Showing papers in "Reviews in Mineralogy & Geochemistry in 2013"

Diamonds and the Geology of Mantle Carbon

Abstract: ### Introduction Earth’s carbon, derived from planetesimals in the 1 AU region during accretion of the Solar System, still retains similarities to carbon found in meteorites (Marty et al. 2013) even after 4.57 billion years of geological processing. The range in isotopic composition of carbon on Earth versus meteorites is nearly identical and, for both, diamond is a common, if volumetrically minor, carbon mineral (Haggerty 1999). Diamond is one of the three native carbon minerals on Earth (the other two being graphite and lonsdaleite). It can crystallize throughout the mantle below about 150 km and can occur metastably in the crust. Diamond is a rare mineral, occurring at the part-per-billion level even within the most diamondiferous volcanic host rock although some rare eclogites have been known to contain 10–15% diamond. As a trace mineral it is unevenly distributed and, except for occurrences in metamorphosed crustal rocks, it is a xenocrystic phase within the series of volcanic rocks (kimberlites, lamproites, ultramafic lamprohyres), which bring it to the surface and host it. The occurrence of diamond on Earth’s surface results from its unique resistance to alteration/dissolution and the sometimes accidental circumstances of its sampling by the volcanic host rock. Diamonds are usually the chief minerals left from their depth of formation, because intact diamondiferous mantle xenoliths are rare. Diamond has been intensively studied over the last 40 years to provide extraordinary information on our planet’s interior. For example, from the study of its inclusions, diamond is recognized as the only material sampling the “very deep” mantle to depths exceeding 800 km (Harte et al. 1999; McCammon 2001; Stachel and Harris 2009; Harte 2010) although most crystals (~95%) derive from shallower depths (150 to 250 km). Diamonds are less useful in determining carbon fluxes on Earth because they provide only a small, …

Serpentinization, Carbon, and Deep Life

Abstract: The aqueous alteration of ultramafic rocks through serpentinization liberates mantle carbon and reducing power. Serpentinization occurs in numerous settings on present day Earth, including subduction zones, mid-ocean ridges, and ophiolites and has extended far into Earth’s history, potentially contributing to the origins and early evolution of life. Serpentinization can provide the energy and raw materials to support chemosynthetic microbial communities that may penetrate deep into Earth’s subsurface. Microorganisms may also influence the composition and quantity of carbon-bearing compounds in the deep subsurface. However, conditions created by serpentinization challenge the known limits of microbial physiology in terms of extreme pH, access to electron acceptors, and availability of nutrients. Furthermore, the downward transport of surface carbon and subsequent mixing with calcium-rich fluids at high pH contributes to the precipitation and immobilization of carbonate minerals. The following chapter will explore the physiological challenges presented by the serpentinite environment, data from studies of serpentinite-hosted microbial ecosystems, and areas in need of further investigation. ### Physical and chemical consequences of serpentinization Serpentinization is an alteration process of low-silica ultramafic rocks, characteristic of the lower oceanic crust and upper mantle. These rocks are rich in the minerals olivine and pyroxene. Water-rock reactions result in the oxidation of ferrous iron from olivine and pyroxene, resulting in the precipitation of ferric iron in magnetite (Fe3O4) and other minerals, and in the release of diatomic hydrogen (H2). At low temperatures (< ~150 °C) the reaction results in extremely high pH, commonly above 10. The combination of H2 and CO2 or CO under highly reducing conditions leads to formation of methane and other hydrocarbons through Fischer-Tropsch Type (FTT) synthesis (McCollom and Seewald 2001; Charlou et al. 2002; Proskurowski et al. 2008; McCollom 2013). Serpentinization also results in volume changes in the altered materials, making serpentinites less dense than …

Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time

Abstract: Earth is unique among the terrestrial planets in our solar system in having a fluid envelope that fosters life. The secrets behind Earth’s habitable climate are well-tuned cycles of carbon (C) and other volatiles. While on time-scales of ten to thousands of years the chemistry of fluids in the atmosphere, hydrosphere, and biosphere is dictated by fluxes of carbon between near surface reservoirs, over hundreds of millions to billions of years it is maintained by chemical interactions of carbon between Earth’s interior, more specifically the mantle, and the exosphere (Berner 1999). This is because of the fact that the estimated total mass of C in the mantle is greater than that observed in the exosphere (Sleep and Zahnle 2001; Dasgupta and Hirschmann 2010) and the average residence time of carbon in the mantle is between 1 and 4 Ga (Sleep and Zahnle 2001; Dasgupta and Hirschmann 2006). But how did Earth’s mantle attain and maintain the inventory of mantle carbon over geologic time? And is the residence time of carbon in the mantle, as constrained by the present-day fluxes, a true reflection of carbon ingassing and outgassing rates throughout Earth’s history? Also, when in the planet’s history did its mantle carbon inventory become established and how did it change through geologic time? The answers to these questions are important because of carbon’s importance in a number of fields of Earth sciences, such as the thermal history of Earth [e.g., trace-volume carbonated melt may extract highly incompatible heat-producing elements from great depths (Dalou et al. 2009; Dasgupta et al. 2009b; Grassi et al. 2012)], the internal differentiation of the mantle and core [carbon influences element partitioning in both carbonate-silicate (e.g., Blundy and Dalton 2000; Dasgupta et al. 2009b; Dalou et al. 2009; Grassi et …

Deep Carbon Emissions from Volcanoes

Abstract: Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO2 concentrations (Berner 1991). Atmospheric CO2 concentrations are therefore stabilized over long timescales by this feedback mechanism (Zeebe and Caldeira 2008). This process may have played a role (Feulner et al. 2012) in stabilizing temperatures on Earth while solar radiation steadily increased due to stellar evolution (Bahcall et al. 2001). In this context the role of CO2 degassing from the Earth is clearly fundamental to the stability of the climate, and therefore to life on Earth. Notwithstanding this importance, the flux of CO2 from the Earth is poorly constrained. The uncertainty in our knowledge of this critical input into the geological carbon cycle led Berner and Lagasa (1989) to state that it is the most vexing problem facing us in understanding that cycle. Notwithstanding the uncertainties in our understanding of CO2 degassing from Earth, it is clear that these natural emissions were recently dwarfed by anthropogenic emissions, which have rapidly increased since industrialization began on a large scale in the 18th century, leading to a rapid increase in atmospheric CO2 concentrations. While atmospheric CO2 concentrations have varied between 190–280 ppm for the last 400,000 years (Zeebe and Caldeira 2008), human activity has produced a remarkable increase …

