Marine Dolomite Research Articles

Dynamic Climate Influence on Magnesium Isotope Variation in Saline Lacustrine Dolomite: A Case Study of the Qianjiang Formation, Jianghan Basin

The investigation of magnesium (Mg) isotopes in dolomite has mainly focused on marine dolomite environments, leaving a significant gap in the understanding of their dynamics within lacustrine settings, especially in saline lake basins. In this study, a total of 16 sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, whole-rock analysis for major and minor elements, and isotopic measurements including δ18Ocarb, δ13Ccarb, δ26Mgdol, and δ26MgSi. In addition, two intact cores were subjected to detailed analysis on the centimeter scale. Sedimentation models were established to elucidate dolomite formation under contrasting climatic conditions, specifically humid climates with a significant riverine Mg input versus relatively dry conditions with a lower Mg input. Furthermore, a quantitative model was developed to assess the magnesium flux and isotopic mass balance within lacustrine systems, simulating the magnesium isotope variations in lake water under different climatic scenarios. The dolomite sample data at a smaller scale (sampling interval ≈ 3~5 mm) demonstrate a consistent trend with the established model, providing additional confirmation of its reliability. Dolomite precipitated under humid climatic conditions exhibits a lower and relatively stable δ26Mgdol, lower δ18O, and higher CIA, indicating higher river inputs and relatively stable Mg isotope values of lake water controlled by river input. Nevertheless, dolomite formed under relatively dry climatic conditions shows a relatively high δ26Mgdol, higher δ18O, and lower CIA, suggesting reduced river inputs and weathering intensity, as well as relatively high magnesium isotope values of the lake water controlled by dolomite precipitation. This study contributes to the understanding of magnesium isotopes in lacustrine dolomite systems.

Resistance of eogenetic dolomites to geochemical resetting during diagenetic alteration: A case study of the lower Qiulitage Formation of the Late Cambrian, Tarim Basin

Rare earth elements (REEs), carbon, oxygen and Mg isotopes have been widely used to explain the origin of dolomite, but dolomite may undergo complex diagenetic alteration during long geological processes, and whether the geochemical signals of precursor dolomite can be preserved is still unknown. In order to further explore the formation mechanism of dolomite and the influence of diagenetic alteration processes on geochemical parameters, this paper takes the thick-layer dolomite of the Lower Qiulitage Formation of the Late Cambrian in the Tazhong uplift area as an example and divides the dolomite into two categories through petrologic observation. One category is fabric-retentive dolomite (FRD), including algal clotted dolomite, algal psammitic or psammitic grain dolomite, and aphanocrystalline dolomite; the other category is fabric-destructive dolomite (FDD), including crystalline dolomite (CD) and siliceous dolomite (SD). Cation ordering, unit cell parameters, major/trace elements, REEs as well as C–O–Mg isotopes of the pure matrix powders of different types of dolomites are further analyzed. The results show that the dolomites are near-source/far-source seepage reflux type and sabkha type eogenetic origin, and the magnesium ions mainly come from Late Cambrian seawater; dolomite crystals of different sizes are the result of the precursor dolomite suffering varying degrees of recrystallization during the early diagenesis period, inheriting the original geochemical characteristics; hydrothermal alteration occurs in the late stage of diagenesis, with limited magnesium ions in the hydrothermal fluid. Its impact on the dolomite is mainly reflected in local dolomite distortion and a large amount of aphanocrystalline silica and coarse-grained quartz cement filling, which fails to obviously reset the geochemical information of the matrix dolomite. The above results demonstrate that, under favorable conditions, early genetic marine dolomites may resist geochemical resetting over time intervals of more than 400 million years.

The covariation between Mg[sbnd]C isotopes and trace elements in dolostones: Implications for reconstructing the seawater chemistry

Magnesium isotopes of marine dolomites have the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between MgC isotopes and trace element contents. Our modeling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1 ‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.

