Sedimentary Dolomite Research Articles

Dissolved silicon as a beneficial factor for biomineralization of disordered dolomite by a halophilic cyanobacterium

The genesis of sedimentary dolomite has long been a mystery in Earth science. It has been shown that the formation of low-temperature disordered dolomite, a crucial precursor of sedimentary dolomite, can be catalyzed by various heterotrophic microorganisms through organic mineralization. However, the potential role of cyanobacteria, which are common and cosmopolitan photoautotrophs in sedimentary environments, in the precipitation process of disordered dolomite has been largely underestimated. Moreover, recent emphasis on the beneficial influence of dissolved silicon (Si) in directly enhancing the Mg content in CaMg carbonates under oversaturated conditions has emerged. Nevertheless, the effect of dissolved Si on the formation of microbially-induced disordered dolomite remains poorly understood. To examine the influence and mechanisms of dissolved Si on the biomineralization of CaMg carbonates by cyanobacteria, biomineralization experiments were conducted using a halophilic cyanobacterium (Synechococcus elongatus FACHB-410) in solutions with varying concentrations of dissolved Si (ranging from 0 to 2 mM). Our findings revealed that the presence of dissolved Si resulted in higher Mg levels in the bio-precipitated CaMg carbonates compared to Si-free systems, with disordered dolomite formation observed at 2 mM Si. Importantly, an amorphous Mg-silicate phase formed prior to the crystallization of CaMg carbonates during the biomineralization process, intimately associated with the grains of CaMg carbonates. Subsequent abiotic carbonation experiments demonstrated its catalytic role in the nucleation of CaMg carbonates. These observations suggest that, in addition to the direct catalytic effect of dissolved Si, the positive role of Si in the biomineralization of disordered dolomite by cyanobacteria may also be attributed to the templating role of the neoformed amorphous Mg-silicate. These results have important implications for understanding the co-occurrence of Mg-silicate and dolomite observed in geological records.

Characteristics and geological significance of carbon and oxygen isotopes of the Permian Lucaogou Formation dolomite in the southern Junggar Basin, northwestern China

Anomalously positive δ13C values in ancient dolomites are very rare. Dark gray argillaceous rocks of the lacustrine sediments of the Permian Lucaogou Formation are important source rocks in the Junggar Basin, and dolomites of varying thicknesses from 10 cm to 150 cm are often interspersed in argillaceous rocks. Based on the study of petrographic sections, this paper systematically analyzes the carbon and oxygen isotopes of dolomite and discusses the causes of abnormally high carbon isotope values and their significance in reconstructing paleoenvironment and paleoclimate. The results show that carbon isotope values are abnormally high in the dolomite of Lucaogou Formation, and the δ13C value is between +3.2 ‰ PDB and +19.6‰ PDB, with an average of +9.7‰ PDB. The δ18O values range from -17.4‰ PDB to -1.7‰ PDB, with an average of -8.1‰ PDB. From the lower part to the upper part of the Lucaogou Formation, the carbon isotope value gradually increases and becomes increasingly positive, and the carbon isotope of the dolomite deposited near the shore is more positive than that of the dolomite deposited far from the shore. The anomalously positive δ13C of the dolomite is mainly caused by microbial methanogenesis, with some contribution from evaporation. Microorganisms are mainly distributed at the redox interface. Evaporation controls the salinity and fluctuation of the redox interface in sedimentary water. The positive deviation difference in carbon isotopes between nearshore and offshore sedimentary dolomites may be related to the location of the redox interface during deposition. Together, the petrographic features and carbon and oxygen isotope signatures of the sections reflect the gradual evolution of the paleolake from a hydrologically open environment to a hydrologically closed one and the possible transition of the paleoclimate from a relatively warm to an arid condition, which is possibly a geochemical response to global climate change in the Permian period.

Sedimentary dolomite in Western Australia and the dolomite problem: Genesis of channel and playa uranium deposits

ABSTRACTQuaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions.

