Petrographic Characteristics Research Articles

Recycled crystal population proving complex plumbing systems: an archetypal example from Cretaceous alkaline magmatism in SE Brazil

Porphyritic alkaline dykes from the Serra do Mar province (SE Brazil) carry clinopyroxene macrocrysts with a complex magmatic history. Clinopyroxene is the main macrocryst phase from the Bom Repouso dyke, representing different populations. Based on petrographical features, mineral chemistry and equilibrium relationships, the clinopyroxene populations are classified as antecrysts (green cores and colourless cores and mantles) and phenocrysts (Ti-augite mantle/rims). Calculated compositions for the liquid in equilibrium with the macrocryst cores indicate mafic (colourless cores) and felsic (green cores) alkaline melts. These were used as poles for mixing-model curves, revealing a hybrid origin for the host matrix. Clinopyroxene barometry reveals key insights into the plumbing systems of the Serra do Mar province, with the Bom Repouso dyke as an archetypal example of these transcrustal systems. All target bodies record a crystallization history over a continuous range of pressures (>8 kbar up to the surface). Some preferred levels for magma storage are observed: c. 0–0.5, 1–2.5, 3–4 and 5.5–7 kbar. Our work highlights the complex magmatic history of the Serra do Mar alkaline occurrences, encompassing several open-system effects. A thorough petrographical examination is necessary to enhance our comprehension of these magmas, while petrogenetic models based on whole-rock compositions may lead to biased interpretations.

Compositional and textural evidences of Na-bentonite authigenesis in the Pellegrini lake deposit (Neuquen basin, Argentina)

Textural and compositional variations of Na-bentonites from the Pellegrini Lake deposit (Neuquén Basin) have been established, with special emphasis on petrographic features in order to better understand the genetic pathways in their formation. In this region, two bentonite beds named “white bentonite” and “green bentonite” are mined mainly for the oil and foundry industries. The genesis of these beds was studied using petrological (petrography, SEM and TEM), mineralogical (DRX, FT-IR, DTA-TGA, 27Al MAS NMR) and geochemical (ICP-AES) data from two quarries named COR and CA. The bentonites mainly consist of Na-montmorillonite, with lesser amounts of quartz, plagioclase, potassium feldspar, illite, zeolite, barite and gypsum. Mineralogical and petrographical evidence indicates that the authigenic smectite was formed by devitrification of unstable glass shards in a shallow coastal marine environment. Green bentonite would be the result of several episodes of volcanic ash-fall input from multiple eruptions with a higher degree of alteration uppermost in the bed. In this case, the alteration would have occurred in a reducing environment as evidenced by the greenish colouration of these bentonites, the Ce anomaly and the presence of pyrite. Meanwhile, white bentonite would have formed in a less restricted environment than green bentonite, as evidenced by the existence of intercalations of silt-sized grains and a higher amount of detrital grains in the clay groundmass transported probably by fresher waters. The presence of celestine, barite, halite and gypsum indicates evaporitic conditions after smectite formation. The observed secondary gypsum and iron staining of surfaces are much later processes. Green bentonite samples indicate a trachyandesite composition of the parent material, while the white bentonite suggests a parent material most related to a rhyodacite/dacite composition showing that the characteristics of the volcanism might have changed with time.

Constraints on the impact history of the Apollo 16 landing site: Implications of soil‐like breccia noble gas records

AbstractThe Apollo 16 regolith breccia sample suite provides a record of lunar regolith formation from the basin‐forming epoch (~3.9 Ga) through to a time of declining impactor flux (~2 Ga). These rocks have been characterized into three groups: the “ancient,” “young,” and “soil‐like” regolith breccias on the basis of their petrographic characteristics, and, in the case of the “ancient” and “young” regolith breccias, noble gas inventory. This study investigates the as‐yet unexamined noble gas records of the “soil‐like” regolith breccias to understand more recent regolith evolution processes that occurred at the Apollo 16 landing site. The range of gas concentrations measured for each noble gas in these samples is comparable to those previously reported for the local Apollo 16 soils. The “soil‐like” regolith breccias were found to be more gas rich than the gas poor “young” and “ancient” regolith breccias, consistent with them having formed from comparatively mature soil(s). Our results further confirm the scientific value of lunar regolith breccias and bulk regolith samples as probes of the impact history and the space environment of the lunar surface across a wide range of time.

