Metamorphic and metasomatic convergence of basic igneous rocks and lime‐magnesia sediments of the precambrian of North‐western Queensland∗

With increasing intensity of metamorphism, and particularly metasomatism, it becomes more and more difficult to separate amphibolites in North‐western Queensland into ortho‐ and para‐amphibolites, and a stage is reached at which the two are indistinguishable. Field evidence, carefully sought, has enabled the origin of some apparently identical rocks to be determined, and laboratory methods were applied to these rocks to see if grounds for the separation of amphibolites of unknown origin could be determined. None of the six methods tried — mineralogy (including feldspar twinning), chemical analysis (including TiO2 content), spectrography, rock magnetism — proved to be as successful as the field method in distinguishing the two rock groups; some methods used with apparent success elsewhere, such as feldspar twinning or chemical analyses, are found to be unsuccessful here, which suggests that though the problem is world‐wide, solutions are only of local validity.     

Depositional and soil history along the Lower Macleay River,New South Wales

Extensive terrace and flood plain deposits occur along the Lower Macleay River. A sequence of terraces from oldest to youngest was named: Madron, Corangula, Mungay, Mooneba, Belgrave and Macleay deposits (contemporary). Basal sediments in the Mooneba terrace were dated by radiocarbon analysis at 3,280 ± 55 years; basal sediments of the Mungay terrace were dated at 6,425 ± 105 years. The Madron and Corangula terraces are considered very much older than the Mungay. The flood plain consists of two early cycles of aggradation buried under 23m of estuarine sediment, which in turn is overlain by up to 6m of alluvium. The estuarine sediments were dated at 8,530 ± 200 years at elevation —4m relative to mean sea level. The base of the overlying Smithtown alluvium was dated at 3,295 ± 95 years. A general chronology is presented for the Lower Macleay valley, and a sequence of terrace soils is discussed.     

Nonstoichiometry and growth of some Fe carbides

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe–Ni–S–C, Fe–Ni–C, and in Fe–C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)₃C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe–C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe₇C₃, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe–C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1–1.5 GPa, complex intergrowths involving Fe–C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.     

Reynolds stress and sand transport over a foredune

Reynolds shear stress (RS = –u′ w′) and sand transport patterns over a vegetated foredune are explored using three‐dimensional velocity data from ultrasonic anemometers (at 0 · 2 and 1 · 2 m) and sand transport intensity from laser particle counters (at 0 · 014 m). A mid‐latitude cyclone on 3–4 May 2010 generated storm‐force winds (exceeding 20 m s^–1) that shifted from offshore to obliquely alongshore. Quadrant analysis was used to characterize the spatial variation of RS quadrant components (Q1 through Q4) and their relative contributions were parameterized using the flow exuberance relation, EX_FL = (Q1 + Q3)/(Q2 + Q4). The magnitudes of RS and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics. A ‘flow exuberance effect’ was evident such that Q2 (ejection‐like) and Q4 (sweep‐like) quadrants (that contribute positively to RS) dominated on the beach, dune toe, and lower stoss, whereas Q1 and Q3 (that contribute negatively to RS) dominated near the crest. This exuberance effect was not expressed, however, in sand transport patterns. Instead, Q1 and Q4, with above‐average streamwise velocity fluctuations (+u′), were most frequently associated with sand transport. Q4 activity corresponded with most sand transport at the beach, toe, and stoss locations (52, 60, 100%). At the crest, 25 to 86% of transport was associated with Q1 while Q4 corresponded with most of the remaining transport (13 to 59%). Thus, the relationship between sand transport and RS is not as straightforward as in traditional equations that relate flux to stress in increasing fashion. Generally, RS was poorly associated with sand transport partly because Q1 and Q4 contributions offset each other in RS calculations. Thus, large amounts of transport can occur with small RS. Turbulent kinetic energy or Reynolds normal stresses (u′², w′²) may provide stronger associations with sand transport over dunes, although challenges exist on how to normalize and compare these quantities. Copyright © 2013 John Wiley & Sons, Ltd.     

Classification of kangaroo habitat distribution using three GIS models

Abstract

In this paper, we compare three techniques for mapping wildlife habitat, termed BIOCLIM, CART and a new classification method based on nonparametric techniques. These techniques model dependent map layers of species distributions, where the areas to be mapped are large and the plot data is sparse. The techniques recognise pattern in the (independent) plot data, available to natural resource managers. In this case, the independent data set comprised 12 climate surfaces, that attempt to represent the range of temperature and precipitation important in determining the habitat of kangaroos across Australia. With this particular data set, the CART (decision tree) model was most accurate, but more time consuming to initialise. The relative performance of these models depends on the quality of the data set, and skill of the GIS analyst. Where possible, GIS analysts should implement all available methods, and compare output.     

The characteristics and source of laminated mud at Lake Barrine, Northeast Australia

In a small (1 km²) crater lake in the Australian upland tropics (17°15′S, 145°38′E, 721 m a.s.l.), laminated mud has accumulated throughout the past 5 ka. It is limited to a central region below 50–67 m of water where it is protected by a monimolimnion. The contents of the laminae (e.g., total lake products, organic detritus, diatoms, pollen grains, vivianite crystals) distinguish two contrasting types, namely detritus-rich and detritus-poor. Judged against radiocarbon dates over long sequences, these couplets are not annual. Gravimetric, mineralogical and chemical observations on the mud, and on suspended organic and inorganic matter caught through several periods, indicate some diagenetic changes across the mud–water interface. Diatom analyses do not assist in relating mud type to source or period of origin. The most likely mechanism of lamination is that continuous deposition of clay, with small and varied detrital accompaniment, is periodically enhanced by greater detrital input. This occurs in some, but not all, periods of winter isothermy when ground temperatures fall to zero and below on several consecutive nights. This leads to the stirring of sediments in water shallower than 50 m and their relocation in the deeper meromictic zone. Fine variations in, e.g., diatom content, partly independent of lamination type, probably contain a wealth of detail about the lake’s environmental history.     

The compressibility and high pressure structure of diopside from first principles simulation

The structure of diopside (CaMgSi₂O₆) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch–Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.