A non-classical crystallization mechanism of microbially-induced disordered dolomite

Non-classical crystallization pathways, involving the formation of complex precursors prior to nucleation, have been observed during the calcification process of invertebrate skeletons and shells. However, these pathways were rarely documented in microbially-induced carbonate precipitation. In this study, we presented experimental evidence demonstrating that a halophilic bacterium (Halomonas sp. strain JBHLT-1) catalyzed the biomineralization of disordered dolomite through an amorphous calcium-magnesium carbonate (ACMC) with a stoichiometry near that of dolomite. During the ACMC formation, strain JBHLT-1 significantly enhanced the incorporation of Mg2+ ions compared to abiotic ACMC that was synthesized without any microbial biomass. In the stage of ACMC crystallization, the presence of microbial biomass reduced the dissolution-reprecipitation rate and induced bias towards a solid-state reaction pathway. This observation was supported by the negligible loss of structural Mg2+ ions in the presence of cells of strain JBHLT-1. The microbially-induced growth of disordered dolomite appeared to follow a spherulitic growth mechanism, as evidenced by its hierarchical structure composed of coalesced nano-spheres, eventually growing into micron-sized spheroids and dumbbells with extended incubation times. Since disordered dolomite is considered as a crucial crystalline precursor of ordered sedimentary dolomite, the findings of this study have important implications for understanding the formation mechanism of natural dolomite.

Условия образования рудовмещающих нижнесреднепалеозойских осадочных формаций Чаткало-Кураминских гор Узбекистана

The article describes stratiform lead-zinc and polymetallic deposits in Paleozoic sedimentary rocks of the southwestern Tien Shan of the Chatkal-Qurama Mountains using rhythmostratigraphic analysis developed by professor at Tashkent State University V. I. Popov (Popov et al., 1963; Popov, 1980). Tectonic movements play a major role in the formation of new structural elements of the lithosphere and the earth's surface. Large tectonic movements spread over large areas and become regional tectonic rhythms. Considerable phases of tectonic movements were divided into smaller phases. We observed the change in formational composition: the boundary and beginning of the rhythm series were dominated by clastic facies of mobile shallow water (surf-clastic), closer to the beltv — quiet-water or stagnant (Popov et al., 1963; Popov, 1980). We stadied the northwestern slopes of the Qurama ridge in the upper reaches of the Karakiya River. Lithological-facies sections and maps of actual materials were made in the study areas. Early Silurian sediments of the were formed in the conditions of a quiet-water (central-sedimentary) shallow-water basin, zones under the influence of bottom currents (bottom current) and deltaic (underwater deltaic) facies belt; Devonian-Lower Carboniferous sediments were accumulated in wave surf zone, bay (shallow-flood belt) and underwater delta (underwater delta belt). Three ore-bearing units with high contents of gold, copper, lead, zinc and a number of other metals were identified in the Lower Silurian terrigenous formations. Black dolomites of the stagnant zone of a shallow-water bay (shallow-bay facies) with high contents of lead were identified in the Devonian sedimentary formations. The confinement of increased concentrations of polymetals to certain layers and units of sedimentary dolomite rocks enriched in organic matter indicates that the accumulation of dispersed ore matter is syngenetic to the sedimentation process.

Organomineralization of dolomite in hypersaline microbial mats from Qatar sabkhas visualized by TEM & STXM

