Dolomites occur extensively in the lower Cretaceous along syn‐sedimentary fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly fluvial–lacustrine sedimentary ...sequence. Four types of dolomite are identified, associated with hydrothermal minerals such as natrolite, analcime and Fe‐bearing magnesite. The finely‐crystalline dolomites consist of anhedral to subhedral crystals (2 to 10 μm), evenly commixed with terrigenous sediments that occur either as matrix‐supporting grains (Fd1) or as massive argillaceous dolostone (Fd2). Medium‐crystalline (Md) dolomites are composed of subhedral to euhedral crystals aggregates (50 to 250 μm) and occur in syn‐sedimentary deformation laminae/bands. Coarse‐crystalline (Cd) dolomites consist of non‐planar crystals (mean size >1 mm), and occur as fracture infills cross‐cutting the other dolomite types. The Fd1, Md and Cd dolomites have similar values of δ18O (−20·5 to −11·0‰ Vienna PeeDee Belemnite) and δ13C (+1·4 to +4·5‰ Vienna PeeDee Belemnite), but Fd2 dolomites are isotopically distinct (δ18O −8·5 to −2·3‰ Vienna PeeDee Belemnite; δ13C +1·4 to +8·6‰ Vienna PeeDee Belemnite). Samples define three groups which differ in light rare‐earth elements versus high rare‐earth elements enrichment/depletion and significance of Tb, Yb and Dy anomalies. Medium‐crystalline dolomites have signatures that indicate formation from brines at very high temperature, with salinities of 11·8 to 23·2 eq. wt. % NaCl and Th values of 167 to 283°C. The calculated temperatures of Fd1 and Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites are distinctly cooler (81 to 124°C). These results suggest that the dolomites formed from hydrothermal fluid during and/or penecontemporaneous with sediment deposition. Faults and fractures bounding the basin were important conduits through which high‐temperature Mg‐rich fluids discharged, driven by an abnormally high heat flux associated with local volcanism. It is thought that differing amounts of cooling and degassing of these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic and geochemical differences between the dolomites.
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.
Metallic magnesium has been included in the list of Critical Mineral Raw Materials (CRM) for European Union countries since 2010. The territory of the Slovak Republic has large reserves of mineral ...raw materials – magnesite and dolomite, which are the initial source of metal Mg. For technological research, the following raw materials (based on chemical analyses of samples) were chosen: dolomite ore from the Sedlice deposit (SED-1), Trebejov deposit (TR-1) and dolomite ore from the Kraľovany deposit (KRA-1). The second deposit is also located near the operation of a potential customer of laboratory results for the production of metal magnesium, OFZ a.s. The aim of the laboratory technological research was to determine the experimental conditions for obtaining suitable Mg intermediates for metal magnesium preparation. For this purpose, there were performed DTA/TG and XRD analyses to study its behaviour, total mass loss and amount of carbon dioxide after calcination process. By optimizing the annealing tests of dolomite, products were obtained that met two conditions for its subsequent use in the sillicothermal process, namely the molecular ratio of CaO/MgO, content of impurities and the content of CO2. The optimization of calcination and repeated annealing pointed at the suitable conditions of dolomite raw sample processing (temperature of 1 050 °C for 2.5 hours, or 1 100 °C for 2 hours).
Dolomite is a very common carbonate mineral in ancient sediments, but is rarely found in modern environments. Because of the difficulties in precipitating dolomite in the laboratory at low ...temperatures, the controls on its formation are still debated after more than two centuries of research. Two important parameters to constrain the environment of dolomitization are the temperature of formation and the oxygen isotope composition of the fluid from which it precipitated. Carbonate clumped isotopes (expressed with the parameter Δ47) are increasingly becoming the method of choice to obtain this information. However, whereas many clumped isotope studies treated dolomites the same way as calcite, some recent studies observed a different phosphoric acid fractionation for Δ47 during acid digestion of dolomite compared to calcite. This causes additional uncertainties in the Δ47 temperature estimates for dolomites analyzed in different laboratories using different acid digestion temperatures.
To tackle this problem we present here a (proto-)dolomite-specific Δ47-temperature calibration from 25 to 1100 °C for an acid reaction temperature of 70 °C and anchored to widely available calcite standards. For the temperature range 25 to 220 °C we obtain a linear Δ47-T relationship based on 289 individual measurements with R2 of 0.864:∆47CDES70°C=0.0428±0.0020×106T2+0.1481±0.0160Tin Kelvin
When including two isotopically scrambled dolomites at 1100 °C, the best fit is obtained with a third order polynomial temperature relationship (R2 = 0.924):∆47CDES70°C‰=−0.0002×106T23+0.0041×106T22+0.0115×106T2+0.2218.
Applying a calcite Δ47-T relationship produced under identical laboratory conditions results in 3 to 16 °C colder calculated formation temperatures for dolomites (with formation temperature from 0 to 100 °C) than using the (proto-)dolomite specific calibration presented here.
For the synthetic samples formed between 70 and 220 °C we also determined the temperature dependence of the oxygen isotope fractionation relative to the water. Based on the similarity between our results and two other recent studies (Vasconcelos et al., 2005 and Horita, 2014) we propose that a combination of the three datasets represents the most robust calibration for (proto-)dolomite formed in a wide temperature range from 25 to 350 °C.103αCaMg−carbonates−Water=2.9923±0.0557×106T2−2.3592±0.4116
Because of the uncertainties in the phosphoric acid oxygen and clumped isotope fractionation for (proto-)dolomite, we promote the use of three samples that are available in large amounts as possible inter-laboratory reference material for oxygen and clumped isotope measurements. A sample of the middle Triassic San Salvatore dolomite from southern Switzerland, the NIST SRM 88b dolomite standard already reported in other Δ47 studies and a lacustrine Pliocene dolomite from La Roda (Spain).
