Stoichiometric Dolomite Research Articles

Evaporite Carbonate Cycles of the Messinian, Sicily: Stable Isotopes, Mineralogy, Textural Features, and Environmental Implications

ABSTRACT An investigation of the genetic environment of laminar and massive gypsum and intercalated carbonates (Messinian, central Sicily) has been carried out by means of isotopic, textural, and mineralogical analyses. The oxygen and hydrogen isotope compositions of crystal water from the laminar gypsum indicate primary precipitation of the sulfate from marine waters concentrated by evaporation. The isotopic data also show compositional changes due to periodic influx of continental waters into the deposition basin. In the laminar gypsum and interlayered dolostones we find a micritic, near-to or stoichiometric dolomite showing very positive 18O and negative 13C values which are consistent with formation of this carbonate, at a very early diagenetic stage, by precipitation from highly evaporated interstitial waters depleted in SO4-- ions by gypsum cr stallization and concurrent bacterial SO4-- reduction. Open-marine-water incursions probably led to the deposition of carbonate layers overlying the laminar gypsum. These contain abundant aragonite whose textural features and isotopic composition suggest precipitation in shallow and quiet marine waters slightly enriched in 18O by evaporation. The isotopic data of a euhedral and Ca-rich dolomite, formed later than the aragonite, may reflect further influxes of continental waters into the basin. On the basis of 18O and D values, the massive gypsum in the upper portion of the sedimentary sequence seems to have been formed by hydration of anhydrite.

Dissolution kinetics of dolomite: Effects of lithology and fluid flow velocity

The dissolution kinetics of three stoichiometric dolomite specimens (hydrothermal single crystal, microcrystalline sedimentary rock, coarse-grained marble) were studied in aqueous carbonate solutions. Hydrodynamic conditions were controlled through use of a rotating dolomite disk in which one face was exposed to solution and fluid flow regime was defined by spinning rate. The resulting mass transfer properties were uniform across the disk surface. The dissolution experiments were begun at an initially undersaturated condition set by CO 2 at ~ 1 atm dissolved in deionized water. The reaction was followed by measuring concentrations of Ca 2+, Mg 2+, HCO 3 −, and pH over time in a free-drift type of experiment at 0, 15, and 25°C. Dissolution rates for all three samples were similar in form and value; grain size effects were insignificant. Ca/Mg was constant throughout each run at 0.81–0.96. From initial conditions, the dissolution rate decreased as the solution became more saturated. At solution conditions still far from equilibrium (ion activity product = 10 −19), rate dropped off sharply to a very low value. Surface morphology, determined by SEM, showed deep narrow holes in the single crystal, while the rocks dissolved along grain boundaries. These features suggested preferential dissolution of energetically favored sites and surface reaction rate control. Initial rates were used to calculate an apparent activation energy of 32 kJ mol −1 (sedimentary dolomite) and 27 kJ mol −1 (single crystal). Initial dissolution rates at 25°C and pH ~ 4 for all samples varied with spinning speed and ranged from 1–3 μmol m −2 s −1 for laminar flow conditions to almost 3–6 μmol m −2 s −1 as the transition to turbulence began. At lower temperatures, the rate was lower, and increasing spinning velocity had less effect. The strongest spinning rate dependence occurred far from equilibrium, and it became a less important factor as the saturation state increased.

Burial Diagenesis of Allochthonous Carbonates from a Permian Slope Setting, Southeastern New Mexico: ABSTRACT

The Bone Spring formation (Permian-Leonardian) from 3-km (9,850-ft) deep conventional cores in the northern Delaware basin is a laminated, black, mixed terrigenous-carbonate mudstone with thin intervals of gray, coarse carbonate-debris flows. All carbonates have been pervasively dolomitized, and porosity and permeability were reduced to 3-8% and 1 md, respectively, during several diagenetic events related to burial history. Three dolomite generations are defined by fluorescence and cathodoluminescence microscopy and electron microprobe, and are further characterized by means of microsampling for carbon and oxygen isotopes. Over a range of 2 ^pmil (PDB), ^dgr13C decreases steadily from the first to last dolomite generation, reflecting constant mixing of rock carbon and organic carbon as hydrocarbons were evolving in the Bone Spring muds. The first dolomite generation was introduced during the time interval between early postdeposition and burial to approximately 1 km (3,280 ft). Dewatering and compaction of the Bone Spring muds accompanied matrix dolomitization, leaching of metastable grains, fracturing, and silicification of carbonate components. The mean ^dgr18O of the high-stronti m, matrix dolomite is -2.1 ± 0.4 ^pmil (PDB). A second dolomite generation may have been late Ochoan; by this time an additional 1.2 km (3,925 ft) of Guadalupian-Ochoan carbonates and evaporites had been deposited. This fluorescent, nearly stoichiometric dolomite cement has a mean ^dgr18O of -3.0 ± 0.3 ^pmil (PDB), and was coeval with hydrocarbon generation in the Bone Spring. It precipitated from fluids that interacted with the overlying evaporites. Hydrocarbon inclusions are contained in this dolomite, which is succeeded by a generation of anhydrite cement. Sulfur isotopes strongly suggest that remobilized Guadalupian anhydrite-sulfur formed this cement. The last two diagenetic phases, including the third dolomite generation, crystallized at about present burial depth. Coarsely crystalline, luminescent, pore-filling calcian dolomite, containing hydrocarbon inclusions, has a mean ^dgr18O of -5.4 ± 0.6 ^pmil (PDB). Depleted poikilotopic calcite cement (^dgr18O = -10.3 ^pmil, PDB) records the input of Tertiary water. End_of_Article - Last_Page 316------------

