Diagenetic Fluids Research Articles

Veracity of Neoproterozoic negative C-isotope values: The termination of the Shuram negative excursion

There is widespread interest in the Neoproterozoic period of the Earth's history (1000 to 542 Ma) because of unprecedented δ 13C fluctuations to < − 10‰ PDB through thick (> 1000 m) succession of stratigraphically complex sedimentary rocks deposited during tens of millions of years. In contrast, Phanerozoic large negative C-isotope excursions have been interpreted as the result of diagenetic fluid mixing during carbonate stabilization and burial and are less enigmatic due to the excellent biostratigraphic control on their timing and duration. The Ediacaran Nafun Group of Oman (part of the Huqf Supergroup spanning the Cryogenian–Early Cambrian) contains a large δ 13C negative excursion (the Shuram excursion) reaching values as negative as − 12‰ at the base of the Shuram Formation. A steady recovery to positive values occurs over the entire Shuram and half through the overlying Buah Formation, suggesting a duration on the order of tens of My. Based on trace metal, chemostratigraphic and sedimentological analyses, the carbon isotope record obtained from the Buah Formation of northern Oman indicates a systematic and reproducible shift of δ 13C values from − 6‰ to + 1‰ in 1 — a demonstrably diagenetic altered carbonate-cemented siliciclastic facies, and 2 — a least diagenetically altered stromatolitic facies. The identical reproducible isotopic pattern in these time-equivalent sections combined to the presence of exceptionally preserved δ 18O values around − 2 to + 1‰ associated with the most negative δ 13C values rules out isotopic resetting by diagenetic fluids as a mechanism to explain these values. It is concluded that it is possible to retain depositional δ 13C values in demonstrably diagenetically altered carbonates. This raises the issue of the ability to recognize diagenetic alteration of C-isotopic values in Neoproterozoic rocks where a robust time frame to support reproducibility is not available. The results of this study provide strong support to a non diagenetic origin of the negative Shuram C-isotope excursion, believed to be the most profound (in terms of amplitude and duration) in the Earth's history.

流体包裹体研究：进展、地质应用及展望

Fluids are aubiquitous transport medium for heat and matter in most geological process.The presence of fluids in rocks may affect the chemical and physical properties,mineral reaction velocity and heat budget of geological systems,Direct sample of geological fluids could be preserved only in fluid inclusions that were trapped during the growth of their host mineral.Fluid inclusions can provide us unique information for the presence and composition of ancient fluids which can not be obtained by other geochemical methods(e.g.the component and evolution of diagenetic fluid;process of fluid-rock interaction;migration of trace elements in fluid).The study of fluid inclusions thus has become one of the hottest fields in earth sciences,and plays an important role in studies correlated with geological processes.Starting with a general introduction to the fluid inclusion properties and a summarization about the classification of primary-and secondary-fluid inclusions,this paper reveiws the most recent development in analytical methods in the fluid inclusion research field.We also review the current application of fluid inclusions to various fields in earth sciences(e.g.metamorphic fluid,ore-deposit,petroleum geology and biomarker) in the paper.Finally,we give a short outlook on potential future research topics about fluid inclusion studies.

Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions

Variations in the oxygen isotope composition (δ 18O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ 18O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ 18O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ 18O values, (δ 18O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ 18O value. At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ 18O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ 18O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature ( T sw at which the amorphous silica precipitated) and diagenesis temperature ( T diagenesis at which microquartz formed) that reproduce the δ 18O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ 18O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T sw and T diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.