New insights on the origin for rapakivi granites: U-Pb and Lu-Hf in zircon from Alto Candeias Intrusive Suite, Amazonian Craton, Brazil

The Southwestern Amazonian Craton, in Rondônia state, Brazil, comprises multiple orogens with ages ranging from 1.8 to 1.0 Ga, where successive magmatic, metamorphic, and deformational processes reworked ancient rocks and produced new complexes and juvenile continental crust. Identification of the geochronological and petrographic characteristics of granitoids and terranes generated during such events is crucial for the recognition and distinction of their magma sources. A total of 64 samples collected from the Alto Candeias Intrusive Suite outcrops in Rondônia state recognized four different groups: (1) Monte Negro unit (rapakivi granites), (2) Buriti unit (equigranular to porphyritic granodiorites), (3) Campo Novo unit (medium to coarse-grained charnockites), and (4) Jacilândia unit (fine-grained granites). U-Pb geochronological data were obtained for each unit, with the following ages: 1350 ± 5 Ma (Monte Negro unit), 1350 ± 2 Ma (Buriti unit), 1349 ± 3 Ma (Campo Novo unit), 1348 ± 3 Ma, and 1349 ± 1 Ma (Jacilândia unit). The ages of the four granitic units are the same within error. Lu-Hf isotopic results presented in this study indicate that the Alto Candeias Intrusive Suite has the mantle as the magmatic source and that the continental crust coeval to the subduction process. This process led to the formation of I-type granitoids generated in the SW margin of the Amazonian Craton around 1350 Ma during San Ignacio orogeny.

Geochemical characterization and reserve estimation of the economic heavy minerals of Sonadia Island, Bangladesh

The goal of the current study is to understand the elemental concentration, reserve assessment, mineralogical composition, and distribution of heavy minerals in the dune sands, foreshore, and backshore of Sonadia Island, Southeast Bangladesh. The study area yielded a number of drill hole composite sand samples that were used for density and magnetic separation of heavy minerals. These samples were then subjected to petrographic characterization, grain counting, mineralogical analyses using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX), x-ray diffraction (XRD), and bulk sand geochemistry using X-ray fluorescence (XRF). The results of heavy liquid separation show that the total heavy mineral (THM) enrichments in foreshore sand (about 4.86%), backshore sand (about 9.03%), and dune sand (about 22.28%) are significantly higher. After characterizing the sand samples from Sonadia Island based on the abundance of each mineral species, the average content values relative to total economic heavy minerals (TEHM) were determined to be 38.14% ilmenite, 5.74% magnetite, 51.52% garnet, 1.01% zircon, 3.57% rutile, and 0.01% monazite. An SEM–EDX examination reveals that the elemental concentration of ZrO2 in zircon is 66.60%, while the average TiO2 content in rutile is ~ 95%, whereas TiO2 is 45% and the total Fe2O3 is 45.70% in ilmenite. Major phases including amphibole (tremolite) 45.9%, muscovite 15.6%, quartz 12.6%, garnet (almandine) 10.9%, dolomite 7.9%, ilmenite 4.1%, and kaolinite 1.3% are also identified by the XRD pattern. There are an estimated 19,794 tons of garnet in the foreshore sand, 47,324 tons in the backshore sand, and 276,868 tons in the dune sand within the reserve, making it the most prevalent heavy mineral. Ilmenite is the second most common heavy mineral, with quantities detected in the foreshore (15,016 tons), backshore (30,487 tons), and dune sand (215,092 tons).

Characteristics and geological significance of carbon and oxygen isotopes of the Permian Lucaogou Formation dolomite in the southern Junggar Basin, northwestern China