Coherent transformations in calcian dolomites

Many sedimentary calcian dolomites commonly display a modulated microstructure when viewed with the transmission electron microscope. This structure has been attributed to a disordered incorporation of excess calcium in the dolomite structure. We describe here two more highly ordered metastable superstructures (γ and δ dolomite) that are considered intermediate between disordered calcian dolomite as formed by dissolution-reprecipitation processes and—probably as a result of diagenesis—decomposition to stable stoichiometric dolomite and calcite. Small platelike regions of coherent calcite are observed in dolomite from the Devonian Lost Burro Formation in southeastern California and in heat-treated calcian dolomite. These observations document the evolution of sedimentary carbonates from metastable to stable phases under favorable kinetic conditions.

Rock Composition, Dolomite Stoichiometry, and Rock/Water Reactions in Dolomitic Carbonate Rocks

Dolomitic carbonate rock units from a variety of Phanerozoic localities and depositional environments throughout North America were evaluated with respect to their dolomite content and dolomite composition by X-ray diffraction analysis. Dolomite compositions range from 48 to 57 mole% $$CaCO_{3}$$ with well-defined modes at 51% (nearly stoichiometric dolomite) and 55% (distinctly calcian dolomite) $$CaCO_{3}$$. Values of percentage of dolomite also exhibit a pronounced, albeit less well-defined, bimodal distribution with modes at 97% (dolostones) and 20% (dolomitic limestones) dolomite. Similar distributions characterize most other published data on Phanerozoic dolomitic carbonates. These data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite: (1) dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution-calcite and dolomite precipitation reactions, and (2) dolostones originate in diagenetically open systems during th...

Experimental Hydrothermal Dedolomitization: ABSTRACT

Hydrothermal replacement of dolomite by calcite has been examined through experimental reactions carried out in teflon-lined stainless steel bombs, at temperatures ranging from 50° to 200°C (122° to 392°F), at End_Page 456------------------------------ equilibrium vapor pressure, and for durations of days to months. Solution compositions were varied to examine the effects of ionic strength, Mg2+/Ca2+ ratio, and SO42- concentration on the direction and/or rate of reaction. The starting solid for these experiments was a stoichiometric and well-ordered synthetic dolomite; several experiments were also seeded with solid reagent-grade calcium carbonate (calcite). In pure CaCl2 solution (0.1 M) replacement was complete in less than one week at temperatures of 100°C (212°F) and greater. Replacement was minor after several weeks at 50°C (122°F). Longer term experiments at 50° are still underway. A series of experiments established that dedolomitization only occurs at Mg2+/Ca2+ ratios of less than 1/10 (total ionic strength = 0.1) at all temperatures from 100° to 200°C (212° to 392°F). Because this reaction proceeds far faster than the reverse reaction (dolomitization), we believe this result establishes the most precise calcite/dolomite phase boundary to date. The presence of SO42- was found to strongly inhibit the dedolomitization reaction, even a concentrations far less than in seawater. Seeding the reaction with a small amount of calcite had little effect on the rate of reaction, suggesting that nucleation is not rate-limiting in dedolomitization. Finally, increasing ionic strength from 0.01 up to 0.5 increased the rate of reaction. End_of_Article - Last_Page 457------------

Selected geochemical factors influencing diagenesis of eocene carbonate rocks, peninsular Florida, U.S.A.

The geochemical significance of three selected ions (Mg 2+, Na +, and Sr 2+) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg 2+ to facilitate dolomitization ( Mg Ca ratios 1 ). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity. Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr 2+ on the average and calcite has 418 ppm Sr 2+. The Sr 2+ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo 3 and 195 ppm for the dolomite with 44–50 mole-% MgCO 3. The difference in the Sr 2+ concentrations of the two dolomite groups indicates the higher mole-% MgCO 3 dolomite recrystallized in a less saline environment than the lower mole-% MgCO 3 dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment. The more nearly stoichiometric dolomite (44–50 mole-% MgCO 3) has less scatter when mole-% MgCO 3 is plotted against Sr 2+ and Na +. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO 3 (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.