A clumped isotope diagenetic framework for the Ediacaran dolomites: Insights to fabric‐specific geochemical variabilities

AbstractWhile marine dolomites formed under near surface conditions have been considered to be potentially reliable archives of past oceanic conditions, this interpretation comes with significant challenges because diagenetic alteration frequently produces diverse fabrics with large geochemical variability. It has been suggested that the Ediacaran dolomites in South China (Hamajing Member, Dengying Formation) recorded the oceanic conditions present at the time they formed, yet these dolomites are composed of five different fabrics (stromatolitic, micritic, oolitic, saddle dolomites and fibrous–radial dolomite cements) and show large variations in multiple geochemical isotope proxies (carbon, oxygen, clumped, magnesium and the sulphur of carbonate‐associated sulphate). This study establishes a paragenetic sequence for these dolomites by combining the clumped and the oxygen isotopic compositions, thereby assessing whether they are geochemically representative of the original seawater. Using this diagenetic framework, the micritic and stromatolitic dolomites show a closed‐system behaviour (low water–rock ratios; <0.3) and are largely resistant to the hydrothermal alteration during late diagenesis. In contrast, the ooid and cement fabrics have been affected by the hydrothermal fluid precipitating saddle dolomite in the open‐system condition with the high stimulated water–rock ratios (>1). Furthermore, in a closed‐system environment, the elevated δ24Mg and δ34S values in the stromatolitic dolomite reflect the isotopic Rayleigh fractionation that enriches the 26Mg and 34S through rock‐buffered recrystallization, coupled with microbial sulphate reduction. These results demonstrate that the complex signals in early marine dolomite should be carefully evaluated when used as a palaeoproxy.

A new age, provenance and tectono-sedimentary setting of the middle Dengying Formation of the terminal Ediacaran in the western Yangtze Block, South China

The widespread terminal Ediacaran Dengying Formation in the western Yangtze Block, South China, is dominated by thick marine dolomite successions deposited in shallow-water platform environments. However, a distinctive succession of mixed clastic and carbonate rocks occurs widely in the middle Dengying Formation, representing sedimentary environment variations. The absolute age of the mixed sedimentary succession is poorly constrained, and tectonic and sedimentary settings remain controversial. In this study, we report a new zircon U–Pb age of 547.3 ± 4.8 Ma from a volcanic ash bed intercalated within a mudstone succession (ca. 40–100 cm thick) in the middle Dengying Formation in the western Yangtze Block. This age is consistent with previously published ages of the middle Dengying Formation or its equivalents in the Yangtze Block and in other blocks worldwide. Field investigations show that the mudstone succession overlies above an unconformity resulted from the lower Dengying Formation weathering and shows a deepening upwards depositional succession representing a period of transgression. Mineralogical analyses show that the mudstones are composed of clay minerals (ca. 68.7%, illite and illite/smectite mixed layers), quartz (24.2%) and minor K-feldspar, indicating a high compositional maturity. Redox sensitive element results (e.g., Mo and U) indicate an increasing anoxic setting upwards. Both elemental geochemical and Lu-Hf-O isotopic analyses suggest that the siliciclastic sediments were moderately weathered (average CIA = 71) and were derived from felsic-intermediate rocks, such as granites and granodiorites (or geochemically-equivalent rocks). We suggest that the mixed sedimentary succession represents as a short-lived, regionally-correlated sea level change event, rather than a tectonic uplift event. Our geochemical data reveal that the mudstone succession accumulated most likely in a passive margin setting, supporting the proposition that the Yangtze Block was located on the periphery of the Gondwana supercontinent during the Ediacaran Period.

Lithium isotope composition of Ediacaran dolostones from the Nuccaleena and Doushantuo formations

The end of the Cryogenian glaciations undoubtedly affected the chemistry of the Neoproterozoic oceans, with potential consequences for the evolution of life; the duration and extent of this influence are poorly constrained. Lithium (Li) isotopes in carbonates can be used to investigate past weathering events and riverine input into the oceans. Here, we report the Li isotope (δ7Li) composition of the Ediacaran cap dolostones (Marinoan) from the Nuccaleena Formation, South Australia and the Doushantuo Formation, South China to investigate changes in weathering during the aftermath of the Marinoan glaciation. The origin of dolomite formation is still hotly debated, and the inability to precipitate dolomite at ambient temperatures (the ‘dolomite problem’) has not yet been resolved. The dominant hypothesis for the presence of marine dolomite is that it is of secondary origin, owing to diagenetic replacement of calcium carbonate, but a second hypothesis is that it forms as a result of primary marine deposition; both are plausible hypotheses. Using carbon isotopes (δ13C) and ratios of manganese and strontium (Mn/Sr), we suggest that diagenesis may not have significantly altered the δ7Li composition of the primary dolomite, but this cannot be discounted without further diagenetic modelling. As a result, we cannot infer the δ7Li composition of the Ediacaran oceans, as further work must be done to address the impact of diagenesis on the δ7Li values. Nevertheless, this dataset contributes to a Li isotope chemostratigraphic record of the Proterozoic, which is a key element to understanding the emergence of complex life.

