower limits on parcel‐sizes that could be called representative elementary volumes once a tolerable probability has been chosen for specified deviations from the mean for all parcels.

Primary fabric ellipsoids in sandstones: implications for depositional processes and strain analysis

When measuring strains in deformed clastic rocks, geologists usually assume one or more of the following: (1) objects were initially circular; (2) objects were initially non-circular but had uniform orientations; (3) object populations initially had a fabric, but that this fabric had a symmetrical relationship to bedding; or that (4) initial fabrics are recognizable even after straining. To evaluate these assumptions, we measured 43 fabric ellipsoids in non-strained, poorly sorted sandstones from four depositional settings: DSDP core 174 (fan and abyssal plain deposits off the coast of Oregon); the Cretaceous Great Valley sequence, California (fore-arc deposits): the Cretaceous Pigeon Point Formation, California (accreted and slumped turbidites); and crossbedded sandstones from intracontinental basins in California and Australia. Our results indicate the following: (1) in two-dimensional cuts, individual grains have variable but usually small axial ratios (<3/1) and grains with larger axial ratios are more often, but not always, oriented at small angles to bedding; (2) averaged ratios and orientations of populations of grains in three dimensions define non-spherical fabric ellipsoids, but with small axial ratios (average principal ratios = 1.31:1.14:1); (3) these fabric ellipsoids show a wide range of shapes; and (4) orientations of fabric ellipsoid XY planes have highly variable orientations and are not parallel to bedding or cross-bedding. These results indicate that fabric ellipsoids measured in deformed sandstones must be corrected for the presence of primary fabrics when attempting to calculate strains. However, because of the variable orientations and shapes of the primary fabric ellipsoids and lack of relationship to bedding, strains can, at best, only be bracketed by multiplying final ellipsoids measured in strained samples by reciprocal primary fabric ellipsoids having a variety of orientations and shapes. These data and microstructures in grains and matrix cements also argue against significant compaction during burial in these clast-supported, sand-rich units and instead suggest that the primary fabrics largely reflect grain packing processes during deposition, and/or slumping and packing in the case of the Pigeon Point samples.

Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California

Paterson, S.R. and Tobisch, O.T. 1993. Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California. Tectonophysics, 222: 135–149. Quantitative fabric, structural, and microstructural analyses of pre-lithification folds, foliations, and lineations formed by slumping of turbidite sequences in the Cretaceous Pigeon Point Formation, California, provide a useful comparison with strain and microstructures developed in lithified and tectonically deformed turbidites. Our results indicate the following: 1. (1) multiple generations of folds, cleavages, and lineations can develop prior to any post-lithification tectonic deformation 2. (2) individual grains in sandstones have variable axial ratios, but the ratios and orientations of large populations of grains define fabric ellipsoids with small axial ratios ( ave. = 1.25:1.13:1) 3. (3) phyllosilicate grains define moderate flattening fabrics (reflecting 20–40% shortening or volume loss), with the intensity of alignment partly controlled by the percent of quartz and feldspar grains 4. (4) the fabric ellipsoids in sand-rich layers largely reflect deposition and slumping: pre- and post-slump compactions did not occur, in sand-rich units but did align clay particles in mud-siltstone units, and 5. (5) intra-grain microstructures in quartz and feldspar (e.g., undulose extinction, subgrains) are inherited or recycled features rather than representing effects of post-lithification strains. Our data also suggest that prelithification slumping occurred by pervasive grain rotation and grain boundary sliding in saturated sands with some local movement of material along bedding horizons. A likely model for the folding and associated fabrics is that buckling and fold-hinge flattening drove fluid expulsion, which in turn caused local grain-scale realignment, transposition of bedding, and the development of an axial planar cleavage in the hinge zones. Continued fluid flow was channeled by the clay-silt layers in fold limbs and axial plane cleavege in fold hinges.

Radioimmunocytochemical distribution of neutral endopeptidase (enkephalinase E.C.3.4.24.11) at the ultrastructural level in the rat median eminence.

Neutral endopeptidase (E.C.3.4.24.11) was visualized at the ultrastructural level in the external zone of the rat median eminence by using 125I-labelled IgG of a monoclonal serum. A precise analysis of the localization of the immunolabelling, which appears in the form of individual stray silver grains, was undertaken. Among the 1,045 grains counted, 82% were localized over membrane appositions involving nerve endings only and nerve endings plus tanycytes. The difference between the real and a randomly generated population of grains was statistically significant. Our results provide morphological arguments in support of the view of a paracrine action of neuropeptides present in the median eminence especially enkephalins but possibly, substance P, angiotensin, cholecystokinin and neurotensin. These neuropeptides are known to be inactivated by neutral endopeptidase. The action of these peptides may be exerted on nerve endings (autocrine or paracrine) but an intervention on tanycytes cannot be excluded.