Deep insight into the low-temperature mineralization mechanism of dolomite in sediments has remained elusive. This issue is popularly termed “The Dolomite Problem” due to its multifactorial nature. Dolomite has been observed to mineralize in the exopolymeric substances produced by microbial mat communities (Bontognali et al. 2013), where productivity is high. One working hypothesis suggests that degrading organic matter in hypersaline environments releases the necessary component ions, increasing saturation with respect to dolomite (DiLoreto et al. 2019, Dupraz et al. 2009, Petrash et al. 2017). Other models suggest a dissolution-reprecipitation reaction of calcite to dolomite (Rivers 2023). High-resolution micro-spectroscopy techniques (such as transmission electron microscopy, TEM; and scanning transmission X-ray microscopy, STXM) can be used to determine chemical changes in crystals nucleating in a matrix, however to date very little studies have focused on observing dolomite mineralization at the nano-scale. The present study investigates microbial mats collected from hypersaline salt flats in the Persian gulf at micro- to nano-meter scales using high-spatial and -energy resolution TEM (Thermo Scientific Talos 200X at the Canadian Centre for Electron Microscopy) and STXM (PolLux Beamline at the Swiss Light Source at Paul Scherrer Institut), specifically to see changes in carbonate mineralization due to interactions with organic matter. C, Ca and O elemental maps of carbonate crystals were obtained with EDXS (energy-dispersive X-ray spectroscopy) in TEM. These crystals were also indexed by SAED (selected area electron diffraction in TEM). Fine spectral signatures (near-edge X-ray absorption fine structures, or NEXAFS) at the C K-edge (280-290 eV) and Ca L2,3-edge (344-356 eV) in STXM were used to determine the chemical identity of carbonate minerals and surrounding organic matter of the microbial mats. The results of the study show that dolomite nucleates in close association with the organic matter of the mats, where degradation is highest (defined in our adjacent study as the increase in C:N ratio). In TEM, polycrystalline dolomite is seen mineralizing in the matrix of the microbial mat organic material (Fig. 1). In STXM, the identity of the carbonate mineral changes from calcite on the outside to dolomite on the inside of the microbial mat particle (Fig. 2). In addition, our microsensor observations of elevated H2S concentrations, surface oxygenation from oxygenic phototrophy, high reduction potential, high organic carbon, high Mg:Ca ratio and high organic matter degradation (by C:N ratio) in each of the studied microbial mats confirms that the ideal dolomite mineralization conditions according to models of the dolomite problem are present in each case.

Low-Temperature Synthesis of Disordered Dolomite and High-Magnesium Calcite in Ethanol–Water Solutions: The Solvation Effect and Implications

How dolomite [CaMg(CO3)2] forms is still underdetermined, despite over a century of efforts. Challenges to synthesizing dolomite at low temperatures have hindered our understanding of sedimentary dolomite formation. Unlike calcium, magnesium’s high affinity toward water results in kinetic barriers from hydration shells that prevent anhydrous Ca–Mg carbonate growth. Previous synthesis studies show that adding low-dielectric-constant materials, such as dioxane, dissolved sulfide, and dissolved silica, can catalyze the formation of disordered dolomite. Also, polar hydrophilic amino acids and polysaccharides, which are very common in biomineralizing organisms, could have a positive role in stimulating Mg-rich carbonate precipitation. Here, we show that disordered dolomite and high-magnesium calcite can be precipitated at room temperature by partially replacing water with ethanol (which has a lower dielectric constant) and bypassing the hydration barrier. Increasing the ethanol volume percentage of ethanol results in higher Mg incorporation into the calcite structure. When the ethanol volume percentage increases to 75 vol %, disordered dolomite (>60 mol % MgCO3) can rapidly precipitate from a solution with [Mg2+] and [Ca2+] mimicking seawater. Thus, our results suggest that the hydration barrier is the critical kinetic inhibitor to primary dolomite precipitation. Ethanol synthesis experiments may provide insights into other materials that share similar properties to promote high-Mg calcite precipitation in sedimentary and biomineral environments.

Heritage Stone 8. Formation of Pinolitic Magnesite at Quartz Creek, British Columbia, Canada: Inferences from Preliminary Petrographic, Geochemical and Geochronological Studies

Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.

