Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T _s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr₂O₃ at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.     

Deformation mechanisms of deeply buried and surface-piercing Late Pre-Cambrian to Early Cambrian Ara Salt from interior Oman

We compared microstructures of Late Pre-Cambrian to Early Cambrian Ara Salt diapirs from the deep subsurface (3.5–5 km) of the South Oman Salt Basin and from surface-piercing salt domes of the Ghaba Salt Basin. Laterally, these basins are approximately 500 km apart but belong to the same tectono-sedimentary system. The excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms, spanning from sedimentation to deep burial, and via re-activated diapir rise to surface piercement. Microstructures of gamma-irradiated and etched thin sections indicate dislocation creep and fluid-assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface. Microstructures from the surface are characterised by large ‘old’ subgrain-rich crystals. These ‘old’ grains are partly replaced by ‘new’ subgrain-free and subgrain-poor crystals, which show gamma irradiation-decorated growth bands and fibrous microstructures, indicative of pressure solution creep and static recrystallisation, most likely due to surface piercement and exposure. Using subgrain size piezometry, the maximum differential stresses for the subsurface salt is 1.7 MPa and those for the surface-piercing salt is 3.4 MPa, the latter value displaying the high stress conditions in the diapir ‘stem’ as the salt rises on its way to the surface.     

Disentangling Shallow and Deep Processes Causing Surface Movement

Understanding and predicting surface movement is important both technically and for social reasons. The shallow processes contributing to subsidence include construction works, peat oxidation, clay compaction, and groundwater withdrawal; deep causes are hydrocarbon and salt production. We describe an inversion procedure we have devised to disentangle the deep and shallow causes of surface movement. It employs a Bayesian inversion scheme, using forward models and other ‘a priori’ information about shallow and deep compaction. Parameter estimation thus takes place at two different depths, thereby disentangling the deep and shallow compaction processes responsible for surface movement. The uncertainty in the surface measurements and ‘a priori’ estimates is naturally incorporated. Furthermore, spatial and temporal correlations can be taken into account through inclusion of the covariance matrix. The inversion scheme is demonstrated for two synthetic cases. The first combines a compacting gas field and a compacting shallow peat layer. We demonstrate that assumptions on the shape of the subsidence bowl are not necessary. We also show how neglecting either deep or shallow causes of subsidence can produce spurious results. The advantage of using the ‘a priori’ estimates of the compaction and the covariance matrix obtained by Monte Carlo simulations is demonstrated with a second synthetic example involving two polders and different depths of their water table. A robust solution is obtained for each polder unit, while a simpler (and faster) ‘a priori’ estimate based on the expected average clay thickness fails to reproduce the actual compaction. Monte Carlo simulations can also be applied to compaction in depleting gas reservoirs. Information on spatial correlations is often available, even when the absolute values of the ‘a priori’ compaction data are quite uncertain. Explicitly incorporating such ‘a priori’ known spatial correlations improves the result significantly.     

Geospatial contour mapping of shear wave velocity for Mumbai city

Shear wave velocity is one of the most important input parameter in the analysis of geotechnical earthquake engineering problems, particularly to estimate site-specific amplification factor and ground response study. Dynamic in situ tests such as spectral analysis of surface waves (SASW) or multichannel analysis of surface waves (MASW) are very expensive. Also due to lack of specialized personnel, these tests are generally avoided in many soil investigation programs. Worldwide, several researchers have developed correlations between the SPT ‘N’ value and shear wave velocity ‘V _s’, which are useful for determining the dynamic soil properties. In the present study, more than 400 numbers of soil borehole data were collected from various geotechnical investigation agencies, government engineering institutes and geotechnical laboratories from different parts of Mumbai city, which is financial capital of India with highest population density. In this paper, an attempt has been made to develop the correlation between the SPT ‘N’ value and shear wave velocity ‘V _s’ for various soil profile of Mumbai city and compared with other existing correlations for different cities in India. Using Geographical Information System (GIS), a geospatial contour map of shear wave velocity profile for Mumbai city is prepared with contour intervals of 25 and 50 m/s. The scarcity of database or maps of shear wave velocity profile for Mumbai city will make the present geospatial contour maps extremely useful and beneficial to the designer, practitioners for seismic hazard study involved in geotechnical earthquake engineering.     

