Showing papers in "Elements in 2011"

Water Management Challenges Associated with the Production of Shale Gas by Hydraulic Fracturing

Abstract: Development of unconventional, onshore natural gas resources in deep shales is rapidly expanding to meet global energy needs. Water management has emerged as a critical issue in the development of these inland gas reservoirs, where hydraulic fracturing is used to liberate the gas. Following hydraulic fracturing, large volumes of water containing very high concentrations of total dissolved solids (TDS) return to the surface. The TDS concentration in this wastewater, also known as “flowback,” can reach 5 times that of sea water. Wastewaters that contain high TDS levels are challenging and costly to treat. Economical production of shale gas resources will require creative management of flowback to ensure protection of groundwater and surface water resources. Currently, deep-well injection is the primary means of management. However, in many areas where shale gas production will be abundant, deep-well injection sites are not available. With global concerns over the quality and quantity of fresh water, novel water management strategies and treatment technologies that will enable environmentally sustainable and economically feasible natural gas extraction will be critical for the development of this vast energy source.

Ferruginous Conditions: A Dominant Feature of the Ocean through Earth's History

Abstract: The reconstruction of oceanic paleoredox conditions on Earth is essential for investigating links between biospheric oxygenation and major periods of biological innovation and extinction, and for unravelling feedback mechanisms associated with paleoenvironmental change. The occurrence of anoxic, iron-rich (ferruginous) oceanic conditions often goes unrecognized, but refined techniques are currently providing evidence to suggest that ferruginous deep-ocean conditions were likely dominant throughout much of Earth's history. The prevalence of this redox state suggests that a detailed appraisal of the influence of ferruginous conditions on the evolution of biogeochemical cycles, climate, and the biosphere is increasingly required.

When the Continental Crust Melts

Abstract: Partial melting of the continental crust has long been of interest to petrologists as a small-scale phenomenon. Mineral assemblages in the cores of old, eroded mountain chains that formed where continents collided show that the continental crust was buried deeply enough to have melted extensively. Geochemical, experimental, petrological and geodynamic modelling now show that when the continental crust melts the consequences are crustal-scale. The combination of melting and regional deformation is critical: the presence of melt on grain boundaries weakens rocks, and weak rocks deform faster, influencing the way mountain belts grow and how rifts propagate. Tectonic forces also drive the movement of melt out of the lower continental crust, resulting in an irreversible chemical differentiation of the crust.

How Does the Continental Crust Get Really Hot

Abstract: There is widespread evidence that ultrahigh temperatures of 900–1000 °C have been generated in the Earth's crust repeatedly in time and space. These temperatures were associated with thickened crust in collisional mountain belts and the production of large volumes of magma. Numerical modelling indicates that a long-lived mountain plateau with high internal concentrations of heat-producing elements and low erosion rates is the most likely setting for such extreme conditions. Preferential thickening of already-hot back-arc basins and mechanical heating by deformation in ductile shear zones might also contribute to elevated temperatures.

Mine Wastes: Past, Present, Future

Abstract: Mine wastes are unwanted, currently uneconomic, solid and liquid materials found at or near mine sites. Volumetrically they are one of the world's largest waste streams, and they often contain high concentrations of elements and compounds that can have severe effects on ecosystems and humans. Multidisciplinary research on mine wastes focuses on understanding their character, stability, impact, remediation and reuse. This research must continue if we are to understand and sustainably manage the immense quantities of historic, contemporary and future mine wastes, given the trend to exploit larger deposits of lower-grade ores.

Mine Waters: Acidic to Circmneutral

Abstract: Acid mine waters, often containing toxic concentrations of Fe, Al, Cu, Zn, Cd, Pb, Ni, Co, and Cr, can be produced from the mining of coal and metallic deposits. Values of pH for acid mine waters can range from −3.5 to 5, but even circumneutral (pH ≈ 7) mine waters can have high concentrations of As, Sb, Mo, U, and F. When mine waters are discharged into streams, lakes, and the oceans, serious degradation of water quality and injury to aquatic life can ensue, especially when tailings impoundments break suddenly. The main acid-producing process is the exposure of pyrite to air and water, which promotes oxidative dissolution, a reaction catalyzed by microbes. Current and future mining should plan for the prevention and remediation of these contaminant discharges by the application of hydrogeochemical principles and available technologies, which might include remining and recycling of waste materials.

