Grain growth kinetics of dolomite,magnesite and calcite: a comparative study

The rates of grain growth of stoichiometric dolomite [CaMg(CO₃)₂] and magnesite (MgCO₃) have been measured at temperatures T of 700–800°C at a confining pressure P _c of 300 MPa, and compared with growth rates of calcite (CaCO₃). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing (HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 μm, respectively, for CaMg(CO₃)₂, MgCO₃, and CaCO₃, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing, with mean values reaching 3.9, 5.1, and 27 μm for CaMg(CO₃)₂, MgCO₃, and CaCO₃, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (≃5 × 10⁻⁵ μm³/s) at T = 800°C is less than that for magnesite by a factor of ~30 and less than that for calcite by three orders of magnitude. Variations in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries, with implications for deformation processes, rheology, and reaction kinetics of carbonates.     

Matrix rheology effects on reaction rim growth I: evidence from orthopyroxene rim growth experiments

The rim-forming reaction quartz + olivine = orthopyroxene is used to investigate the effect of matrix rheology on rim growth rates. Orthopyroxene rim growth around olivine grains in quartz matrix is compared to rim growth around quartz grains in an olivine matrix. At constant P–T , within one single capsule, orthopyroxene rims grow faster around quartz clasts in olivine matrix than around olivine clasts in quartz matrix. Fourier transform infra-red spectra indicate that the entire samples are water saturated because of water adsorption on the reactant grain surfaces. The increased orthopyroxene growth rates in olivine matrix as opposed to quartz matrix are interpreted in terms of matrix rheology, where in the two different matrix-inclusion arrangements the olivine matrix behaves 'softer' and the quartz matrix 'more rigid'. The strain energy associated with accommodation of the negative reaction volume is higher for the quartz than the olivine matrix and reduces the free energy that drives orthopyroxene rim growth. Growth textures in both kinds of orthopyroxene rims indicate that the diffusivity of MgO slightly exceeds the diffusivity of SiO₂. The relative mobility of MgO and SiO₂ at given P , T , f H₂O seems to be controlled by energy minimization during orthopyroxene growth at the compressive Ol/Opx interface. Our experiments provide evidence for two previously overlooked effects relevant to rim growth reactions in metamorphic rocks: (i) diffusivity along chemical potential gradients to reaction sites is a function of rheology and (ii) the relative diffusivity of components during reaction rim or corona growth is a function of local volume changes at the rim's interfaces.     

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 CO₂ at ~ 1 atm dissolved in deionized water. The reaction was followed by measuring concentrations of Ca²⁺, Mg²⁺, HCO₃^?, 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.     

Self-limiting growth on dolomite: Experimental observations with in situ atomic force microscopy

In situ Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) studies on dolomite (101?4) were performed during exposure to supersaturated aqueous solutions (supersaturated in dolomite, calcite, aragonite, vaterite, huntite and magnesite) at pH = 9 at various Ca²⁺/Mg²⁺ aqueous ion activity ratios. At high saturation ratios, rapid growth of a single layer (∼3 Å thick) of a carbonate followed by much slower growth of a second layer was observed. Growth of the second layer was highly inhibited, suggesting that the first layer was essentially self-limited, and inhibited further layer-by-layer growth. The growth of the first layer was observed over a wide range of Ca²⁺/Mg²⁺ ratios, suggesting that the dolomite surface is favorable to formation of a range of Ca-Mg carbonates. LFM data revealed contrast in the tip-surface frictional forces on the first grown layer, but this contrast was only observed in layers grown from middle to high Ca²⁺/Mg²⁺ solutions. Thus, LFM may have detected or responded to differences in the structure and/or composition between the first layer relative and the dolomite substrate. Dissolution of the first layer occurred from significantly supersaturated solutions relative to ordered stoichiometric dolomite permitting an estimate of the excess interfacial strain energy of up to 10 mJ/m².     

Two stage growth of microdiamond in UHP dolomite marble from Kokchetav Massif, Kazakhstan

The abundance and morphology of microdiamond in dolomite marble from Kumdy‐kol in the Kokchetav Massif, are unusual; a previous study estimated the maximum content of diamonds in dolomite marble to be about 2700 carat ton^?1. Microdiamond is included primarily in garnet, and occasionally in diopside and phlogopite pseudomorphs after garnet. They are classified into three types on the basis of their morphology: (1) S‐type: star‐shaped diamond consisting of translucent cores and transparent subhedral to euhedral very fine‐grained outer parts; (2) R‐type: translucent crystals with rugged surfaces; and (3) T‐type: transparent, very fine‐grained crystals. The S‐type is the most abundant. Micro‐Laue diffraction using a 1.6‐µm X‐ray beam‐size demonstrated that the cores of the star‐shaped microdiamond represent single crystals. In contrast, the most fine‐grained outer parts usually have different orientations compared to the core. Laser–Raman studies indicate that the FWHM (Full Width at Half Maximum) of the Raman band of the core of the S‐type diamond is slightly larger than that for the outer parts. Differences in morphology, crystal orientations, and in the FWHM of the Raman band between the core and the fine‐grained outer‐parts of S‐type microdiamond suggest that the star‐shaped microdiamond was formed discontinuously in two distinct stages.     

