Search Results (50)

Rare earth elements (REEs) play a critical role in provenance tracing and the environmental reconstruction of the Earth. However, systematic investigations into the geochemical behavior and fractionation mechanisms of REEs during halite crystallization in brine–salt systems remain limited. This study reports new trace element and REE data for Late Cretaceous halites from the Thakhek Basin, Laos. Ratios of Sr/Ba, Sr/Cu, and V/Cr indicate a marine origin for the halites, which formed under hot climatic and oscillating oxidizing–anoxic redox conditions. Both primary and secondary halites display uniform Post-Archean Australian Shale (PAAS)-normalized REE distribution patterns, characterized by relative enrichment in medium rare earth elements (MREE) and depletion in light (LREE) and heavy rare earth elements (HREE). Similar REE patterns are also observed in halites from other modern and ancient, continental and marine salt basins worldwide. These observations suggest that the influences of parent brine composition and external provenance supplies on REE fractionation are negligible, given the consistent source, salinity, and redox conditions recorded in these halites. Accordingly, REE fractionation in halite was largely controlled by crystallographic effects, with aqueous MREE preferentially incorporated into halite crystals during deposition. In addition, the relatively lower Zr/Hf ratios in secondary halites compared to primary halites further validate the utility of the Zr/Hf ratio for distinguishing authigenic halite from salt modified by diagenesis, weathering, dissolution, or recrystallization. While our results establish a fundamental REE distribution pattern for halite, further research is needed to better constrain the underlying fractionation mechanisms of REEs in evaporite minerals within brine–salt systems. Full article

Lake Van (Eastern Turkey), the world’s largest soda lake, represents a unique geochemical environment characterized by high alkalinity (pH about 9.7) and a complex hydrochemistry, driven by deep hydrothermal input and extreme evaporative processes. This article evaluates mineralogy, minor elements, and Rare Earth Element (REE) geochemistry of coastal stromatolites from 11 sites, to discriminate between endogenous chemical signals and terrigenous contamination. Results identify two distinct lithological groups: a chemically pure authigenic end-member (CaCO₃ > 85%), overprinted by a significant siliciclastic detrital contribution, rich in SiO₂, Al₂O₃, and Fe₂O₃. Authigenic samples successfully preserve the primary hydrothermal signature, exhibiting marked Heavy Rare Earth Element (HREE) enrichment and superchondritic Y/Ho ratios (=35), inherited from the stability of dissolved dicarbonate complexes, such as [REE(CO₃)₂]⁻, which favor HREE solubility and uptake into the carbonate lattice. Conversely, the significant detrital contribution is highlighted by a robust correlation between REE and lithogenic proxies (Al-Si-Fe). Furthermore, the non-CHARAC behavior observed in Y/Ho and Zr/Hf twin pairs effectively distinguishes biogenic-chemical precipitation from detrital inputs. These results highlight the effectiveness of REE geochemistry as a proxy to filter out lithogenic overprints and accurately isolate the primary hydrochemical record of carbonate stromatolites. Full article

The southeastern Iberian Peninsula, particularly the Águilas Arc within the Neogene Volcanic Province (NVP), represents a promising geothermal domain with complex tectonics and geology. The Palomares Fault Zone (PFZ), a key shear structure initiated during the Late Miocene, acts as a conduit for fluid migration, promoting mineralization and potential anomalies of rare and critical metals through fluid–rock interaction. This study investigates such interactions in the southernmost Águilas Arc, focusing on the El Arteal fault segment within the eastern PFZ strand. Mineralogical, geochemical, and hydrogeological analyses were performed using XRD, SEM, and ICP-MS techniques. Results reveal six mineral assemblages (MA) within the fault segment where the fault gouge samples were characterized by cataclastic textures and the occurrence of authigenic minerals, including halite, kaolinite, illite, paragonite, goethite, hematite, gypsum, barite, celestine, and quartz. Geochemical data indicate enrichment signatures in large-ion lithophile elements (LILE) and minor chalcophile and light rare-earth elements (LREE). Two thermal hydrofacies with alkaline metals enrichment were identified in wells and mine shafts: (1) Na⁺SO₄²⁻ and (2) Na⁺Cl⁻, where the latter exhibits high Na⁺ and Cl⁻ concentrations toward deeper sectors. These findings suggest multiple stages of fluid–rock interaction controlled by temperature: an early phase dominated by epithermal mineralization, followed by late-stage circulation of hypersaline fluids. This evolution provides an abnormal geochemical signature that is unique in the Aguilas Arc Geothermal System. Full article