Early dolomitization and subsequent hydrothermal modification of the middle Permian Qixia Formation carbonate in the northwest Sichuan Basin

High-quality dolomite reservoir are widely developed in Permian Qixia Formation in northwest Sichuan Basin. Understanding the genesis of dolomite is significant to predicting high-quality reservoir distribution. Based on the petrographical and geochemical characteristics, three types of dolomites and one type of dolomite cement were identified in the Qixia Formation in northwest Sichuan Basin: micritic to fine-crystalline dolomites (D1), medium-crystalline dolomite (D2), medium-to coarse-crystalline dolomite (D3) and saddle dolomite cement.Type D1 dolomites are characterized by micritic to fine-sized crystals, lamellar structure, generally dull to no luminescence, and relatively high strontium (Sr) concentrations. These characteristics suggest that type D1 dolomites were initially formed during the penecontemporaneous phase. The wide range of depleted δ18O values, slightly higher 87Sr/86Sr ratios and slightly positive europium (Eu) anomalies indicate that type-D1 dolomites were subsequently subjected to hydrothermal modification. Type D2 dolomite is characterized by unimodal middle-sized crystals and crystal texture dominated by nonplanar crystals. It usually occurs as thin or thick layers in the study area. The δ18O and δ13C values of type D2 dolomite generally fall within the estimated Middle Permian marine dolomite and show a significantly positive offset compared to those of coeval seawater. The 87Sr/86Sr ratios of type D2 dolomite are slightly higher than the coeval seawater. These features combined with the flat rare earth element (REE) profile and relatively high Sr concentrations (both are similar to the host limestone) indicate that type D2 dolomites first precipitated in the slightly evaporated Middle Permian seawater during the near-surface to shallow burial phase. Type D3 dolomites are mainly characterized by interlocking mosaic crystals with crystal sizes varying from 300 μm to 900 μm. It mainly occurs as patches in type D2 dolomites or as a thick layer sandwiched by type D2 dolomites. These features combined with its high 87Sr/86Sr ratios, δ18O depletion and positive Eu anomalies (which are very similar to those of the saddle dolomites) demonstrate that it is formed by hydrothermal recrystallization upon type D2 dolomites. Saddle dolomite generally occurs as the filling cement of pores, vugs, and fractures and is usually combined with matrix dolomite to form zebra dolomite and hydrothermal dolomite breccia. It has a curve crystal face, demonstrates wavy extinction under cross-polarization and red luminescence under a cathodoluminescence microscope. These features, together with the highest Th, most negative δ18O value and highest 87Sr/86Sr ratio compared to those of all types of dolomites in the study area as well as the positive Eu anomalies suggest that it should have been formed by hydrothermal fluid precipitation.Type D1 and type D3 dolomites are volumetrically minor and the pores are poorly developed. Saddle dolomite cemented and filled the pores and vugs of matrix dolomite, resulting in reservoir damage. Type D2 dolomites constitute the main body of high-quality reservoir in Qixia Formation. It precipitated in the slightly evaporated penesaline Middle Permian seawater during the near-surface to shallow burial phase. Potential high-quality reservoirs may occur in the shoal around the limited inter-shoal sea.

Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data

The marine shale in the Sinian Doushantuo Formation of Qinglinkou outcrop section is well developed, but the current characterization methods for outcrops are unsatisfactory. In this paper, the data of outcrop in the field study area were collected by Unmanned Aerial Vehicle, then processed and interpreted by oblique photography technology combined with manual investigation. Subsequently, we established a quantitative geological knowledge database of the shale formations and carried out the typical section of anatomy analysis. The results showed that the high-precision image information captured by unmanned aerial vehicle oblique photography technology can be well coupled with a three-dimensional coordinate system. The three-dimensional digital model was used to characterize the lithologic assemblage, thickness and distribution characteristics of the target reservoir. Based on this digital model, we established the three-dimensional lithology and the total organic carbon models of the outcrop area. The spatial distribution characteristics of interbedding between marine dolomite and shale in the outcrop area were displayed, and the distribution of total organic carbon was revealed under lithological constraints. The models are beneficial for the analysis and prediction of the lithology and total organic carbon, which is of great significance to the understanding of shale gas sweet spots. Cited as: Yin, S., Feng, K., Nie, X., Chen, Q., Liu, Y., Wang, P. Characterization of marine shale in Western Hubei Province based on unmanned aerial vehicle oblique photographic data. Advances in Geo-Energy Research, 2022, 6(3): 252-263. https://doi.org/10.46690/ager.2022.03.08

Application of U-Pb dating technique in the study of hydrothermal activities of dolomite reservoir: A case study on 3<sup>rd</sup> member of Yingshan Formation in Gucheng area, Tarim Basin, NW China

古城地区是塔东探区的重点勘探区块，下奥陶统鹰三段是塔东探区首要的勘探层系。探寻多期热液作用改造后的优质白云岩储层是扩大研究区勘探成果的关键。本文在储层岩石学特征判识的基础上，应用U-Pb同位素定年技术，结合碳氧锶镁同位素和稀土元素分析测试，分析了热液性质，明确了热液作用发生的确切时期及期次，探讨了热液作用对白云岩储层的影响。研究结果表明，古城地区鹰三段白云岩储层发育准同生-浅埋藏海水成因的粉晶-中晶白云岩及热液作用产物。热液作用可划分为二期：第一期为中-晚奥陶世贫镁热液，表现为鞍状白云石充填缝洞及重结晶次生加大的粗晶白云石，U-Pb测定年龄为464±12Ma到473.9±9.1Ma，氧同位素值比早奥陶世正常海相白云石明显偏负，锶同位素比值明显高于早奥陶世海水值，稀土元素Eu正异常，锶、铁、镁元素含量低；第二期为晚奥陶世-早志留世钙质热液，表现为方解石充填缝洞及对先存白云石的溶蚀，U-Pb测定年龄为448±15Ma到457.4±6.6Ma，氧锶同位素组成严重偏离同时期正常海相白云石范围，轻稀土元素明显富集，Eu正异常，锶、钙元素含量高，锰元素含量低。第二期热液作用是白云岩储层溶蚀的关键，同期NNE向断裂带作为其输导系统，是优质储层发育的有利部位，为有利区带预测提供依据。;The dolomite in 3^rd member of Yingshan Formation in Gucheng area is a significant exploration field in the east of Tarim Basin. High-quality dolomite reservoir reworked by hydrothermal activities is the key to expand exploration. Based on petrographic characteristics, this paper analyzed hydrothermal properties, clarified the precise dating and stages of hydrothermal process, and discussed hydrothermal effects on dolomite reservoir using U-Pb dating technique, combined with analysis of carbon and oxygen, strontium, magnesium isotopes and rare earth elements. The study shows that micrite-medium grained dolomites taken place in paracontemporaneous-shallow buried phase and hydrothermal products in research area. There are two stages of hydrothermal processes. The first stage hydrothermal process is very low in magnesium content in the Middle-Late Ordovician, at the result of saddle dolomite filling in fractures/vugs and coarse dolomite formed by recrystallization aged 464±12Ma~473.9±9.1Ma by U-Pb dating with oxygen isotope negative compared with normal marine dolomite, strontium isotope values higher than Early Ordovican sea values and positive Eu anomalies. The contents of strontium, ferrum and magnesium are relatively low. The second stage hydrothermal process is high in calcium content in the Late Ordovician-Early Silurian, at the result of calcite filling in fractures/vugs and dissolution of existing dolomites, aged 448±15Ma~457.4±6.6Ma by U-Pb dating with significantly negative oxygen isotope and enriched LREE and positive Eu anomalies. The strontium content is high while manganese content is low. Migration pathway of the second stage hydrothermal process as a key of dolomite reservoir dissolution is NNE fracture zones developing high-quality reservoir, which provides a basis for the prediction of favorable zones.