Carbonate melts and carbonatites

Abstract: Carbonatites are familiar to students of petrology as rare igneous rocks formed predominantly of carbonate, whose only modern expression is a single active volcano that erupts strongly alkaline carbonate lavas with no direct match in Earth’s geological record (see Lengai movie in the electronic version of this chapter or on the MSA RiMG website). Based on their Sr-Nd-Pb isotopic data, stable isotopic compositions, noble gases, and experimental phase equilibria, they are derived from the mantle, showing almost no sign of contamination by the crust. As liquids, carbonate melts have remarkable physical properties, which set them apart from the alkaline silicate melts with which they are often temporally associated. They show very high solubilities of many elements considered rare in silicate magmas, and they have the highest known melt capacities for dissolving water and other volatile species like halogens at crustal pressures. They are highly efficient transport agents of carbon from the mantle to the crust, remaining mobile over extraordinary ranges of temperature, and their very low viscosity should enhance connectivity along grain boundaries in the mantle where they are implicated in geochemical enrichment processes related to metasomatism. Most carbonatites have unambiguous origins in the mantle and the limit to their depth is not known, but the likelihood that they may exist in the lower mantle (Kaminsky et al. 2009, 2012; Stoppa et al. 2009) needs to be appraised since they may exert a fundamental control on the mobility and long-term storage of deep carbon in Earth. Ultimately the stability of carbonate melt is an extension of the stability of carbonate minerals (Hazen et al. 2013a,b) subject critically to the mantle oxidation state (Luth 1993; Frost and McCammon 2008); carbonate-melts have also been predicted in the oceanic low-velocity zone and deep mantle (Hauri et al. 1993 …

Laboratory Simulations of Abiotic Hydrocarbon Formation in Earth’s Deep Subsurface

Abstract: In recent years, methane and other light hydrocarbons with an apparently abiotic origin have been identified in an increasing number of geologic fluids on Earth. These compounds have been found in a variety of geologic settings, including seafloor hydrothermal systems, fracture networks in crystalline rocks from continental and oceanic crust, volcanic gases, and gas seeps from serpentinized rocks (e.g., Abrajano et al. 1990; Kelley 1996; Sherwood Lollar 2002, 2008; Fiebig et al. 2007, 2009; Proskurowski et al. 2008; Taran et al. 2010b). Understanding the origin of these compounds has significant implications for range of topics that includes the global carbon cycle, the distribution of life in the deep subsurface (Gold 1992), and the origin of life (Martin et al. 2008). There are even claims that abiotic sources are major contributors to global hydrocarbon reservoirs (Gold 1993; Glasby 2006; Kutcherov and Krayushkin 2010; Sephton and Hazen 2013). While most experts are highly skeptical of such broad claims, it seems possible that at least some petroleum and gas reservoirs could contain hydrocarbons with an abiotic origin. Conceptually, there are two potential major sources of abiotic hydrocarbons to fluids in Earth’s crust. First, abiotic hydrocarbons could migrate to the crust from deeper sources within Earth, through processes such as convective transport, grain boundary diffusion, or release of magmatic volatiles. Second, abiotic hydrocarbons could form in situ within the crust through reduction of inorganic carbon sources. Potential substrates for carbon reduction include CO2 and CO in circulating fluids, and carbon-bearing solids such as carbonate minerals and graphite. In either case, the ultimate source of the inorganic carbon may be primordial (i.e., from the mantle) or recycled from Earth’s surface. This paper summarizes some of the recent laboratory experimental studies conducted to investigate potential …

Carbon Mineralization: From Natural Analogues to Engineered Systems

Abstract: Carbon mineralization sequesters CO2 by reaction of alkaline earth metal bearing silicate and hydroxide minerals with CO2 to form stable carbonate minerals. Seifritz (1990) proposed harnessing this natural process as a method for sequestration of anthropogenic CO2. It was first studied in detail as an industrial process by Lackner et al. (1995), which is often referred to as “mineral carbonation.” Much of this early research aimed to capitalize on the globally abundant natural deposits of ultramafic and mafic rocks, which are rich in alkaline earth metals, in addition to the long-term stability of the resultant carbonate minerals (Lackner et al. 1995). More recently, other process routes have been investigated that rely on feedstocks other than naturally occurring minerals (e.g., industrial wastes) as a source of cations for carbonate precipitation. Therefore, we use the more general term “carbon mineralization” to refer to any process that sequesters CO2 as a solid carbonate phase. The main advantages of carbon mineralization as a CO2 storage method are that the reactions are thermodynamically favored, the carbonation processes can be readily controlled and manipulated, and the resulting product is benign and stable over geological time. We begin this review with an overview of the fundamental processes that are relevant to carbon mineralization, which provides a basic framework in which to understand CO2 sequestration strategies based on carbon mineralization. We next discuss natural analogues to engineered systems, focusing on (1) exhumed hydrothermal systems in peridotite that have formed listvenite (magnesite + quartz) and soapstone and (2) shallow subsurface peridotite weathering processes and related alkaline springs that form carbonate veins, surficial travertine deposits, and hydromagnesite–magnesite playas. The propensity to form carbonate minerals in these ultramafic terranes reflects the thermodynamic instability of Mg-silicate minerals in the presence of CO2. …

Speciation and Transport of Metals and Metalloids in Geological Vapors

Abstract: Geological aqueous fluids operate in a wide range of temperatures (from 0 to 1000 °C) and depths (from Earth surface to ~10s km), over which the physical-chemical properties of water and water-salt-gas systems and, consequently, their capacities to dissolve minerals and to transport chemical elements are very different. The principal types of geological fluids are illustrated in Figure 1, showing the domains of the liquid, vapor, and supercritical fluid phases in the water phase diagram as a function of temperatures ( T ) and pressures ( P ) typical of Earth’s crust. Understanding the impact of these different fluid phases on geological processes requires knowledge of mineral solubility and metal speciation and partitioning among the different fluid phases. The degree of this knowledge also significantly varies across this T-P range, as roughly illustrated by the three fields identified in Figure 1. In the domain of moderate-temperature aqueous solutions (light-gray field, labeled 1) a large amount of data exists about the nature and stability of major metal complexes, and thermodynamic models have been developed over ~50 years for predicting ore mineral solubilities. By contrast, it is only recently that new insights were obtained into the speciation and transport of economic metals in low-density vapor and fluid phases typical of hydrothermal-magmatic deposits (white field, labeled 2 in Fig. 1), but a number of questions still remain concerning the role of the vapor phase in metal transfers. Similarly, our current knowledge is still insufficient in the domain of high-pressure fluids typical of subduction zones (dark-gray field, labeled 3 in Fig. 1), for which very little is known about the effect of the major ligands like chloride, sulfur, carbon or silica on metal mobility. These lacks hamper our understanding of the geological impact of all these fluids in the continuity of metal and volatile transfers through …