The Permian Xiashihezi Formation in the Ordos Basin is a typical tight sandstone gas reservoir, which is characterized by low porosity and strong heterogeneity. Diagenesis plays a crucial role in controlling reservoir quality. However, the multiple phases and types of diagenetic processes throughout geological history make the compaction mechanisms highly complex. This study employed a high-temperature and high-pressure diagenesis simulation system to conduct geological simulation experiments. Typical reservoir samples from the 2nd Member of the Permian Xiashihezi Formation were selected for these simulations. The experiments replicated the diagenetic evolution of the reservoirs under various temperature, pressure, and fluid conditions, successfully reproducing the diagenetic sequences. The diagenetic sequence included early-stage porosity reduction through compaction, early carbonate cementation, quartz overgrowth, chlorite rim formation, feldspar dissolution, and late-stage illite and quartz cementation. Mechanical compaction is the primary factor reducing reservoir porosity, exhibiting a distinct four-stage porosity reduction pattern: (1) continuous burial stage (>4000 m); (2) stagnation stage of burial (3900 m–4100 m); (3) the secondary continuous burial stage (>5000 m); (4) tectonic uplift stage (3600 m). The experiments confirmed that the formation of various authigenic minerals is strictly controlled by temperature, pressure, and fluid chemistry. Chlorite rims formed in an alkaline environment enriched with Fe²⁺ and Mg²⁺ (simulated temperatures of 280–295 °C), effectively inhibiting quartz overgrowth. Illite appeared at higher temperatures (>300 °C) in platy or fibrous forms. Feldspar dissolution was noticeable upon injection of acidic fluids (simulated organic acids), providing material for authigenic quartz and kaolinite. The key mineral composition significantly impacts reservoir diagenesis. The dissolution released Mg²⁺ and Fe²⁺ ions, crucial for forming early chlorite rims in the overlying sandstones, confirming the importance of inter-strata interactions in “source-facies coupling.” Through physical simulation methods, this study deepened the understanding of the diagenetic evolution and compaction mechanisms of tight sandstones. This provides significant experimental evidence and theoretical support for predicting “sweet spot” reservoirs in the area. Full article

Here, we report the discovery of silica- and sulfur-enriched deposits forming on the seafloor off Zannone Island (western Mediterranean Sea), where hydrothermal activity is ongoing. Our multidisciplinary investigation reveals that these deposits form through the interplay between hydrothermal processes and microbial activity. The deposits result from a dynamic equilibrium involving microbial mediation, sedimentation, and episodic lithification, driven primarily by two mineralization pathways: silica and sulfur precipitation. This study provides new insights into the bio-sedimentary processes shaping authigenic crusts in shallow submarine hydrothermal settings, contributing to a broader understanding of mineralization in marine environments influenced by both geological and biological factors. Full article

The Aptian–Albian interval represents a significant cooling phase within the Cretaceous “hothouse” climate, marked by dynamic climatic fluctuations. High-resolution continental records are essential for reconstructing terrestrial climate and ecosystem evolution during this period. This study examines a lacustrine-dominated succession of the Shahezi Formation (Lishu Rift Depression, Songliao Basin, NE Asia) to access paleo-weathering intensity and paleoclimate variability between the Middle Aptian and Early Albian (c. 118.2–112.3 Ma). Multiple geochemical proxies, including the Chemical Index of Alteration (CIA), were applied within a sequence stratigraphic framework covering four stages of lake evolution. Our results indicate that a hot and humid subtropical climate predominated in the Lishu paleo-lake, punctuated by transient cooling and drying events. Periods of lake expansion corresponded to episodes of intense chemical weathering, while two distinct intervals of aridity and cooling coincided with phases of a reduced lake level and fan delta progradation. To address the impact of potassium enrichment on CIA values, we introduced a rectangular coordinate system on A(Al₂O₃)-CN(CaO* + Na₂O)-K(K₂O) ternary diagrams, enabling more accurate weathering trends and CIA corrections (CIA_corr). Uncertainties in CIA correction were evaluated by integrating geochemical and petrographic evidence from deposits affected by hydrothermal fluids and external potassium addition. Importantly, our results show that metasomatic potassium addition cannot be reliably inferred solely from deviations in A-CN-K diagrams or the presence of authigenic illite and altered plagioclase. Calculations of “excess K₂O” and CIA_corr values should only be made when supported by robust geochemical and petrographic evidence for external potassium enrichment. This work advances lacustrine paleoclimate reconstruction methodology and highlights the need for careful interpretation of weathering proxies in complex sedimentary systems. Full article

