Mechanisms of fluid flow and fluid–rock interaction in fossil metamorphic hydrothermal systems inferred from vein–wallrock patterns, geometry and microstructure

Comparison of mass transfer patterns, geometry and microstructures developed within and around veins allows the interpretation of processes of fluid flow during deformation, metamorphism and mineralization. A classification of vein types based on the degree of interaction with wallrock (using petrological, geochemical or isotopic indicators) can be used to identify a range of processes, from closed system behaviour in which the vein mass is derived from local wallrock, through to open system behaviour in which the vein mass is derived externally. Microstructural characteristics, such as wallrock selvages, multiple growth events recorded by vein seams and vein crystal morphology, also help to constrain mass transfer patterns during vein formation. We present a range of processes for vein formation, including: (i) the formation of closed system fibrous veins by dissolution–precipitation creep, including varieties in which tensile failure is not required; (ii) pressure‐ or kinetically dependent closed system segregation veins in which transfer of soluble components from wallrock to vein leaves behind a residual selvage; (iii) similar vein–selvage patterning, but with mass imbalances between vein and wallrock requiring fluid advection through both interconnected fracture networks and in the surrounding permeable rock; and (iv) the proposed formation of veins by fluid ascent in mobile hydrofractures, in which isotopic or chemical disequilibrium within and around the vein suggests that the crack and fluid within it moved essentially as one. The postulate of rapid fluid and mass transfer via such mobile hydrofractures has implications for the release of volatiles from metamorphic terrains. Also, consideration of a broad range of possible vein‐forming mechanisms is highly desirable when dealing with mineral deposits found in deformed, metamorphosed rocks, because closed system veining may produce patterns that, if erroneously recognized as being open systems, could lead to false interpretations of the role of tectonic fracturing in ore genesis.     

Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability ( k ) of tectonically active continental crust decreases with depth ( z ) according to log  k  ≈ −14–3.2 log  z , where k is in m² and z in km. Other independently derived, crustal-scale k – z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k – z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log  k  ≈ −11.5–3.2 log  z , suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k – z curves. Several lines of evidence suggest geologically rapid (years to 10³ years) decay of high-permeability transients toward background values. Crustal-scale k – z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.     

Quartz recrystallization and fluid flow during contact metamorphism: a cathodoluminescence study

Cathodoluminescence (CL) images of quartz grains in the Appin Quartzite from the aureole of the Ballachulish Igneous Complex (Scotland) reveal a textural complexity that we interpret in the light of published models of the evolution of the contact aureole. Five distinct generations of quartz can be discriminated in CL. The oldest of these is a dark luminescing mottled quartz (Type 1 quartz) that occurs in the centres of pre‐existing grains, in samples collected from 210 m to 0.1 m from the contact. Dark mottled quartz is interpreted to be unrecrystallized material and has a regional metamorphic CL spectral signature. The onset of contact metamorphism resulted in grain growth visible in CL as a series of fine‐scale alternating bands of bright and dark luminescing material (Type 2 quartz), which we attribute to infiltration of repeated pulses of small amounts of H₂O along grain boundaries. Close to the intrusion, a subgrain‐scale network of intragranular, bright luminescing features could have resulted from either intragranular microcrack‐controlled infiltration of H₂O at high temperatures or intergranular cracking followed by grain growth (Type 3 quartz). Broad bands of bright material on grain boundaries in samples that are inferred to have undergone partial melting are interpreted as quartz crystallized from the melt phase (Type 4 quartz). The final stage in the textural development is marked by a series of aligned fractures, detected in CL by nonluminescing material (Type 5 quartz) and corresponding closely with trails of fluid inclusions. These fractures are interpreted as the pathways for late‐stage, low‐temperature, retrogressive fluids.     

Application of boron isotopes to the understanding of fluid–rock interactions in a hydrothermally stimulated oil reservoir in the Alberta Basin, Canada

