Diagenesis Of Sedimentary Rocks Research Articles

Fluid flowrates and compositions and water–rock interaction in the Hikurangi margin forearc, New Zealand

The decrease in interseismic coupling and slip rate deficit from south to north, from locked to slipping, of the Hikurangi margin is attributed from interpretation of geophysical data to more fluid-saturated sediments in the north. To assess this interpretation, fluid flowrates were measured in springs of the subaerial forearc, structures correlated with flowrates and the fluids geochemically and isotopically evaluated to understand their origins and source depths. Extrapolations from on-ground measurements show that ∼95% of aqueous fluids and ∼98% of gas are expelled to the surface in the slipping zone, within the northern subaerial forearc. The relative fluid residence time, from the equilibration of CH4 and CO2 and measured ascent rates, is 3x shorter in the slipping zone suggesting less constricted migration pathways than in the locked zone. Fluids are expelled not only from the sediments that produce Na–Cl fluids and thermogenic gas, but also from the oceanic igneous mafic slab that releases high-Cl fluids and abiotic CH4 from serpentinization, giving rise to Na–Ca–Cl fluids. Chloride-rich surface emissions consist of ∼70% by volume Na–Cl and ∼30% Na–Ca–Cl fluids. Higher rates of fluid expulsion in the north coincide with higher permeability and greater production of aqueous fluids from clay dehydration, serpentinization and a higher degree of diagenesis in sedimentary rocks. Mantle gas, subduction interface fluids and porewaters have the most direct access to the surface in the slipping Center, where the degree of water–rock interaction and homogeneity of water compositions suggest more recent fluid generation than other sectors in the subaerial forearc. Porewaters and thermogenic gas equilibrate at 110° + 20 °C with a slight increase in temperature from south to north. Overall, ∼120 Tg/a water is subducted in the Hikurangi margin and <0.5 Tg/a CH4 is being emitted to the atmosphere. This study shows the added impact of waters of serpentinization on the overall fluid compositions, aqueous fluid subduction budget, extent of fluid distribution in the oceanic slab and, indirectly, the temperature estimate of 185° + 25 °C for fluids ascending from the subduction interface in the Hikurangi margin.

Mineralogy, Geochemistry and Genesis of Agate—A Review

Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (&gt;90 ppm), B (&gt;40 ppm) and U (&gt;20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors.

Persistence of Grenvillian dominance in Laurentian detrital zircon age systematics explained by sedimentary recycling: Evidence from detrital zircon double dating and detrital monazite textures and geochronology

Abstract Grenvillian ages dominate Neoproterozoic to Paleozoic detrital zircon (DZ) populations across eastern Laurentia and persist through the present. The persistence of this dominance is inferred to result from recycling of DZ grains ultimately sourced from exceptionally Zr-rich and zircon-fertile Grenvillian granitoids. Pennsylvanian arenites of the Appalachian Basin (eastern United States) exhibit DZ U-Pb age distributions that are nearly identical to those of Neoproterozoic to Cambrian strata, and contain detrital diagenetic monazite grains formed via metamorphism or diagenesis of sedimentary rocks in the source region. Detrital zircon (U-Th)/He ages are mostly 475–300 Ma, yielding lag times [Δt = U-Pb age − (U-Th)/He age] of 500–1000 m.y. and 1200–2400 m.y. for Grenvillian and Paleoproterozoic to Archean DZ grains, respectively. Detrital monazite Th-Pb ages are comparable to (U-Th)/He cooling ages, reflecting formation of monazite during Paleozoic regional metamorphism of Neoproterozoic to Cambrian strata that reset the (U-Th)/He systematics of Grenvillian DZ grains within those metasediments. These results are either consistent with or prove recycling. Incorporation of other geological constraints permits definition of at least three (and potentially five) recycling events and their timing following initial post-Grenvillian exhumation and erosion (the “great Grenvillian sedimentation episode”). Recycling events include dispersal of post-Grenvillian sediment during deposition of Neoproterozoic to Cambrian strata (formation of the “Great Unconformity”: cycle 1), subsequent erosion of metamorphosed Neoproterozoic to Cambrian strata generating detritus for the Pennsylvanian arenites sampled here (cycle 2), and modern erosion of those arenites (cycle 3). Pancontinental river systems facilitated dispersal of sediment of ultimate Grenvillian age during or after each cycle.

花粉化石の明度変化からみた堆積岩の続成作用 - 基礎試錐「小国」の例 -( 堆積盆の熱履歴を探る(その1))

花粉化石の明度変化からみた堆積岩の続成作用 - 基礎試錐「小国」の例 -( 堆積盆の熱履歴を探る(その1))

Goethite as a potential source of magnetic nanoparticles in sediments

Goethite is an important iron-bearing mineral found in nearly all types of soils and sediments that typically occurs as particles of nanoscale crystallites. Here we show that poorly crystalline nanophase goethite can undergo dehydroxylation and alteration to produce strongly magnetic Fe-oxide nanoparticles. After moderate reductive heating, synthetic oriented aggregates of nanogoethite were converted to stoichiometric sub-micron magnetite. In contrast, oxidative heating produced only nanocrystalline hematite. We present a novel characterization of the products of nanogoethite alteration, which represents an important pathway for the production of both superparamagnetic and stable single-domain magnetic particles in a wide variety of sedimentary environments, including soils and lacustrine and marine settings in which goethite is commonly present. In nature, the mechanism of magnetite formation observed in our experiments could contribute to the widespread phenomenon of magnetic enhancement in many soil types, and could be responsible for secondary magnetizations often observed to accompany diagenesis in sedimentary rocks. Thus, nanogoethite may be an important precursor of fine magnetic particles and magnetic remanence carriers in the environment, particularly in soils and catchment areas affected by wildfire as well as in sediments subjected to deep burial diagenesis and low-grade metamorphism.