In order to clarify the pore evolution and coupling characteristics with hydrocarbon charging in the deep-buried ultra-tight sandstone reservoirs of the second member of Xujiahe Formation (hereinafter referred to as the Xu 2 Member) on the eastern slope of the Western Sichuan Depression, this study integrates burial history and thermal history with analytical methods including core observation, cast thin section analysis, scanning electron microscopy, carbon-oxygen isotope analysis, and fluid inclusion homogenization temperature measurements. The Xu 2 Member reservoirs are predominantly composed of lithic sandstones and quartz-rich sandstones, with authigenic quartz and carbonates as the main cementing materials. The reservoir spaces are dominated by intragranular dissolution pores. The timing of reservoir densification varies among different submembers. The upper submember underwent compaction during the Middle-Late Jurassic period due to the high ductility of mudstone clasts and other compaction-resistant components. The middle-lower submembers experienced densification in the Late Jurassic period. Late Cretaceous tectonic uplift induced fracture development, which enhanced dissolution in the middle-lower submembers, increasing reservoir porosity to approximately 5%. Two distinct phases of hydrocarbon charging are identified in the Xu 2 Member. The earlier densification of the upper submember created unfavorable conditions for hydrocarbon accumulation. In contrast, the middle-lower submembers received hydrocarbon charging prior to reservoir densification, providing favorable conditions for natural gas enrichment and reservoir formation. Three sweet-spot reservoir development patterns are recognized: paleo-structural trap + (internal source rock) + source-connected fracture assemblage type, paleo-structural trap + internal source rock + late-stage fracture assemblage type, and paleo-structural trap + (internal source rock) + source-connected fracture + late-stage fracture assemblage type. Full article

The Sanandaj–Sirjan Zone and Zagros Fold–Thrust Belt in Iran host numerous Mediterranean-type karst bauxite deposits; however, their formation mechanisms and critical raw material potential remain ambiguous. This study combines mineralogical and geochemical analyses to explore (1) the formation of authigenic minerals, (2) the role of microbial organic processes in Fe cycling, and (3) the assessment of their critical raw materials potential. Mineralogical analyses of the Late Cretaceous Daresard and Middle–Late Permian Yakshawa bauxites reveal distinct horizons reflecting their genetic conditions: Yakshawa exhibits a vertical weathering sequence (clay-rich base → ferruginous oolites → nodular massive bauxite → bleached cap), while Daresard shows karst-controlled profiles (breccia → oolitic-pisolitic ore → deferrified boehmite). Authigenic illite forms via isochemical reactions involving kaolinite and K-feldspar dissolution. Scanning electron microscopy evidence demonstrates illite replacing kaolinite with burial depth enhancing crystallinity. Diaspore forms through both gibbsite transformation and direct precipitation from aluminum-rich solutions under surface conditions in reducing microbial karst environments, typically associated with pyrite, anatase, and fluorocarbonates under neutral–weakly alkaline conditions. Redox-controlled Fe-Al fractionation governs bauxite horizon development: (1) microbial sulfate reduction facilitates Fe³⁺ → Fe²⁺ reduction under anoxic conditions, forming Fe-rich horizons, while (2) oxidative weathering (↑Eh, ↓moisture) promotes Al-hydroxide/clay enrichment in upper profiles, evidenced by progressive total organic carbon depletion (0.57 → 0.08%). This biotic–abiotic coupling ultimately generates stratified, high-grade bauxite. Finally, both the Yakshawa and Daresard karst bauxite ores are enriched in critical raw materials. It is worth noting that the overall enrichment appears to be mostly driven by the processes that led to the formation of the ores and not by the chemical features of the parent rocks. Divergent bauxitization pathways and early diagenetic processes—controlled by paleoclimatic fluctuations, redox shifts, and organic matter decay—govern critical raw material distributions, unlike typical Mediterranean-type deposits where parent rock composition dominates critical raw material partitioning. Full article

