Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.     

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680‐m‐thick formation is composed of a stromatolite–dolomite–‘red bed’ sequence formed in a complex combination of shallow‐marine and non‐marine, evaporitic environments. Dolomite‐collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by δ¹³C values from +7·1‰ to +11·6‰ (V‐PDB) and δ¹⁸O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ¹³C values ranging from +9·0‰ to +11·6‰ and δ¹⁸O values of 20·0–25·7‰. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water‐derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high δ¹³C values reflect a combined contribution from both global and local carbon reservoirs. A ¹³C‐rich global carbon reservoir (δ¹³C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in ¹³C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.     

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA)

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ¹³C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ¹³C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ¹³C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ³⁴S) and silica (δ¹⁸O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.     

Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ¹⁸O values from ?13·7 to ?9·7‰ VPDB, δ¹³C values from ?2·7 to + 1·5‰ VPDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (T_h) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ¹⁸O values (?14·2 to ?9·3‰ VPDB), lower δ¹³C values (?3·0 to ?0·7‰ VPDB), higher ⁸⁷Sr/⁸⁶Sr ratios (≈ 0·7100) and higher T_h values (171–209 °C), which correspond to trapping temperatures (T_r) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ¹⁸O values (?18·5 to ?14·3‰ VPDB), variable δ¹³C values (?11·3 to ?1·2‰ VPDB) and high T_h values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).     

Genesis of island dolostones

Cenozoic ‘island dolostones’ are found on islands throughout the oceans of the world. Due to their geological youth and lack of deep burial, these dolostones provide an opportunity to resolve some of the mysteries surrounding the dolomite problem. In island dolostone bodies, which are of variable size and variable dolomitization, the petrographic and geochemical properties of the dolostones are characterized by geographic and stratigraphic variations. In the larger island‐wide dolostone bodies, like those found on Grand Cayman, there are progressive increases in mole %Ca (%Ca_mean: 53·9 to 57·6%), depletion of the heavier ¹⁸O and ¹³C isotopes (δ¹⁸O_mean: 3·6 to 2·1‰ _VPDB; δ¹³C_mean: 3·1 to 1·4‰ _VPDB), and changes from fabric‐retentive to fabric‐destructive fabrics and a decrease in the amount of dolomite cement from the coastal areas towards the centres of the islands, similar to the Little Bahama Bank. These changes define geographically concentric zones that parallel the coastlines and reflect geochemical modification of the dolomitizing fluid through water–rock interactions, mixing with meteoric water and the changes in the rate and flux of seawater as it flowed from coasts to island interiors. The pattern of dolomitization, however, is not consistent from island to island because geographic and stratigraphic variations, specific to each island, influenced groundwater flow pattern (for example, geometry and size of the islands; the porosity and permeability of the precursor limestone), the duration of the dolomitization reaction, and other factors. The geographic extent of dolomitization and variation in dolomite stoichiometry of island dolostones may be comparable to the reaction stages established in high‐temperature laboratory experiments.     

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.     

Meteoric diagenesis and dedolomite fabrics in precursor primary dolomicrite in a mixed carbonate–evaporite system

Dedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate‐evaporite successions. In these settings, elevated Ca²⁺/Mg²⁺ ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north‐eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non‐pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north‐eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative δ ¹⁸O and low δ ¹³C values (averages of −13·6‰_VPDB and 0·5‰_VPDB, respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of −6·0‰_VPDB and 1·2‰_VPDB, respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non‐pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs.     

Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ¹⁸O and δ¹³C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ¹⁸O=–2·2, δ¹³C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ¹⁸O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.     

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

Abstract Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ¹⁸O = 0·9 ± 1·0‰, N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ¹⁸O = 2·1 ± 0·7‰, N = 48) than the hypersaline backreef dolomite (δ¹⁸O = 3·6 ± 0·9‰, N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.     

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures

Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea‐level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite‐rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre‐scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite‐filled fenestrae; (ii) dolomicrite with bacterium‐like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa‐like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm‐like remains occur in voids between the filaments. The dolomite consistently shows negative δ¹³C values (down to ?11·3‰) and very positive δ¹⁸O (mean value 7·9‰) that suggest formation as primary precipitate with a substantial contribution of organic CO₂. Very negative δ¹³C values (down to ?31·6‰) of early diagenetic calcite associated with the dolomite suggest contribution of CO₂ originating by anaerobic methane oxidation. The shale‐normalized rare earth element patterns of Sutera stromatolites show features similar to those in present‐day microbial mats with enrichment in light rare earth elements, and M‐type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation.     