Recycling, Reuse and Rehabilitation of Mine Wastes

Abstract: If we want to ensure a sustainable future for the human race, we must learn to prevent, minimize, reuse and recycle waste. Reuse of mine wastes allows their beneficial application, whereas recycling extracts resource ingredients or converts wastes into valuable products. Yet, today, many of the proposed reuse and recycling concepts for mine wastes are not economic. Consequently, the great majority of mine wastes are still being placed into waste storage facilities. Significant research efforts are required to develop cost-effective reuse and recycling options and to prevent the migration of contaminants from rehabilitated waste repositories in the long term.

Tourmaline Isotopes: No Element Left Behind

Abstract: Tourmaline typically forms where crustal rocks interact with migrating hydrous fluids or silicate melts, and its isotopic composition provides a reliable record of the isotopic composition of the fluids and melts from which it crystallized. Minerals of the tourmaline supergroup are exceptional in their physical robustness and chemical variability, and they allow us to extract a uniquely broad range of isotopic information from a single mineral. The chemical variability of tourmaline confronts us with the difficulty of deciphering an extremely complex mineral system, but it also presents us with a geochemical recorder of half the periodic table, a breadth of representation that is unparalleled among minerals. Plate tectonic–scale geochemical cycles, local and regional fluid–rock interactions, magmatic–hydrothermal systems, ore-forming processes, and ages of tourmaline formation have all been reconstructed using this unique isotopic broadband recorder.

Iron in microbial metabolisms

Abstract: Microbes are intimately involved in the iron cycle. First, acquisition of iron by microorganisms for biochemical requirements is a key process in the iron cycle in oxygenated, circumneutral pH environments, where the solubility of Fe(III) (oxyhydr)oxides is extremely low. Second, a number of aerobic (using O2) and anaerobic (living in the absence of O2) autotrophic bacteria gain energy for growth from the oxidation of dissolved and solid-phase Fe(II) compounds to Fe(III) (oxyhydr)oxides. Third, heterotrophic Fe(III)-reducing bacteria close the chemical loop by reducing solid-phase Fe(III) minerals back to dissolved and solid-phase Fe(II). Together these metabolic processes control the partitioning of the Earth's fourth most abundant crustal element, and they are additionally tied to the cycling of several major nutrients (e.g. carbon, oxygen, nitrogen, sulfur) and trace elements (e.g. phosphorus, nickel) in modern and ancient environments.

Tourmaline: A Geologic DVD

Abstract: Tourmaline is an eye-catching mineral, but even more importantly, it has played a significant role in the evolution of scientific thought and, more recently, has been recognized as a medium for recording geologic information, not unlike a DVD. With its plethora of chemical constituents, its wide range of stability from conditions near the Earth9s surface to the pressures and temperatures of the upper mantle, and its extremely low rates of volume diffusion, tourmaline can acquire a chemical signature from the rock in which it develops and can retain that signature through geologic time. As a source as well as a sink for boron, tourmaline is nature9s boron recorder. Tourmaline can be used as a geothermometer, provenance indicator, fluid-composition recorder, and geochronometer. Although long prized as a gemstone, tourmaline is clearly more than meets the eye.

Melted Rocks under the Microscope: Microstructures and Their Interpretation

Abstract: Recognising the former presence of melt in rocks which have undergone cooling and exhumation over millions of years following regional metamorphism commonly relies on the correct interpretation of grain-scale structures visible only under the microscope. The evolution of these structures during prograde melting and, later, retrograde cooling can be understood using concepts derived from experimental simulation and materials science.

Iron in Earth Surface Systems: A Major Player in Chemical and Biological Processes

Abstract: As an essential nutrient and energy source for the growth of microbial organisms, iron is metabolically cycled between reduced and oxidized chemical forms. The resulting flow of electrons is invariably tied to reactions with other redox-sensitive elements, including oxygen, carbon, nitrogen, and sulfur. Therefore, iron is intimately involved in the geochemistry, mineralogy, and petrology of modern aquatic systems and their associated sediments, particulates, and porewaters. In the geological past, iron played an even greater role in marine geochemistry, as evidenced by the vast deposits of Precambrian iron-rich sediments, the “banded iron formations.” These deposits are now being used as proxies for understanding the chemical composition of the ancient oceans and atmosphere.