Palygorskite is assumed to be the predominant clay mineral in playa-lakes, where it may be detrital or authigenic in origin. Discriminating between detrital and authigenic clays is crucial to elucidate paleoenvironmental conditions in lacustrine deposits. This study provides insight into the sedimentary evolution of clay minerals from source, lacustrine Miocene marlstones and mudstones, to sink, represented by three recent hyperalkaline playa-lakes in Central Spain. XRD, TEM, and AEM analyses show concomitant detrital and authigenic palygorskites in the three playa-lakes. The inherited palygorskites exhibit degradation features, larger widths, and common and ideal compositions, in contrast to neoformed particles. The latter are narrower. Depending on the hydrochemical composition of each playa-lake, neoformed palygorskites are enriched in a different octahedral cation (Al₂O₃, MgO, and Fe₂O₃). Iron-rich palygorskites are only formed in association with authigenic saponites in one of the playa-lakes. The same effect of magnesium competition between smectite and palygorskite is observed in Miocene mudstones, where palygorskite is relatively enriched in iron. In hyperalkaline, seasonal playa-lakes lying in the vicinity, slight physicochemical differences play a crucial role in the crystallochemical composition of authigenic palygorskites, highlighting the use of this mineral as a geochemical proxy. Full article

This study aimed to develop a geochemical database by thoroughly analyzing groundwater and sediments from coastal aquifers of southwest Bangladesh. Moreover, we investigated the source of sediment deposition and the mechanisms behind the presence of arsenic and salinity in groundwater. The seasonal distribution patterns of arsenic among the shallow and deep coastal aquifers were found to be 45.12 µg/l and 20.65 µg/l during dry and wet seasons, respectively. Moreover, the groundwater salinity distribution ranged from 3262.88 mg/l to 1930.88 mg/l during the dry and wet seasons. Cored sediment samples showed fine to medium sands of 92%, with silt and clay particles. The petrographic study of authigenic and heavy minerals revealed that the mineral grains were subangular to angular, indicating their textural immaturity of coastal sediments. The reactivity of goethite (FeOOH) and siderite (FeCO₃) minerals suggests that the aquifers were subjected to slightly oxidized to moderately reducing conditions, with ORP values ranging from +50.40 mv to −149.5 mv. Such redox conditions could potentially result in the enrichment and mobility of arsenic in the groundwater. Although arsenic concentrations in deep aquifers are relatively low, higher salinity values are found in both shallow and intermediate coastal aquifers. Full article

The Late Permian coal measures in eastern Yunnan, western Guizhou, and central Guangxi are significantly enriched in critical metals that could serve as important supplements to conventional critical metal deposits in China. This study collected previous geochronological and geochemical data from the Late Permian coal measures to evaluate the distribution characteristics and enrichment factors of critical metals. Moreover, metallogenic models for critical metals were also developed. The results showed that Late Permian coal measures in Yunnan, Guizhou, and Guangxi provinces exhibited abnormal enrichment in Nb, Zr, and rare earth elements (REY, or REE if Y is excluded). The Emeishan mafic rocks and intermediate-felsic volcanic ash from the Truong Son orogenic belt underwent chemical weathering, with Nb and Zr selectively preserved in situ in the form of heavy minerals (e.g., rutile, zircon, and anatase), which subsequently led to the enrichment of Nb and Zr in bauxite and Al-claystone at the bottom of the Late Permian coal measures. Intermediate-felsic volcanic ash from the Emeishan large igneous province (ELIP) and the Truong Son orogenic belt supplied Nb, Zr, and REY for the middle and upper parts of the Late Permian coal measures. The intermediate-felsic mineral material of the coal measures in the intermediate zone, outer zone, and outside zone of ELIP are derived mainly from the ELIP, the mixture from ELIP and the Truong Son orogenic belt, and the Truong Son orogenic belts, respectively. Nb, Zr, and REY were leached by acidic aqueous solutions and from the parting and roof into underlying coal seams, where they deposited as authigenic minerals or adsorbed ions on organic matter during early coalification. Full article