The Chemistry of Carbon in Aqueous Fluids at Crustal and Upper-Mantle Conditions: Experimental and Theoretical Constraints

Abstract: Carbon can be a major constituent of crustal and mantle fluids, occurring both as dissolved ionic species (e.g., carbonate ions or organic acids) and molecular species (e.g., CO2, CO, CH4, and more complex organic compounds). The chemistry of dissolved carbon changes dramatically with pressure ( P ) and temperature ( T ). In aqueous fluids at low P and T , molecular carbon gas species such as CO2 and CH4 saturate at low concentration to form a separate phase. With modest increases in P and T , these molecular species become fully miscible with H2O, enabling deep crustal and mantle fluids to become highly concentrated in carbon. At such high concentrations, carbon species play an integral role as solvent components and, with H2O, control the mobility of rock-forming elements in a wide range of geologic settings. The migration of carbon-bearing crustal and mantle fluids contributes to Earth’s carbon cycle; however, the mechanisms, magnitudes, and time variations of carbon transfer from depth to the surface remain least understood parts of the global carbon budget (Berner 1991, 1994; Berner and Kothavala 2001). Here we provide an overview of carbon in crustal and mantle fluids. We first review the evidence for the presence and abundance of carbon in these fluids. We then discuss oxidized and reduced carbon, both as solutes in H2O-rich fluids and as major components of miscible CO2-CH4-H2O fluids. Our goal is to provide some of the background needed to understand the role of fluids in the deep carbon cycle. ### Carbon in aqueous fluids of crust and mantle Numerous lines of evidence indicate that carbon may be an important component of crustal and mantle fluids. Fluid inclusions provide direct samples of carbon-bearing fluids from a range of environments. Carbon species in fluid …

Structure, Bonding, and Mineralogy of Carbon at Extreme Conditions

Abstract: The nature and extent of Earth’s deep carbon cycle remains uncertain. This chapter considers high-pressure carbon-bearing minerals, including those of Earth’s mantle and core, as well as phases that might be found in the interiors of larger planets outside our solar system. These phases include both experimentally produced and theoretically predicted polymorphs of carbon dioxide, carbonates, carbides, silicate-carbonates, as well as very high-pressure phases of pure carbon. One theme in the search for possible high P-T , deep-Earth phases is the likely shift from sp 2 bonding (trigonal coordination) to sp 3 bonding (tetrahedral coordination) in carbon-bearing phases of the lower mantle and core, as exemplified by the graphite-to-diamond transition (Bundy et al. 1961; Davies 1984). A similar phenomenon has been documented in the preferred coordination spheres of many elements at high pressure. For example, silicon is ubiquitously found in tetrahedral coordination in crustal and upper mantle minerals, but adopts octahedral coordination in many high-pressure phases. Indeed, the boundary between Earth’s transition zone and lower mantle may be described as a crystal chemical shift from 4-coordinated to 6-coordinated silicon (Hazen and Finger 1978; Finger and Hazen 1991). Similarly, magnesium and calcium commonly occur in octahedral 6-coordination in minerals at ambient conditions, but transform to 8- or greater coordination in high-pressure phases, as exemplified by the calcite-to-aragonite transformation of CaCO3 and the pyroxene-to-perovskite and post-perovskite transformations of MgSiO3 (Murakami et al. 2004; Oganov and Ono 2004). Consequently, a principal focus in any consideration of deep-Earth carbon minerals must include carbon in higher coordination, and even more complex bonding at more extreme conditions that characterize the interiors of larger planets. We briefly review theoretical methods used to examine dense carbon-bearing minerals, focusing on first-principles or ab initio approaches. To compute the energies, one can choose one among …

Carbon in the Core: Its Influence on the Properties of Core and Mantle

Abstract: Earth’s core is known to be metallic, with a density of about 9.90 Mg·m−3 at the core-mantle boundary and as such is substantially denser than the surrounding mantle (5.56 Mg·m−3 at the core-mantle boundary; Dziewonski and Anderson 1981). Comparison with cosmic abundances suggests that the core is predominantly Fe with around 5% Ni (Allegre et al. 1995; McDonough 2003) and 8–12% of one or more light elements (Birch 1952). The latter conclusion comes from the observation that the core is appreciably less dense than pure Fe or Fe-Ni alloys under any plausible core temperature conditions (Stevenson 1981). The nature of the light element (or elements) has been the subject of considerable speculation, because of its bearing on Earth’s overall bulk composition, the conditions under which the core formed, the temperature regime in the core, and possible ongoing interactions between core and mantle. Any element with substantially lower atomic number than iron ( z = 26) would have the required effect on core density, but it must also be of high cosmic abundance and it must be soluble in liquid Fe under both the conditions of core formation and those of the outer core. A review of the likely contributors to the core density deficit (Wood 1993) concluded that S and C were the most likely candidate elements and acknowledged that Si, which is extensively soluble in Fe at low pressures, could also conceivably be present. More recently, arguments have been put forward in favor of H (Okuchi 1997) and O (Rubie et al. 2004) as major “light” elements in the core. Although the presence of any of these other elements would not exclude C from the core, dissolution of most of them in liquid Fe require specific compositions of accreting planetesimals and specific conditions of core formation. In …

Geochemistry of Wellbore Integrity in CO2 Sequestration: Portland Cement-Steel-Brine-CO2 Interactions