Anomalously positive δ13C values in ancient dolomites are very rare. Dark gray argillaceous rocks of the lacustrine sediments of the Permian Lucaogou Formation are important source rocks in the Junggar Basin, and dolomites of varying thicknesses from 10 cm to 150 cm are often interspersed in argillaceous rocks. Based on the study of petrographic sections, this paper systematically analyzes the carbon and oxygen isotopes of dolomite and discusses the causes of abnormally high carbon isotope values and their significance in reconstructing paleoenvironment and paleoclimate. The results show that carbon isotope values are abnormally high in the dolomite of Lucaogou Formation, and the δ13C value is between +3.2 ‰ PDB and +19.6‰ PDB, with an average of +9.7‰ PDB. The δ18O values range from -17.4‰ PDB to -1.7‰ PDB, with an average of -8.1‰ PDB. From the lower part to the upper part of the Lucaogou Formation, the carbon isotope value gradually increases and becomes increasingly positive, and the carbon isotope of the dolomite deposited near the shore is more positive than that of the dolomite deposited far from the shore. The anomalously positive δ13C of the dolomite is mainly caused by microbial methanogenesis, with some contribution from evaporation. Microorganisms are mainly distributed at the redox interface. Evaporation controls the salinity and fluctuation of the redox interface in sedimentary water. The positive deviation difference in carbon isotopes between nearshore and offshore sedimentary dolomites may be related to the location of the redox interface during deposition. Together, the petrographic features and carbon and oxygen isotope signatures of the sections reflect the gradual evolution of the paleolake from a hydrologically open environment to a hydrologically closed one and the possible transition of the paleoclimate from a relatively warm to an arid condition, which is possibly a geochemical response to global climate change in the Permian period.

Dolomite formation in the Miocene Kardiva platform, Maldives archipelago: a tale of closed-system and open-system dolomitization by current pumping of seawater

Dolomites from two IODP Expedition 359 sites on the northeastern margin of the Miocene Kardiva platform, Maldives archipelago, were examined to explore the question of open versus closed system dolomitization in a drowned carbonate platform. The uppermost approx. 130 m of platform margin carbonate at site U1465 contains less than or equal to 12% dolomite except for five, meters-thick intervals with up to 65% dolomite. All U1465 dolomites consists of decimicron-sized euhedral cement crystals and mimetically replaced peloids and coralline red algal clasts. The abundance and the petrographic features of these dolomites are similar to periplatform and slope dolomite in many other settings that have been interpreted as the product of hydrologically closed-system diagenesis. In contrast, the recovered platform margin deposits at site U1469 are greater than 99% dolomite. Those dolomites are partially fabric retentive with fine-to-medium crystalline, planar subhedral to euhedral crystal mosaics of replacive dolomite and dolomite cement, all with Sr contents that average 256 ppm. Their characteristics are comparable to Neogene platform carbonates universally interpreted to signify hydrologically open-system dolomitization. Sr-isotope ages indicate Miocene dolomitization at both sites after platform drowning, and 18Odolomite values are compatible with dolomitization by cold (10⁰ to 15⁰ C) seawater when the platform margin was, on average, approx..400 m below sea level (mbsl). A current pumping mechanism for the advection of seawater into the top of the platform at site U1469 is proposed and tested with a computational fluid-flow simulation. Current pumping occurs when strong ocean bottom currents flow over sedimentary bedforms and generate lateral pressure differences along the sediment-water interface. The pressure differentials drive seawater through the underlying sediments. The flow simulation shows that the ocean currents that swept large sediment sand waves over and off the drowned Kardiva platform for many millions of years could have vigorously pumped Miocene seawater to sub-seafloor depths of many tens of meters . Mass balance considerations suggest complete dolomitization of the upper 20 m of the platform within 500 Ky or less. The current pumping mechanism could drive dolomitization and mineralogical stabilization below hiatal surfaces, or very slowly accumulating sediments, in any marine setting characterized by strong bottom currents. The greater acidity of Mioc ene seawater relative to younger seawater possibly made the pumping, by any means, of Miocene seawater of normal salinity an effective dolomitizing agent at site U1469 and in undrowned Miocene platforms globally.

Detrital K-feldspar as a novel archive of continental crustal evolution using coupled in situ Rb[sbnd]Sr dating and Pb isotope analysis