How Porosity Increases During Incipient Weathering of Crystalline Silicate Rocks

Weathering of bedrock to produce porous regolith, the precursor to biologically active soil and soluble mineral nutrients, creates the life-supporting matrix upon which Earth’s Critical Zone—the thin surface layer where rock meets life—develops (Ollier 1985; Graham et al. 1994; Taylor and Eggleston 2001). Water and nutrients locked up in low porosity bedrock are biologically inaccessible until weathering helps transform the inert rock into a rich habitat for biological activity. Weathering increases the water-holding capacity and nutrient accessibility of rock and regolith by increasing porosity and mineral surface area, affecting the particle-size distribution, and enhancing ecosystem diversity (Cousin et al. 2003; Certini et al. 2004; Zanner and Graham 2005). Especially in areas where soils are thin and climate is dry, the water stored in weathered rock is essential to ecosystem productivity and survival (Sternberg et al. 1996; Zwieniecki and Newton 1996; Hubbert et al. 2001; Witty et al. 2003). Removal of soluble material during weathering decreases the concentrations of major elements such as Ca, Na, and Mg and the overall mass of the solid, decreasing the bulk density and increasing porosity. These chemical and physical changes result in decreased uniaxial compressive strength and elastic moduli of the rock and increased infiltration of water through the weathered rock (Tugrul 2004). Porosity in intact bedrock is comprised of inter- and intra-granular pores developed during (re-) crystallization in igneous and metamorphic rocks or diagenesis in sedimentary rocks. As we conceptualize it, the conversion of low-permeability bedrock to regolith generally begins due to the transport of meteoric water into protolith through the large-scale fractures that are present as a result of regional tectonic factors or exhumation (Wyrick and Borchers 1981; Molnar et al. 2007). In zones near the fractures, water can infiltrate into the low-porosity rock …

The Molecular-Scale Fundament of Geothermal Fluid Thermodynamics

Chemical interactions between fluids and minerals play a central role in numerous geological processes. In a hydrothermal environment, such reactions are usually referred to as fluid-rock interaction or water-rock interaction and lead to alteration of rocks by changing the mineral assemblages and compositions. During fluid-rock interaction the fluid’s chemical composition also changes, some components being depleted upon secondary mineral formation and others being enriched due to primary mineral dissolution. An example is the interaction between rocks at the seafloor and heated seawater during hydrothermal convection at mid-ocean ridges. There, the rocks are altered, extracting Mg and SO4 from the circulating seawater and releasing metals such as Cu into the fluid. The chemically modified seawater is released back into the ocean at black smoker sites and diffuse discharge sites. This process contributes to buffering the chemical composition of the oceans over geological time periods. In sedimentary systems, the evaporation of aqueous fluids in marine and intracontinental settings leads to the formation of evaporites from complex, multicomponent brines. The diagenesis of sedimentary rocks is closely related to mineral dissolution and precipitation reactions in the presence of pore fluids that often flow over long distances in aquifers. The majority of ore deposits have formed by the interaction of hot hydrothermal fluids with rocks, dissolving the ore constituents as trace elements from a large volume of source material, transporting and focusing them during hydrothermal fluid flow and eventually precipitating them in concentrated form in a smaller rock volume due to chemical precipitation. The precipitation may result from the chemical effects of the reactions between the fluid and the rock. For example, the pH may increase by neutralization of acid fluids, a process that is often key to Pb-Zn sulfide mineralization in limestone and marble. Changes in the physico-chemical conditions (e.g., a temperature decrease) …

Structural Arrangement of Sedimentary Organic Matter: Nanometer-Scale Spheroids as Evidence of a Microbial Signature in Early Diagenetic Processes