Abstract: ### Geochemistry and wellbore integrity in CO2 sequestration Effective geologic sequestration of CO2 requires long-term storage with very low leak rates. Injection wells are an obvious leakage pathway for CO2 because they perforate the confining caprock. In addition, sequestration sites are likely to use monitoring wells to assess performance and, in the case of depleted oil and gas fields, may contain 10s to 1000s of older operating and abandoned wells. All wells may have leakage pathways due to faulty construction or other defects. However, it is the subject of this chapter to consider whether geochemical reactions induced by CO2 could result in damage to wells and the development of leaks. This concern is based on the thermodynamic incompatibility of CO2-saturated fluids with the Portland cement and steel used to prevent fluid migration to the surface. Portland cement is an alkaline substance with pH > 12.5 and is not in equilibrium with CO2-bearing fluids (pH < 6). Low-carbon steel used as well casing is subject to aggressive corrosion by carbonic acid. As a result, well integrity has been a central issue in risk analysis of sequestration sites (Gasda et al. 2004; IPCC 2005; Viswanathan et al. 2008; Nordbotten et al. 2009). At the outset, it is important to bear in mind that geochemical reactions alone do not yield insight into wellbore integrity, which is governed primarily by the effective permeability of the Portland cement seal and the mechanical integrity of the system. Thus the significance of chemical reactions must be considered with respect to their impact to changes in permeability or in strength. There is an unfortunate tendency in the literature to not distinguish between reaction and impact. Often CO2 reactions are described as “degradation of cement” or “corrosion of cement” without adequately defining what these terms …

High-Pressure Biochemistry and Biophysics

Abstract: By the end of the 19th century British and French oceanographic expeditions had shown that life exists in the deepest ocean trenches. Since then, microorganisms have been found to thrive in diverse environments characterized by a wide range of pressure-temperature-composition ( P - T - X ) conditions (Rothschild and Mancinelli 2001). The range of physicochemical conditions under which microbial life has been observed has continued to expand with greater access to extreme environments and greatly improved tools for sampling and assessing the diversity and physiology of microbial communities. This exploration now includes examination of subseafloor and continental subsurface settings—key goals of the Deep Life Directorate within the Deep Carbon Observatory (DCO) Program. Bacterial metabolic activity has been described at temperatures as low as −40 °C (Rivkina et al. 2000; Price and Sowers 2004; Panikov and Sizova 2007; Collins et al. 2010) and a methanogen has been cultured at 122 °C under hydrostatic pressure (Takai et al. 2008). Moreover, bacteria can withstand ionizing radiation levels up to 30,000 grays (Rainey et al. 2005), and can grow over a pH range between 0 and 12.5 (Takai et al. 2001; Sharma et al. 2012), at salinities up to 5.2 M NaCl (Kamekura 1998), and at hydrostatic pressures up to 130 MPa (Yayanos 1986). Bacterial survival has also been demonstrated up into the GPa range (Sharma et al. 2002; Vanlint et al. 2011). Because of the difficulty to access deep pressure-affected environments compared to most other extreme environments, less is known about deep-sea and deep-continental microbial communities and their physiological adaptation to high hydrostatic pressure, even though high-pressure environments are more voluminous in nature than other extreme environments. Our current knowledge about life at high pressure currently derives from studies of deep-sea microorganisms that possess adaptations for growth …

Nature and Extent of the Deep Biosphere

Abstract: In the last three decades we have learned a great deal about microbes in subsurface environments. Once, these habitats were rarely examined, perhaps because so much of the life that we are concerned with exists at the surface and seems to pace its metabolic and evolutionary rhythms with the overt planetary, solar, and lunar cycles that dictate our own lives. And it certainly remains easier to identify with living beings that are in our midst, most obviously struggling with us or against us for survival over time scales that are easiest to track using diurnal, monthly or annual periods. Yet, research efforts are drawn again and again to the subsurface to consider life there. No doubt this has been due to our parochial interests in the resources that exist there (the water, minerals, and energy) that our society continues to require and that in some cases are created or modified by microbes. However, we also continue to be intrigued by the scientific curiosities that might only be solved by going underground and examining life where it does and does not exist. But really, is life underground just a peculiarity of most life on the planet and only a recently discovered figment of life? Or is it actually a more prominent and fundamental, if unseen, theme for life on our planet? Our primary purpose in this chapter is to provide an incremental assembly of knowledge of subsurface life with the aim of moving us towards a more complete conceptual model of deep life on the planet. We aim to merge the consideration of the subsea-floor and the continental subsurface because it is only through such a unified treatment that we can reach a comprehensive view of this underground life. We also provide some thoughts on a way forward with what we …

Carbon in Silicate Melts

Abstract: Silicate melts are the main agent for transporting carbon from Earth’s interior to the surface. The carbon concentration in the atmosphere and the size of the carbon reservoir in oceans, sediments, and biomass are ultimately controlled by the balance between carbon removal through weathering, burial in sediments, and subduction on one hand and volcanic degassing on the other hand (e.g., Berner 1994). Carbon emissions from volcanoes may have ended the Neoproterozoic “snowball-Earth” glaciation (Hoffman et al. 1998) and they have been invoked as a potential mechanism that could link flood basalt eruptions to mass extinction events (Beerling 2002). In Earth’s deep interior, the strong partitioning of carbon into silicate melts relative to solid minerals may contribute to melting in the seismic low-velocity zone of the upper mantle and in the transition zone (e.g., Dasgupta and Hirschmann 2010; Keshav et al. 2011). The formation of some highly silica-undersaturated melts is likely related to the effects of carbon dioxide on melting in the mantle (Brey and Green 1975). While carbon is usually less abundant than water in magmas erupting at Earth’s surface, the lower solubility of carbon dioxide (either molecular or as CO32−) in silicate melts implies that it is primarily carbon dioxide that controls the nucleation of bubbles, which is an important aspect of eruption dynamics (e.g., Holloway 1976; Papale and Polacci 1999). In the lower mantle, more reduced carbon species may be dominant in silicate melts, which may behave in a different way than carbon dioxide (e.g., Kadik et al. 2004). Data on the solubility and speciation of carbon in silicate melts (in a broad sense, covering superliquidus liquids, supercooled liquids and glasses) and its effect on melt properties are therefore essential for understanding a wide range of phenomena in the Earth system. Under typical …