Here we present in situ RbSr and Pb isotopic analysis of K-feldspar from the Shap Granite, (Cumbria, UK) and detrital K-feldspar from the associated Shap Wells Conglomerate Formation, to investigate the potential of the detrital K-feldspar record as an archive of crustal composition. We use a unique, collision cell – multi collector inductively coupled plasma tandem mass spectrometer to make the in situ isotope measurements, using ‘chemical resolution’ of 87Rb+ and 204Hg+ isobaric interferences on 87Sr+ and 204Pb+, achieved with SF6 and NH3 reaction gases, respectively. Using this technique, we successfully demonstrate that the majority of detrital K-feldspar RbSr ages, as well as initial Sr and Pb isotope ratios, from the Shap Wells Conglomerate are within uncertainty of those for parental Shap Granite K-feldspar (398 ± 1 Ma, n = 6). Some detrital K-feldspar grains record perturbed RbSr ages, reflecting post depositional alteration events. The majority of samples with disturbed RbSr ages carry distinct petrographic features evident in SEM imaging, prior to in situ dating, which indicates a means to help screen for reliable ages.We further illustrate the potential value of combined RbSr and Pb isotopic information preserved in K-feldspar to estimate the timing and chemical composition of juvenile continental crust formed in the past. Where UPb has been fractionated during juvenile crust formation, the initial Pb isotope ratio provided by the K-feldspar yields model crustal U/Pb (diagnostic of tectonic setting) and extraction age. The contrast in the U/Pb composition of juvenile crust formed in intraplate or subduction settings allows the model U/Pb ratios of the detrital K-feldspars to be used to constrain the style of juvenile crust formation. Additionally, the initial 87Sr/86Sr isotope ratio measured in detrital K-feldspar can be combined with the calculated crustal extraction age and RbSr K-feldspar age to model the Rb/Sr of the juvenile crustal protolith.

Origin of the Ca-phosphate inclusions in Ivory Coast and Australasian Muong-Nong-type tektites

Tektites are reduced (Fe2+) glasses formed by the quenching of molten material ejected from Earth’s surface as a result of a hypervelocity impact. The vast majority of tektites are usually homogeneous glasses, but rare samples containing mineral inclusions can provide insights about the source material, sample thermal history, and tektite formation process. Tektites from two distinct strewn fields presenting Ca-phosphate inclusions detected from anomalous magnetic susceptibility were studied: one sample from the Ivory Coast tektite (ICT) field ejected at 1.07 Ma from the Bosumtwi crater (10.5 km in size) in Ghana and two Muong Nong type samples from the Australasian tektite field (MN-AAT) ejected at 0.79 Ma from a crater possibly situated in southeast Asia. In ICT, Ca-phosphate inclusions are systematically embedded in lechatelierite (SiO2 glass). In MN-AAT Ca-phosphate are either embedded in lechatelierite or in Fe-rich glass forming schlieren. Multiscale petrographic characterization using correlative microscopy associating scanning electron microscopy, microprobe and, transmission electron microscopy reveals that rounded inclusions in ivoirite are composed of acicular Ca-phosphates (merrillite) embedded in an amorphous P-rich glass. In MN-AAT, inclusions consist mostly of single droplets of Fe-Mg rich Ca-phosphate (structurally related to apatite), but few droplets often forming an emulsion texture show a complex assemblage of apatite, magnetite, pyroxene, and spinel growing from a Pt-rich nucleus. Diffusion profile around lechatelierite domains reveals maximum temperatures greater than 2200–2400 °C in the impact plume of the Australasian tektite and the Ivory coast tektite. Heating time is of the order of seconds-tens of seconds rather than minutes as previously suggested (20 s for MN-AAT and 5 s for ICT). The number, the density, and the fact that inclusions are entirely crystallized in MN-AAT support relatively slow cooling rates (<200 °C/h), in comparison with the faster cooling rates (>2000 °C/h) indicated by the precipitation of amorphous P-rich glass in ICT. In both impact events, ejecta that had been heated to high temperatures did not remain in the vapor plume for an extended period of time and landed rapidly (within tens of seconds) at a relatively high temperature (>1000 °C) on the Earth’s surface.Phosphate inclusions systematically embedded in lechatelierite in ICT provide clues about the source material. It suggests that the parent material for these silica-rich inclusions is not conventional detrital quartz. Rather, parts of lechatelierite domains may be inherited from a biogenic source that could be consistent with tropical soil (source of the phosphor) and its biomass (silica of plant origin). The reduction process that tektites record during their formation may be explained by superficial material since forests can contain a sizable mass of carbon that can reduce iron in tektites and produce platinoid-rich metallic nuclei and the Fe3+/ΣFe gradient recorded by the dendritic spinels.