Molybdenum isotopic composition of modern and Carboniferous carbonates

We investigate the redox-sensitive isotope system of molybdenum (Mo) in marine carbonates to evaluate their potential as archive of the Mo isotopic composition of coeval seawater. We present Mo isotope data ( δ 98/95Mo) of modern skeletal and non-skeletal carbonates as well as a variety of precipitates from the mid and late Carboniferous. The external reproducibility is determined by repeated analyses of two commercially available carbonate standards. The resulting uncertainty of the low concentration samples is ± 0.1‰ (2 σ). Analysis of modern ooid sands from the Bahamas shows a consistently heavy Mo isotopic composition ( δ 98/95Mo between 2 and 2.2‰), approaching modern mean seawater values ( δ 98/95Mo = 2.3‰ ± 0.1‰ (2 σ)). This suggests that isotope fractionation during Mo uptake into non-skeletal carbonate precipitates is small. In contrast, modern skeletal carbonates show variable isotopic compositions (0.1 to 2.2‰) which suggests a biologically controlled fractionation process. The varying Mo signatures found in Carboniferous cement phases point to a strong response to local changes in fluid composition from which they precipitated. Overall, we recognized three important factors to cause an offset relative to ocean water: Mo derived from skeletal components, input of detrital Mo and admixture of light, hydroxide derived Mo via diagenetic fluids. All of these factors cause a lighter Mo isotopic composition relative to seawater. Due to the apparent small isotope fractionation during Mo uptake into non-skeletal carbonates, their δ 98/95Mo closely reflects the ambient fluid composition. From these results we conclude that carbonates represent a promising new tool to characterize the water mass Mo isotopic composition of marine paleo-environments.

Using in situ REE analysis to study the origin and diagenesis of dolomite of Lower Paleozoic, Tarim Basin

In situ REE concentrations of various dolomites from Tarim Basin were obtained by LA-ICP-MS analysis, and the data were normalized to standard seawater (Seawater Normalized=SWN). Most of the samples have a ΣREE range of less than 20 ppm. All samples show similar REESWN distributions with heavy REE depletion, and positive Ce anomaly, which indicates that they have the same dolomitization fluids (seawater). According to the origin and diagenetic process of dolomite, two types of dolomite are determined and described as follows: 1) syndepositional dolomite, with the highest REE concentrations (more than 20 ppm), the cores of which are more enriched in REE compared with their cortexes, indicating that they underwent the dolomitization of calcareous sediments by hypersaline and subsequent diagenesis decreased the REE content of the cortex because of the low REE concentration of the diagenetic fluids; 2) diagenetic dolomite, which can be subdivided into four groups. (1) burial dolomite which has higher REE concentrations than limestone, but lower than syndepositional dolomite. This shows that pore fluids with high salinity dolomitized the pre-existing limestone; (2) void filling dolomite which has the similar REE patterns with the matrix dolomite. In addition, the Eu anomaly is not obvious, suggesting that the dolomitization fluids originated from the diagenetic fluids; (3) recrystallized dolomite, whose REE concentration was obviously decreased, indicating that the REE concentration was decreased during the recrystallization processes; and (4) hydrothermal altered void-filling dolomite, which has the lowest REE concentration, but obvious positive Eu anomaly, reflecting its hydrothermal activity related origin. Thus, the diverse REE signatures, which were recorded in different dolomites, retain the information of their formation conditions and subsequent diagenetic processes. In situ REE analysis of dolomite is an effective probe into the origin and diagenetic process of dolomite.

Nanostructural and Geochemical Features of the Jurassic Isocrinid Columnal Ossicles

Calcite isocrinid ossicles from the Middle Jurassic (Bathonian) clays in Gnaszyn (central Poland) show perfectly pre− served micro− and nanostructural details typical of diagenetically unaltered echinoderm skeleton. Stereom pores are filled with ferroan calcite cements that sealed off the skeleton from diagenetic fluids and prevented structural and geochemical alteration. In contrast with high−Mg calcite skeleton of modern, tropical echinoderms, the fossil crinoid ossicles from Gnaszyn contain only 5.0–5.3 mole% of MgCO3. This low Mg content can be a result of either a low temperature environ− ment (ca. 10C) and/or low Mg/Ca seawater ratio. Both conditions have been proposed for the Middle Jurassic marine en− vironment. Occurrence of Mg−enriched central region of stereom bars of Jurassic columnal ossicle of Chariocrinus an− dreae is consistent with the concept of magnesium ions involvement in earliest growth phases of calcium carbonate biominerals. Key wor ds: Echinodermata, Crinoidea, calcite, nanostructure, geochemistry, AFM, NanoSIMS, Jurassic.