A urolith of biogenic dolomite—another clue in the dolomite mystery

A male Dalmatian, Canis familiaris, produced uroliths of almost pure dolomite, 3–8 mm across, in his urinary bladder in less than 8 months at 38°C and about 1 atm. The X-ray diffractogram identified the predominant mineral as dolomite, and the sharp (01.5) peak showed it is ordered dolomite, not the disordered form, protodolomite.Geochemically and biologically plausible causes include (1) renal, respiratory, or metabolic alkalosis, (2) infection by urease-producing (urea-splitting) fungi or bacteria and (3) infection by uric acid-fermenting bacteria. Hematological, bacteriological, urological and geochemical considerations most strongly implicate infection by either anaerobic, urease-producing bacteria or anaerobic, uric acid-fermenting bacteria.The physical and chemical conditions of this urinary system more closely approximate modern and inferred ancient carbonate depositional settings than most previous laboratory experiments, especially in terms of temperature, pressure, total salinity and, possibly, biota. The presence of urease-producing and/or uric acid-fermenting bacteria in urea- and/or acid-containing sediment, such as fecal pellets and algal mats, could promote formation of authigenic dolomite or other carbonates.     

Matrix rheology effects on reaction rim growth II: coupled diffusion and creep model

Chemical reactions and phase changes generally involve volume changes. In confined settings this will cause a mechanical deformation of the matrix that surrounds the reaction sites where the volume change takes place. Consequently, mineral reactions and the mechanical response of the rock matrix are coupled. A companion paper in this issue illustrates this coupling with experiments where quartz and olivine react to form enstatite reaction rims under ambient conditions of 1 GPa and 1000 °C. It has been demonstrated that for identical run conditions, the thickness of the reaction rims depends on whether quartz grains are embedded in an olivine matrix or olivine grains are included in a quartz matrix. The experimental conditions, the nature of the results, and the large volume change of the reaction (?6%) leave only viscous creep as a viable matrix response to the reaction progress. A model is developed for this reaction, which combines diffusion of chemical components through the growing rim and viscous creep of the matrix. The resulting rate law for reaction rim growth in spherical geometry shows that the progress rate is proportional to the reaction overstepping and controlled by the slower of the two competing processes; either diffusion or creep. If diffusion is rate limiting the usual linear proportionality between rim growth and results. However, if viscous creep is rate limiting, then the reaction rates are reduced and may become effectively creep controlled. With respect to the experiments in the companion paper it is inferred that the effective viscosity of the two matrix materials, i.e. polycrystalline quartz and olivine, differ by approximately one order of magnitude with the quartz being the stronger one. The absolute values of the inferred viscosities correspond well to published flow laws. The rheological properties of natural rocks are well within the parameter range for which significant mechanical control on reaction rim growth is expected. This implies that for the interpretation of natural reaction rims and corona structures both diffusion and mechanical control need to be considered. In addition the mechanical effect also needs to be considered when interdiffusion coefficients are retrieved from rim growth experiments. This should also be considered for geospeedometry analyses. Furthermore, the control on reaction rate because of slow creep of the matrix is expected to be even more important, compared to the experiments, under colder crustal conditions and may contribute substantially to the frequent observation of only partially completed reactions. We suggest that this phenomenon is referred to as ‘mechanical closure’, which may be an important mechanism in the kinetic displacement of the boundaries between the stability fields of phase assemblages.     

A tem microstructural study of dolomite with curved faces (saddle dolomite)

Electron diffraction, analytical electron microscopy, and high voltage, high resolution electron microscopy have been used to investigate crystal defects in calcium-rich saddle dolomites having pronounced curvature of the faces. Results show that branching, ribbon-like defects in these so-called saddle dolomites are thin, coherent laths of calcitic material. The ribbons are profuse and explain the characteristic calcium excess found in most saddle dolomites. Because the lattice spacings of calcite are between 3.8% and 6.7% larger than the corresponding lattice spacings of dolomite, a calcitic ribbon causes local distortion of the host dolomite. The branching ribbons have a predominant {10¯14} orientation and are generally present in high density. They may represent the source of crystal distortion that ultimately manifests itself on the macroscopic scale. The calcitic ribbons form during growth from aqueous solution, although they have features in common with similar defects found in carbonatite carbonates. This fine-scale intergrowth microstructure may be a variant of even finer-scale modulated structures found in other sedimentary calcian dolomites.     