Characteristics and primary mineralogy of fibrous marine dolomite cements in the end-Ediacaran Dengying Formation, South China: Implications for aragonite–dolomite seas

Fibrous marine cements are widely distributed in the terminal Ediacaran Dengying Formation of South China and are an important record for reconstructing paleoseawater chemistry. However, the morphological characteristics and filling sequence of these cements, and their formation mechanism and paleoceanographic significance, are poorly understood. Our investigation of five outcrop sections in the northern upper Yangtze region allows four types of fibrous dolomite cement to be identified: fascicular fast dolomite (FFD), radial fast dolomite (RFD), radial slow dolomite (RSD), and fascicular slow dolomite (FSD), where “fast” and “slow” refer to the optical character of cement crystals. FFD cements possess original aragonitic mineralogy, whereas RFD cements had high-Mg calcitic precursors that subsequently underwent mimetic dolomitization via syntaxial replacement. RSDs and FSDs are primary marine precipitates, whereas fibrous dolomite cements with length-fast optical characteristics (FFDs and RFDs) are of secondary origin. These fibrous cements are widely distributed laterally in the Dengying Formation, although the RSDs and FSDs occur only in the second member of this formation. Therefore, the seawater chemistry of the terminal Ediacaran period in the upper Yangtze area was dominated by an aragonite sea, with a regional short-lived “dolomite sea” during the late depositional stage of the second member of the Dengying Formation, from which dolomite may have directly precipitated.

Early-stage marine dolomite altered by hydrothermal fluids in the Middle Permian Maokou Formation in the eastern Sichuan Basin, Southern China

Deciphering the dolomitization process is the key to understanding the formation of massive dolostones on shallow-marine platforms, it also improves our understanding of dolostone reservoirs. Hydrothermal fluids are suggested to be a potential mechanism for dolomitization; however, determining whether hydrothermal fluids are responsible for dolomitization or the recrystallization of an early dolostones is poorly understood. The dolomite of Maokou Formation (Guadalupain) in the eastern part of the Sichuan Basin is a good case study on the hydrothermal dolomitization and recrystallization. Based on petrographic observation, the massive dolostone (MD) consists of fine-crystalline dolomite (D1), fine to medium crystalline dolomite (D2) and coarse-crystalline dolomite (D3), and the patchy dolostone (PD) which is composed of medium to coarse dolomite with an euhedral to subhedral texture. Since the fine-crystalline dolomite is cross-cut by low amplitude stylolites, and is composed of euhedral dolomite with a dull red luminescence, dolomitization is interpreted to have occurred in near surface condition. While the Fe, Mn, and ∑REE concentrations, δ13CVPDB values, 87Sr/86Sr ratios, and REE Pattern of D1 are comparable to those of the coexisting limestone, and the δ18OVPDB values are slightly higher than that of the coeval limestone, indicating that the dolomitization fluid of D1 was derived from Middle Permian seawater. In contrast, zebra fabrics and saddle dolomites are common in D2 and D3, and their geochemical characteristics and cathluminescence are quite different from those of D1 but they are similar to those of the saddle dolomites. Besides, the fluid inclusions in D2 and D3 have high homogenization temperatures and salinities, which are also similar to those of the saddle dolomite. Moreover, the D2 shows an inequigranular texture, and there are no obvious boundaries between D1, D2 and D3. Therefore, the D2 and D3 were recrystallized upon D1 by hydrothermal fluids related to the Emeishan magmatism. The patchy dolostone exhibits a non-stoichiometric composition and low degree of cation ordering, suggests rapid and incomplete dolomitization. The dolomitization of PD is interpreted to be the replacement caused by hydrothermal fluids as the PD exhibits negative δ18OVPDB values, high homogenization temperatures and salinity, positive Eu anomalies and REE Patterns similar to those of the saddle dolomites. The porosity and stoichiometry of the D1 are higher and lower, respectively, than those of D2 and D3, and the vugs are almost entirely filled. Thus, the hydrothermal fluids were destructive for the formation of the dolostone reservoir of Maokou Formation in the study area.