Petrographic signature of the gravel fraction from late Quaternary glacigenic sediments in the Ross Sea (Antarctica): Implications for source terranes and Neogene glacial reconstructions

The Ross Embayment is a key region to study the dynamics of the ice sheets during colder and warmer than present climatic conditions, because both the East and West Antarctic Ice sheets shed into the Ross Sea. Numerical modeling and reconstructions of the paleo ice flows during the Last Glacial Maximum show variable contribution of East and West Antarctic Ice sheets based on a variety of proxies. In this study, we present the first petrographic and minero-chemical investigation of the gravel-sized fraction of Last Glacial Maximum subglacial-glacimarine sediments collected with piston cores in a W–E transect across the Ross Sea. The clast petrographic features are compared with outcropping geology to individuate the sediment source regions. The gravel content of the glacigenic diamictite was classified on the basis of petrographic and minero-chemical features, and three main petrofacies were identified. They reflect changes in the basement geology of the source regions, allowing the reconstruction of the paleo ice flow pattern and their comparison with scenarios built up with other datasets. Moreover, the comparison with the Oligocene to Pleistocene glacigenic sediments provided information about the changes of the gravel signature across the Ross Sea and the erosion history of the source regions during Cenozoic.

Grain textural bias in detrital single-mineral provenance studies

Detrital single-mineral geochemistry and geochronology are strong tools in provenance studies and indicate great potentials in addressing issues in earth sciences. Various biases (both natural and artificial) exist objectively and may mislead provenance interpretations. Both the sedimentary sorting process and hand-picking in-laboratory processing may lead to analyzed grain textural (e.g., size and shape) variability and thus may introduce biases in single-mineral provenance analysis. Here, we take the Mesozoic–Cenozoic Qaidam basin, northeastern Tibet, as an example to investigate the relationship between single-mineral grain texture and detrital zircon geochronological and detrital tourmaline, rutile and garnet geochemical data and to explain how grain texture affects detrital single-mineral provenance interpretations. Results indicate that Precambrian zircons take less proportions in coarse (>125 μm), subrounded and high aspect ratio (>2) fractions than Phanerozoic zircons. Parent rock lithology discrimination results of detrital tourmaline and garnet in different grain size fractions show significant differences. Zr-temperature values of detrital rutile have an increasing trend with increasing grain size. The geochemistry of detrital tourmaline, rutile and garnet shows no dependence with grain aspect ratio and roundness. We suggest that inheritance of grain texture features from parent rocks is the major reason. Detrital zircons from recycled (meta)sedimentary rocks tend to be smaller and more rounded than those from igneous rocks. Detrital tourmaline, rutile and garnet grains from different parent rock types vary in size. Grain textural bias may cause the underestimated contributions of the Qilian Shan to the Cenozoic Qaidam basin if small detrital zircons were not involved in the analysis. Quantitative description of the source-to-sink system of the Cenozoic Qaidam is also influenced by grain textural bias. This study highlights the underestimated grain textural bias in single-mineral provenance studies. We suggest that a comprehensive understanding of potential sedimentary sources, depositional processes, sample petrographic features and laboratory analysis procedures is important to reliable provenance interpretations and to related implications in earth sciences.

Adsorption behavior of helium in quartz slit by molecular simulation

Due to the multiple influences of unique physicochemical properties of helium, petrographic characteristics and temperature and pressure conditions, little is known about the helium adsorption behaviors in minerals and rocks at geological conditions. Based on the grand canonical Monte Carlo simulations, this study revealed the adsorption characteristics of pure helium and the competitive adsorption of binary mixtures with different proportions of methane and helium under geological temperature and pressure conditions in quartz slit model. Molecular simulation of pure helium shows that physical adsorption of helium exists in mineral surfaces, which indicates a preservation mechanism of helium in helium source rocks. Binary mixtures simulations indicate that the adsorption capacity of methane in quartz is stronger than that of helium, and the competitive adsorption of methane increases with decreasing burial depth. This means that during the upwards migration processes of natural gas, the adsorbed helium that distributed in the migration pathway will be gradually displaced by methane, then concentrate in the hydrocarbon gases and subsequently accumulate together in favorable traps to form helium-rich natural gas reservoirs. Our results provide a molecular-scale insight into the preservation and accumulation of helium in helium source rocks and are significant for assessing the helium resource potential.