The incongruent solution of dolomite

Two-mineral solubility diagrams are presented for the system calcite: calcium-rich dolomite which show the effect of calcite on the solubility of calcium-rich dolomite and conversely. These diagrams illustrate the incongruent solution of calcium-rich dolomite at low temperatures and the evolution of solutions of calcite and dolomite in a convenient way. They also show how calcium-rich dolomite may appear to be more soluble than calcite under some conditions.     

Chemical kinetics, speleothem growth and climate

The morphology and stratigraphy of speleothems are controlled by parameters that depend on climate. These are the water supply rates feeding the speleothem, e.g. a stalagmite, the growth rates dependent on the chemical kinetics of calcite precipitation and the supersaturation of the solution from which calcite is precipitated. To elucidate the basic principles of speleothem growth, a physical-chemical model of calcite precipitation is used to estimate growth rates under various geologically relevant conditions. Furthermore, we present a model that allows the computation of the growth history of stalagmites, i.e. their morphology and stratigraphy under varying climatic conditions. This enables us to see how climatic signals are inscribed into stalagmites. Owing to the counter-balancing effects of some parameters, it is not possible to read climatic conditions backwards from the morphology and stratigraphy of a speleothem in a simple way, but a basic understanding of the growth of speleothems can be a helpful supporting tool in the interpretation of palaeoclimatic records.     

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The dolomite problem has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite.We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase.Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.     

Mineralogy,nucleation and growth of dolomite in the laboratory and sedimentary environment: A review

Dolomite [CaMg(CO₃)₂] forms in numerous geological settings, usually as a diagenetic replacement of limestone, and is an important component of petroleum reservoir rocks, rocks hosting base metal deposits and fresh water aquifers. Dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of Ca²⁺ and Mg²⁺ cations interspersed with anion layers normal to the c‐axis. Dolomite has symmetry, lower than the (CaCO₃) symmetry of calcite primarily due to Ca–Mg ordering. High‐magnesium calcite also has symmetry and differs from dolomite in that Ca²⁺ and Mg²⁺ ions are not ordered. High‐magnesium calcite with near‐dolomite stoichiometry (≈50 mol% MgCO₃) has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high‐magnesium calcite. Many dolomites display some degree of cation disorder (Ca²⁺ on Mg²⁺ sites and vice versa), which is detectable using transmission electron microscopy and X‐ray diffractometry. Laboratory syntheses at high temperature and pressure, as well as studies of natural dolomites show that factors affecting dolomite ordering, stoichiometry, nucleation and growth include temperature, alkalinity, pH, concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of the carbonate being replaced, and surface area available for nucleation. In spite of numerous attempts, dolomite has not been synthesized in the laboratory under near‐surface conditions. Examination of published X‐ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near‐ambient conditions by microbial mediation are unsubstantiated. These laboratory products show no evidence of cation ordering and appear to be very high‐magnesium calcite. Elevated‐temperature and elevated‐pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high‐magnesium calcite formation. It remains to be demonstrated whether microbial‐mediated growth of very high‐magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high‐temperature experiments.     

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.     

Direct physical evidence of dolomite recrystallization

The possibility of recrystallization is a long‐standing barrier to deciphering the genetic origin of dolomites. There is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event. Results from 65 new high‐temperature dolomite synthesis experiments (1 m , 1·0 Mg/Ca ratio solutions at 218°C) demonstrate dolomite recrystallization affecting stoichiometry, cation ordering and nanometre‐scale surface texture. The data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization, which occur by various dissolution–precipitation reactions. During the first phase (induction period), no dolomite forms despite favourable conditions. The second phase (replacement period) occurs when Ca‐rich dolomite products, with a low degree of cation ordering, rapidly replace calcite reactants. During the replacement period, dolomite stoichiometry and the degree of cation ordering remain constant, and all dolomite crystal surfaces are covered by nanometre‐scale growth mounds. The third phase (primary recrystallization period), which occurs in the experiments between 97% and 100% dolomite, is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering. The end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat, laterally extensive layers. The fourth phase of the reaction (secondary recrystallization period) occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time. Inferences of recrystallization, in natural dolomite, based on cation order or stoichiometry of dolomite, usually depend on assumptions about the precursor dolomite subjected to recrystallization. If it is assumed that the experimental evidence presented here is applicable to natural, low‐temperature dolomites, then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization.     