Platform-Margin Trajectory as a Control on Syndepositional Fracture Patterns, Canning Basin, Western Australia

Abstract Syndepositional fractures are an important feature of high-relief, reef-rimmed carbonate systems and exert a profound control on many facets of platform evolution including: the timing and frequency of platform-margin collapse events, generation of an early diagenetic fluid flow system, and subsequent karst and enhanced-permeability development. In the Devonian Reef Complexes of the Canning Basin, syndepositional fracturing increases significantly with progradation, with a twofold increase in fracture intensity observed between the most retrogradational and progradational carbonate platforms. This study demonstrates a statistically significant relationship between syndepositional fracture patterns and variations in stratigraphic architecture, approximated here by platform-margin trajectory. In the platform-margin, syndepositional fracturing varies systematically with platform-margin trajectory, regardless of proximity to regional tectonic elements, whereas in the platform interior, tectonically active settings consistently display syndepositional fracture intensities significantly higher than predicted by platform-margin trajectory alone. The results presented here suggest a first-order relationship between long-term changes in stratigraphic architecture and syndepositional fracture development in high-relief carbonate platforms and imply that whereas external regional tectonic drivers may enhance early fracturing, they are not required.

Venezuelan sandstone caves: a new view on their genesis, hydrogeology and speleothems

Caves in arenites of the Roraima Group in Venezuela have been explored on the Chimantá and Roraima plateaus (tepuis). Geological and geomorphological research showed that the most feasible method of caves genesis was the winnowing and erosion of unlithified or poorly lithified arenites. The unlithified arenitic beds were isolated by well-cemented overlying and underlying rocks. There is a sharp contrast between these well-lithified rocks and the loose sands which form the poorly lithified to unlithified beds. They are only penetrated by strongly lithified pillars which were cemented by vertical finger flow of the diagenetic fluids from the overlying beds. Such finger flow is only typical for loose sands and soils where there is a sharp difference in hydraulic conductivity. The pillars exhibit no signs of further dissolution. The caves form when the flowing water accesses the poorly lithified beds through clefts/crevices. Collapse of several superimposed winnowed-horizons can create huge subterranean spaces. Futher upward propagation of the collapses can lead to large collapse zones which are commonly observed on the tepuis. Dissolution is also present but it probably plays neither a trigger role, nor a volumetrically important role in the cave-forming processes. The strongest dissolution/reprecipitation agent is condensed atmospheric moisture which is most likely the main agent contributing to growth of siliceous speleothems. As such, it can be active only after, but not before the cave is created. Siliceous speleothems are mostly microbialites except for some normal stalactites, cobweb stalactites and flowstones which are formed inorganically. They consist of two main types: 1. fine-laminated columnar stromatolite formed by silicified filamentous microbes (either heterotrophic filamentous bacteria or cyanobacteria) and 2. a porous peloidal stromatolite formed by Nostoc-type cyanobacteria. The initial stages of encrusted shrubs and mats of microbes were observed, too, but the surrounding arenitic substrate was intact. This is strong evidence for the microbial mediation of silica precipitation.

The Transfiguration continental red-bed Cu–Pb–Zn–Ag deposit, Quebec Appalachians, Canada

The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive, grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ13C and from 14.7‰ to 21.3‰ δ18O. The negative δ13C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite and galena vary from −19‰ to 25‰ δ34S, as measured on mineral concentrates by a conventional SO2 technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ34S values, from −21‰ to 25‰. Negative and positive δ34S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret similarly high δ34S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation. The positive correlation between Cu and Fe3+/Fe2+ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is tentatively attributed to the Late Silurian Salinic extensional event.

Diagenetic fluids evolution and genetic mechanism of tight sandstone gas reservoirs in Upper Triassic Xujiahe Formation in Sichuan Basin, China

The reservoirs of the Upper Triassic Xujiahe Formation in Sichuan Basin have the characteristics of low compositional maturity, low contents of cements and medium textural maturity. The general physical properties of the reservoirs are poor, with low porosity and low permeability, and there are only a few reservoirs with medium porosity and low permeability in local areas. Based on the diagenetic mineral association, a diagenetic sequence of cements is established: early calcites (or micrite siderites) → first quartz overgrowth→chlorite coatings→dissolution of feldspars and debris→chlorite linings→second quartz overgrowth (quartz widen or filled in remain intergranular pores and solution pores)→dissolution→third quartz overgrowth (quartz filled in intergranular and intragranular solution pores)→intergrowth (ferro) calcites→dolomites→ferro (calcites) dolomites→later dissolution→veins of quartz and calcites formation. Mechanical compaction is the main factor in making the reservoirs tight in the basin, followed by the second and third quartz overgrowth. In a long-term closed system, only feldspars and some lithic fragments are dissolved by diagenetic fluids, while intergranular cements such as quartz and calcit are not dissolved and thus have little influence on the porosity of the Xujiahe Formation. This is the third factor that may have kept the sandstones of Xujiahe Formation tight finally. The hydrocarbon was extensively generated from organic materials after the second quartz overgrowth, and selectively entered favorable reservoirs to form tight sandstone gas reservoirs.

