Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica

Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are (1) consistent development of a discordant, pervasive S₃ axial-plane foliation; (2) re-orientation of S₃ axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D₃ static thermal overprint; and (4) relatively common development of retrogressive, E-W-trending, mylonitic shear zones. Peak metamorphic conditions in excess of 800°C at 900 ± 100 M Pa (9 kbar) were attained at one locality following, but probably close to the time of D₂ folding. D₃ took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 ± 50°C and pressures of 700 MPa (7kbar) were attained in an amphibolite-facies event, presumably associated with the widespread granulite to amphibolite-facies metamorphism and intense deformation involved in the formation of the Rayner Complex at about 1100 Ma. The area was subsequently subjected to near-isothermal uplift. Rb-Sr isotopic data indicate that the pervasive D₃ fabric developed at about 2400–2500 Ma, and this age can be further refined to 2456⁺⁸_-5 Ma by concordant zircon analyses from a syn-D₃ pegmatite. All zircons were affected by only minor (<7–10%) Pb loss and/or new zircon growth during the Rayner event at about 1100Ma. Thus the 450–850 μg/gU concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite at 522 ± 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions. A good correlation is found between Rb-Sr total-rock ages and rock fabric. U-Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.     

Wollastonite--Scapolite Assemblages as Indicators of Granulite Pressure-Temperature-Fluid Histories: The Rauer Group, East Antarctica

Mid-Proterozoic ( 1000 Ma) granulite facies calc-silicates fromthe Rauer Group, East Antarctica, contain grossular-wollastonite-scapolite-dinopyroxene( + quartz or calcite) assemblages which preserve symplectiteand corona textures typically involving the growth of secondarywollastonite. The textures include (1) wollastonite rims betweenquartz and calcite; (2) wollastonite-plagioclase rims and intergrowthsbetween quartz and scapolite; (3) wollastonite-scapolite-clinopyroxeneinter-growths replacing grossular; and (4) wollastonite-plagioclasesymplectites replacing grossular or earlier symplectites (3). Reactions between grossular, scapolite, wollastonite, calcite,quartz, anorthite, and vapour, have been modelled in the CaO-Al₂O₃SiO₂-H₂O-CO₂and more complex systems using the internally consistent data-setof Holland & Powell (1990). Reactions producing scapoliteand wollastonite consume vapour as temperature increases (i.e., carbonation), in agreement with the results of Moecher &Essene (1990). These calc-silicates can therefore behave asfluid sinks under high-grade conditions. Conversely, they maybe important fluid sources on cooling and contribute to theformation of post-metamorphic CO₂rich fluid inclusions in isobaricallycooled granulites. P-T-_CO2 diagrams calculated for typical phase compositions (e.g., garnet, scapolite) demonstrate that the observed texturesare a record of near-isothermal decompression at 800–850 C, consistent with P—rpath determinations based on otherrock types from the Rauer Group. For example, texture (2) resultsfrom crossing the reaction Scapolite + Quartz = Wollastonite + Plagioclase + V on decompression, at 6. 5–7 kb, 820 C, and a_CO2 of0–4–0–5. Furthermore, correlations betweenmodes of product phases (e. g., wollastonitexlinopyroxene) andreactant garnet composition preclude open-system behaviour inthe formation of these textures, consistent with post-peak vapour-absentreactions such as Grossular + Calcite + Quartz = Wollastonite + Scapolite occurring on decomposition at high temperatures (>800C). Reaction textures developed in calc-silicates from other granuliteterranes often involve the formation of grossular ( + quartz calcite) as rims on wollastonite-scapolite, or replacementof wollastonite by calcite-quartz. These textures have developedprincipally in response to cooling below 780–810 C andmay be signatures of near-isobaric cooling. Infiltration ofhydrous fluid is not a necessary condition for the productionof garnet coronas in wollastonite-scapolite granulites. ^*Present address: Department of Earth Sciences, University ofMelbourne, Parkville, Victoria 3052, Australia     

Experimental evidence for rapid biotic and abiotic reduction of Fe (III) at low temperatures in salt marsh sediments: a possible mechanism for formation of modern sedimentary siderite concretions

Fe (III) reduction is a key component of the global iron cycle, and an important control on carbon mineralization. However, little is known about the relative roles and rates of microbial (biotic) iron reduction, which utilizes organic matter, versus abiotic iron reduction, which occurs without carbon mineralization. This paper reports on the capacity for salt marsh sediments, which typically are rich in iron, to support abiotic reduction of mineral Fe (III) driven by oxidation of sulphide. Sediment was reacted with amorphous FeS under strictly anaerobic conditions at a range of temperatures in biotic and abiotic microcosm experiments. Fe (III) reduction driven by sulphide oxidation occurs abiotically at all temperatures, leading to Fe (II) and elemental sulphur production in all abiotic experiments. In biotic experiments elemental sulphur is also the oxidized sulphur product but higher bicarbonate production leads to FeCO₃ precipitation. Abiotic reduction of Fe (III) occurs at rates that are significant compared with microbial Fe (III) reduction in salt marsh sediments. The solid phases produced by coupled abiotic and biotic reactions, namely elemental sulphur and FeCO₃, are comparable to those seen in nature at Warham, Norfolk, UK. Furthermore, the rates of these processes measured in the microcosm experiments are sufficient to generate siderite concretions on the rapid time scales observed in the field. This work highlights the importance of abiotic Fe (III) reduction alongside heterotrophic reduction, which has implications for iron cycling and carbon mineralization in modern and ancient sediments.     