Tectonic interpretations of systematic variations in quartz c-axis fabrics across the Thompson Belt

The Thompson Belt is a linear NE-trending belt of Archean and early Proterozoic rocks forming the boundary between the Archean Superior and early Proterozoic Churchill provinces. The gneisses in the Belt record deformation which took place during the early Proterozoic Hudsonian orogeny. Quartz c-axis fabrics were measured from gneisses collected along a transect across the western portion of the Thompson Belt. Well developed quartz c-axis fabrics were found in virtually all gneisses, even in coarse-grained, quartz-poor and weakly lineated rocks. Several distinctive c-axis fabric types were found to occur throughout the area and can be related to specific deformation events. Different c-axis fabric types can be linked to the activation of separate glide systems within quartz at different stages of the orogeny. Polar asymmetry within fabrics that should be symmetric, if they had formed from a population of grains with random orientations, are evidence that the present c-axis fabric has overprinted an earlier preferred crystallographic orientation, c-axis fabric variations in the Thompson Belt show similarities to variations observed in the Saxony granulite region. Results of this study indicate that quartz c-axis fabric analysis can contribute to the knowledge of the regional deformation in the Thompson Belt. Regional quartz c-axis variations may prove to be useful for studying the deformation history of other high-grade gneissic terrains.

Sublimation and reformation of Icy grains in the primitive solar nebula

We examine the frictional heating, sublimation, and recondensation of grains free-falling into the solar nebula from a surrounding interstellar cloud. The amount of water ice sublimated varies over a wide range—from over 90% of the grain mass at 30 AU from the nebular center to less than 10% beyond 100 AU. We further conclude that essentially all of the water sublimated eventually recondenses, because the cold nebular gas beyond 10 AU is able to hold only a small fraction as vapor. The expansion of the sublimating gas from the grain surface and abundance of cold grains implies that most of the gas returns to the solid phase near nebular ambient temperatures (∼50 K). Such a process could lead to at least two populations of grains: (1) essentially unaltered interstellar grains which did not sublimate due to drag or accretion shock heating and (2) a component comprised of water ice cocondensed, after heating, with more volatile gases at nebular ambient temperatures, yielding volatile-rich amorphous phases. Component (2) may be by far the most abundant in the portion of the outer solar nebula where Triton and Pluto formed; component (1) may be much more important for comets.

Infrared studies of H II regions: the Sharpless regions S148, 184, 198, 206 and 269

The results of a combined near-infrared and IRAS mapping study of five H II regions, for which fairly complete information at optical and radio wavelengths previously existed, are presented. The stellar content at the core of each nebula is study from near-infrared observations, while an analysis of extended near-infrared emission zones showed these to arise mainly form ionized gas (S148 and S206) or through scattering and thermal emission from dust grains surrounding a massive young star (S269-IRS2). Analysis of IRAS data suggests that two different populations of grains contribute to the observed 12 to 100 μm fluxes, and there are modelled with a small sized, PAH-type, grain component plus the standard interstellar silicate grains.

Direct dating of Plio-Quaternary pumices by 40Ar/39Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give 40Ar/39Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the 40Ar/39Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25ℵ. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.

Simulated fissioning of uranium and testing of the fission-track dating method

A computer program (FTD-SIM) faithfully simulates the fissioning of 238U with time and 235U with neutron dose. The simulation is based on first principles of physics where the fissioning of 238U with the flux of time is described by N s = λ f 238 Ut and the fissioning of 235U with the fluence of neutrons is described by N i = σ 235U φ. The Poisson law is used to set the stochastic variation of fissioning within the uranium population. The life history of a given crystal can thus be traced under an infinite variety of age and irradiation conditions. A single dating attempt or up to 500 dating attempts on a given crystal population can be simulated by specifying the age of the crystal population, the size and variation in the areas to be counted, the amount and distribution of uranium, the neutron dose to be used and its variation, and the desired ratio of 238U to 235U. A variety of probability distributions can be applied to uranium and counting-area. The Price and Walker age equation is used to estimate age. The output of FTD-SIM includes the tabulated results of each individual dating attempt (sample) on demand and/or the summary statistics and histograms for multiple dating attempts (samples) including the sampling age. An analysis of the results from FTD-SIM shows that: (1) The external detector method is intrinsically more precise than the population method. (2) For the external detector method a correlation between spontaneous track count, N s , and induced track count, N i , results when the population of grains has a stochastic uranium content and/or when the counting areas between grains are stochastic. For the population method no correlation can exist. (3) In the external detector method the sampling distribution of age is independent of the number of grains counted. In the population method the sampling distribution of age is highly dependent on the number of grains counted. (4) Grains with zero-track counts, either in N s or N i , are in integral part of fissioning theory and under certain circumstances must be included in any estimate of age. (5) In estimating standard error of age the standard error of N s and N i and φ must be accurately estimated and propagated through the age equation. Several statistical models are presently available to do so.