Carbon Mineralogy and Crystal Chemistry

Abstract: Carbon, element 6, displays remarkable chemical flexibility and thus is unique in the diversity of its mineralogical roles. Carbon has the ability to bond to itself and to more than 80 other elements in a variety of bonding topologies, most commonly in 2-, 3-, and 4-coordination. With oxidation numbers ranging from −4 to +4, carbon is observed to behave as a cation, as an anion, and as a neutral species in phases with an astonishing range of crystal structures, chemical bonding, and physical and chemical properties. This versatile element concentrates in dozens of different Earth repositories, from the atmosphere and oceans to the crust, mantle, and core, including solids, liquids, and gases as both a major and trace element (Holland 1984; Berner 2004; Hazen et al. 2012). Therefore, any comprehensive survey of carbon in Earth must consider the broad range of carbon-bearing phases. The objective of this chapter is to review the mineralogy and crystal chemistry of carbon, with a focus primarily on phases in which carbon is an essential element: most notably the polymorphs of carbon, the carbides, and the carbonates. The possible role of trace carbon in nominally acarbonaceous silicates and oxides, though potentially a large and undocumented reservoir of the mantle and core (Wood 1993; Jana and Walker 1997; Freund et al. 2001; McDonough 2003; Keppler et al. 2003; Shcheka et al. 2006; Dasgupta 2013; Ni and Keppler 2013; Wood et al. 2013), is not considered here. Non-mineralogical carbon-bearing phases treated elsewhere, including in this volume, include C-O-H-N aqueous fluids (Javoy 1997; Zhang and Duan 2009; Jones et al. 2013; Manning et al. 2013); silicate melts (Dasgupta et al. 2007; Dasgupta 2013; Manning et al. 2013); carbonate melts (Cox 1980; Kramers …

Geochemistry of Geologic Carbon Sequestration: An Overview

Abstract: Over the past two decades there has been heightened concern about, and an improving scientific description of, the impacts of increasing carbon dioxide concentrations in Earth’s atmosphere. Despite this concern, the global rate of addition of carbon dioxide to the atmosphere by the burning of fossil fuel, now approaching 10 Gton C/yr, continues to increase, and at an accelerating rate (Fig. 1a). Although many still hope and believe that carbon emissions can be arrested at near the current rates, and decreased over the remainder of the 21st century, there is as yet little evidence that this is going to occur. The driver for carbon emissions is a globally increasing demand for energy, and the fact that energy can be produced relatively inexpensively and with well-developed technology by burning coal, oil and natural gas. Given that the focus on fossil fuel energy is not lessening to an appreciable degree (Fig. 1b), it is not only prudent, but necessary to have the technology to reduce the carbon emissions associated with fossil fuel burning. This reduction can potentially be accomplished with large-scale carbon capture and storage, where carbon dioxide would be captured from the flue gases of electric power generation facilities, purified, compressed, and injected underground as a supercritical fluid into porous geologic rock formations (Oelkers and Cole 2008). To be effective in reducing carbon accumulation in the atmosphere, this injected or “stored” CO2 must remain underground for thousands of years with only insignificant amounts of leakage back to the surface (Benson and Cook 2005). To date, a significant number of large CO2 injection demonstrations and more modest pilot tests have been linked to either Enhanced Oil (EOR) or Gas Recovery (EGR) operations such as at the Weyburn EOR site in Canada, the In Salah site in Algeria and the …

Experimental Perspectives of Mineral Dissolution and Precipitation due to Carbon Dioxide-Water-Rock Interactions

Abstract: Interactions between CO2 and H2O fluids and the rocks that host them are of significance for Geological Carbon Storage (GCS) for several reasons. These interactions determine the amount of CO2 that can be trapped in solution and in minerals. The petrophysical properties of reservoir and cap rocks, especially porosity and permeability, are also affected. Carbon storage in fluids and minerals, coupled with potential changes to the petrophysical properties of rocks, have a direct bearing on the long-term effectiveness of GCS. Many potential reservoir rocks contain a range of minerals that may react at very different rates. In particular, carbonate minerals are widespread minor components of sedimentary rocks and react much more rapidly than silicates, while clay minerals are often much more reactive than minerals such as quartz or alkali feldspars. It follows that the evolution of pore fluid composition in a reservoir into which CO2 is injected may be strongly influenced by kinetic factors. Dissolution of fast-reacting minerals may be limited by the transport of reactants to the mineral surface, while minerals whose surfaces react only slowly may persist out of equilibrium with pore fluid for extended periods. Some reactions, such as congruent dissolution, proceed until the reacting mineral is in equilibrium with the pore fluid, but other, incongruent reactions may involve unstable reactants which never attain equilibrium with the pore fluid, resulting in very extensive mineralogical transformations over time. This chapter examines interactions between CO2 and H2O fluids and the rocks and minerals that comprise GCS reservoirs, as well as the caprocks that seal these reservoirs, from the perspective of laboratory experiments. Laboratory experiments determine thermodynamic and kinetic parameters and can identify fluid-rock reactions and processes that may have been previously unknown or unappreciated. Experimental studies of equilibrium and kinetic aspects …

Pore Scale Processes Associated with Subsurface CO2 Injection and Sequestration

Abstract: The injection of CO2 into the Earth’s subsurface drives the fluid-rock system into “far-from- equilibrium” conditions, which means that the fluxes that return the system to equilibrium are nonlinearly related to the generalized driving forces (e.g., chemical affinities and gradients in the fluid pressures and chemical potentials). The nonlinear response results in emergent structures and self-organization (Prigogine 1968, 1980; Ortoleva 1994; Lasaga 1998; Jamtveit and Meakin 1999), of which the reactive infiltration instability is a well-known example in the Earth Sciences that is directly applicable to CO2 injection and caprock integrity (Ortoleva et al. 1987; Hoefner and Fogler 1988; Steefel and Lasaga 1994; Steefel and Maher 2009). Flow, solute transport, colloid transport, mineral dissolution and mineral precipitation combine within the mechanical framework of the porous medium to generate precipitate structures in individual pores correlated over many thousands to millions of pores, and immiscible fluid structures with fractal geometry over scales from millimeters to kilometers (Feder 1988). The focus in this chapter is on processes, emergent and otherwise, taking place at the pore scale, defined here as the scale where individual grains and fluid interfaces can be resolved. The chief focus of research in recent years has been to improve the understanding of the nonlinear dynamics specifically associated with biogeochemical and microbially induced processes coupled to physical processes such as flow and diffusion at the pore scale. When coupled, whether in field-scale applications, bench-scale experiments, or in models, emergent behavior may result, including changes in permeability, diffusivity, and reactivity. The pore scale is an important scale to analyze these emergent behaviors given that it provides the architecture within which the smaller scale (nano and molecular scale) processes occur. For the individual processes to interact, transport is typically necessary and the resulting behavior …