Oxygen isotope and Mg/Ca composition of Late Viséan (Mississippian) brachiopod shells from SW Iberia: Palaeoclimatic and palaeogeographic implications in northern Gondwana

Latest Viséan brachiopod shells ( Gigantoproductus sp.) from the Pedroches basin of SW Spain were sampled in order to obtain chemical and isotopic evidence that may help, first, to assess if primary geochemical and isotopical signals are preserved in these shells and, second, to reconstruct temporal variations in palaeoceanographic and palaeoclimatic conditions in northern Gondwana during this time span. Microtexture, cathodoluminescence (CL) and ultrastructure (under SEM) of the brachiopods were examined to avoid sampling of shell portions altered after deposition. Shell portions classified as non-luminescent (NL) and non-luminescent to slightly luminescent (NL–SL) present geochemical signatures within the ranges considered normal for Recent brachiopods that have not undergone significant diagenetic modification. These include low to undetectable Fe and Mn contents, high Sr values, and δ 18O values similar to those of carbonates in equilibrium with seawater. On the other hand, valve areas that are luminescent (L) or moderately luminescent with luminescent portions (ML–L) present higher Fe and Mn contents, lower Sr concentrations, and lower δ 18O values than those corresponding to NL and NL–SL shells. This suggests a greater diagenetic alteration of these portions and that they acted as microzones of reaction with the diagenetic fluids. Temporal trends through the studied interval show a spread in δ 13C values much lower than that observed for contemporaneous samples (up to 1.6‰), thus obscuring the δ 13C global signal at the time-scale resolution of this study. In contrast, the δ 18O record of brachiopod calcite ( δ 18O c) shows an overall increase of ∼ 1‰ (relative to V-PDB) from the oldest to the youngest samples. This is accompanied by a ∼ 30% decrease in Mg/Ca ratios of the brachiopod calcites, confirming that at least part of this shift in δ 18O c was related to cooling. Combination of Mg/Ca and δ 18O c proxies established in this paper, although not full-proof for brachiopods yet, allowed for the calculation of a temperature drop of ∼ 3.3 °C and a change in seawater δ 18O w that becomes ∼ 0.4‰ more positive during this interval. The data are consistent with regional seawater cooling that accompanied an increase in continental ice volume, over a relatively short time period of < 4 Myr. Comparison of the Carboniferous isotopic data from Spain with data compiled from Palaeotethyan basins suggests that this positive event in δ 18O c during the Brigantian (latest Viséan) may be correlated worldwide. This synchronous global shift provides evidence for the onset of the Carboniferous glaciation in Gondwana during the Brigantian.

Datação de Processos Diagenéticos em Arenitos-Reservatório de Hidrocarbonetos: Uma Revisão Conceitual

Geochronology has became a powerfull tool for diagenetic studies in siliciclastic rocks as it supply parameters for the definition of their depositional or diagenetic ages. This development occurred particularly in the last decades, with analytical techniques improvements and application of oil exploration. Sandstones are important hydrocarbon reservoir-rocks in many basins of the world. Sandstones devoid of fossils with defined ages lack precise chronostratigraphic definition. Such lack of definition limits the precision of the generation-migration-accumulation models developed for the exploration and exploitation of these reservoirs. However, many sandstones contain diagenetic minerals which represent the possibility of dating the deposition of these sandstones, or episodic diagenetic fluid migration through radiometric 40K-40Ar, 40Ar-39Ar and 87Rb-87Sr methods. In the present work, we show that these methods have been successfully applied to samples of reservoirsandstones all over the world. The improvement of the radiometric methods applied to sedimentary rocks yielded significant results concerning depositional or diagenetic ages.

