Ophiolites are suites of temporally and spatially associated ultramafic, mafic, and felsic rocks that are interpreted to be remnants of ancient oceanic crust and upper mantle. Ophiolites show significant variations in their internal structure, geochemical fingerprints, and emplacement mechanisms. These differences are controlled by (1) the proximity, when formed at the magmatic stage, to a plume or trench; (2) the rate, geometry, and nature of ocean-ridge spreading; (3) mantle composition, temperature, and fertility; and (4) the availability of fluids. The oceanic crust preserved in ophiolites may form in any tectonic setting during the evolution of ocean basins, from the rift-drift and seafloor spreading stages to subduction initiation and terminal closure. An ophiolite is emplaced either from down-going oceanic lithosphere via subduction-accretion or from the upper plate in a subduction zone through trench-continent collision. Subduction zone tectonics is thus the most important factor in the igneous evolution of ophiolites and their emplacement into continental margins.
Much of our understanding of ocean ridges has come from the collection and analysis of glasses recovered from ridge axes. However, applying the resulting methodologies to ophiolite complexes is not straightforward because ophiolites typically experience intense alteration during their passage from ridge to subduction zone to mountain belt. Instead, immobile element proxies for fractionation indices, alkalinity, mantle temperature, mantle flow and subduction addition may be used to classify ophiolite lavas and fingerprint the precise setting of the ridge at which an ophiolite formed. The results can help us recognise and interpret past spreading centres and so make plate tectonic reconstructions.
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.
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.
The rare earth elements (REEs) are all around us, not only in nature but in our everyday lives. They are in every car, computer, smartphone, energy-efficient fluorescent lamp, and color TV, as well as in lasers, lenses, ceramics, and more. Scientific applications of these elements range from tracing the provenance of magmas and sediments to studying body structures with magnetic resonance imaging. The realization that we need rare earths for so many applications, but that their supply is effectively restricted to several mining districts in China, has brought these elements to the headlines and created a critical-metals agenda. Here we introduce the REE family: their properties, minerals, practical uses, and deposits. Potential sources of these elements are diverse and abundant if we can overcome the technical challenges of rare earth mining and extraction in an environmentally and socially responsible way.
Although the rare earth elements have been thought by many to be immobile in hydrothermal fluids, we have known since the first attempts to separate them in the early nineteenth century that they are soluble in aqueous solutions. Driven by a need to isolate individual REEs for industrial applications, and more recently to explore for them, we have started to develop an understanding of their solubility and speciation in hydrothermal fluids. This knowledge is allowing us to understand the processes that promote their transport in the Earth's crust, their concentration, and their fractionation.
Ophiogites are a newly documented host of diamonds on Earth. Abundant diamonds have indeed been separated from peridotites and chromitites of ophiolites in China, Myanmar, and Russia. In addition, diamond grains have recently been discovered in chromite from the Cretaceous Luobusa ophiolite (Tibet) and the early Paleozoic Ray-lz ophiolite (polar Urals, Russia). These diamonds are accompanied by a wide range of highly reduced minerals, such as Ni-Mn-Co alloys, Fe-Si and Fe-C phases, and moissanite (SiC); these have been found as either mineral separates or inclusions in diamonds and indicate growth under superreducing conditions. The diamond-bearing chromite grains likely formed near the mantle transition zone and were then brought to shallow levels in the upper mantle to form podiform chromitites in oceanic lithosphere. Because these diamond grains occur widely in peridotites and chromitites of many ophiolites, we refer to them as ophiolite-hosted diamonds. It is possible that such diamonds may be common in the upper oceanic mantle.
Carbon dioxide capture and sequestration (CCS) in deep geological formations has recently emerged as an Important option for reducing greenhouse emissions. If CCS is Implemented on the scale needed to make noticeable reductions in atmospheric CO2, a billion metric tons or more must be sequestered annually-a 250 fold Increase over the amount sequestered today. Securing such a large volume will require a solid scientific foundation defining the coupled hydrologic-geochemical-geomechanical processes that govern the long-term fate of CO2 In the subsurface. Also needed are methods to characterize and select sequestration sites, subsurface engineering to optimize performance and cost, approaches to ensure safe operation, monitoring technology, remediation methods, regulatory overview, and an institutional approach for managing long-term liability.
Apatite is a superb mineral by which to investigate the nature of fluids that have passed through and altered a rock (metasomatic processes). Its ubiquity allows it to act as a reservoir for P. F, Cl, OH, CO2, and the rare earth elements. It is also a powerful thermochronometer and can be chemically altered by aqueous brines (NaCL-KCL-CaCl2-H2O), pure H2O, and aqueous fluids containing CO2, HCl, H2SO4, and/or F. Thus, apatite is the perfect tracker of metasomatic fluids, providing information on the timing and duration of metasomatism, the temperature of the fluids, and the composition of the fluids, all of which can feed back into the history of the host rock itself.