Mineral chemistry, petrography and crystallization conditions of the Middle Eocene Kazıkbeli Pluton (Eastern Pontides, NE Türkiye)

The Eastern Pontides host a diverse suite of plutonic rocks spanning a wide range of ages and compositions. Among these, the Middle Eocene Kazıkbeli pluton, located in the Kürtün district of Gümüşhane, stands out due to its distinctive petrological characteristics. This study aims to unravel the petrological implications of petrographic and mineral chemical data to constrain the physicaydınçakırochemical conditions (temperature, pressure, oxygen fugacity) under which the Kazıkbeli magma crystallized and was emplaced. By integrating mineral chemical data, we seek to quantify emplacement pressure, crystallization temperature, and oxygen fugacity. A comprehensive understanding of the genetic relationships and physicochemical properties of the Kazıkbeli pluton rocks, as determined through geological, petrographic and mineral chemistry, is crucial for elucidating the geological evolution of the Eastern Pontides. The Kazıkbeli Pluton exhibits a predominant NE-SW orientation and encompasses an area of roughly 46 km². Modal mineralogical analysis reveals a compositional spectrum ranging from gabbroic diorite to monzogranite, with granodiorite and tonalite being the most prominent rock types. Textural variations encompass fine- to medium-grained, porphyritic, poikilitic, and occasionally graphic textures. The primary mineral assemblage of the pluton comprises plagioclase, orthoclase, quartz, amphibole, biotite, and Fe-Ti oxides. Accessory minerals include zircon, apatite, sphene, and allanite. Plagioclases are labradorite to oligoclase (An26 to An66) in composition. K-feldspar minerals exhibited an orthoclase composition (Or80 to Or97). All amphiboles belong to the calcic amphibole field and exhibit a magnesio-hornblende (Mg#=0.63-0.73) composition. Biotites crystallized as solidified melt products with compositions between annite and phlogopite endmembers, plotting close to the magnesium-rich (Mg#=0.52-0.58) end of the phlogopite solid solution series. Calculated crystallization temperatures derived from amphibole and biotite data range from 712°C to 824°C. Pressure estimations calculated using amphibole-plagioclase, amphibole and biotite suggest a range of 0.04 to 2.06 kbar. Oxygen fugacity (ƒO2) values calculated using amphibole and biotite fall between -12.5 and -16.1. Amphibole-based water content estimations indicate a range of 3.7% to 5.7% for the pluton. Biotite compositions within the studied Kazıkbeli pluton rocks exhibit characteristics suggestive of a potential mantle origin. Geobarometric calculations based on mineral chemistry data with geological and petrographic features indicate the emplacement of the Kazıkbeli Pluton at relatively shallow depths within the crust (~1 to 8 km).

Subdividing a Cretaceous Carbonate Reservoir in the East Mediterranean Sea, Egypt, Based on Petrographical Characteristics: A Case Study

Subdividing a Cretaceous Carbonate Reservoir in the East Mediterranean Sea, Egypt, Based on Petrographical Characteristics: A Case Study

Understanding the extreme differentiation of granitic magmas through elemental geochemistry and Fe isotope signatures

The petrogenesis of extremely fractionated igneous rocks is closely linked to the evolution of the continental crust and the formation of rare metal mineralization. Extremely fractionated magmas evolve via fractional crystallization, hydrothermal evolution, and volatile-rich fluid–melt interaction, but the specific effects of these magmatic and magmatic–hydrothermal processes remain unclear. To understand the mechanisms by which extremely fractionated granitic magmas evolve, we characterized the geochemical and Fe isotope signatures of a suite of granites and pegmatites from the Baishitouquan (BST) granitic pluton in northwest China. The BST granitic system evolved from a high SiO2 and volatile-poor system to a low SiO2 and volatile-rich system as it transitioned from an evolutionary stage dominated by magmatic processes to one dominated by magmatic–hydrothermal processes. This transition is supported by changes in the degree of melt polymerization (NBO/T) and other geochemical indicators of fluid activity (e.g., TE1,3 (REE tetrad effect), Zr/Hf, Nb/Ta, and F/Cl ratios), suggesting that extremely fractionated granites form as a result of a combination of fractional crystallization and volatile-rich fluid–melt interactions. The bulk-rock samples of the BST pluton exhibit significant Fe isotope variability, with δ56Fe values ranging from 0.40 ± 0.02‰ in the least evolved leucogranite (Zone-a) to 0.65 ± 0.02‰ in the most evolved topaz albite granite (Zone-e). Notably, the increase in bulk-rock δ56Fe values during magma evolution is accompanied by a decrease in SiO2 contents, which is opposite to the general trend during granitic magma evolution. Given the correlations between δ56Fe values and indicators of magmatic differentiation (e.g., Fe2O3T, Rb, (Na + K)/(Mg + Ca), and (La/Lu)N), the variability in bulk-rock Fe isotope composition of granites is likely due to changes in magma composition, with garnet removal being the most likely mechanism fractionating Fe isotopes. Rhyolite-MELTS modeling, coupled with geochemical and petrographic characterization, indicate that the high concentrations of F and H2O in felsic magmas likely play critical roles in structurally modifying granitic melts by dramatically reducing their viscosity and enhancing fluid flow. The distinct geochemical–isotopic features of granites can, therefore, be used as tracers for identifying magmatic–hydrothermal processes and, potentially, rare metal mineralization.