Pathways,motivations and challenges: contemporary Tuvaluan migration to New Zealand

The global climate change literature suggests that at some point in the future it is possible that low-lying areas may be drowned due to rising sea levels, and a number of Pacific Ocean states could be particularly vulnerable. It is increasingly evident that the emerging scenario of environmental change has compounded more established push–pull factors leading to migration, and has provided a new imperative for migration decision-making. By interrogating the proposition that there is a clear distinction between the policy categories of ‘migrants’ and ‘refugees,’ we examine a hitherto un-defined group of Tuvaluan migrants whose movement is in response to environmental change and possible climate change effects. The paper examines Tuvaluan migration to New Zealand through various immigration schemes for permanent residency, with an emphasis on the International/Humanitarian stream.     

A comparative analysis of several vulnerability concepts

A comparative analysis of six vulnerability models aims to identify differences and similarities between several approaches towards understanding vulnerability. The analysis yields a set of characteristics for explaining the condition of vulnerability (multiple contexts, multiple dimensions, temporal variability, multiple scales and scale-interdependency). In addition, ‘adaptation’ and ‘adaptive capacity’ are identified as key elements of vulnerability. The results of the analysis are put into a wider context not only of vulnerability but also of resilience and risk research. It is demonstrated that ‘adaptation’ and ‘adaptive capacity’ serve as hinges not only for conceptualising vulnerability but between ‘vulnerability’ and ‘resilience’ alike. Based on the results of the comparative analysis, a model of vulnerability focussing on the household and community scale is developed, which displays the key findings of this work.     

Nature of the Moyar and Bhavani shear zones,with a note on its implication on the tectonics of the southern Indian Precambrian shield

The Moyar, Bhavani and Palghat-Cauvery shear zones have figured prominently in tectonic and metamorphic syntheses in the southern Indian Precambrian shield. Implicit in these studies is the assumption that in the Moyar shear zone in particular, transport has a large strike-slip component, with a dextral displacement of as high as 70 km. Detailed structural investigations in several key sectors and reconnaissance over a large terrane cast doubt on several aspects of the accepted tectonic model. Both the Moyar and the Bhavani shear zones are steeply-dipping thrusts, with the Moyar shear in particular characterized by a predominantly dip-slip transport. Such a movement on a subvertical plane striking EW could in no way rotate the northerly ‘trends’ to EW ones as believed by many worker. Further, the Moyar and Bhavani shear zones are neither as extensive nor as pervasive as envisaged. Veering of the ‘trends’ in southern Karnataka, northern Tamil Nadu and northern Kerala is an inherent feature of the superposed fold systems here. Isoclinal folds with axial planes of diverse attitudes, overprinted by upright folds of varying tightness, have resulted in this change in ‘trend’.     

A model for multiphase flow and transport in porous media including a phenomenological approach to account for deformation—a model concept and its validation within a code intercomparison study

Multiphase flow processes in unsaturated cohesive soils are often affected by deformation due to swelling and shrinking as a result of varying water contents. This paper presents a model concept which is denoted ‘phenomenological’ in terms of the processes responsible for soil deformation, since the effects of deformation on flow and transport are only considered by constitutive relations that allow an adaptation of the hydraulic properties. This new model is validated in a detailed intercomparison study with two state-of-the-art models that are capable of explicitly describing the processes relevant for the deformation. A ‘numerical experiment’ with a state-of-the-art reference model is used to produce ‘measurement data’ for an inverse-modelling-based estimation of the model input parameters for the phenomenological concept.     