Formation of the first oxidized iron in the solar system

For fayalite formation times of several thousand years, and systems enriched in water by a factor of ten relative to solar composition, 1 μm radius olivine grains could reach 2 mole% fayalite and 0.1 μm grains 5 mole% by nebular condensation, well short of the values appropriate for precursors of most chondrules and the values found in the matrices of unequilibrated ordinary chondrites. Even 10 μm olivine crystals could reach 30 mole% fayalite above 1100 K in solar gas if condensation of metallic nickel‐iron were delayed sufficiently by supersaturation. Consideration of the surface tensions of several phases with equilibrium condensation temperatures above that of metallic iron shows that, even if they were supersaturated, they would still nucleate homogeneously above the equilibrium condensation temperature of metallic iron. This phenomenon would have provided nuclei for heterogeneous nucleation of metallic nickel‐iron, thus preventing the latter from supersaturating significantly and preventing olivine from becoming fayalitic. Unless a way is found to make nebular regions far more oxidizing than in existing models, it is unlikely that chondrule precursors or the matrix olivine grains of unequilibrated ordinary chondrites obtained their fayalite contents by condensation processes. Perhaps stabilization of FeO occurred after condensation of water ice and accretion of icy planetesimals, during heating of the planetesimals and/or in hot, dense, water‐rich vapor plumes generated by impacts on them. This would imply that FeO is a relatively young feature of nebular materials.     

Crisis in policy, policy in crisis: understanding environmental discourse and resource-use conflict in northern Nigeria

This article explores the emergence and persistence of crisis narratives in the policy environment of twentieth century Nigeria. It finds that environmental crisis narratives have a well-established and traceable historical lineage, initially, as continuity between colonial and post-colonial policy discourse, but more recently with donor agencies gaining increasing importance in influencing and shaping debate. Counter-narratives are identified, but these lack historical 'legitimacy', and often sit uncomfortably alongside donor-funded initiatives. In light of weak governance systems that impede the development and maintenance of robust data-sets, the article argues that flawed 'conventional wisdom' regarding environmental processes and people–environment linkages will combine to produce a policy environment that could actually constrain rather than enable the improvement of rural livelihoods.     

Separated by common ground? Bringing (post)development and (post)colonialism together

The relationship between mainstream development policy (and perhaps also development studies) and postcolonial theorists has often been characterized as a dialogue of the deaf. Rather like in the old 'debates' between adherents of modernization and neo-Marxist theories, the protagonists are often thought to be talking at or past one another, rather than with each other. This paper reassesses some firmly held views on both sides of the schism. On the one hand, many official development agencies appear to promote business as usual (often quite literally, as a recent War on Want report attests in the case of the UK's DFID using its aid budget to promote profitable opportunities for British corporations). On the other hand, some postcolonial purists rely on surprisingly modernist, totalizing discursive techniques while claiming post-structural credentials, or baulk at the prospects of practical engagement. Discrepancies between theory, discourse, policy and practice are not the preserve of one side. However, the middle ground is firmer and better trodden than most believe. Considerable progress has been made and the paper assesses examples of productive engagement and concludes with suggestions for carrying forward the challenges.     

Fluid inclusion microthermometry for P–T constraints on normal displacement along the Median Tectonic Line in Northern Besshi area,Southwest Japan

The Median Tectonic Line (MTL) is a first‐order tectonic boundary that separates the Sanbagawa and Ryoke metamorphic belts. Documented large‐scale top‐to‐the‐north normal displacements along this fault zone have the potential to contribute to the exhumation of the Sanbagawa high‐pressure metamorphic belt. Fluid inclusion analyses of vein material formed associated with secondary faults within the Sanbagawa belt affected by movement on the MTL show normal movement was initially induced under temperatures greater than around 250°C. Microstructures of quartz and K‐feldspar comprising the vein material suggest a deformation temperature of around 300°C, supporting the results of fluid inclusion analyses and suggesting deformation at depths of around 10 km. The retrograde P–T path of the Sanbagawa metamorphic rocks and the estimated isochore of the fluid inclusions do not intersect. The semi‐ductile structures of surrounding rocks and lack of evidence for hydrothermal metamorphism around the veins imply the temperature of the rocks was similar to that of the fluid. These observations suggest fluid pressure of the veins was lower than lithostatic pressure close to the MTL.     

Particle shape: a review and new methods of characterization and classification

Shape is a fundamental property of all objects, including sedimentary particles, but it remains one of the most difficult to characterize and quantify for all but the simplest of shapes. Despite a large literature on the subject, there remains widespread confusion regarding the meaning and relative value of different measures of particle shape. This paper re‐examines the basic concepts of particle shape and suggests a number of new and modified methods which are widely applicable to a range of sedimentological problems; it is shown that the most important aspects of particle form are represented by the I/L ratio (elongation ratio) and S/I ratio (flatness ratio). A combination of these two ratios can be used to classify particles in terms of 25 form classes. A method of obtaining a quantitative measure of particle roundness using simple image analysis software is described, and a new visual roundness comparator is presented. It is recommended that measurements of both roundness and circularity (a proxy measure of sphericity) are made on grain images in three orthogonal orientations and average values calculated for each particle. A further shape property, irregularity, is defined and a classification scheme proposed for use in describing and comparing irregular or branching sedimentary particles such as chert and coral.     

In situ formation of palisade bodies in calcium,aluminum-rich refractory inclusions

Abstract— It has been suggested that palisade bodies—shells of spinel found within some calcium, aluminum-rich inclusions (CAIs) and the phases the shells enclose—are intact mini-CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies formed instead in situ while their host inclusions were crystallizing. The evidence includes observations of spinel-lined cavities and glass-filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: (1) formation of vesicles in a largely molten inclusion; (2) nucleation of spinel upon and/or adherence to vapor-melt interfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to one proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (i.e., dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near-polar sections through palisade bodies and therefore do not make a genetic distinction between the two features.