Crustal Melting and the Flow of Mountains

Abstract: As the continental crust thickens during mountain building, it can become hot enough to start melting, leading to a profound reduction in its strength. Melt-weakened crust can flow outward or upward in response to the pressure gradients associated with mountain building, and may be transported hundreds of kilometres laterally as mid-crustal channels. In the Himalayan–Tibetan system, melting began about 30 million years ago, and widespread granite intrusion began at 20–23 Ma. Geophysical data indicate that melt is present beneath the Tibetan plateau today, and deeply eroded mountain belts preserve evidence for melt-enhanced ductile flow in the past. Flow of partially molten crust may limit the thickness and elevation of mountain belts and has influenced the deep structure of continents.

Tourmaline as a Petrologic Forensic Mineral: A Unique Recorder of Its Geologic Past

Abstract: Tourmaline is nature's perfect forensic mineral. From a single grain, the full geological past of its host rock can be reconstructed, including the pressure–temperature path it has taken through the Earth and the changing fluid compositions it has encountered. Tourmaline is able to provide this record owing to its compositional and textural sensitivity to the environment in which it grows, and is able to preserve this record because element diffusion in its structure is negligible. Furthermore, tourmaline has an exceptionally broad stability range, allowing it to record conditions in igneous, sedimentary, metamorphic, and hydrothermal settings. As our mineralogical and geochemical tools advance, we are able to interrogate tourmaline's memory with increasing precision, making tourmaline a truly powerful indicator of conditions in the Earth.

Tourmaline as a Recorder of Ore-Forming Processes

Abstract: Tourmaline occurs in diverse types of hydrothermal mineral deposits and can be used to constrain the nature and evolution of ore-forming fluids. Because of its broad range in composition and retention of chemical and isotopic signatures, tourmaline may be the only robust recorder of original mineralizing processes in some deposits. Microtextures and in situ analysis of compositional and isotopic variations in ore-related tourmaline provide valuable insights into hydrothermal systems in seafloor, sedimentary, magmatic, and metamorphic environments. Deciphering the hydrothermal record in tourmaline also holds promise for aiding exploration programs in the search for new ore deposits.

Geochemistry and Mineralogy of Solid Mine Waste: Essential Knowledge for Predicting Environmental Impact

Abstract: Large volumes of waste rock and mine tailings are stored at mine sites. Predicting the environmental impact of these wastes requires an understanding of mineral–water interaction and the characterization of the solid materials at the microscopic scale. The tendency of mine wastes to produce acid or neutral drainage containing potentially toxic metals generally reflects the ratio of primary sulfide to carbonate minerals and the trace element concentrations inherited from the ore deposit, as well as any ore processing that may have created new compounds. Whether potentially toxic elements are released to surface water, groundwater, or bodily fluids (in the case of ingestion or inhalation) depends on the host mineral and the possibility of sequestration by secondary minerals.

Mine Wastes and Human Health

Abstract: Historical mining and mineral processing have been linked definitively to health problems resulting from occupational and environmental exposures to mine wastes. Modern mining and processing methods, when properly designed and implemented, prevent or greatly reduce potential environmental health impacts. However, particularly in developing countries, there are examples of health problems linked to recent mining. In other cases, recent mining has been blamed for health problems but no clear links have been found. The types and abundances of potential toxicants in mine wastes are predictably influenced by the geologic characteristics of the deposit being mined. Hence, Earth scientists can help understand, anticipate, and mitigate potential health issues associated with mining and mineral processing.

Iron minerals in marine sediments record chemical environments

Abstract: Post-depositional chemical reactions involving iron are important in shallow-marine sediments. They play a significant role in governing the types of minerals that precipitate in such settings. The level of iron supply to marine sediments creates contrasting chemical pathways, each producing distinctive mineral assemblages. An understanding of these processes not only offers insights into past sedimentary environments on Earth but also a greater appreciation of the nature of mineral–water–bacteria interactions throughout the shallow-marine realm.