Abstract Nanometer-scale spheroidal objects (20–200 nm wide) have been widely encountered in geological samples, while their origin remains unclear. First described as “nannobacteria” (Folk 1993), they are more likely associated with various bacterial debris. In order to explore their potential role in degradation and preservation of organic matter (OM) in rocks, this research investigates a range of recent and fossil occurrences of various microbial imprints using electron and atomic force microscopy. Selected samples illustrate different stages in OM cycle and diagenesis, i.e., from production to accumulation before and after lithification. More specifically, effects of oxygen- and light-depleted conditions, which occur naturally at the water–sediment interface, have been evaluated in microbial mats using a specifically tailored laboratory strategy. Recent sediments reveal a close association between a large occurrence of nanometer-scale spheroids and the degree of degradation due to microbial activity. Wide nanometer-scale spheroids appear in fossilized microbial mats and in microbial mats under experimental diagenesis. These samples show that nanometer-scale spheroids are associated with well-preserved OM (e.g., intact bacteria). This corresponds to reduced physicochemical degradation processes. This observation suggests that nanometer-scale spheroids, i.e., in the size range of nannobacteria, occur concomitantly with a significant microbial degradation shown by numerous organic debris (e.g., cell walls). Both recent and fossil samples from anoxic settings show a close association of nanometer-scale spheroids with extracellular polymeric substances (EPS) and bacterial cell walls, and not with algal cell walls or other organic compounds. Because they occur concomitantly with well-preserved organic constituents, nanometer-scale spheroids indicate a reduced level of OM degradation in the diagenetic processes. Therefore, nanometer-scale spheroids would be a microbial evidence of early diagenesis in sedimentary rocks.

A geometrical model of diagenesis using percolation theory

We study the restructuring process of diagenesis in sedimentary rocks using a geometrical model of percolation theory. The competing processes of cementation and dissolution of diagenesis are modelled by growth of vacant sites and culling of occupied sites in a percolation model depending on a threshold parameter m. We numerically study the percolation thresholds of this model for different values of m and compare them with the corresponding thresholds in the Bootstrap and ordinary percolation problems.

Die Genese devonischer Vorlandsedimente am NW-Rand des Rheinischen Massivs -- Zement- und Porenraumentwicklung

Die Genese devonischer Vorlandsedimente am NW-Rand des Rheinischen Massivs -- Zement- und Porenraumentwicklung

Transport properties and diagenesis in sedimentary rocks: The role of micro-scale geometry

In this paper we propose a new way to determine the permeability of many sedimentary rocks using fractal analysis of pore-scale geometry. We present experimental evidence that the fractal properties of sedimentary rocks evolve due to fluid-rock interactions that occur during diagenesis. A numerical model of pore-mineral, reaction-limited growth is constructed to study this evolution. Model results indicate a quantitative relation between fractal dimension and crystal growth rates at the pore-grain interface. The model and experiments together suggest that the state of diagenetic alteration, as well as the permeability of many sedimentary rocks, can be partly assessed from measurements of pore geometry alone and that trends in diagenesis and transport properties are linked via the evolving pore structures.

Tempered microdolomites in crinoids: A new criterion for high-grade diagenesis

Microdolomite inclusions in crinoids of the Middle Triassic “Trochitenkalk” stage from north and central Germany were formed during diagenesis in a closed system and normally consist of 52–55 mole % CaCO3 (48-45 % MgCO3). In areas influenced by thermal activity these calcium microdolomites are tempered to stoichiometric dolomites. Thus, the microdolomite composition can be used as a new criterion for assessing the degree of diagenesis in sedimentary rocks. As an effect of the Upper Cretaceous intrusive bodies of the Bramsche-Vlotho-Solling Massifs, which influenced the overlying sediments for 1–10 million years, a microdolomite composition of d(104)=2.889 A (~51 mole % CaCO3) is associated with an illite crystallinity of 350 (Hbrel; <2μm). The composition of the microdolomites is more sensitive to short term heating (for example: ascending thermal waters near the faults of the Hessen grabens) than is illite crystallinity.

Elements of the Diagenesis of Sedimentary Rocks

ABSTRACTThree of the most important elements of the diagenesis of sedimentary rocks are consolidation, porosity reduction, and pressure dissolution or cementation. These are discussed qualitatively with special emphasis on the central role of contact formation in controlling both the driving forces and the kinetics of morphological change. It is shown, in particular, that the equilibrium forces would drive the rocks towards collapse of their porosity earlier in diagenesis were they not prevented by kinetic barriers. At greater depths these barriers are effectively removed by pressure concentration at the small contacts. We propose an explicit, qualitative mechanism leading to the observed morphologies and identify specific problems for quantitative analysis.

Pressure Solution During Diagenesis

Pressure is durable problem in geology. It has engendered continuing arguments on its mechanism, thermodynamic basis, and geo­ logical significance. Recently, it has drawn renewed attention both for its important role in the diagenesis of sedimentary rocks and for its relation to rock deformation. Pressure is defined as process by which grains dissolve at intergranular or intercrystalline contacts. This process is presumably related to the higher solubility under nonhydrostatic stress at the contacts than at free grain surfaces. The process often, although not always, is accompanied by reprecipitation at adjacent free grain surfaces. Nevertheless, it is not justifiable to treat pressure as recrys­ tallization process. Two common types of pressure are recognized: intergranular pressure and stylolitization. The former, occurring at individual grain contacts, was first described by Sorby (1863) based on experiments and theoretical predictions by Thompson ( l862a,b). The latter produces a surface or contact that is marked by an irregular and interlocking penetration of the two sidcs, and is supposedly formcd diagenctically by differential vertical movement under pressure, accompanied by solution (Bates & Jackson 1980). Here we use the term for surface that involves an of grains (the aggregate stylolite of Park & Schot 1968), originating exclusively by dissolution under nonhydrostatic stress.