On the Origins of Deep Hydrocarbons

Abstract: Deep deposits of hydrocarbons, including varied reservoirs of petroleum and natural gas, represent the most economically important component of the deep carbon cycle. Yet despite their intensive study and exploitation for more than a century, details of the origins of some deep hydrocarbons remain a matter of vocal debate in some scientific circles. This long and continuing history of controversy may surprise some readers, for the biogenic origins of “fossil fuels”—a principle buttressed by a vast primary scientific literature and established as textbook orthodoxy in North America and many other parts of the world—might appear to be settled fact. Nevertheless, conventional wisdom continues to be challenged by some scientists. The principal objectives of this chapter are: (1) to review the overwhelming evidence for the biogenic origins of most known deep hydrocarbon reservoirs; (2) to present equally persuasive experimental, theoretical, and field evidence, which indicates that components of some deep hydrocarbon deposits appear to have an abiotic origin; and (3) to suggest future studies that might help to achieve a more nuanced resolution of this sometimes polarized topic. ### Types of hydrocarbons Deep hydrocarbons include a rich diversity of organic chemical compounds in the form of petroleum deposits, including oil and gas in various reservoirs, bitumen in oil sands, coal and clathrate hydrates. The major gaseous hydrocarbons are the alkanes methane (natural gas, CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). Liquid components of petroleum include a complex mixture primarily of linear and cyclic hydrocarbons from C5 to C17, as well as numerous other molecular species, while solid hydrocarbons include such broad categories as paraffin waxes (typically from C18 to C40). In addition, mature coal deposits sometimes hold a suite of unusual pure crystalline hydrocarbon phases and other organic minerals …

Why Deep Carbon

Abstract: All chemical elements are special, but some are more special than others. Of the 88 naturally occurring, long-lived elements on Earth, carbon stands alone. As the basis of all biomolecules, no other element contributes so centrally to the wellbeing and sustainability of life on Earth, including our human species. The near-surface carbon cycle profoundly affects Earth’s changeable climate, the health of ecosystems, the availability of inexpensive energy, and the resilience of the environment. No other element plays a role in so diverse an array of useful solid, liquid, and gaseous materials: food and fuels; paints and dyes; paper and plastics; abrasives and lubricants; electrical conductors and insulators; thermal conductors and insulators; ultra-strong structural materials and ultra-soft textiles; and precious stones of unmatched beauty. No other element engages in such an extraordinary range of chemical bonding environments: with oxidation states ranging from −4 to +4, carbon bonds to itself and more than 80 other elements. Carbon’s chemical behavior in Earth’s hidden deep interior epitomizes the dynamic processes that set apart our planet from all other known worlds. Past consideration of the global carbon cycle has focused primarily on the atmosphere, oceans, and shallow crustal environments. A tremendous amount is known about these parts of Earth’s carbon cycle. By contrast, relatively little is known about the deep carbon cycle (Fig. 1). Knowledge of the deep interior, which may contain more than 90% of Earth’s carbon (Javoy 1997), is limited (Table 1). Basic questions about deep carbon are poorly constrained:

Caprock Fracture Dissolution and CO2 Leakage

Abstract: Caprocks are impermeable sedimentary formations that overlie prospective geologic CO2 storage reservoirs. As such, caprocks will be relied upon to trap CO2 and prevent vertical fluid migration and leakage. Natural and industrial analogues provide evidence of long-term performance of caprocks in holding buoyant fluids. However, the large volumes of CO2 that must be injected and stored to meaningfully reduce anthropogenic greenhouse gas emissions will exert unprecedented geomechanical and geochemical burdens on caprock formations due to elevated formation pressures and brine acidification. Caprocks have inherent vulnerabilities in that wellbores, faults and fractures that transect caprock formations may provide conduits for CO2 and/or brine to leak out of the intended storage formation. As a result, a critical criterion for CO2 storage reservoir siting assessments will be to predict and reliably quantify the risk of leakage through caprock formations. We use “flow paths” as a catchall term for any fluid conduit through caprocks including pore networks, fractures and faults along with any combination of the three elements. It is useful to assess leakage rates through flow paths in terms of their individual transmissivity, T [m4], which is the product of the permeability and the cross-sectional area of the flow path. Darcy’s law can be used to relate these intrinsic flow path characteristics and the hydraulic potential (pressure) gradient to determine a volumetric flow rate, Q , or a leakage rate for the individual flow path: ![Formula][1] (1) Where P is the hydraulic potential [Pa], z is the depth [m], μ is the fluid viscosity [Pa s] and A [m2] is the cross-sectional area of the flow path perpendicular to flow, and A equals the product of average fracture aperture and fracture length normal to the flow direction. Predicting leakage potential, however, is extremely complex because assessments … [1]: /embed/mml-math-1.gif

Thermodynamic Modeling of Fluid-Rock Interaction at Mid-Crustal to Upper-Mantle Conditions

Abstract: Fluid-rock interaction is important in a wide range of settings in the middle crust to the upper mantle. Fluids are liberated in the crust during prograde metamorphism in convergent-margin settings. At greater depth, devolatilization of subducting lithosphere plays a major role in global element cycling, metasomatic alteration of the mantle, and the genesis of arc magmas. Mafic magmas produced in these settings may stall in the lower crust and liberate volatiles that metasomatize surrounding rocks and trigger production of silicic magmas of crustal derivation. Mounting evidence points to an important role for metasomatic alteration by saline brines in the genesis of some lower-crustal granulite-facies metamorphic terranes. Unlike shallow geothermal systems or low-grade metamorphic environments, there has been only limited progress in modeling the fluid-rock interaction that characterizes the deeper geologic systems enumerated above. Such phase-equilibrium models require as input the chemical potential, μ i μ i ≡ ( ∂ G i ∂ n i ) P , T (1) of the participating phases or species i , where G i is the Gibbs free energy of i , n i is the number of moles of i , P is pressure, and T is temperature. At the P and T of interest, μ i = μ i ∘ + R T l n a i (2) Where μ° is standard state chemical potential, a is activity, R is the gas constant, and T is absolute temperature. It is challenging to quantify the right-hand terms in Equation (2) at P and T ranging from the middle crust to the upper mantle. This is chiefly because of (1) limitations in our quantitative knowledge of key properties of relevant solutes and H2O, and (2) uncertainty in how properly to model solute activities at the requisite conditions. As a consequence, aqueous geochemists have been discouraged from plying their trade in the numerous deep geologic systems in which fluid-rock interaction may play a major role in the Earth’s chemical and physical evolution. …