Stable isotopic composition of dinosaur eggshells and pedogenic carbonates in the upper cretaceous seonso formation, South Korea: Paleoenvironmental and diagenetic implications

Stable isotopic compositions of the pedogenic calcites and calcitic dinosaur eggshells analyzed from the Seonso Formation in southern Korea provide information on the paleoenvironmental conditions that dominated the region during the Upper Cretaceous as well as on the nature of the diagenetic fluids. The δ 18O values of the calcites recovered from the paleosols (−15.46 to −26.22 ‰ VPDB) indicate that they were precipitated and/or altered under a high temperature regime, whereas the δ 13C values (−3.88 to −7.72 ‰ VPDB) suggest that these carbonates received contributions from hydrothermal fluids during shallow burial and possibly from C 3-type vegetation. The δ 18O and δ 13C of partially to well-preserved calcite eggshells are significantly different from those of the paleosols, indicating a departure from the earlier environmental conditions. The observed shift in δ 18O values between the dinosaur eggshells (−3.43 to −14.09 ‰) and pedogenic calcites indicates different environmental and thermal conditions and suggesting two possible scenarios; either that the timing of egg laying by the dinosaur and the soil formation were not synchronous, or that the dinosaur consumed water from the rivers and evaporated ponds. In contrast, isotopic compositions of vein calcites cross-cutting paleosols (−19.06 to −21.72 for δ 18O, and −3.78 to −4.79 for δ 13C) reflect their precipitation under hydrothermal conditions. The narrow range in the δ 13C values of the eggshells (−7.04 to −8.69 ‰) reflect a uniform source for carbon; mostly representing fresh water charged with CO 2 from hydrothermal sources as dissolved carbon.

Early diagenesis of bone and tooth apatite in fluvial and marine settings: Constraints from combined oxygen isotope, nitrogen and REE analysis

Fossil bones and teeth of Late Pleistocene terrestrial mammals from Rhine River gravels (RS) and the North Sea (NS), that have been exposed to chemically and isotopically distinct diagenetic fluids (fresh water versus seawater), were investigated to study the effects of early diagenesis on biogenic apatite. Changes in phosphate oxygen isotopic composition ( δ 18O PO4), nitrogen content (wt.% N) and rare earth element (REE) concentrations were measured along profiles within bones that have not been completely fossilized, and in skeletal tissues (bone, dentine, enamel) with different susceptibilities to diagenetic alteration. Early diagenetic changes of elemental and isotopic compositions of apatite in fossil bone are related to the loss of the stabilizing collagen matrix. The REE concentration is negatively correlated with the nitrogen content, and therefore the amount of collagen provides a sensitive proxy for early diagenetic alteration. REE patterns of RS and NS bones indicate initial fossilization in a fresh water fluid with similar REE compositions. Bones from both settings have nearly collagen-free, REE-, U-, F- and Sr-enriched altered outer rims, while the collagen-bearing bone compacta in the central part often display early diagenetic pyrite void-fillings. However, NS bones exposed to Holocene seawater have outer rim δ 18O PO4 values that are 1.1 to 2.6‰ higher compared to the central part of the same bones ( δ 18O PO4 = 18.2 ± 0.9‰, n = 19). Surprisingly, even the collagen-rich bone compacta with low REE contents and apatite crystallinity seems altered, as NS tooth enamel ( δ 18O PO4 = 15.0 ± 0.3‰, n = 4) has about 3‰ lower δ 18O PO4 values, values that are also similar to those of enamel from RS teeth. Therefore, REE concentration, N content and apatite crystallinity are in this case only poor proxies for the alteration of δ 18O PO4 values. Seawater exposure of a few years up to 8 kyr can change the δ 18O PO4 values of the bone apatite by > 3‰. Therefore, bones fossilized in marine settings must be treated with caution for palaeoclimatic reconstructions. However, enamel seems to preserve pristine δ 18O PO4 values on this time scale. Using species-specific calibrations for modern mammals, a mean δ 18O H2O value can be reconstructed for Late Pleistocene mammalian drinking water of around − 9.2 ± 0.5‰, which is similar to that of Late Pleistocene groundwater from central Europe.