Diagenesis, Geochemistry, and Origin of a Precambrian Dolomite: the Beck Spring Dolomite of Eastern California

The Beck Spring Dolomite is a platform carbonate sequence dominated by cryptalgal laminites with some stromatolites and grainstones and is inferred to have been deposited in shallow subtidal to intertidal environments. Cavity structures, where the cements occur, include birdseyes, laminoid fenestrae, stromatactis, sheet cracks, and neptunian dikes. Four types of synsedimentary-early diagnetic fibrous dolomite are distinguished. Type A forms isopachous fringes consisting of length-slow acicular to columnar pseudopleochroic crystals with fine, substrate-parallel color banding. The dolomite is mostly nonluminescent with some bright blotches, and inclusion trains, define acicular crystallites within the columnar crystals. Type B consists of length-slow columnar crystals with distinctive bladed relic structures, defined by inclusions and nonluminescence, surrounded by inclusion-free and brightly luminescent dolomite. Type C consists of bladed to wedge-shape pseudopleochroic crystals with undulose extinction (mostly fascicular optic); inclusion trains define acicular crystallites within crystals, and forms mammillated crusts and botryoids. Type D fibrous dolomite forms thin layers of acicular crystals, with a distinctive geometry of the cement layers thickening into the corners of cavities. Types B and C contain relic structures indicating replacement, but types A and D have fabrics which could be primary. The depositional components (pisolites and micrite) and some fibrous dolomites have oxygen isotopic values around -2 per mil versus PDB, compatible with a marine origin. However, Na (around 220 ppm) and Sr (around 65 ppm) contents are very low, suggesting precipitation from dilute waters. Meteoric-marine mixing-zone dolomitization of CaCO 3 sediments and early cements. Or primary marine precipitation of poorly ordered calcian dolomite and early diagenetic stabilization to stoichiometric dolomite. The dolomite spar has very negative values of delta 18 O (-9.6 per mil versus PBD) and is more depleted in Na (34 ppm) and Sr (47 ppm), reflecting precipitation from meteoric waters of very low ionic strength during burial diagenesis. A replacement origin for the dolomite of the depositional components and fibrous cements can be reconciled with the lack of obliterative replacement fabrics invoking an original high-Mg calcite mineralogy.--Modified journal abstract.

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.

Electron optical investigation of sedimentary dolomites

Transmission electron microscope studies demonstrate a structural distinction between stoichiometric and ancient calcian dolomites of sedimentary origin. A modulated or tweed structure similar in appearance to spinodal microstructures characterizes ancient calcian dolomites, whereas stoichiometric dolomites are homogeneous. Recent calcium-rich dolomites lack a modulated structure, but have a heterogeneous microstructure with high densities of growth defects. Electron and optical diffraction and lattice imaging suggest a fluctuation in interplanar spacing in a coherent lattice. In addition, lattice imaging also reveals discontinuous faults throughout the modulated structure. Ordering and possibly incipient exsolution are suggested, ‘c’ reflections in SAD patterns of calcian dolomites document a new superstructure in at least part of the lamellar structure and raise questions of different ordering states throughout the structure.

Dolomite Nonstoichiometry; Its Relation to Carbonate-Rock Fabric: ABSTRACT

Nearly 300 samples of carbonate rocks representing all pre-Tertiary periods (except Cambrian) were analyzed for their dolomite stoichiometry by X-ray peak displacement. These samples represent a wide variety of fabric types (0 to 100% micrite, and boundstone), and a wide variety of depositional environments (shelf edge, subtidal shelf interior, shore zone, and deep marine). The percent calcium carbonate in the dolomite lattice ranged from 48.67 to 57.93, and was fairly uniformly distributed over the range 50.0 to 56.0%. There does not appear to be any clear relation through time and over wide geographic areas between the percent calcium carbonate in the dolomite lattice and (1) facies; (2) total percent dolomite; (3) degree of recrystallization; (4) spar-crystal size; (5) total fossil content; (6) percentages of gastropods, brachiopods, bryozoans, and echinoids; (7) percent insoluble residue; and (8) percent visible porosity (in thin section). There does appear to be a trend to more nearly stoichiometric dolomite (50% calcium carbonate) with increasing age, but this generalization has many exceptions. Results from samples of geologically related suites do show systematic facies-related patterns, but the trends vary and even reverse from suite to suite. We conclude that dolomite nonstoichiometry may be a useful parameter in facies analysis of geologically related (time and space) sets of samples, but it shows no unambiguous facies-related trends over the geologic record. End_of_Article - Last_Page 431------------