Transformation of protodolomite to dolomite proceeds under dry-heating conditions

The genesis of sedimentary dolomite remains an unresolved issue. Protodolomite has been considered as a metastable precursor for some sedimentary dolomites. Through laboratory experiments, much has been learnt about the transformation of protodolomite into dolomite under hydrothermal conditions mimicking those in open diagenetic systems. However, it is still unclear whether such mineral transformation could proceed in closed diagenetic systems, in which the supply of externally-derived fluids is often limited. Here through dry-heating experiments we demonstrated that low-temperature protodolomite converts into dolomite in the absence of external fluid. The starting materials for the recrystallization reactions included two types of protodolomite: biotic protodolomite and its abiotic counterpart. Biotic protodolomite was synthesized by means of a halophilic bacterium at 30°C. Since the synthesis of abiotic protodolomite normally requires higher temperatures than biotic ones, the abiotic protodolomite samples used herein were prepared at 60°C and 100°C. These protodolomites were spherical in shape and composed of nano-globular subunits. Our protodolomite samples contained considerable structural water in the range of 1.4-7 wt%. The water content of protodolomites was linearly correlated with their synthesis temperature, that is, biotic protodolomite had a higher amount of water than its abiotic counterparts. The protodolomite samples were then dry-annealed at temperatures of 100 to 300°C for two months. The results indicated that the rate of protodolomite-to-dolomite transformation was higher in the reactors using biotic protodolomite than those using abiotic protodolomites. This conversion was likely triggered by the dehydration of structural water within protodolomite. The resulting dolomite mostly retained spherical morphology, whereas its nanosized subunits tended to become rhombohedral. Calcite neoformation was also found to accompany the dolomite formation. Our findings suggest that structural water within protodolomite is an overlooked internal fluid and it might have an impact on the genesis of sedimentary dolomite during burial diagenesis.

Dissolved silica-catalyzed disordered dolomite precipitation

AbstractThere is a great abundance of sedimentary dolomite in the Proterozoic and Lower Paleozoic, but examples of primary dolomite are scarce in the Cenozoic. This discrepancy suggests a poorly understood but dramatic shift in the geochemical system that inhibited dolomite formation. Previous research on microbial-mediated dolomite formation demonstrated that microbial activity could promote disordered dolomite precipitation through the catalytic role of polysaccharides. However, the microbial-mediated model cannot explain some of the Precambrian dolomite for which there is no evidence of microbial origin. Here, we present an abiotic mechanism with dissolved silica catalyzed dolomite precipitation that provides new insight into this long-lasting “dolomite problem.” In this study, we demonstrate that the presence of 1–2 mM of aqueous Si(OH)4 in high Mg:Ca ratio solutions at room temperature will promote disordered dolomite precipitation (with up to 48.7 mol% MgCO3) and inhibit aragonite formation. Dissolved silica in solution also promotes Mg incorporation into the Ca-Mg carbonates. Dissolved silica possesses low-dipole moment and dielectric constant similar to hydrogen sulfide, dioxane, polysaccharide, and exopolymeric substances (EPS), which are catalysts in previously established room-temperature dolomite synthesis. The molecules with low-dipole moment adsorbed on the dolomite surface can lower the dehydration energy barrier of a surface Mg2+-water complex and promote dolomite nucleation and growth. This study provides a new model for abiotic sedimentary dolomite formation, which is likely to be responsible for the significant amount of primary dolomite in Earth history.

A New Model for Formation of Lacustrine Primary Dolomite by Subaqueous Hydrothermal Venting

AbstractAlmost 230 years after it was first described, formation of the mineral dolomite remains enigmatic. Dolomitic rocks are abundant throughout the geological record, and most are thought to form by replacement of limestone. Recent work has identified substantial volumes of high‐temperature primary dolomite deposited in Cretaceous lacustrine rift basins in Northern China. Here we provide the missing conceptual understanding of the fundamental processes that may have formed such dolomitic sediments by combining an analysis of the physics and chemistry of sub‐lacustrine hydrothermal systems with new data on the mineralogy and geochemistry of the dolomites. This novel mechanism accounts for systematic changes in the characteristics of the dolomite sequence that intimately link with the evolution of convection within the fault damage zone and venting of brine‐rich mixtures to the lake basin, and provides a new model to be tested against dolomitic sediments in high‐enthalpy settings such as rifting basins.