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

Formation of modern dolomite in hypersaline pans of the Western Cape,South Africa

Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km² in size. The δ¹⁸O_PDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ¹³C_PDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The ⁸⁷Sr/⁸⁶Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca²⁺) ions with relatively minor amounts of Ca²⁺ derived from the chemical weathering of bedrock.     

Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

In past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic-rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense-clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro-infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ¹⁸O_VPDB from −3.2 to −1.76‰ and for δ¹³C_VPDB from −2.62 to −3.78‰. Inferred δ¹⁸O_SMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene.     

Traces of brine and hydrocarbon migration in carbon,oxygen, and sulfur isotopic compositions of reservoir rocks in the Talakan petroleum field,Southwestern Yakutia

Dolomites from the productive Osa horizon (upper subformation of the Lower Cambrian Bilir Formation) in the Talakan petroleum field show a prominent 1–2‰ decrease in δ¹⁸O (from 23–24 to 21–22‰), which presumably marks a zone of relatively high water/rock ratios. Productive boreholes are characterized by moderate δ³⁴S values (from 25.1 to 30.6‰) and negative correlation between δ³⁴S in anhydrite and δ¹⁸O in associated dolomite, which points to a partial sulfate reduction during catagenesis. In nonproductive borehole, δ³⁴S values increase significantly (from 31.4 to 35.6‰) and show positive correlation with δ¹⁸O in dolomite. Rocks recovered by nonproductive borehole possibly recrystallized during early diagenesis, and, correspondingly lost their permeability and capacity to form pores. Limestones and dolomites of the Osa horizon have a carbon isotopic composition within the range of normal marine carbonates (δ¹³C = 0 ± 1 ‰), which does not indicate a significant role of organic matter in postsedimentary recrystallization of carbonate sediments. A positive δ¹³C excursion up to 4.5‰ recorded in the lower subformation of the Bilir Formation presumably occurred at the sedimentation stage under conditions of high rates of bioproductivity and organic matter burial in sediments.     

Dedolomitization and calcite cementation in the Middle Devonian Winnipegosis Formation in Central Saskatchewan,Canada

Limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the Winnipegosis Formation on the east‐central margin of the Elk Point Basin where the overlying Prairie Evaporite deposits have been removed. This type of crystalline limestone is interpreted as dedolomite, based on petrographic observations. The δ¹⁸O and δ¹³C values of the Winnipegosis dedolomite vary from ?12·8‰ to ?11·9‰ VPDB (Vienna Pee Dee Belemnite) and from ?0·5‰ to +1·7‰ VPDB, respectively; both values are significantly lower than those for the corresponding dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of the dedolomite are significantly higher, between 0·7082 and 0·7087. The spatial distribution and geochemical data of the Winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the Prairie Evaporite anhydrite during the latest Mississippian to the Early Cretaceous when the basin was subjected to uplift and erosion. The Winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite, including the occurrence of dedolomite restricted along fractures and adjacent areas, dolomite patches ‘floating’ in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts. However, the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the Winnipegosis dedolomite. Coarsely to very coarsely crystalline, subhedral to euhedral calcite cement is restricted in the dedolomite. The petrographic features, isotopic compositions and homogenization temperatures, coupled with the burial history of the Winnipegosis Formation, constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during Late Cretaceous to Neogene time. The more negative C‐isotopic signatures of the calcite cement (?5·3‰ to ?2·3‰ VPDB) probably reflect a hydrocarbon‐derived carbon.     

Formation of siderite-Mg-calcite-iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England

Concretions cemented mainly by siderite, Mg-calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate-reducing bacteria are active. δ¹³C values ranging from ?3 to ?11·8% (mean ?5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ¹⁸O values ranged from ?6·4% to + 0·8% (mean ?1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to-sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg-calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.     

Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO₃), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO₂ and K contents are highly correlated. δ¹⁸O‐ and δ¹³C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in ¹⁸O and ¹³C between the two minerals. The correlation coefficient between δ¹⁸O and δ¹³C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO₃^?/CO₃^2? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.     

Petrography and geochemistry of early-stage, fine- and medium-crystalline dolomites in the Middle Devonian Presqu'ile Barrier at Pine Point, Canada

The petrography and geochemistry of fine- and medium-crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal fluids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back-barrier facies in the Elk Point Basin. Fine-crystalline dolomite has δ¹⁸Ο values between ?1·6 to –3·8‰ PDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine-crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor. Medium-crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has δ¹⁸O values between ?3·7 to ?9·4‰ PDB (mean ?5·9‰ PDB) and ⁸⁷Sr/⁸⁶Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.     

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ¹⁸Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ¹³C values together with ⁸⁷Sr/⁸⁶Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ¹⁸Ο and δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ¹⁸Ο, δ¹³C and ⁸⁷Sr/⁸⁶Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.