Seafloor-precipitated carbonate fans in the Neoproterozoic Rainstorm Member, Johnnie Formation, Death Valley Region, USA

Cm-sized carbonate seafloor fans occur in the Neoproterozoic Rainstorm Member of the Johnnie Formation, Death Valley, USA. The fans formed in a mixed carbonate-clastic succession near storm wave base at the base of parasequences on a storm-dominated ramp. Petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis. Although not directly dated, the Rainstorm Member preserves a large magnitude negative carbon isotopic anomaly (down to − 11‰ PDB) tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 Ma and 550 Ma. Thus, the age distribution of seafloor aragonite fans in Neoproterozoic strata appears more widespread than previously thought, occurring in strata significantly younger than the last widespread Neoproterozoic glaciation. Rainstorm Member carbonate fans and oolitic units (representing time-correlative shallower environments) record similar carbon isotope ratios during the negative carbon isotopic anomaly. The carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well-mixed rather than stratified during the late Neoproterozoic isotopic anomaly. In addition, the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon (on a m- to cm-scales) suggests that the local source of alkalinity required for fan growth was derived from a well-mixed reservoir, likely seawater, rather than local diagenetic fluids. Increased alkalinity and the presence of inhibitors to carbonate nucleation (perhaps Fe 2+ under anoxic conditions) likely fostered precipitation of aragonite crystal fans on the seafloor.

Pb-Pb age of sedimentary phosphorite reworking in Lower Riphean carbonate sediments, the Satka Formation of Southern Urals

The mineral composition and U-Pb and Rb-Sr systematics of phosphorites from the Satka Formation of Lower Riphean carbonates, the Burzyan Group of Southern Urals, are studied. Phosphorites occurring as small lenses between stromatolite layers are composed largely of fluorapatite with admixture of detrital quartz, feldspars, illite, and chlorite. Phosphorite samples have been subjected to stepwise dissolution in 1 N (fraction L-1) and 2 N (fraction L-2) HCl. As is established, the maximum apatite content is characteristic of fraction L-1, while fraction L-2 is enriched in products of dolomite and sulfide dissolution and in elements leached from siliciclastic components. The Sr content in the Satka apatites (280–560 ppm) is substantially lower as compared with that in unaltered marine apatite. The 87Sr/86Sr “initial ratio in the phosphorites studied (0.71705–0.72484) and host dolomites from the lower part of the Satka Formation is significantly higher than in the Early Riphean seawater that indicates a reset of the Rb-Sr original systems in sediments. The Pb-Pb age of 1340 ± 30 Ma (MSWD = 6.4) estimated based on 7 data points characterizing fractions L-1 and L-2 is younger than the formation time of overlying Burzyan sediments, being consistent, within the error range, with date of the Mashak rifting event recorded at the Early-Middle Riphean boundary. The comparative U-Pb characteristics of two soluble fractions (L-1 and L-2) and silicate residue of phosphorites show that epigenetic redistribution of Pb and U was characteristic of the phosphorite horizon only. The initial Pb isotope composition and μ (238U/204Pb) estimated according to model by Stacey and Kramers for the early diagenetic fluids in carbonate and phosphate sediments of the Satka Formation suggest that they were in isotopic equilibrium with erosion products of the Taratash crystalline complex.

Dolomitization of the late Cretaceous–Paleocene platform carbonates, Gölköy (Ordu), eastern Pontides, NE Turkey

Late Cretaceous–Paleocene Ağıllar Formation in the eastern Pontide zone, NE Turkey, is composed of shallow-marine platform carbonates that were buried until uplift in Late Paleocene time. The formation, which is represented by 100 m thick reefal limestones around Gölköy, is completely dolomitized. On the basis of petrographic and geochemical features, four different replacive and cement dolomite phases are present. The replacive dolomites, which are cut by low-amplitude stylolites, include 1) fine crystalline planar-s dolomite (Rd1) and 2) coarsely crystalline planar-s(e) dolomite (Rd2). Two types of dolomite cements were developed in minor amounts: 1) a coarse dolomite cement (Cd1) that mostly fills molds/vugs and fractures in Rd1 dolomites, and 2) rim dolomite cement (Cd2) that commonly develops on the space-facing surfaces of Rd2 dolomite. The replacive dolomites are nearly stoichiometric (Ca 49–51 Mg 48–50) and have δ 18O values of − 3.5 to − 1.6‰ VPDB, δ 13C values of − 1.1 to 0.8‰ VPDB, and 87Sr/ 86Sr ratios of 0.707104 to 707458. Sr, Na, Mn and Fe concentrations of replacive dolomites are 102–171 ppm, bdl–371 ppm, 244–9941 ppm and 2643–8951 ppm, respectively. Petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from Late Cretaceous–Paleocene seawater and/or seawater slightly modified by water–rock interaction process, and then recrystallized by meteoric waters at increasing burial depths and temperatures. Compared with replacement dolomites, Cd1 dolomites yield lower δ 18O values (− 4.9 to − 5.3‰ VPDB), similar δ 13C values (− 1.2 to − 0.5‰ VPDB) and higher 87Sr/ 86Sr ratios (0.707660–0.707609). The Cd1 dolomites overlie the low-amplitude stylolites and have homogenization temperatures between 94 and 112 °C, indicating that they were deposited from basinal brines. Although replacive dolomites and dolomite cements have similar Na (bdl–413 ppm), Mn (259–6990 ppm) and Fe (2184–6690 ppm) concentrations, the cements have lower Sr contents (63–114 ppm). Like replacive dolomites, Cd1 dolomite is nearly stoichiometric (Ca 49–50 Mg 47–49), but Cd2 dolomite is slightly Ca-rich (Ca 51–52 Mg 47–48). The Cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite.