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic ArchaeaMethanosarcina barkeri: Implications for sedimentary dolomite formation

AbstractA correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.

Reservoir quality of tight oil plays in lacustrine rift basins: Insights from early Cretaceous fine-grained hydrothermal dolomites of the Erlian Basin, NE China

There has been increasing recognition of fine-grained dolomitic sediments in continental rift basins, but research has tended to focus on their petrography, geochemistry and genesis, rather than the pore characteristics that determine tight oil and gas reservoir quality. This paper presents a classification of pore types for dolomitic sediments influenced by hydrothermal processes from the early Cretaceous Erlian Basin, and examines relations between pore types to infer underlying genetic processes and controls as a precursor to predicting their distribution. We recognize three pore types: (1) primary porosity, which includes inter-crystalline pores between hydrothermal minerals within granules and nano-scale pores within fine matrix; (2) secondary dissolutional porosity, including inter-crystalline dissolved pores in granules, mouldic pores in laminae and bandings and selectively dissolved pores in argillaceous matrix, as well as large vugs in breccias; and (3) fracture porosity, including low-angle dissolved fractures, bedding-parallel microfractures and high-angle tectonic fractures. Both hydrothermal and hydrocarbon-related fluids caused significant dissolution, generating secondary pores that form the largest volume of reservoir storage. Channeling of aggressive fluids via low-angle fractures localized dissolution in the matrix and significantly enhanced permeability. Vertical fractures have a lesser impact on porosity but contribute to improved pore network connectivity. These relationships reflect dolomite accumulation within a tectonically-active extensional setting, with associated fault and fracture networks providing conduits for hydrothermal fluids. These fluids are responsible for formation of the dolomitic reservoir rocks and their diagenesis and may result in a characteristic suite of pore types. The high heat flux ensured rapid maturation of source rocks and may have played an important role in generating additional dissolutional porosity. Given the tight nature of reservoirs in such lacustrine systems, a systematic understanding of the nature and evolution of the pore network is key to identifying productive “sweet spots” in the vicinity of faults for de-risking future hydrocarbon exploration and also in developing such settings for geothermal energy.

Prograde and retrograde metasomatic reactions in mineralised magnesium-silicate skarn in the Cu-Au Ertsberg East Skarn System, Ertsberg, Papua Province, Indonesia

The 2.7–2.9 Ma Ertsberg East Skarn System (EESS) is a world-class Cu-Au skarn that formed within and adjacent to an intrusion within a paleodepth of 0.5 km and >2.5 km. Its economic mineralisation developed by sustained reaction of magmatic fluid with contact metamorphosed siliciclastic and carbonate rocks at the margin of the adjacent Ertsberg quartz monzodiorite intrusion. Based on high-resolution mineral mapping, chemical analysis and thermodynamic calculations, the multistage formation processes of the exoskarn components of the EESS are examined in the context of changing pressure, temperature, fluid composition and fluid phase. We show that contact metamorphism of dolomitic sediments occurred at 51 ± 5 MPa, between 700 °C and 800 °C and in the presence of a H2O-CO2-fluid containing ~10 to ~70 mol% CO2. This prograde metamorphism formed a forsterite + diopside + calcite + phlogopite + spinel assemblage. Such forsterite-dominated skarns account for ~55 vol% of the EESS exoskarns. Rare pargasite (previously unrecognized in this deposit) formed locally in the metamorphosed carbonate sequence where the protolith was composed of supratidal evaporites with dolomitic carbonate and interlayered calc-silicate rocks. The subsequent flux of a lower pressure magmatic gas containing SO2(g) caused sulphate metasomatism. This high temperature gas alteration of the metamorphic assemblage also caused skarn Cu-Fe-sulphide mineralisation. The influx of a SO2 gas through fracture permeability occurred at a temperature between ~600 and 700 °C and caused calcite to be replaced by anhydrite, with the coupled release of H2S(g). This in-situ release of H2S(g) scavenged trace Cu from the gas phase to deposit Cu-Fe-sulphides, which make the economic value of the distinct. We demonstrate that the formation of metal sulphides within forsterite skarns of the Ertsberg East Skarn System required a minimum flux of ~1,050 Mt SO2(g) and show that volcanic degassing may have occurred over a time span of ~3,900 years. As the system waned, the ambient fluid resulted in partial retrograde serpentinization of olivine and diopside without carbonation, and at temperatures sufficiently high to preserve anhydrite.