Rock Joint Surfaces Measurement and Analysis of Aperture Distribution under Different Normal and Shear Loading using GIS

Summary The geometry of the rock joint is a governing factor for joint mechanical and hydraulic behaviour. A new method for evaluating the aperture distribution, based on measurement of joint surfaces and three dimensional characteristics of each surface, is developed. This method allows one to determine and visualize the aperture distribution under different normal stresses and shear displacements, which is difficult to observe experimentally. A new laser scanner system is designed and developed for joint surface measurements. Special attention is paid to both surfaces’ data gained by measurements and processing, such as x-y coordinate table modification, data referencing, and matching between upper and lower surfaces. The surfaces of an artificial joint in granite are measured, processed, analyzed and three dimensional approaches are carried out for surface characterization. Parameters such as “asperity’s heights”, “slope angles”, and “aspects” distribution at micro scale, local concentration of elements and their spatial localization at local scale are determined by Geographic Information System (GIS). These parameters are used for joint surfaces matching and its real behavior quantitative analysis. The upper surface is brought down to make contact with the lower surface and the distance between the two surfaces is evaluated from the joint mean experimental aperture, which is obtained from normal and shear tests. Changes of aperture distribution at different normal stresses and various shear displacements are visualized and interpreted. Increasing normal load causes negative changes in aperture frequency distribution which indicates high joint matching. However, increasing shear displacement causes a rapid increase in the aperture and positive changes in the aperture frequency distribution, which could be due to un-matching, surface anisotropy and spatial localization of contact points with proceeding shear. Author’s address: Mostafa Sharifzadeh, Department of Mining, Metallurgy and Petroleum Engineering, Amirkabir University of Technology, Hafez 424, Tehran 15875-4413, Iran     

The neighborhood council: Where ‘top-down’ engages with ‘bottom-up’

The paper examines the neighborhood council from the perspective of its contribution to town planning. Within the context of planning in Israel, two distinct models of neighborhood council and their relationship to the municipal planning administration in two different towns are considered. Their potential as a new arena of planning at the neighborhood level is evaluated, including the ability to mediate between ‘bottom-up’ and ‘top-down’ planning processes, and to offer a planning framework that combines local with professional knowledge.     

New political violence in Africa – Secular sectarianism in Sierra Leone

Mary Douglas (1993) has indicated some of the shared concerns that keep voluntary enclaves together as ‘social movements’. The present paper describes a rather different kind of enclave politics, being concerned with the ‘new violence’ of groups enclaved by social exclusion and force. The paper focuses on the emergence and subsequent development of dissidents in the civil war in Sierra Leone. The RUF was at the outset a tiny but conventional guerrilla force, modelled on one of the militias in the Liberian civil war, and seeking to overthrow a decayed neo-patrimonial ‘one-party’ regime. Gaining little popular support, its members became marooned in forested isolation on the Liberia-Sierra Leone border. Recruits were mainly school children seized by force and ‘converted’ through having to take part in atrocities against rural civilians. Unable to return to the larger society on pain of summary execution, captives have had little option but to adjust to the political fantasies of a violent and unstable leadership. They were enclaved by force. Indicating the predictive strength of Douglas' arguments about the cultural dynamics of the enclave, the subsequent atrocity-drenched story of the RUF suggests that it is not the content of belief that determines institutional culture but vice versa. To survive, the enforced enclave develops the concerns of the classic ‘sect’ - a doctrinaire ‘rationality’ (expressed in crude acts of ‘subtractive’ violence), rejection of magic, decision making by lots, preoccupation with defection, otiose leadership style. If the rest of the world is to come to terms with violent enclave organizations such as the RUF and Algerian GIA it may have to pay more careful attention to the way enclave institutions think. They may not respond ‘rationally’ to the kinds of incentives offered by mainstream groups organised around hierarchical and/or individualist systems of social accountability. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nomospace and genospace: A spatial matrix

We are concerned here with the notion of space as a primary condition for building social relations. From this stand, we have created two analytical matrixes that can help us characterize two modes: that of ‘being-there-in-space’ and that of ‘being-of-the-space’; the nomospace and the genospace. While the nomospace is characterized by ‘cold’ connotations, which are ideally defined as a result of a logical option, the genospace is mainly characterized by emotional or ‘warm’ connotations, which originate from a feeling of shared nature and common destiny. We believe that these matrixes could help us to identify the meanings of some behaviours and, therefore, make us more capable of interpreting many dynamics that occur in modern societies. The importance of geography is justified by the existing relationship between these behaviours and their proper spatial form. This revised version was published online in July 2006 with corrections to the Cover Date.     