Boron isotope ratios of reservoir minerals and fluids can be a useful geothermometer and monitor of fluid–rock interactions. In Cold Lake oil sands of northern Alberta, there is a large variation in δ¹¹B of the produced waters generated during steam injection and recovery of oil and water. The higher temperature waters (～ 200 °C) have isotopically light δ¹¹B values (+ 3‰ to + 14‰) and high B contents (～150 p.p.m.). It is inferred that the range of δ¹¹B values of the hydrothermal fluids results from reaction with the reservoir rock, and is a function of the temperature of the fluid–rock interaction. The distinct B geochemistry of the produced waters can be used to show that there is no detectable mixing of the oil recovery waters with the regional formation waters or shallow groundwater aquifers containing potable water. Examination of B isotope ratios of reservoir minerals, before and after steam injection, allows the evaluation of sources of B in the reservoir. The only significant phase containing B is pumice. It shows generally positive δ¹¹B values before steam injection and negative values after steam, with δ¹¹B as low as ? 28‰. Other possibly reactive phases include clay minerals and organic matter, but their abundance is not great enough to impact on the isotopic composition of the produced waters. This information makes it possible to evaluate the boron isotope fractionation equation derived from experimental data ( Williams LB (2000) Boron isotope geochemistry during burial diagenesis. PhD Dissertation. The University of Calgary, Alberta, Canada; Williams LB, Hervig RL, Holloway JR, Hutcheon I (2001a) Boron isotope geochemistry during diagenesis: Part 1. Experimental determination of fractionation during illitization of smectite. Geochimica et Cosmochimica Acta, in press). The results show that the fractionation curve predicts the difference between δ¹¹B of the pumice and hydrothermal fluids in the Cold Lake reservoir. This not only indicates that the reservoir fluids have approached boron isotope equilibrium with the reservoir rock, but also shows that B isotopes provide a useful geothermometer for hydrothermally stimulated oil reservoirs.     

Rates of retrograde metamorphism and their implications for crustal rheology

The rate of reaction of a natural hornblende garnet granulite with water under a range of mid- to lower crustal conditions has been investigated experimentally. In runs of between 7 and 84 days small but measurable amounts of water were consumed, and sheet silicates (300°C, 300 MPa and 400°C, 400 MPa) and/or secondary actinolite (400°C, 400 MPa and 500°C, 500 MPa) were observed to have grown. When normalized to the surface area of the starting materials, hydration rates were in the range of 2–5 × 10⁻⁸ g m⁻² sec⁻¹. These reaction rates imply that a film of water that infiltrated a planar crack with a half width of 100 μm would be completely consumed within c. 100 years. These results imply that where water infiltrates the crust along faults or underlying shear zones in response to a deformation, it will remain as a free phase for only a finite period of time, which in some cases will be less than the repeat time for major earthquakes in the fault system. Thus, the rheology of fault zones and shear zones is likely to be cyclical, with the zone becoming stronger with time as water is consumed, and then weakened by infiltration of water after each rupture.     

Devolatilization‐induced pressure build‐up: Implications for reaction front movement and breccia pipe formation

Generation of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction‐induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build‐up: (i) The efficiency of the devolatilizing reaction front (pressure build‐up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic‐rich shales owing to methane generation in the contact aureole.     

Quality assessment of teaching: inputs,processes and outputs

Abstract

The nature of quality in teaching and learning is outlined, as a prelude to a discussion of the processes of quality assessment to teaching being employed by the Higher Education Funding Council for England. Despite claims that the latter defines quality as ‘fitness for purpose’ there are strong indications that ‘quality as standard’ is what is generally being applied. The implications of this procedure are discussed, both generally and for English university departments of geography.     

太平洋上的航海者——南岛语族岩画中的“船”形研究

南岛语族是一个造船技术高超、航海知识丰富的海洋民族,船既是他们驰骋太平洋的重要交通工具,也是他们岩画中经常出现的一个重要文化元素。在南岛语族分布区中,多种风格的"船"形岩画,不仅反映了这个民族丰富的海洋文化内涵;同时,也显示了岩画创造者在几个世纪之前的民族迁徙与文化扩散。     

Scotland's coast: Understanding past and present processes for sustainable management

The coastline of Scotland is extremely long and varied. It comprises most of the major types of coastal environment and includes machair, a sand dune system that is unique to the western seaboard of the British Isles. Such variety has its roots in the geology and sea level inheritance of Scotland and demonstrates that long‐term processes still exert a profound influence on coastal form and functioning. Understanding how these processes affect the coast and influence such diversity is essential to efficient coastal management, although it is also imperative that the vision, strategy and political will to deliver such management are also in place.     