In this issue of Elements we explore the characteristics, potential causes, and implications of episodic magmatism in arcs. A comparison of U-Pb bedrock and detrital zircon ages in arcs with independent calculations of volumetric magma addition rates (MARs) indicates that the former nicely track the episodic temporal histories of arc magmatism but not MARs. MAR estimates indicate that 100-1000 times more magmatism is added to continental arcs during flare-ups than during lulls and result in plutonic/volcanic ratios of >30/1. Episodic arc magmatism may result from external forcing on arc systems caused by events outside the arc and/or from internal cyclic processes driven by feedback between linked tectonic and magmatic processes within the arc. Along and across arc strike, changes and asymmetries in magmatic, tectonic, and geochemical histories provide important constraints for evaluating these poorly understood driving mechanisms.
Apatite is ubiquitous in igneous, metamorphic, and sedimentary rocks and is significant to more fields of study than perhaps any other mineral. To help understand why, one needs to know apatite's structure, composition, and crystal chemistry. Apatite has a robust hexagonal atomic framework based on two distinct metal-cation sites (M1, M2), a tetrahedral-cation site (T), and an anion column along four edges of the unit cell. These cation and anion sites can, among them, incorporate more than half of the long-lived elements in the periodic table, giving rise to the "apatite supergroup," which contains over 40 mineral species. The structure and composition impart properties that can be technologically, medically, and geologically very useful.
A survey of the global carbon reservoirs suggests that the most stable, long-term storage mechanism for atmospheric CO, is the formation of carbonate minerals such as calcite, dolomite and magnesite. The feasibility Is demonstrated by the proportion of terrestrial carbon bound in these minerals: at least 40,000 times moire carbon Is present In carbonate rocks than In the atmosphere. Atmospheric carbon can be transformed Into carbonate minerals either ex situ, as part of an Industrial process, or In situ, by Injection Onto geological formations where the elements required for carbonate-mineral formation are present. Many challenges In mineral carbonation remain to be resolved. They Include overcoming the slow kinetics of mineral-fluid reactions, dealing with the large volume of source material required and reducing the energy needed to hasten the carbonation process. To address these challenges, several pilot studies have been launched, Including the CarbFix program in Iceland. The aim of CarbFix Is to inject CO, Into permeable basaltic rocks in an attempt to form carbonate minerals directly through a coupled dissolution-precipitation process.
Nickel laterite ores account for over 60% of global nickel supply. They are the product of intensive deep weathering of serpentinites under humid tropical conditions. Nickel is concentrated to over 1.0 wt% and is hosted in a variety of secondary oxides, hydrous Mg silicates and smectites. The formation, mineralogy and grade of the deposits are controlled by the interplay of lithology, tectonics, climate and geomorphology. Most deposits have a multi-phase development, evolving as their climatic and/or topographic environment change. The richest deposits (>3 wt% Ni) formed where oxide-rich regoliths were uplifted and Ni leached downwards to concentrate in neo-formed silicates in the saprolite.
There is widespread evidence that ultrahigh temperatures of 900-1000 degrees 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.
Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe2+ to Fe3+. Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H-2. The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.
Apatite may be a minor constituent in magmatic rocks but it is a powerful research tool because it is ubiquitous and it incorporates magmatic water, halogens, S. C, and trace elements including Sr, U, Th, and the rare earth elements. Recent advances in experimental and analytical methodologies allow geologists to analyze apatite textures and compositions In great detail. This information improves understanding of the behavior of volatiles and trace elements both in terrestrial igneous melts and their related fluids and in extraterrestrial bodies, such as the Moon and Mars. With more research, the petrological power of apatite can only increase with respect to understanding eruptive, pluton-building, and mineralizing magmatic systems.
Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion-reaction process in which a hydrous species diffuses inwards and "catalyzes" structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution-reprecipitation process can produce similar textures. The reacted domains,usually nave lower trace element contents and may contain pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology.
As a source of strategic commodities for high technologies, the deposits of rare earth elements (REEs) in China are a world-class phenomenon. The combination of the world's largest accumulation of REEs in the Bayan Obo deposit and the low cost of mining the extremely valuable heavy REEs from residual deposits makes China almost a monopoly producer. Research on a range of Chinese deposits shows that not only hypogene but also secondary processes create economic REE deposits. These deposits have characteristic REE distribution patterns, which range from primary light REE enrichment in carbonatites from the Himalayan Mianning-Dechang orogenic belt and in metamorphosed carbonatite and polyphase mineralization at Bayan Obo, through unusual flat REE patterns in carbonatites from the Qinling orogenic belt, to strong secondary heavy REE enrichment in residual clays from southern China.
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.
The world's biggest Phanerozoic magmatic arcs formed above subduction zones and comprise the products of continuous magma emplacement into the crust over periods of up to 500 My. However, the intensity of magmatic activity can vary significantly. Punctuated magmatic events lasting from 5 to 20 My can dwarf the volume of magmas generated through the remainder of an arc's history: these high-volume events are called "flare-ups" and can completely rebuild an arc's crust. In arcs formed on continental lithosphere, flare-ups typically correlate with regional structural events that shorten and/or thicken the crust. Geochemical and isotopic signatures show that these high magmatic addition rate events involve similar to 50% recycled upper-plate crust and mantle lithosphere; the remaining similar to 50% comes from the mantle wedge.