Feldspar concentrations in lower Cambrian limestones of the Moroccan Atlas: Pyroclastic vs authigenic processes

Carbonate strata containing abundant euhedral feldspar, usually considered as authigenic, are characteristic of some lower Cambrian exposures in the Atlas Mountains, Morocco. Impure limestones are abundant in the Issendalenian Amouslek Formation of the western Anti-Atlas, and in the Banian Lemdad Formation of the southern High Atlas. Some of their limestone beds contain up to 40% acid insoluble residue, of which feldspar comprises as much as 65% in some samples, the remainder consisting of detrital and authigenic quartz, apatite, pyrite, and clay minerals. The host limestones are bioclastic tempestites, ooidal–oncoidal–bioclastic shoal complexes, archaeocyathan-microbial peri-reef settings (mainly flanks but not reef cores), and microbial reefs. A volcanic origin is adopted for the feldspars based on: (i) fluctuations in feldspar concentration paralleling bedding, but unrelated to host-facies (except for the aforementioned archaeocyathan-microbial reef cores that were controlled by turbidity); (ii) the local association with glassy fragments (with feldspars embedded in glass shards); and (iii) the scattered occurrence of gradational lithofacies from silty tuff to tuffitic limestone. The mineral and chemical composition of the host-rock and diagenetic fluids was the determining factor for the kind of feldspar preservation. Although the primary type of K–feldspar replacement is albitization, secondary albite subsequently suffered from illitization, chloritization, and/or replacement by calcite. A direct implication of this work is the possible geochronological dating of both volcanic eruptions and archaeocyath- and trilobite-bearing host-rocks based on radiometric analyses within unaltered K–feldspar pyroclasts extracted after etching.

Spherical and columnar, septarian, 18O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

AbstractCalcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2·5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non‐compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low‐Mn, Mg‐rich calcite, whereas the later generation is a low‐Mg, Mn‐rich calcite. The change in chemistry is broadly consistent with a time (burial)‐related transition from oxic to sub‐oxic/anoxic conditions close to the sediment–water interface. Geochemical features of all types of cement were controlled by the sulphate‐poor environment and by the absence of bacterial sulphate reduction. All types of cement present have δ13C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V‐PDB), and highly variable and highly depleted δ18O (down to 14‰ Vienna Standard Mean Ocean Water, V‐SMOW). The late generation of cement is most depleted in both 13C and 18O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, 18O‐depleted, reducing, ice‐meltwaters accumulated in the underlying coal‐bearing sediments during the Permian deglaciation. The invariant δ13C distribution across core‐to‐rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba‐bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift‐bound, lacustrine depositional systems.

Geochemistry of some banded iron-formations of the Archean supracrustals, Jharkhand-Orissa region, India

Banded iron-formations (BIF) form an important part of the Archean supracrustal belts of the Jharkhand-Orissa region, India. Major, trace and REE chemistry of the banded iron-formation of the Gandhamardan, Deo Nala, Gorumahisani and Noamundi sections of the Jharkhand-Orissa region are utilized to explore the source of metals and to address the thermal regime of the basin floor and the redox conditions of the archean sea. Hydrothermal fluids of variable temperatures might have contributed the major part of the Fe and other trace elements to the studied banded iron-formations. Diagenetic fluids from the sea floor sediments and river water might have played a subdued role in supplying the Fe and other elements for the banded iron-formations.