Rock Art and Ritual: An Archaeological Analysis of Rock Art in Arid Central Australia

Rock art researchers throughout the world have explicitly or implicitly invoked ritual as an activity associated with the production of rock art but the articulation between the structure and composition of rock art assemblages and ritual behaviour remains poorly understood. Anthropologist, Roy Rappaport (1999) argued that all ritual, whatever the content or focus, has a universal structure. We review this proposition in the context of ritual studies and propose a method aimed at assessing the potential for identifying ritual structure in rock art assemblages. We discuss an archaeological analysis undertaken on the rock art assemblages in arid Central Australia, which aimed at distinguishing such a ritual structure among engraved assemblages, likely to have a Pleistocene origin, as well as more recent painted, stencilled and drawn assemblages. This analysis, despite its limitations, provides the foundation for developing a model, which will enhance the understanding of the relationship between the production of rock art and ritual.     

《古代的交互作用:欧亚大陆的东部与西部》述评

最近 ,英国剑桥大学麦克唐纳研究所出版《古代的交互作用 :欧亚大陆的东部与西部》文集 ,它集中展示了欧亚大陆的考古及东西文化交流的最新成果。 1998年以来 ,剑桥大学举办了 3次大型国际会议 ,内容涉及欧亚大陆史前学、人类学、语言和文化。本书为 2 0 0 0年“史前后期欧亚草原的开发”会议文集。由著名考古学家伦福儒教授作序 ,收入不同国家学者的文章 2 1篇 ,全面介绍了欧亚草原新石器晚期至铁器时代初的考古发现和的研究成果 ;另有部分文章涉及该区域内的植物考古学与人种学的Y染色体DNA分析     

Beyond the identification of formation processes: Behavioral inference based on traces left by cultural formation processes

Formation processes are generally considered as a negative factor for behavioral inference in archaeology. The concept of formation processes, however, can potentially be far more useful for behavioral inference in archaeology than simply identifying spurious variability in the archaeological record. Physical traces left by cultural formation processes convey a certain kind of behavioral information which may not be otherwise available. Variables related to site occupation, such as the occupation span, the number of occupation episodes, and the location of activities, can be inferred from characteristics of refuse deposits, or refuse structure.     

Spatial interaction and security: a review and case study of the Syrian refugee crisis

ABSTRACT

Sir Alan Wilson's ideas have been highly influential in the modelling of phenomena including migration, transport and economics. Latterly, research has explored the application of similar ideas to new problems at larger scales. Many of these studies relate to global challenges with significant policy implications. Here, we present an example in the form of original empirical work concerning forced migration associated with the current Syrian refugee crisis. We employ a spatial interaction framework to examine the flows of migrants fleeing Syria and the characteristics which influence their choice of destination country. In line with the broader literature, we find that shorter distances, economic prosperity, and cultural similarity (e.g. shared language) attract forced migrants, as does the probability of being granted asylum; a finding with potential implications for policy. Contrary to expectation, we find little influence for levels of security in potential host nations (e.g. absence of terrorism).     

Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians,Slovakia

Hydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low‐grade Variscan basement host. Early siderite precipitated from homogeneous NaCl‐KCl‐CaCl₂‐H₂O brines with minor CO₂, while immiscible gas–brine mixtures are indicative of the superimposed barite, quartz–tourmaline and quartz–sulphide stages. The high‐salinity aqueous fluid (18–35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300°C. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo‐Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140–300°C) and oxygen isotope ratios of the parental fluids (4–10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock‐buffered hydrothermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas–water mixtures, variable salinities and strongly fluctuating P–T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6–14 km (1.7–4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub‐stages of the north‐Gemeric veins, and up to 16 km (1.6–4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz–tourmaline stage of the south‐Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U‐Pb‐Th, ⁴⁰Ar/³⁹Ar and K/Ar geochronology applied to hydrothermal muscovite–phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.     

The demographic transition then and now: processes, perspectives, and analyses

Fifty years have passed since the post-World War II development of demography as an academic field. During this time, one of the central focuses of research has been the study of demographic and fertility transitions. The authors review a selection of research developments and analytic issues that have appeared in the literature. After presenting, in roughly chronological order, the general development of this research work, they raise questions concerning theory and methodology. In doing so, they argue that some research directions have been overemphasized to the neglect of others.     

Regeneration processes in Northern Ireland: The public sector and partnership structures

The peripheral location of Northern Ireland as an Objective 1 region in the EU and consequences in terms of competitiveness and economic disadvantage are discussed. The need for high levels of public expenditure to at least partly offset these effects is argued; this is particularly apparent in the mechanisms which have been adopted to promote urban regeneration. The dominance of central government in the planning and development process is highlighted together with recent policy initiatives that place increasing emphasis upon partnerships as a delivery mechanism. The paper examines various forms of partnership arrangements highlighting how structures and actors differ depending upon local circumstances and objectives.