Ultraviolet extinction curves derived from IUE data

Data from the International Ultraviolet Explorer (IUE) have been used to derive extinction curves for several stars chosen because of unusual diffuse band strength or location within dense clouds. Both the strength and position of the 2200 A extinction maximum were assessed, as well as the level of extinction in the far-ultraviolet. A variety of combinations of these features were found, ranging from near or complete absence of a 2200 A bump (HD 29647), to low far-ultraviolet extinction (Rho Oph), to unusually steep, far-ultraviolet extinction (HD 169454). Anomalous extinction apparently shows up frequently in stars associated with nebulosity or dense clouds. The apparent independence of 2200 A and far-ultraviolet extinction supports earlier suggestions that these two features are created by distinct populations of grains.

The recrystallization of heavily-drawn Doped Tungsten Wire

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47Tm, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced «110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the «110» drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.

The relation between microfabric and strain in a progressively deformed quartzite sequence from Central Australia

In the Ormiston Nappe Complex, west of Alice Springs, central Australia, a deformed zone up to 0.7 km thick is developed in the sedimentary Heavitree Quartzite. The deformed zone is adjacent to a major thrust fault and is defined by mylonitic foliation, which is parallel to the thrust plane and by isoclinal folds. Recognition of original detrital quartz grains allows strain ellipsoids to be measured across the zone. The strain generally plots in the flattening field and many specimens show pure flattening strain. The mylonitic foliation is an axial-plane structure to the folds and is parallel to the XY-plane of the strain ellipsoid. A quartz elongation lineation may be present within the foliation and is parallel to the principal extension direction (the X- axis ) of the strain ellipsoid. Strain is accommodated principally by intracrystalline plastic deformation of the quartz grains. In detail the strain is not homogeneous and may vary even between adjacent grains of the same specimen. Quartz optic axis fabrics reflect this strain inhomogeneity. If the strain ellipsoid is an oblate spheroid, c- axes lie in small-circle girdles about the principal shortening axis (the Z- axis ). With general triaxial strain, the c- axes lie in a great-circle girdle or girdles which intersect the foliation parallel to the intermediate strain axis (the Y- axis ) and lie symmetrically about the Z- axis . A random population of grains from a specimen often shows a composite c-axis pattern between these two types. With approach to the thrust there is an increase in the amount of strain within the specimens. The increasing strain correlates with an increase in the degree of c- axis preferred orientation of the deformed detrital grains and in the amount of new strain-free grains present in the deformed quartzite. Adjacent to the thrust the quartzite is completely composed of polygonal new grains. The new grains probably formed under syntectonic conditions caused by movement along the thrust. The bulk of the new grains developed by increasing misorientation between the subareas of an initially polygonized old grain. There is no evidence of any marked host control on new-grain orientation, but new grain c- axis plots are generally similar to the corresponding old-grain plots from the same specimen.

Recrystallization textures in metals and alloys

When a rod is drawn into wire, a bar is forged, or a sheet is rolled, the grains acquire a preferred orientation or deformation texture. When any of these are subsequently heated to a temperature high enough to create a new population of grains, this population also possesses a texture, usually though not always different from the deformation texture. The process of creation of a population of new grains is called recrystallization (since there is no change of phase, this usage differs from geologists' common usage), the treatment is called annealing and the texture is termed an annealing texture. Most annealing textures are statistically diffuse, but some (especially the (100) [001] cube texture in copper and its alloys) can be so sharp as to verge on single-crystal status. There has long been conflict between two categories of interpretation: oriented nucleation hypotheses and oriented growth hypotheses. The former assert that nuclei are generated only in particular orientations and all then grow; the latter assert that nucleation is random and that only certain orientations are able to grow fast. For a long time, attempts to decide between these were based on statistical arguments, depending on close analysis of pole figures before and after annealing. There is plenty of experimental evidence pointing to mechanisms that lead to oriented nuclei (polygonization, strain-induced grain-boundarymig-ration); and to mechanisms for oriented growth, based on the differential segregation of soluble impurities to various kinds of grain boundaries. It has become quite clear that to get an understanding of

The interrelationship between cosmic dust and the microwave background

Attention is given to the radiation of microwaves by charged dust in space. Presently-used particle distributions do not restrict the presence in space of large numbers of small (r<10−6 cm) silicate grains, but it is shown that such densities (ϱ≈10−25–10−26 g cm−3) of small grains would produce a microwave background with an energy density of the same order of magnitude as the energy density of the (presumed) cosmological 3 K background. Limits set by the isotropy of the latter are: ϱ(HI clouds)≲10−26, ϱ(Galactic plane)≲10−30, ϱ(Halo)≲10−32, ϱ(Local Group)≲10−34 g cm−3. These limits imply that either there is a cutoff in particle distributions atr≃10−6 cm, or that the density of silicate grains in space has been generally overestimated, or that cosmic rays have broken up a lot of grains so that they now form a population of grains of very small size (≃10−7 cm) which are difficult to detect by conventional methods. One way to look for the latter population is by studying expected distortions of the 3 K spectrum to the short wavelength side of the portion hitherto observed (grains may have a size distribution able to give an approximate black-body curve for radiation from larger grains of 10−6 cm size), and by testing the effective energy density of the 3 K field in other galaxies.