Iron Transport from the Continents to the Open Ocean: The Aging-Rejuvenation Cycle

Abstract: The biogeochemical cycle of iron plays a key role in the ocean by delivering bioavailable iron that controls plankton productivity. Transport through the iron cycle occurs mainly as nanoparticulate (oxyhydr)oxides, which are physically and chemically intermediate between aqueous and particulate forms and can be directly or indirectly bioavailable. Iron nanoparticles transform with time to more stable forms by increased crystallinity, aggregation and growth, and they also alter to other nanominerals. These age transformations can be inhibited or reversed. The resulting aging–rejuvenation cycle first produces stability during long-distance transport and then reverses the process such that bioavailable and labile iron can be produced and delivered to the open ocean.

Organizing melt flow through the crust

Abstract: Melt that crystallizes as granite at shallow crustal levels in orogenic belts originates from migmatite and residual granulite in the deep crust; this is the most important mass-transfer process affecting the continents. Initially melt collects in grain boundaries before migrating along structural fabrics and through discordant fractures initiated during synanatectic deformation. As this permeable porosity develops, melt flows down gradients in pressure generated by the imposed tectonic stress, moving from grain boundaries through outcrop-scale vein networks to ascent conduits. Gravity then drives melt ascent through the crust, either in dikes that fill ductile-to-brittle–elastic fractures or by pervasive flow in planar and linear channels in belts of steep structural fabrics. Melt may be arrested in its ascent at the ductile-to-brittle transition zone or it may be trapped en route by a developing tectonic structure.

Water: Is There a Global Crisis?

Abstract: Providing safe drinking water to the world's 6.9 billion and growing population is one of the greatest challenges of the century. Consideration of the global water cycle, however, shows that the available renewable freshwater resources exceed the current human demand by roughly a factor of 10. Scarcity results from the uneven spatial and temporal distribution of water. Over-withdrawal of surface water and groundwater has led to depletion of water resources and environmental damage in some regions. In many developing countries, inadequate sanitation is a major cause of disease. These problems can be solved through the improved management of water infrastructure and water resources, advances in technology, and a valuation of water that reflects its importance to society. The role of Earth scientists in addressing the global water crisis is crucial. Indeed, resource monitoring, development of novel waste-water treatment technologies, and determination of the quantities of water that can be withdrawn without causing adverse effects on the environment will be essential for the efficient management of global water resources in the future.

Tourmaline the Indicator Mineral: From Atomic Arrangement to Viking Navigation

Abstract: Tourmaline sensu lato has been known for at least two thousand years, and its unique combination of physical properties has ensured its importance to human society, from technical devices (such as a possible Viking navigational aid and early piezoelectric gauges in the 20th century) to attractive and popular gemstones. The chemical diversity and accommodating nature of its structure combine to make tourmaline a petrogenetic indicator for the wide range of rocks in which it occurs. Recent advances in understanding the structure, site assignments, and substitution mechanisms have led to a new nomenclature for the tourmaline supergroup minerals. Eighteen species have been described to encapsulate the chemical variety found in this intriguing structure.

Waste Streams of Mined Oil Sands: Characteristics and Remediation

Abstract: The bitumen found in the oil sands of northern Alberta, Canada, represents a significant oil resource. This bitumen is extracted either from mined ore or by using in situ methods. The water-based extraction of mined ore creates large volumes of mineral suspensions that are stored in tailings ponds. Remediation of fine tailings has presented challenges. Several new treatment technologies promise to accelerate the remediation process and at the same time recover more water for use in the extraction process. As a world-class oil reserve, and the only commercially developed oil sand deposit, the Alberta oil sands represent an important future oil source, the magnitude of which will depend to some extent on our ability to limit environmental impacts.

Is the Crucible Reproducible? Reconciling Melting Experiments with Thermodynamic Calculations

Abstract: Experimental studies and thermodynamic modelling have advanced our understanding of partial melting in the crust and have provided a frame-work for the interpretation of migmatites, residual granulites and granites. Each approach has advantages and pitfalls, and each is more appropriate than the other for investigating particular aspects of the melting process. A comparison of these two approaches may be useful because, together, they potentially give more information. A comparison of a small number of experiments with model calculations using equivalent bulk compositions shows important consistencies between the results, especially regarding the overall topologies of key melting equilibria. Despite this, several significant differences between the two approaches remain, though the sources of these differences are difficult to determine.