Primordial Origins of Earth’s Carbon

Abstract: It is commonly assumed that the building blocks of the terrestrial planets were derived from a cosmochemical reservoir that is best represented by chondrites, the so-called chondritic Earth model. This view is possibly a good approximation for refractory elements (although it has been recently questioned; e.g., Caro et al. 2008), but for volatile elements, other cosmochemical reservoirs might have contributed to Earth, such as the solar nebula gas and/or cometary matter (Owen et al. 1992; Dauphas 2003; Pepin 2006). Hence, in order to get insights into the origin of the carbon in Earth, it is necessary to compare: (i) the elemental abundances and isotopic compositions of not only carbon, but also other volatile elements in potential cosmochemical “ancestors,” and (ii) the ancestral compositions with those of terrestrial volatiles. This approach is the only one that has the potential for understanding the origin of the carbon in Earth but it has several intrinsic limitations. First, the terrestrial carbon budget is not well known, and, for the deep reservoir(s) such as the core and the lower mantle, is highly model-dependent (Dasgupta 2013; Wood et al. 2013). Second, the cosmochemical reservoir(s) that contributed volatile elements to proto-Earth may not exist anymore because planet formation might have completely exhausted them (most of the mass present in the inner solar system is now in Venus and Earth). Third, planetary formation processes (accretion, differentiation, early evolution of the atmospheres) might have drastically modified the original elemental and isotopic compositions of the volatile elements in Earth. Despite these limitations, robust constraints on the origin(s) of the carbon in Earth can be deduced from comparative planetology of volatile elements, which is the focus of this chapter. Carbon in the cloud of gas and dust from which the solar system formed was probably mainly in the …

Thermodynamics of Carbonates

Abstract: The goal of geologic CO2 sequestration is not just to pump large volumes of supercritical carbon dioxide into underground repositories but to keep it there for hundreds to thousands of years, preferably in chemically bound form. The permanence of CO2 storage in geological repositories is important since large leakage rates would diminish the CO2 abatement achieved with carbon capture and sequestration (CCS). Effective permanent geologic CO2 storage depends ultimately on the interactions of the supercritical CO2 with the minerals and fluids present in the host underground repositories and their caprock sealing (Xu et al. 2005; Kharaka et al. 2006, 2010; Benson and Cole 2008). Migration of supercritical CO2 within these geological repositories is controlled by confinement/trapping of CO2 in the porous structure as well as solubility of CO2 in the fluids (brine and hydrocarbons) already present in the storage formation (Cole et al. 2010; Doughty 2010). In deep saline aquifers, the dissolution of CO2 in water generates carbonic acid, which in turn reacts with minerals such as clay, mica and feldspar and carbonates present in the reservoir rocks to generate cations and carbonate/bicarbonate ions (Kaszuba et al. 2003, 2005; Kharaka et al. 2006; Ketzer et al. 2009; Cole et al. 2010; Doughty 2010). Finally, precipitation of metal carbonate occurs either by direct or indirect reaction of the CO2 with other minerals and organic matter present in the reservoirs (Garcia et al. 2012; Kharaka et al. 2010; Shao et al. 2010, Xu et al. 2004, 2005). It is essential to understand and predict the chemical reactions and stability of mineral phases formed under sequestration conditions as these could affect the migration of CO2 and the seal …

Capillary Pressure and Mineral Wettability Influences on Reservoir CO2 Capacity

Abstract: The movement of injected CO2 through permeable pore networks determines its distribution and stability within reservoirs used for carbon sequestration (Fig. 1), and this process is dependent on capillary interactions with the displaced brine (IPCC 2005; Benson and Cole 2008). Capillary phenomena controlling the distribution of CO2 and reservoir brine depend on pore size (from mm to nm), wetting and interfacial properties. The pressure of the usually nonwetting CO2 phase relative to that of the native brine is the capillary pressure, P c , which in combination with pore size, wettability, and interfacial properties determines the saturation of each fluid phase. In typically reservoirs used for geologic carbon sequestration, CO2 exists as a supercritical (sc) fluid because pressures and temperatures associated with storage depths exceed the critical point values for CO2 (7.38 MPa, 31.1 °C). Thus, predicting carbon sequestration in reservoirs require understanding of interfacial tension, mineral surface wettability, and the capillary pressure dependence on saturation, for interactions between scCO2, brine, and reservoir mineral surfaces. Investigations into these aspects of scCO2 behavior, especially at elevated pressures and temperatures characteristic of geologic carbon sequestration, have largely only recently begun. Given the fairly early stage of research, many models for CO2 transport in reservoirs rely heavily on the better understood capillary characteristics of other immiscible fluid pairs such as air-water, oil-water, and oil-gas, supplemented with properties of scCO2. However, a growing number of experimental studies are being published on the behavior of scCO2 under reservoir conditions, leading to better understanding of C sequestration while also giving rise to new questions. In this chapter, we review aspects of the current (mostly laboratory-based) understanding of equilibrium capillary interactions between scCO2 and brines, and their implications for scCO2 behavior in …