(Mis)adventures in Rumsfeld space

Ewald’s recent genealogy of constructions of risk in Western societies argues that the 1980s saw an important paradigm shift to the “precautionary principle”. Critical scholars have taken up this idea as a lens through which to interpret the Bush administration’s ‘war on terror’. I argue that 11 September 2001 actually brought about qualitative changes to this paradigm. Bush’s pre-emptive doctrine is driven, and perhaps even more importantly, continually justified to the US population, by what might be called the “trans-precautionary principle”, a move from “decisionism” to “actionism”, and a new radicalization of the way fear is produced and managed. Donald Rumsfeld’s famous typology of different articulations of knowledge and ignorance offers an excellent analytical window onto the connections between ignorance, fear and geopolitical action in this new regime. In the latter part of the essay, his four modes of knowledge/ignorance are arrayed, for heuristic purposes, in an abstract spatial grid organized along dimensions of the specificity and possession of knowledge. This allows a ‘mapping’ of some of the Bush administration’s more controversial strategies in the ‘war on terror’, as a set of different pathways through Rumsfeld Space.     

Impact of various synthetic vortices on cyclone track prediction

Sensitivity experiments are conducted for three cases of cyclones for investigating the impact of different vortex initialization schemes on the structure and track prediction of the cyclone using India Meteorological Department’s Limited Area Model. The surface wind and pressure profiles generated using Holland and Rankine initialization schemes differ from each other. These different generated profiles are compared with the actual data and the root mean square error (RMSE) was calculated between them. In case of the Holland vortex, ‘b’ is found to be equal to 1.5 and 2.0 respectively for two cases of very severe cyclonic storms in the Arabian Sea, namely 6–10 June 1998 and 16–20 May 1999 and 2.25 for the severe cyclonic storm in the Bay of Bengal. The ‘α’ parameter in Rankine’s scheme was found to be 0.5 for two cases and 0.4 for the third system. This shows that cyclones differ even if they attain the same intensity. The values of these parameters i.e. ‘b’ and ‘α’ are used for generating the synthetic wind data for individual cyclones and the same is used in the data assimilation system. The analysis and forecast generated for the above cases using the Holland scheme show that the simulated structure has characteristics closer to the actual storm; however, the Rankine scheme shows a weaker circulation. The mean track error for three cases in the Holland scheme is 93, 149, 257 and 307 km in 12-, 24-, 36- and 48-h forecast. The mean track errors for the Rankine scheme are 152, 274, 345 and 327 km, respectively, for the same period.     

War on Terror,work in progress: security,surveillance and the configuration of the US workfare state

This paper evaluates the impact of recent shifts in the geopolitical outlook of the United States—specifically the ‘Global War on Terror’ and its domestic ramifications—on the prior project of reconfiguring the US state to an after-Fordist ‘workfare’ paradigm. In particular, the paper attempts to situate recent developments in the reconfiguration of the American welfare state within the context of debates over the ‘exceptional’ nature of US politics post-9/11. The extent to which this has had a bearing on work practices, welfare provision and workfarism is investigated, citing examples from the post-9/11 era. On the basis of such examples it has been suggested that the project of reconfiguring the US welfare state and labour relations does overlap and intersect with the emergency practices that the US state has instituted domestically as part of the War on Terror. However, the reconfiguration of the US welfare state towards a neoliberal or workfarist paradigm is, we argue, largely part of the ‘normal’ (as opposed to exceptional) fabric of US politics. The project of reconfiguring the US welfare state is ongoing and largely autonomous (although not entirely removed) from the use of exceptional practices in relation to the US security state. Investigation of this relationship, we assert, provides a potentially fruitful ‘new direction’ for both critical geopolitics and political economic geography, and grounds for interaction between the two.     

