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

Steven B. SIMON  Lawrence GROSSMAN 《Meteoritics & planetary science》1997,32(1):61-70

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.  相似文献   

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

TETSUZO FUKUNARI  SIMON R. WALLIS  TOSHIAKI TSUNOGAE 《Island Arc》2011,20(3):426-438

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.  相似文献   

Formation of the first oxidized iron in the solar system     

Lawrence GROSSMAN  Alexei V. FEDKIN  Steven B. SIMON 《Meteoritics & planetary science》2012,47(12):2160-2169

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.  相似文献   

Syntectonic carbonate veins from the Barbados accretionary prism (ODP Leg 110): record of palaeohydrology     

PETER VROLIJK  SIMON M. F. SHEPPARD† 《Sedimentology》1991,38(4):671-690

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The Solubility of Alumina in Orthopyroxene Coexisting with Garnet in FeO-MgO--Al2O3--SiO2 and CaO--FeO--MgO--Al2O3--SiO2   总被引：1，自引：0，他引：1  

HARLEY  SIMON L. 《Journal of Petrology》1984,25(3):665-696

The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al₂O₃ in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al₂O₃—SiO₂(FMAS). These results have been extended into the CaO—FeO—MgO—Al₂O₃—SiO₂(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al₂O₃ contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters G^o_y= –5436 + 2.45T cal mol^–1,and W^M1_FeA1= –920 cal mol^–1. The volume change ofreaction, V^o, the entropy change, S^o₉₇₀ and the enthalpy changeH^o_1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term W^ga_CaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al₂O₃ contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.  相似文献   

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

DAVID SIMON 《The Geographical journal》2006,172(1):10-21

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.  相似文献   

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

SIMON MILLIGAN  TONY BINNS† 《The Geographical journal》2007,173(2):143-156

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.  相似文献   

Particle shape: a review and new methods of characterization and classification   总被引：3，自引：0，他引：3  

SIMON J. BLOTT  KENNETH PYE 《Sedimentology》2008,55(1):31-63

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.  相似文献   

Role of glaciohydraulic supercooling in the formation of stratified facies basal ice: Svínafellsjökull and Skaftafellsjökull, southeast Iceland     

SIMON J. COOK  ZOE P. ROBINSON  IAN J. FAIRCHILD  PETER G. KNIGHT  RICHARD I. WALLER  IAN BOOMER 《Boreas: An International Journal of Quaternary Research》2010,39(1):24-38

Cook, S. J., Robinson, Z. P., Fairchild, I. J., Knight, P. G., Waller, R. I. & Boomer, I. 2009: Role of glaciohydraulic supercooling in the formation of stratified facies basal ice: Svínafellsjökull and Skaftafellsjökull, southeast Iceland. Boreas, 10.1111/j.1502‐3885.2009.00112.x. ISSN 0300‐9483. There is need for a quantitative assessment of the importance of glaciohydraulic supercooling for basal ice formation and glacial sediment transfer. We assess the contribution of supercooling to stratified facies basal ice formation at Svínafellsjökull and Skaftafellsjökull, southeast Iceland, both of which experience supercooling. Five stratified basal ice subfacies have previously been identified at Svínafellsjökull, but their precise origins have not been determined. Analysis of stratified basal ice stable isotope compositions (δ¹⁸O and δD), spatial distribution and physical characteristics demonstrates that two subfacies present at both glaciers are consistent with supercooling. These ‘supercool’ subfacies account for 42% of stratified facies exposed at Svínafellsjökull, although estimates at Skaftafellsjökull are precluded by limited basal ice exposure. Owing to their high debris contents, supercooling‐related facies contribute a debris flux of 4.8 to 9.6 m³ m^?1 a^?1 at Svínafellsjökull (83% of the stratified facies debris flux). Other stratified subfacies, formed by non‐supercooling processes, account for 58% of the stratified basal ice at Svínafellsjökull, but only contribute a debris flux of 1.0 to 2.0 m³ m^?1 a^?1 (17% of the stratified facies debris flux). We conclude that supercooling has a significant role in glacial sediment transfer, although in stratified basal ice formation its role is less significant at these locations than has been reported elsewhere.  相似文献   

Multiple generations of hibonite in spinel-hibonite inclusions from Murchison     

Steven B. SIMON  Lawrence GROSSMAN  Andrew M. DAVIS 《Meteoritics & planetary science》1997,32(2):259-269

Abstract— Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered, in addition to many spinel-hibonite spherules, several hibonite-rich inclusion fragments in which the hibonite has wider ranges in TiO₂ contents (e.g., 0.07–8.6 wt% in one inclusion and 2–10 wt% in another) than previously observed within single inclusions. In these inclusions, there are sharp contacts between texturally early, Ti-poor hibonite and relatively late, Ti-rich hibonite, and the two types occur in complex intergrowth textures that are not consistent with crystallization from a melt in a single-stage cooling event. One inclusion has, in addition to relatively TiO₂-rich hibonite, some that is virtually TiO₂-free but contains ～1 wt% MgO and ～1.5 wt% SiO₂. Instead of the common substitution of Mg + Ti for 2Al, Mg coupled with Si in this case, probably reflecting crystallization from an unusual, Ti-free silicate liquid. Ion microprobe analyses of Ti-rich and Ti-poor hibonite from this inclusion yield quite similar trace-element patterns and Mg-isotopic compositions. The results are most consistent with formation of Ti-rich hibonite from Ti-poor hibonite by addition of Mg and Ti to the latter by exchange with a hot, Ti-rich liquid. That this occurred without a resolvable change in the Mg-isotopic composition requires that the Ti-rich, second generation of hibonite formed <2.5 times 10⁵ yr later than the Ti-poor hibonite. Observations of spinel-hibonite spherules and spherule fragments in our sample suite provide additional evidence that, despite the claims by Greenwood et al. (1994), spinel-hibonite spherules crystallized from individual, molten droplets, as earlier suggested by Macdougall (1981) and MacPherson et al. (1983).  相似文献