Molecular Simulation of CO2- and CO3-Brine-Mineral Systems

Abstract: Atomistic simulations—molecular dynamics (MD) and Monte Carlo (MC) simulations, ab initio and density functional theory (DFT) calculations—have proved useful in gaining insight into the molecular basis of fundamental processes in aquatic geochemistry, such as solvation, ion pair formation, adsorption, molecular diffusion, and the energetics of mineral phases (Rotenberg et al. 2007; Bickmore et al. 2009; Hamm et al. 2010; Kerisit and Liu 2010; Hofmann et al. 2012; Stack et al. 2012; Wallace et al. 2013). Key strengths of these simulations are their ability to examine the behavior of individual atoms (where spectroscopic and other experimental methods probe the average behavior of large numbers of molecules) and to allow constraints that would be difficult or impossible to impose in the laboratory. These features make atomistic simulations powerful tools for elucidating the manner in which collective phenomena arise from molecular scale properties in geochemical systems. The range of length and time scales probed by atomistic simulations (from angstroms to tens of nanometers and from femtoseconds to microseconds, continuously expanding with advances in the availability and sophistication of computational resources) makes them ideally suited to complement several spectroscopic techniques, including X-ray, neutron, and nuclear magnetic resonance approaches. A well known limitation of the methods described in the present chapter, particularly in the case of classical mechanical (MD and MC) simulations, is the approximate nature of the models that are used to describe interatomic forces. In the simplest of these simulations, bond lengths and angles are fixed; inter-atomic interactions are modeled as the sum of two-body interactions that depend only on the identity of the interacting atoms and the distance between them; and chemical bonds are not allowed to break or form during a simulation (Allen and Tildesley 1987; Frenkel and Smit 2001). The choice of force …

The Deep Viriosphere: Assessing the Viral Impact on Microbial Community Dynamics in the Deep Subsurface

Abstract: All regions of Earth’s biosphere that we have studied—the waters of Earth’s oceans, the soil beneath our feet, and even the air we breathe—teem with viruses. Viral particles are among the smallest biological entities on the planet, with the average viral particle measuring about 100 nm in length: a size so small that five thousand viruses, lined end to end, would fit across the thickness of a human fingernail. What they lack in size, though, they compensate with sheer abundance. If we were to line up all the viruses in the ocean, they would stretch across the diameter of the Milky Way galaxy one hundred times (Suttle 2007). Those viruses are responsible for up to 1023 infections per second in the oceans (Suttle 2007). With each new infection, viruses can have a profound impact on their hosts: they can alter the structure of a microbial population, break up cellular biomass into its constituent organic matter, or introduce new genes into their hosts. Through this activity, viruses play a role in top-down as well as bottom-up processes, and can potentially alter the course of evolution. The importance of viruses in the surface oceans is now well recognized, and research is increasingly dedicated to improving our understanding of their role in important marine processes. The viral role in the deep subsurface, however, is rarely considered. Deep within the crust and sediment below the ocean, viruses may play a profound role in altering biogeochemical cycles, structuring microbial diversity, and manipulating genetic content. Yet many questions remain unanswered: Are certain species or strains in the deep subsurface more susceptible to viral infection than others? What role do viruses play in driving natural selection and evolution in the deep biosphere? Is it more common for viruses to persist as protein-bound virion particles, or do …

Thermodynamics of Organic Transformations in Hydrothermal Fluids

Abstract: Hydrothermal fluids obtain organic compounds through diverse pathways. In submarine systems organic compounds are already dissolved in seawater that is heated and transformed into hydrothermal fluids through water-rock reactions. Microbes inhabiting hydrothermal systems produce metabolites that enter the fluids, and cells can be carried into the reaction zones by circulating fluids and pyrolyzed. Analogous sources of organic compounds can be anticipated in continental systems with the possible addition of novel plant- and soil-derived organic compounds from the surface. In addition, hydrothermal systems possess large potentials for abiotic organic synthesis that may add a novel suite of compounds. When sedimentary rocks are present, ancient biogenic organic matter can be mobilized or transformed by hot fluids. These transformations accompany the generation of petroleum, coal, and other fossil fuels, suggesting that expectations for hydrothermal transformations can be built on those that occur in sedimentary basins. Likewise, some types of ore deposition are accompanied by transformations of organic compounds, and metal-organic complexes may be involved in enhancing the transport of metals in ore-forming and other crustal fluids. With these thoughts in mind, this review starts with an inventory of the types of organic compounds found in hydrothermal systems and some ways that hydrothermal organic compounds are transformed. Methane can be generated biotically and abiotically from organic or inorganic reactants, and since it lacks a carbon-carbon bond, some researchers would not consider it to be an organic compound. Nevertheless, more data exist for methane in hydrothermal fluids than for any organic compound that fits the definition. Methane has been quantified in continental and submarine hydrothermal fluids, fumarolic gases associated with hydrothermal systems, oil-field brines, deep fluids in sedimentary basins and igneous basement rocks, fluids associated with active serpentinization, and fluid inclusions in minerals from ore deposits, sedimentary basins, and deep crustal settings (recent examples include: …

Carbon Mineral Evolution

Abstract: The discovery of the extreme antiquity of specific minerals through radiometric dating (e.g., Strutt 1910), coupled with Norman L. Bowen’s recognition of a deterministic evolutionary sequence of silicate minerals in igneous rocks (Bowen 1915, 1928), implies that Earth’s crustal mineralogy has changed dramatically through more than 4.5 billion years of planetary history. Detailed examination of the mineralogical record has led to a growing realization that varied physical, chemical, and biological processes have resulted in a sequential increase in diversification of the mineral kingdom. This diversification has been accompanied by significant changes in the near-surface distribution, compositional range (including minor and trace elements), size, and morphology of minerals (Ronov et al. 1969; Nash et al. 1981; Zhabin 1981; Meyer 1985; Wenk and Bulakh 2004; Hazen et al. 2008, 2009, 2011). Variation in Earth’s mineralogical character thus reflects the tectonic, geochemical, and biological evolution of Earth’s near-surface environment (Bartley and Kah 2004; Hazen et al. 2009, 2012; Grew and Hazen 2010a,b; McMillan et al. 2010; Krivovichev 2010; Grew et al. 2011; Tkachev 2011). The mineral kingdom’s evolutionary narrative shares many features with the increased complexity inherent within other evolving systems, including the nucleosynthesis of elements and isotopes, the prebiotic synthesis of organic molecules, biological evolution through Dar-winian natural selection, and the evolution of social and material culture (Hazen and Eldredge 2010). In particular, well-known biological phenomena such as diversification, punctuation, and extinction appear to be common traits within a wide range of complex, evolving systems. Perhaps more than any other element, carbon exemplifies these processes of “mineral evolution.” Four episodes outline major events in the mineral evolution of carbon: (1) the synthesis of the first mineral, likely diamond, and perhaps a dozen other “ur-minerals” …