Response of Mg isotopes to dolomitization during fluctuations in sea level: Constraints on the hydrological conditions of massive dolomitization systems

Marine dolomitization processes are characterized by complex variations in hydrological conditions and pore-fluid chemistry. Deposition of massive dolomitized carbonates on the Comanche Platform of south Texas, USA, during the Albian coincided with multiple fluctuations in sea level, thereby providing an ideal setting to study the response of magnesium isotopes to dolomitization during eustatic sea-level change. In this study, we conducted Mg, C, and O isotope analyses and complimentary mineralogical and petrographic investigations of dolomitized massive carbonates of the Albian Edwards Group in the Comanche Platform. Petrographic observations indicate that the carbonate rocks in the studied units were not altered by deep burial diagenesis or hydrothermal fluids. Based on our petrographic observations and the trace element and C–O isotope data, we infer the dolomites were formed via syn-depositional dolomitization during a period of low sea level. The δ26Mg values of the dolomites increase rapidly from −2.5‰ to −1.8‰ in the basal part of the unit, reflecting a change in fluid chemistry caused by dolomitization in a restricted marine environment. Subsequently, δ26Mgdolomite values gradually decrease back to their initial value of approximately −2.5‰ due to seawater replenishment during transgression. The observed variability in dolomite δ26Mg values reflects changes in the connectivity of the platform with the open ocean during marine transgressions. However, the δ26Mgdolomite values do not vary with the high-frequency eustatic sea-level change recorded in the lithological variations, indicating uniform hydrologic conditions of the massive dolomitization system despite the hydrodynamic variations in the sedimentary environment. Therefore, we propose that massive dolomitization systems are mostly fluid-buffered and, as a result, Mg isotopes of dolomites can be used to trace changes in the paleo-marine environment, such as basin connectivity.

Controls on formation and alteration of early diagenetic dolomite: A multi-proxy δ44/40Ca, δ26Mg, δ18O and δ13C approach

The full potential of the dolomite Ca isotope proxy only unfolds when combined with data of the other main elements (C, O, Mg) in the crystal lattice of Mg-carbonates. Data presented here reveal the level of complexity inherent to dolomite precipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation. Well-constrained Precambrian to Pleistocene dolomites were investigated, representing three characteristic formation and alteration environments: (i) sabkha, (ii) altered marine and (iii) lacustrine/palustrine dolomites. Primary sabkha dolomites with typically low cation ordering degree (COD), high δ13C and low δ44/40Ca values contrast with recrystallized sabkha dolomites with relatively high COD, low δ13C and high δ44/40Ca values. Both δ13C and δ44/40Ca values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium Ca isotope fractionation conditions. Primary sabkha dolomites display Δ44/40Cadolomite-fluid ranging from −0.4 to −1.3‰, whereas recrystallized dolomite is approaching isotopic equilibrium (Δ44/40Cadolomite-fluid ∼ 0‰). Using the fractionation factor deduced from the sabkha dolomite data set, recrystallized Precambrian dolomite points to a δ44/40Caseawater/pore fluid of about 1.2‰ (SRM 915a), a value that is distinctively lower compared to previously suggested ones and modern seawater. Altered marine dolomites display evidence for meteoric overprint, indicated by δ18O values as low as −4.62‰. Both Mg and Ca isotope signatures correlate with δ18O values of altered marine dolomites, whereas δ13C values lack correlation with the other isotope systems. We propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and Ca, Mg and O isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint. Lacustrine/palustrine dolomites display a correlation between the isotope values of C, Mg and Ca. These dolomites are formed during pulses of marine ingression in swamp, playa and lake environments and are thus characterized by water-logged conditions (anaerobic) and saline, sulfate-rich fluids. Bacterial-sulfate reduction induces dolomite formation and leads to lower δ13C and δ44/40Ca as well as to higher δ26Mg values. The Ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested. Taking COD and dolomite stoichiometry into consideration, the here documented multi-proxy isotope approach is promising and provides benchmarks against which proxy signals can be calibrated and tested. Interestingly, even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research.