Organic Chemistry of Carbonaceous Meteorites

Abstract: The early Solar System contained a wide range of abiotic organic compounds. As the Solar System evolved, these organic molecules were incorporated into planetesimals and eventually planetary bodies, such as the parent bodies of meteorites. One particular class of meteorites, the carbonaceous meteorites, contains a large variety of extraterrestrial organic compounds. These compounds represent a record of the chemical reactions and conditions in the early Solar System. Different formation mechanisms and sources (interstellar, nebular or parent body) contributed to the inventory of meteoritic organic molecules. Their subsequent delivery to the early Earth may have contributed the first prebiotic building blocks of life.

Groundwater: A Resource in Decline

Abstract: Around the world, groundwater sources are in decline due to overpumping and pollution. History informs us that as water supplies are lost so are civilizations. Such was the case with the Garamantian civilization, which thrived in the western Libya desert from 500 BCE to 400 CE, then disappeared when the groundwater ran out. Present-day mining of groundwater from large aquifers in the United States, North Africa, and China illustrates this problem. In less than a century, pressures from food production and population growth are leading to declines in supplies that appeared to many as inexhaustible. In many countries, there can be no replacement for declining water resources. Food scarcity and health epidemics, leading to societal decline, are likely outcomes as people chase dwindling water supplies.

Hydrogeochemical Processes and Controls on Water Quality and Water Management

Abstract: The chemical constituents in water determine its potability, usability for agriculture and recreation, and interactions with biological systems. Anthropogenic processes have significantly influenced the geochemistry of water in many regions. Physical, chemical, and biological processes control the chemistry and chemical evolution of water in natural and contaminated systems. Advances in our ability to quantify these processes will improve our ability to manage our water resources, help us identify potential sources of contamination, and illuminate potential solutions to water-quality problems. Particularly impressive are the applications of chemical and isotopic tracers, which can track water movement and quantify water fluxes on the surface and in the subsurface. To better address societal needs, future advances will require a holistic approach to interpreting geochemical data.

Chronometry of Meteorites and the Formation of the Earth and Moon

Abstract: The planets of the Solar System grew by collisions, starting with the aggregation of tiny dust particles within the solar nebula and culminating in giant collisions between large planetary bodies. These giant impacts occasionally caused the formation of satellites such as the Earth's Moon. Our understanding of planet formation is based on information from various sources, including meteorites - leftovers from the earliest stages of planet formation - and samples from the Earth and Moon. By combining results from isotopic dating of these materials with dynamic modelling of the solar nebula and planet formation, researchers can reconstruct the accretion and early evolution of planetary bodies during the first [~]100 million years of Solar System history.

The Asteroid–Comet Continuum: In Search of Lost Primitivity

Abstract: Recent results from the Stardust comet sample-return mission have confirmed the idea that there is a continuum between primitive small bodies in the outer main asteroid belt and comets. Indeed, the mineralogy as well as the chemical and oxygen isotope compositions of the dust from comet Wild 2 are very similar to those of carbonaceous chondrites, a class of meteorites allegedly derived from primitive, dark asteroids. Comets no longer represent extremely primitive samples of the early Solar System that are radically different from dark asteroids. We enter a new era in which comets and their siblings, the dark asteroids, are seen as a collection of individual objects whose geology can be studied. The most primitive of these objects, i.e. the ones that escaped thermal metamorphism or hydrothermal alteration, can help us decipher physicochemical processes in the interstellar medium and in the protoplanetary disk from which our Solar System formed.

Tourmaline: The Kaleidoscopic Gemstone

Abstract: With their multitude of colors, gem tourmalines are among the most popular colored gemstones. Spectacular color-zoned tourmalines are valued as gems and crystal specimens, and some complexly zoned crystals contain nearly the entire spectrum of color variation found in the mineral world. The top-quality “neon” blue-to-green, copper-bearing tourmaline, the Paraiba-type , is one of the highest-priced colored gemstones, with values comparable to those of some diamonds. The wide variety and intensity of colors are related primarily to color-producing ions in the structure and to exposure to natural radiation. Gem tourmalines that form in magmatic, pegmatitic environments are most commonly elbaite and fluor-liddicoatite species, and the rarer gem tourmalines that develop in metamorphic rocks are generally dravite–uvite species.