This study demonstrates the necessity to apply (proto-)dolomite specific Δ47-T relationships for accurate temperature estimates of dolomite formation, ideally done at identical acid digestion temperatures to avoid additional uncertainties introduced by acid digestion temperature corrections. In addition, the simultaneous analyses of dolomite reference material will enable a much better comparison of published dolomite clumped and oxygen isotope data amongst different laboratories.
We welcome David W. Morrow's Comment to our article Merino and Canals (2011) (MC-2011 hereafter), in which we presented a new model of burial dolomitization. Morrow raises several questions and sees ...conflicts of some of the model's parts with published experiments.
The origin of sedimentary dolomite is a subject of long-standing enigma that still awaits resolution. Previous studies have shown that room temperature synthesis of abiotic dolomite is rarely ...achieved and primary (proto-)dolomite precipitation is closely associated with microbial activities. In this study, we demonstrate through laboratory carbonation experiments that highly negative-charged clay minerals (as indicated by the values of zetal potential) such as illite and montmorillonite can aid the precipitation of abiotic proto-dolomite under ambient conditions, whereas nearly-neutral charged kaolinite exerts negligible influence on such process. In comparison to montmorillonite, illite has higher surface-charge density, thus is more effective in catalyzing proto-dolomite precipitation. Furthermore, the signal of proto-dolomite in carbonate neoformations is enhanced with increasing concentrations of illite or montmorillonite. On the basis of these results, we suggest that clay minerals catalyze dolomite formation perhaps via electrostatic binding of Mg2+ and Ca2+ ions and simultaneous desolvation of these strongly hydrated cations, a crucial step for dolomite crystallization. The resulting proto-dolomites display the morphologies, textures, and structures similar to those of biogenic dolomite reported before, which are considered precursors of ordered sedimentary dolomite. Therefore, our results offer a possible route to authigenic dolomite found in sedimentary environments.
There 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)
in high Mg:Ca ratio solutions at room temperature will promote disordered dolomite precipitation (with up to 48.7 mol% MgCO
) 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 Mg
-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.
Norbert W. Mitzel
Angewandte Chemie International Edition,
June 11, 2018, Letnik:
57, Številka:
24
Journal Article
Recenzirano
“My worst nightmare is to be reincarnated as an earthworm, olm, lawyer, or tax consultant. My favourite place on earth is the Altipiano delle Pale di San Martino in the Dolomite Mountains of Belluno ......” This and more about N. Mitzel can be found on page 6973 – 6974.
Dolomite is common in carbonate successions. There is an ongoing debate about the origin of ancient massive dolostones. Recent studies suggest widespread syndepositional and early-burial ...dolomitisation by normal and near-normal seawater, in the rock record, than previously recognised. Seawater dolomitisation is ubiquitous in Cenozoic islands but Cretaceous counterparts are rare, except for Resolution and Allison guyots, Mid-Pacific Mountains. We explored their dolomitisation conditions and the implications on our understanding of Cretaceous normal seawater syndepositional and near-surface dolomitisation. Integrating geochemical and borehole data reveals temporal overlap between 87Sr/86Sr ages of dolomites, and K/Ar and 40Ar/39Ar ages of the basement. Dolomites near basaltic basement exhibit 42-times Fe-enrichment compared to normal seawater dolomites. Host-rock thermal modelling suggested that δ18Odol temperatures are ∼18–29 °C higher than concurrent temperatures. Seismic interpretation and seismic facies forward modelling demonstrate a spatial overlap between dolomites and magmatic sills in both guyots. Accordingly, magmatically driven hydrothermal dolomitisation is proposed, which promoted dolomitisation by providing high heat flow, increasing seawater circulation, and overcoming limited dolomitisation potential of Cretaceous seawater. The H2O–CO2–SO2–HCl-rich magmatic-hydrothermal acidic fluids would mix with seawater causing slight modification by leaching Mg and Fe from basaltic basement. Consequently, magmatically active Cretaceous islands had greater potential for seawater dolomitisation than counterparts with ceased magmatism, due to increased heat flux and slight increase in Mg:Ca of dolomitising fluids. Our findings imply that deep-seated island dolostones don't only form by deep cold seawater below calcite saturation depth. Instead, magmatism appears crucial in this study and potentially other locations (e.g. Xisha Islands). The absence of dolostones in other Cretaceous islands supports previous models of reduced effectiveness of massive dolomitisation by Cretaceous normal seawater, questioning interpretations of syndepositional and near-surface dolomitisation. Thus, caution is advised when interpreting these dolomites by considering additional factors (dolomite geometry, geological settings, and dolomitisation age), as they may share characteristics with syndepositional and near-surface normal seawater dolomites.
•Magmatically driven hydrothermal dolomitisation is suggested for the Cretaceous atolls in the Mid-Pacific Mountains.•Magmatically active Cretaceous islands had greater potentiality for seawater dolomitisation than the inactive counterparts.•The model gives an alternative explanation to deep-seated island dolostones in this study and potentially other locations.•The Cretaceous normal seawater dolomitisation appears of reduced effectiveness, supporting previous reactive transport modelling and field studies.•Caution is advised in interpreting these dolomites as they may share characteristics with syndepositional and near-surface early-burial normal marine dolomites.
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