Catalytic effect of microbially-derived carboxylic acids on the precipitation of Mg-calcite and disordered dolomite: Implications for sedimentary dolomite formation

The genesis of dolomite is a contentious issue partly due to the difficulty in its synthesis at ambient temperature. Certain types of microorganisms have been demonstrated to be effective in promoting the precipitation of disordered dolomite, an important precursor of sedimentary ordered dolomite. In contrast to a growing body of research on the catalytic role of microbial exopolymers in the crystallization of disordered dolomite, the role of other microbial exudates (e.g., carboxylic acids) remains unknown. To fill in this knowledge gap, precipitation experiments, mimicking the carbonation process within microbial mats, were conducted in saline solutions containing 0–30 mM succinic acid or citric acid, which are commonly produced by microbes. The starting salinities of experiment solutions were set to 35‰ and 70‰, in order to evaluate the effect of solution salinity on dolomite formation. Our results showed that both succinic acid and citric acid enhanced the incorporation of Mg2+ into growing Ca-Mg carbonates. Solution salinity also played a positive role in enhancing Mg signature in Ca-Mg carbonates. Disordered dolomite with 40.92 mol% MgCO3 was detected under the conditions of 30 mM succinic acid and 70‰ salinity, whereas in other reactors with succinic acid, Mg-calcites formed. Citric acid was more effective in loading Mg2+ into Ca-Mg carbonates compared with succinic acid, as evidenced by the predominant occurrence of disordered dolomite with MgCO3 content ranging from 40.91 mol% to 46.75 mol% in most conditions tested. The results of this study have implications for the formation mechanism of sedimentary dolomite.

New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation

The oceanic magnesium budget is important to our understanding of Earth’s carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ26Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ26Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26Mg incorporation into the oceanic crust, on average by εsolid-fluid ≈ 1.6‰. These new data, along with the constant seawater δ26Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5–2.9 Tmol yr−1; 40–60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation.

Precipitation of protodolomite facilitated by sulfate-reducing bacteria: The role of capsule extracellular polymeric substances

The origin of dolomite has long puzzled geologists. It has recently been documented that sulfate-reducing bacteria (SRB) are capable of catalyzing the formation of protodolomite, a previously proposed precursor of ordered sedimentary dolomite. However, the catalytic mechanism of SRB remains incompletely understood. This experimental study is aimed at probing the effect of capsule extracellular polymeric substances (capsule EPS) from SRB on the crystallization of protodolomite in vivo. The capsule EPS tested herein was isolated from a protodolomite-mediating SRB, Desulfotomaculum ruminis, and added into a solution wherein the degree of oversaturation was close to the growth medium of D. ruminis at stationary phase. The solid products were characterized with X-ray diffraction (XRD), Raman spectroscopy and, scanning and transmission electron microscopy (SEM and TEM). Our results indicated that aragonite emerged in the reactors without capsule EPS, while Ca-Mg carbonates (Mg-calcite and protodolomite) were produced in the systems amended with capsule EPS. The incorporation amount of Mg2+ in Ca-Mg carbonates was enhanced with the increasing concentration of capsule EPS. The predominant occurrence of protodolomite was found in the reactor with 140 mg/L capsule EPS. These resulting protodolomites were spherical in shape, and composed of numerous nano-particles. The catalytic influence of capsule EPS on the precipitation of protodolomite might be attributed to their strong Mg2+ binding capacity, potentially diminishing Mg-hydration, which is a potent inhibitor of protodolomite crystallization. The results of Fourier transformation infrared (FT-IR) spectra showed that Mg2+ was bonded with carboxyl and hydroxyl groups on capsule EPS. This inferred adsorption capacity of capsule EPS was also supported by new calculations of complexation chemistry between Mg-H2O complex and organic compounds present in capsule EPS.

Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China.

The late Ediacaran Dengying Formation (ca. 551.1-538.8 Ma) in South China is one of two successions where Ediacara-type macrofossils are preserved in carbonate facies along with skeletal fossils and bilaterian animal traces. Given the remarkable thickness of carbonate-bearing strata deposited in less than 12.3 million years, the Dengying Formation holds the potential for a relatively continuous chemostratigraphic profile for the terminal Ediacaran stage. In this study, a detailed sedimentological and chemostratigraphic (δ13Ccarb, δ18Ocarb, δ13Corg, δ34Spyrite, and 87Sr/86Sr) investigation was conducted on the Dengying Formation at the Gaojiashan section, Ningqiang County of the southern Shaanxi Province, South China. Sedimentological results reveal an overall shallow marine depositional environment. Carbonate breccia, void-filling botryoidal precipitates, and aragonite crystal fans are common in the Algal Dolomite Member of the Dengying Formation, suggesting that peritidal facies were repeatedly karstified. The timing of karstification was likely early, probably soon after the deposition of the dolomite sediments. The presence of authigenic aragonite cements suggests high alkalinity in the terminal Ediacaran ocean. Geochemical analysis of micro-drilled samples shows that distinct compositions are registered in different carbonate phases, which should be considered when constructing chemostratigraphic profiles representative of true temporal variations in seawater chemistry. Integrated chemostratigraphic data suggest enhanced burial of organic carbon and pyrite, and the occurrence of extensive marine anoxia (at least in the Gaojiashan Member). Rapid basinal subsidence and carbonate accumulation during a time of elevated seawater alkalinity and increased rates of pyrite burial may have facilitated the evolutionary innovation of early biomineralizing metazoans.

Effects of early diagenesis on Mg isotopes in dolomite: The roles of Mn(IV)-reduction and recrystallization

Abstract Sedimentary dolomite plays an important role in global Mg cycling, and Mg isotopes in massive dolostones may be used to infer secular changes in seawater chemistry through geological history. However, sedimentary dolomite is generally regarded as a diagenetic product, and many details about the effects of early diagenesis on the Mg isotope composition of dolomite remain unclear. The mid-Cretaceous (Albian) Soreq and Givat Ye’arim formations near Jerusalem, Israel, contain exceptionally well-preserved massive dolostones, which provide an ideal opportunity to investigate the Mg isotope responses to early diagenesis. Dolomite samples from this section show large variations in δ13C values and Mn contents that are negatively correlated, reflecting degradation of organic matter and mineralization of organic carbon in the Mn(IV) reduction zone within soft sediment during dolomite formation. This is a rare example of a clear link between Mn(IV) reduction and dolomite precipitation based on geochemical evidence in the rock record. The dolomite samples also exhibit large variations in d(104) values and the degree of cation ordering. The latter is negatively correlated with Sr contents, implying that variable degrees of dolomite recrystallization occurred during diagenesis. δ26Mg values of >50 dolomite samples from this section vary from −2.28‰ to −1.78‰, but do not correlate with indicators of organic matter degradation (δ13C values and Mn contents) or dolomite recrystallization (e.g., Sr contents), suggesting that Mg isotopes behave conservatively after initial dolomite (or proto-dolomite) precipitation during the very early stages of diagenesis. We propose that the Mg isotope composition of dolomite formed due to Mn(IV) reduction is buffered by seawater due to the shallowness of the Mn(IV) reduction zone in platform sediments, which is different from the dolomitization associated with bacterial sulfate reduction or methanogenesis, where Mg supply can be diffusion-limited. Furthermore, Mg isotopes in dolomite are robust to resetting by recrystallization during burial. Magnesium isotope compositions of platform dolomite that show variable and negatively correlated δ13C values and Mn contents can, therefore, be considered robust archives for reconstructing paleo-seawater Mg isotope compositions. The variation in δ26Mg values of the dolostones in the Soreq and Givat Ye’arim formations is interpreted to reflect Rayleigh fractionation in response to dolomitization in a restricted water body. Therefore, the lowest δ26Mg value is considered to be the Mg isotope composition of dolomite that was in equilibrium with coeval seawater in the open ocean, and thus the δ26Mg value of Albian seawater was around −0.4‰.