Halogens (Cl, Br, and I) geochemistry in Middle Triassic carbonates: Implications for salinity and diagenetic alteration of I/(Ca + Mg) ratios

As an important family of elements, halogens (Cl, Br, and I) have been widely used to study geological processes, particularly in high-temperature environments, showing great prospects either in fundamental or in applied studies. However, little work has been conducted for sedimentary and low-temperature diagenetic environments re-construction although these elements have the potential in theory. Here we perform a pilot study on this issue using a case study of Middle Triassic marine dolomitization in South China. Our results show that the average Cl concentrations in limestone, dolomitic limestone, calcitic dolomite and dolomite are 88.3 ppm, 114.7 ppm, 121.3 ppm and 142.9 ppm respectively, and the corresponding Br concentrations are 1.15 ppm, 1.42 ppm, 1.59 ppm and 2.13 ppm, respectively. Both the concentrations of Cl and Br increased with the increasing dolomitization degree. The increasing tendency of Cl and Br is similar to the seawater evaporation trajectory before halite precipitation, indicating that dolomitization occurs in fluids with elevated salinity. Meanwhile, the difference of Cl/Br ratios between our carbonate samples and seawater may be ascribed to the input of terrestrial water, which has variable Cl/Br ratios. As for the concentration of I, the mean values sharply decreased from 0.36 ppm in limestone to 0.13 ppm in dolomitic limestone samples, and it further lowered to 0.09 ppm and 0.08 ppm in calcitic dolomite and dolomite samples, respectively. The calculated I/(Ca + Mg) ratios in limestones vary between 0.05 and 0.58 μmol/mol with an average value of 0.29 μmol/mol, indicating that limestone formed in open ocean oxygen-minimum zone (OMZs) or anoxic basin environment. The average I/(Ca + Mg) ratio decreases from 0.29 μmol/mol in limestone to 0.06 μmol/mol in dolomite, serving as an important case that dolomitization decreases I/(Ca + Mg) ratios of carbonates rocks. Halogens in carbonates therefore have promising potential for investigating the sedimentary and diagenetic environments of carbonate rocks.

Formation, diagenesis and palaeoenvironmental significance of upper Ediacaran fibrous dolomite cements

AbstractNeoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high‐resolution geochemical data from well‐preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south‐west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high‐Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length‐slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox‐sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre‐scale spheroidal ankerite and sulphur‐containing dolomite, and intergrown pyrite grains with U‐shaped δ34S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite.

Testing the preservation potential of early diagenetic dolomites as geochemical archives

AbstractEarly marine diagenetic dolomite is a rather thermodynamically‐stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post‐depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U‐Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike‐slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric‐preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto‐Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high‐grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late‐stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically‐complex dolomite archives must be considered with caution.

Sparse spectral attributes to detect hydrocarbons: A case study from deep dolomite reservoirs

Hydrocarbon detection is always one of the most critical sections in geophysical exploration, which plays an important role in subsequent hydrocarbon production. However, due to the low signal-to-noise ratio and weak reflection amplitude of deep seismic data, some conventional methods do not always provide favorable hydrocarbon prediction results. The interesting dolomite reservoirs in Central Sichuan are buried over an average depth of 4500 m, and the dolomite rocks have a low porosity below approximately 4%, which is measured by well-logging data. Furthermore, the dominant system of pores and fractures as well as strong heterogeneity along the lateral and vertical directions lead to some difficulties in describing the reservoir distribution. Spectral decomposition (SD) has become successful in illuminating subsurface features and can also be used to identify potential hydrocarbon reservoirs by detecting low-frequency shadows. However, the current applications for hydrocarbon detection always suffer from low resolution for thin reservoirs, probably due to the influence of the window function and without a prior constraint. To address this issue, we developed sparse inverse SD (SISD) based on the wavelet transform, which involves a sparse constraint of time-frequency spectra. We focus on investigating the applications of sparse spectral attributes derived from SISD to deep marine dolomite hydrocarbon detection from a 3D real seismic data set with an area of approximately [Formula: see text]. We predict and evaluate gas-bearing zones in two target reservoir segments by analyzing and comparing the spectral amplitude responses of relatively high- and low-frequency components. The predicted results indicate that most favorable gas-bearing areas are located near the northeast fault zone in the upper reservoir segment and at the relatively high structural positions in the lower reservoir segment, which are in good agreement with the gas-testing results of three wells in the study area.

Neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites

Neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large Cryogenian glaciations. Several thin dolomite units that have features resembling cap dolomites occur within the Cryogenian-Ediacaran succession of the Adelaide Geosyncline. While each of these dolomite units has distinctive features, they also share many similarities with cap dolomites including: stratigraphic thickness ranging from several centimetres to several metres; impure dolomicrite composition; graded beds with a sharp top that become more dolomite-rich upwards; and evidence of volume expansion in the form of sheet cracks, tepees or other soft- sediment deformation structures. The carbon isotopic compositions for these thin dolomite horizons have values ranging from ∼−5 to +2 per mil VPDB, which overlap those commonly found in both Sturtian and Marinoan cap dolomites. The generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate.Sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground, where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation. Dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds. Volume expansion structures are a product of this near-surface dolomicrite precipitation, brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited (marine) carbonate source for the precipitating dolomite.We suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of Cryogenian- early Ediacaran oceans and caused by alkalinity-producing anoxic bacterial reactions. When sedimentation rates slow sufficiently (e.g. during transgressions), thin dolomicrite horizons are produced. In this hypothesis, cap dolomites may largely be the result of sediment starvation, combined with a possible increase in carbonate saturation during and following the post-glacial transgressions associated with the Sturtian and Marinoan glacial events. This scenario explains most of the sedimentological features of these unusual carbonates. The widespread development of such authigenic carbonate would have profound implications for the interpretation of Precambrian marine carbon isotopic compositions.

The effects of diagenesis on geochemical paleoredox proxies in sedimentary carbonates

Metal and metal isotope records in carbonates have the potential to provide novel insights into ancient ocean–atmosphere redox conditions, paleoenvironmental conditions, and biogeochemical cycling. However, trace element geochemical signatures in carbonates can record either diagenetic or depositional signatures. Here we explore the variability in uranium isotopes, trace metal and rare earth element + yttrium (REY) concentrations in carbonate successions that have undergone several common types of diagenetic alteration. Case studies include the Cryogenian Balcanoona Reef, Australia (marine dolomitization, neomorphism); the Devonian Canning Basin reefs, Australia (burial dolomitization, karstification); the Paleozoic of the Great Basin, USA (high temperature and regional burial dolomitization) and the Carboniferous Waulsortian Limestone, Ireland (Pb-Zn mineralization). In all of the examined cases there are significant heavier and/or lighter shifts in U isotope values between the most petrographically pristine marine depositional components and altered or late-stage diagenetic phases. Although we also found that REY patterns can be overprinted during diagenesis, normalized REY profiles appear to be commonly retained through diagenetic recrystallization, consistent with previous studies. The direction and magnitude of the change in metal isotope systems during diagenesis cannot be generalized between case studies, consistent with carbonate alteration being controlled by a wide range of factors including the composition and source of the alteration fluids. In this light, we build on framework developed from traditional isotope systems and support the view that future work on metal and metal isotope compositions of carbonates should include integrated sedimentological and petrographic analysis. Petrographic work not only represents a screening procedure for sedimentary geochemical work, but can also significantly enhance the paleo-environmental interpretation of metal isotope data.

The Tonian Beck Spring Dolomite: Marine dolomitization in a shallow, anoxic sea

The reason for the abundance of dolomite lithologies in Earth's early geological record compared to modern environments remains contentious. This study provides new insight into this Precambrian “dolomite problem” by revisiting one of the most controversial dolomite localities, the Beck Spring Dolomite, of Death Valley, USA. Consistent with some previous studies, petrographic evidence indicates that although the Beck Spring Dolomite now consists almost entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite. Depositional constituents (microbialites and ooids) were likely originally aragonitic, and early marine length-fast cements (now dolomite) are suggested to have precipitated as high-Mg calcite then replaced syntaxially by dolomite. Based on petrographic and geochemical evidence, we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit, and for the most part, involved syntaxial and mimetic replacement. Further, a length-slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement. This is indicated by the length-slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation. This new evidence for Tonian marine dolomite precipitation reinforces the idea of Precambrian marine environmental conditions, including the chemical composition of seawater, promoting dolomite formation at this time. The trace metal geochemical composition of well-preserved marine components, especially dolomite marine cements, reveals information about redox conditions in this Tonian shallow seawater. In terms of rare earth element geochemistry, the Beck Spring Dolomite has no significant Ce anomaly, and a ubiquitous positive Eu anomaly, consistent with widespread oceanic anoxia during deposition. Furthermore, the relatively low levels of iron and chalcophile elements Co, Cu, Pb and Zn in marine components compared to other Neoproterozoic carbonates suggest euxinic conditions (both anoxic and sulphidic) prevailed during deposition of the Beck Spring Dolomite.