Paleosol-induced early dolomitization with U[sbnd]Pb age constraints and its implications for fluid pathways in ancient sandstone aquifers

In hydrogeology, a key challenge involves identifying patterns within ancient formations to forecast the distribution of fluid pathways and barriers to permeability, as well as comprehending the palaeohydrological system and its changes over time. This study addresses two main research inquiries concerning fluid flow: firstly, the influence of dolomitization induced by paleosols on flow characteristics, and secondly, the implications for fluid flow pattern distribution in continental sedimentary units. The objectives are pursued through: (1) meticulous, high-resolution measurements of porosity and permeability coupled with well-log data from an outcrop and two boreholes; (2) investigation of petrographic features of diagenetic minerals and their ages using the UPb geochronology system; and (3) an integration of these methodologies to grasp rock properties and fluid flow at a broader scale. Findings suggest that early dolomitization in continental sequences significantly affects fluid flow properties across the basin. The development of paleosols triggered early dolomitization, supported by UPb geochronology evidence. Subsequent groundwater migration along hydraulic gradients, influenced by fluctuations in the local aquifer's water table, facilitated the vertical distribution of dolomitization. Dolomitization occurred in residual pores resulting from initial mineral alteration, early lithifying the sediment and preventing mechanical compaction, thus preserving porosity. During migration events, fluids moved vertically along local faults and horizontally through the dolomitized layers of the formation, which likely remained porous at the time, leading to substantial silica mineralization.

Impact of clay minerals on reservoir sandstone properties: comparative study in Colombian eastern cordillera and middle Magdalena valley basins

The aim of this study is to examine how mineralogy influences the petrophysical properties, particularly porosity and permeability, of potential sandstone reservoirs in Colombia. It seeks to comprehensively understand how the presence of clay minerals impacts the overall quality of hydrocarbon reservoirs in the country. Samples of sandstones from reservoirs at various outcrops in the Eastern Cordillera and Middle Magdalena Valley basins were collected. Detailed analysis of mineralogy and petrographic characteristics of the samples was conducted through various analytical techniques such as transmitted light microscopy, X-ray diffraction, and scanning electron microscopy. Porosity and permeability were measured using automated permeametry and porosimetry equipment. The predominant composition of the analyzed reservoir rocks comprises quartz (45-50%), feldspar (35-40%), and clays (10-20%). These rocks were categorized into two distinct groups based on their permeability (K) and porosity (Φ), ranging from 0.009 to 29.220 mD and 1.88 to 20.75%, respectively. The presence of illite correlated with a reduction in both porosity and permeability, highlighting its negative impact on reservoir quality. Conversely, an elevated concentration of kaolinite was associated with favorable porosity and permeability. Samples with feldspar sericitization demonstrated inferior hydrocarbon storage quality. This study provides a deeper understanding of how mineralogy affects the petrophysical properties of sandstone reservoirs in Colombia. These findings are crucial for guiding exploration and production strategies in the Colombian oil industry, especially in challenging geological environments like those studied.