The binational reality of border-crossing cities

In a time of changing border functions city pairs on either side of national state borders have increasingly become objects of attention, because of border crossing contacts and co-operation at the local level. This paper focuses on conceptual aspects of such city pairs, owing their existence to either duplication of an already existing city or to partitioning of a once united city. In search of a proper name that expresses the relations between such partner cities without metaphorical exaggeration, the current vocabulary is reviewed. Especially the notion of the ‘binational city’ receives critical attention. A discussion of selected cases of paired cities in Europe and North-America has been included to underline the importance of local conditions and historical backgrounds with regard to the mutual relations and conditions of co-operation. In conclusion the author states that for the time being ‘border–crossing’ cities seems to be the best name for city pairs of different national backgrounds, the emergence of true ‘binational cities’ being a rather exceptional outcome in a far future. This revised version was published online in July 2006 with corrections to the Cover Date.     

It's a long way to the other geographers and geographic knowledges

Geographic knowledge is usually understood as the knowledge produced by geographers. Yet, it is also produced by people outside of the academia. But as Western science claimed for its exclusive self the status of ‘knowledge’, others have long been denied the production and possession of a true knowledge, and recognised merely the mastering of practical skills. The binary opposition between scientific and other forms of knowledge has been central to the construction of academic geography until the late 1980s, when postmodern thought cast some doubt on the universality of scientific knowledge. This led to critical analyses of academic geography, revealing its situatedness, as well as to a new interest for the geographies of the ‘Others’. Examining how geographers have dealt with other knowledge sets so far, and how they have labelled them, this paper argues that other geographies should be given more attention. This means focusing on the knowledges themselves, and considering implementing a true dialogue between these and academic geography. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analytic hierarchy models for regional groundwater monitoring well allocation in Southeast Asian countries and South Korea

Groundwater has played an important role in economic development in Southeast Asian countries, but some problems caused by nature or human actions such as contamination, over pumping, and land subsidence bring the necessity of more systematic groundwater monitoring wells. The analytical hierarchy process with pairwise comparison was used to allocate and organize the regional groundwater monitoring wells in five regions, Thailand, Cambodia, East/West Malaysia, and South Korea. Five different multi criteria decision models, which were composed of three primary criteria and eight secondary criteria, were developed based on the answers of the questionnaire from 76 groundwater experts in Thailand, 100 in Cambodia, 101 in East Malaysia, 87 in West Malaysia, and 93 in South Korea. It was revealed that the weights of model criteria for each country, which also represent relative importance on groundwater monitoring, were different according to the diverse groundwater situation. The most important factor to determine the number of monitoring well was ‘number of households using only groundwater as a water source’ for Thailand and South Korea, ‘number of contamination sources’ for Cambodia, ‘amount of groundwater use for drinking-water supply’ for East Malaysia, and ‘number of wells with contaminated water’ for West Malaysia.     

High-potassium intrusives from southeastern Papua

This paper presents petrographic, major element, and trace element data from high-potassium ‘shoshonitic’ rocks of Miocene age which intrude Eocene submarine basalts in southeastern Papua. The intrusives fall into two distinct but overlapping groups, a ‘near-saturated’ group ranging from gabbro to syenite with regular petrographic and chemical variations, which is either slightly nepheline normative or quartz normative, and a nepheline normative ‘undersaturated’ group which shows wide variations in texture, modal mineralogy, and chemistry. Biotite-bearing pyroxenites are associated with the intrusives but their genetic relationship to the intrusives is unknown. The intrusion of shoshonitic rocks at the beginning of a period of major tectonic activity in southeastern Papua shows that high-potassium magmas can be generated in areas of active tectonism and may form part of the island arc ‘magmatic’ association.     

Experimental determination of the reaction dolomite + 2 coesite = diopside + 2 CO2 to 6 GPa

 The carbonation reaction CaMg(CO₃)₂ (dolomite)+2SiO₂ (coesite)=CaMgSi₂O₆ (diopside)+2 CO₂ (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction, a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist with carbonate-free, CO₂-bearing eclogite. Received: 25 May 1994/Accepted: 13 June 1995