湖相原生白云石的微生物成因机理探讨*

近年来,随着对微生物白云石模式研究的不断深入,为解释“白云石问题”提供了新思路。前人对微生物白云石成因研究侧重于微生物对未固结沉积物的改造,即有机准同生白云石化作用,这与实验室中以微生物为媒介形成的“有机原生白云石”在成因机理上存在差异。笔者将微生物白云石机理引入湖相原生白云石成因解释中,认为在湖水—沉积物交界处也会发生微生物成因的原生白云石沉淀,即有机原生白云石。湖水与沉积物交界处的微环境存在明显区别,总体可分为有氧和缺氧2种亚环境,不同亚环境中生活有不同的微生物群落。根据湖泊亚环境特性和微生物种类及其在白云石形成过程中所发挥的作用,可以区分出细菌有氧氧化模式、硫酸盐还原模式和产甲烷模式3种微生物白云石模式。不同模式对应于不同的湖泊环境: 细菌有氧氧化模式主要发生于有氧、高Mg/Ca值的咸水/盐湖环境;硫酸盐还原模式主要发生于缺氧、高Mg/Ca值的咸水/盐湖环境;产甲烷模式主要发生于缺氧、低Mg/Ca值的淡水/咸水湖环境。另外,还探讨了pH值变化、SO42-的存在和硫化物对镁水合物脱水的影响以及微生物白云石沉淀的环境因子。对微生物成因的原生白云石模式的深入认识,将为湖相白云石成因研究提供新的理论基础和研究思路。     

Weathering of dolomite in industrial environments

To the exclusion of other major atmospheric pollutants, sulfur dioxide is mainly responsible for attack upon dolomite. This article characterizes Laurel Dolomite on the basis of composition, texture, and porosity; describes the mechanism of the SO₂ reaction with dolomite; and develops two equations for the prediction of the rate of decay of dolomite. Over a period of 120 yr nearly 3.57 mm surface reduction at protected areas and 0.915 mm surface reduction at unprotected surfaces of a building in Louisville was calculated. However, these values could not be verified because measurement of these quantities in the field cannot be made.Dr. Caner-Saltik worked at the University of Louisville during her sabbatical leave from Middle East Technical University, Ankara, Turkey from September 1989 to August 1990 as a Fulbright scholar.     

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P_CO2, and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P_CO2, and pH at 25°C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na⁺ with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca²⁺ in solution will decrease K_ds. At the same time, to the extent that high P_CO2s increase Ca²⁺ levels, K_ds will decrease with CO₂ levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to desorption). This indirect CO₂ effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K_ds.     

Experimentally determined biomediated Sr partition coefficient for dolomite: Significance and implication for natural dolomite

Two strains of moderately halophilic bacteria were grown in aerobic culture experiments containing gel medium to determine the Sr partition coefficient between dolomite and the medium from which it precipitates at 15 to 45 °C. The results demonstrate that Sr incorporation in dolomite does occur not by the substitution of Ca, but rather by Mg. They also suggest that Sr partitioning between the culture medium and the minerals is better described by the Nernst equation (D_Sr^dol = Sr_dol/Sr_bmi), instead of the Henderson and Kracek equation (D_Sr^dol = (Sr/Ca)_dol/(Sr/Ca)_solution. The maximum value for D_Sr^dol occurs at 15 °C in cultures with and without sulfate, while the minimum values occur at 35 °C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 °C, we observed that D_Sr^dol values are greater in cultures with sulfate than in cultures without sulfate, whereas D_Sr^dol values are smaller in cultures with sulfate than in cultures without sulfate at 15 °C.Together, our observations suggest that D_Sr^dol is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the D_Sr^dol. These results have implications for the interpretation of depositional environments of ancient dolomite. The results of our culture experiments show that higher Sr concentrations in ancient dolomite could reflect microbial mediated primary precipitation. In contrast, previous interpretations concluded that high Sr concentrations in ancient dolomites are an indication of secondary replacement of aragonite, which incorporates high Sr concentrations in its crystal lattice, reflecting a diagenetic process.     

白云石重结晶作用及其地质意义

重结晶作用是白云石形成多期次动态系统中一种常见的成岩后生作用。白云石的成因问题，即“白云岩问题”一直困扰着地质学者，其中一个重要原因是白云岩中白云石的重结晶作用导致白云石形成的原始信息被掩盖，主要包括其结构和地球化学特征的变化。本文在调研国内外有关白云石重结晶作用文献基础上，系统梳理了地质历史时期古老地层中白云石和现代沉积白云石的重结晶作用，以及重结晶模拟实验中白云石岩石学及地球化学变化特征，阐述白云石重结晶的主要条件、影响因素及成岩过程中白云石重结晶作用发生的多期次性及主要变化特征，总结了白云石重结晶作用的研究意义，旨在为“白云岩问题”及成岩作用方面的研究提供一定的借鉴和启示。