Dolomitization of Lower Paleozoic Burrow-Fillings

ABSTRACT Dolomite in an upper Ordovician sequence composed of the Irene Bay and Thumb Mountain Formations on Devon Island in the Canadian Arctic Archipelago is confined to burrows. Micron-sized dolomite crystals may have formed in burrows contemporaneous with deposition because of seasonal salinity changes in the Irene Bay-Thumb Mountain shelf lagoon. These penecontemporaneous dolomite crystals formed nuclei for the selective precipitation of late diagenetic dolomite from dilute subsurface solutions. Post-lithification crystal growth during late diagenesis caused idiotopic dolomite fabrics with intercrystalline micrite to become more coarsely crystalline xenotopic fabrics with no intercrystalline micrite. Dolomite crystal growth was accompanied by a progressive decrease in strontium and sodium contents and by a lowering of the amount of excess calcium. A possible rule regarding dolomite compositional variations is that high Mg/Ca solution ratios favor precipitation of more stoichiometric dolomite whereas low Mg/Ca solution ratios favour precipitation of calcium-rich dolomite with the exception that solutions of very low salinities will precipitate stoichiometric dolomite regardless of their Mg/Ca solution ratios.

Origin and Controls on Distribution of Arid Supratidal (Sabkha) Dolomite, Abu Dhabi, Trucial Coast: ABSTRACT

Holocene dolomites in supratidal carbonate beds south of Abu Dhabi are polygenetic. The overall distribution of most authigenic is controlled primarily by a pre-Holocene Pleistocene sand topography. Two types of dolomites are present: (1) a series of disordered calcium-rich dolomites (Ca52Mg48-Ca57Mg43), and (2) a disordered stoichiometric (Ca50Mg50). Dolomitic carbonate muds contain various combinations of calcium-rich species and/or stoichiometric dolomite. Calcium-rich dolomites have two origins. A major part is formed by reaction of aragonite and magnesium calcite with seawater-derived brines (marine dolomite), and a minor but locally significant part is formed by reaction of aragonite and calcite with continental groundwaters. The stoichiometric is detrital and of wind-blown origin. Authigenic dolomites form the major fraction, but locally 25% is detrital dolomite. Distribution of authigenic marine dolomite is controlled by interrelated factors of (1) sedimentary facies, (2) surface relief, and (3) frequency of seawater flooding. These factors are in turn controlled by the pre-Holocene sand topography. Dolomitization is most extensive (to 100% dolomite) in buried, former intertidal sediments composed of an algal-laminated aragonitic mud facies and a burrowed aragonitic mud facies. Rocks in the latter facies are more completely dolomitized, probably because of their greater permeability. An underlying subtidal skeletal aragonitic mud facies is only partly dolomitized (5-15% dolomite). The dolomitized buried facies attain their greatest areal distribution underlying elongate supratidal areas of low relief. These areas, because of their relief and orientation, are frequently flooded with sea-water resulting in input of brines with mCa2+/mMg2+ ratios > 30 for dolomitization. These supratidal areas of low End_Page 332------------------------------ relief are ancestral to tidal channels cut into the underlying Pleistocene sandstone beds. End_of_Article - Last_Page 333------------

Precipitation of calcian dolomites and magnesian calcites in the southeast of South Australia

Magnesian calcites and calcian dolomites have been observed in active simultaneous precipitation from shallow saline waters. The total magnesium precipitating varies with geographic location. "Protodolomite" and stoichiometric dolomite found in suspension in the accumulated lagoonal sediments are discussed at length. Celestite, SrSO ₄ is also found to precipitate along with the carbonate phases. The area appears to be a high-strontium locality. The precipitation is suggested to be controlled by plant photosynthesis.

Subsolidus Phase Relations in the System CaCO3-MgCO3

The solid solubility, disorder, and decomposition relationships in the system CaCO3-MgCO3 were investigated in the pressure range of from 10 to 10,000 bars and at temperatures of from 700° to 1,200° C. A sealed-tube technique was used for the most part in an internally heated pressure vessel designed for rapid operation. The top of the solvus outlining the two-phase field of magnesian calcites and dolomite is at a temperature of 1,075° C. at the composition Ca57Mg43, and above the solvus a single-phase region extends from CaCO3 to CaMg(CO3)2. The solubility gap between CaMg(CO3)2 and MgCO3 is considerably larger, and solubility of Ca++ in MgCO3 and Mg++ in CaMg(CO3)2 is quite limited at 1,100° C. Cation disorder in stoichiometric dolomite becomes observable at approximately 1,000° C. and is complete at approximately 1,200° C., at which temperature CaMg(CO3)2 has the calcite-type structure. There is no evidence for anything but a continuous (higher-order) transformation over this 200° C. interval. Disorder...