Natural factors forming the quality of carbonate raw materials

The article is devoted to the study of natural factors that influence the regularities in the distribution of the quality of carbonate raw materials in the deposits of the Articulation Zone of the Donbass Basin with the Priazovskiy block of the Ukrainian Shield. Paleogeographic conditions of sedimentation and the processes of secondary quality change were established by comparing the lithologic-stratigraphic sections of deposits in different parts of the Articulation Zone: from the west (Novotroitskoe deposit) through the center (Elenovskoe deposit) and to the east (Karakubskoe deposit). The variability in the quality of carbonate raw materials was estimated from the results of statistical processing of the chemical analysis of exploratory samples. Geochemical associations were established for carbonate rocks within the stratigraphic horizons of each deposit. The best in quality of all deposits of the Donbass Articulation Zone are the carbonate rocks of the Cl t c horizon. The conditions for their formation were the most stable. They are characterized by a negative connection of the main quality component (CaO) with all other quality indicators. At the same time, the association of indicators associated with a significant positive connection is retained: SiO2, R2O3, S.The process of forming carbonate strata are accompanied by intense silicification processes that relate to climatic changes in temperature, salinity and depth of the sea basin. This is confirmed statistically, because the significant negative correlation between the main component of quality and silica is established on the horizons of carbonate rocks with an increased silica content. The process of dolomitization of the carbonate stratum took place in two stages. At the beginning of the accumulation of the carbonate layer, the sea basin was characterized by lower temperatures and higher magnesium contents than in the subsequent period. Therefore, the lower stratum of the Novotroitskoe and Elenovskoe deposits include sedimentary dolomites. Later, the conditions changed, so there are no dolomites in the Karakubskoe deposit. The second stage of dolomitization is associated with the post-carbon andesite-trachyandesite magmatic complex. The rocks of this complex are found in the carbonate stratum in the form of interplastic intrusions, dikes and stock-like bodies. This contributed to the appearance of sulphide mineralization, zones of silicification and recrystallization of carbonate minerals in spot-dolomitized limestones. The structural-tectonic factor, which is manifested by intensive leaching processes, exerts the greatest influence on the degradation of the quality of carbonate raw materials. In zones of tectonic disturbances, they have maximum extension areas. The structural transformationsthat began in the Donbas after the early Carboniferous disrupted the primary stratified occurrence and quality of the carbonate thickness. The upper Visean horizons are subject to intensively leached, similar to tectonic,melange.The contribution made by these studies to scientific knowledge lies in the identification of the natural factors that determined not only the quality of carbonate rocks, but also indicators of the development stages of the Donbass Articulation Zone of two major tectonic structures - Donbass and the Ukrainian Shield. The practical significance of the results of the work is connected with the fixed possibility of conducting geological and technological mapping and further construction of varietal maps of the deposits of carbonate raw materials.