Carbonate-siliciclastic deposits of the Lower Uk Subformation (Neoproterozoic) in the stratotype section and its correlation within Bashkirian Mega-Anticlinorium of the Southern Urals

Research subject. Carbonate-siliciclastic deposits of the Lower Uk Subformation were thoroughly studied in the section along the Yuryuzan river (near Ust-Katav town, Chelyabinsk region) and correlated with subformation sections along the Basu, Zilim and Bolshoy Inzer rivers. Material and methods. A detailed description of the stratotype section was carried out; sandstones, siltstones and limestones were studied in thin sections. The sections were correlated with each other. Results. Relationships between the Lower Uk Subformation and the underlying deposits are studied. The petrographic characteristics of rocks are presented. The Lower Uk Subformation sequences are traced between sections of various structural-tectonic zones of the Bashkirian Mega-anticlinorium (BMA). Conclusions. The set of features supports the idea about the presence of a sedimentary hiatus at the base of the formation, even in the most complete sequences (which contain the locally developed Shubino Member of the Minyar Formation). In the stratotype and in other reference sections, the Lower Uk Subformation has a pronounced three-membered structure; it is subdivided into the lower (siliciclastic) Yamashta, middle (substantially carbonate) Akkostyak, and upper (siliciclastic-carbonate) Avdyrdak members. The lithofacies analysis results suggest a change in environments from coastal-marine siliciclastic to shallow-marine siliciclastic-carbonate (middle and inner ramps) due to fluctuations in the relative sea level and, probably, climate change. It was established that the area of the Suleimanovo anticline at latest Riphean corresponded to the distal sedimentary environments in comparison with the southern and western zones (Alatau anticlinorium) of the BMA.

Radiometric and petrographic characterization of El-Yatima granite: Evaluating radiological risks and mineralogical features

The current investigation aims to assess the potential of El-Yatima granitic pluton as a decorative stone by examining its natural radioactivity and mineralogical elements. Specifically, the study aims to determine the concentrations of radionuclides 40K, 232Th, and 226Ra, perform petrographic analyses, and evaluate the associated radiological risks to determine the safety and usability of the granite in construction and decoration, as well as to assess the potential of these granitic rocks as a safe and effective decorative stone for various applications. It has a 12 km2 surface area of 812 m above sea level. It is situated in the Eastern Central Desert. Its hypidiomorphic texture, medium-to-coarse grain size, and predominant composition of K-feldspars, quartz, and plagioclase are observed, along with minute amounts of muscovite and biotite. By employing NaI (Tl) gamma-ray spectroscopy, the natural radionuclide concentrations of 40K, 232Th, and 226Ra in El-Yatima granitic rocks were ascertained. Additionally, radiological concerns were identified for the materials that were examined. The average concentrations of radionuclides 40K, 232Th, and 226Ra are 178 Bqkg−1, 44 Bqkg−1, and 41 Bqkg−1, respectively. The following are the computed values: 117 Bqkg−1, 0.43, 0.32, 53 nGyh−1, average radium equivalent (Raeq), hazard indices (Hin and Hex) and dose rate (D), respectively. When compared to levels typically indicated, it was discovered that its values were within the worldwide standard. The study's findings could be a starting point for radiometric data for upcoming surveillance and epidemiological research.

Discovery of the world longest known intra-till cave (Jura mountains, France): Age and formation processes

We describe an atypical cave network in the Jura mountains (eastern France), currently the only one known worldwide to be entirely excavated within a subglacial deposits. Extending over 600 m of mapped galleries, this natural cavity developed entirely within a large impermeable drumlin shaped during last glacial stage MIS 2 (Wurm). By using lidar topography, petrophysical, sedimentological and geochemical measurements, as well as U-Th dating, this cave is mainly attributed to mechanical erosion processes (piping cave). The network's morphogenesis seems to be characterized by two main phases. Firstly, thin mini-pipes were probably dug along shear planes previously formed within the drumlin. The first mini-pipes may have appeared under the ice cover during the last glaciation in response to differential hydraulic pressures on either side of the drumlin. They may also have appeared later when the drumlin was free from its glacial cover, forming a natural earth dam across a thalweg and obstructing the flow of surface water. A second phase would correspond to the development of mega-pipes, resulting from sequential capture of flows from mini-pipes. Mega-pipes appeared >2500 years ago, as indicated by U-Th dating of one speleothem. In other regions of the world, intra-till caves have been documented, but are <20 m long. This limitation is mainly attributed to instability of the materials constituting subglacial tills, disfavoring a durable existence of galleries without collapses. In the Jura calcareous mountains the subglacial till contains a high proportion of very fine calcite matrix produced by subglacial abrasion, which was indurated by compaction under several hundred meters of ice. These particular petrographic features appear to be required for the creation and the preservation of this long intra-till cave. Similar caves may likely be found in other regions of the world displaying limestone bedrock and where subglacial till with abundant carbonate matrix was formed.