Deep-sea sediments enriched in rare earth elements and yttrium (REY-rich sediments) are widely distributed on the deep-sea floor, and their formation mechanism remains elusive. Although studies have recognized the link between seamounts and REY-rich sediments, in-depth analysis of the specific roles and effects of seamounts in the formation of REY-rich sediments is lacking. In this study, we analyzed surface sediments from the Marcus-Wake Seamounts for grain size, geochemistry, and mineral composition and classified the samples into three types: samples with moderate REY content and dominated by terrestrial detritus; samples with high REY and authigenic mineral content; and samples rich in CaCO₃ but poor in REY. The REY in the sediments of the study area partly originate from Asian dust input and partly from seawater and/or pore water, and are mainly enriched in REY carrier particles including bioapatite fossils and micronodules. The amount of REY carrier particles influences the REY content in the sediments. The current field, primary productivity, weathering process, and depositional environment around seamounts are different from those of abyssal plains, which are conducive to the formation of REY-rich sediments. Strong bottom currents may exist in the southeastern direction of some large seamounts (e.g., Niulang Guyot), leading to the selective accumulation of REY-rich bioapatite fossils and micronodules, resulting in the formation of REY-rich sediments. Full article

An in situ electron microprobe study of detrital minerals yielded important insights into the diagenetic history of the Cretaceous-to-Paleogene flysch sandstones from the Chvalčov site, Rača Unit, Flysch Belt of the Outer Western Carpathians. Detrital titanite and a Fe-Ti mineral (probably ilmenite) were almost completely altered to TiO₂ minerals, which also newly crystallized in intergranular spaces of sandstone. Brookite, anatase, and, exceptionally, rutile were identified by Raman spectroscopy. Authigenic TiO₂ phases show complex composition with occasionally elevated contents of Fe, Nb, Zr, V, Sc, Cr, Al, Y, and/or P, which were likely sourced from altered neighboring heavy minerals. In addition, rare authigenic LREE- and Y-enriched apatite rims were observed on detrital apatite. The remobilization of REE, Y, and HFSE was likely mediated by acidic early diagenetic fluids enriched in fluoride and sulfate anions. The superimposed formation of calcite cement was associated with the dissolution of detrital garnet, feldspars, and quartz. The compositions of detrital apatite and garnet (Alm_60-82Prp_4-30Sps_0-24Grs_0-19) are comparable with those from adjacent parts of the Flysch Belt. Detrital rutile is enriched in Nb, V, Cr, and Zr. Our study illustrates the intensity of diagenetic alteration of detrital minerals in flysch sandstones as well as the usefulness of in-situ electron-microprobe investigations for the recognition of processes influencing heavy minerals in diagenetically altered sediments. Full article

Time-series samples intercepted via three synchronized moored sediment traps, deployed at 1000 m, 2150 m, and 3200 m in the northern South China Sea (NSCS) during June 2009–May 2010, were analyzed to quantify the bioactive trace metal fluxes in sinking particles and investigate their different source contributions. Iron (Fe) primarily originated from lithogenic sources. Manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn) exhibited various degrees of enrichment over their continental crustal ratios. Since the sources of bioactive trace metals in sinking particles can be divided into lithogenic, biogenic, and excess fractions, mass conservation calculations were used to quantify the contribution of each source. The results showed that Fe, Mn, and Co had extremely low biogenic proportions (0.1–3.3%), while Ni, Cu, and Zn had higher proportions (2.7–17.3%), with the biogenic fraction decreasing with the depth. Moreover, excess sources accounted for a significant proportion of Mn (68–75%), Co (34–54%), Ni (60–62%), Cu (59–74%), and Zn (56–65%) in sinking particles at the three sampling depths. The excess fractions of Mn, Co, and Cu in sinking particles can be affected by authigenic particles. This is supported by their similar scavenging-type behavior, as observed via the increase in their fluxes and enrichment patterns with the increasing depth. Furthermore, the excess fractions of Ni, Cu, and Zn may have significant contributions from anthropogenic sources. The variability of Fe in sinking particles was mainly controlled via lithogenic matter. Notably, organic matter and opal were found to be pivotal carriers in the export of excess bioactive trace metals (Mn, Co, Ni, and Cu) via the water column, accompanied with the elevated ballast effect of lithogenic matter with the depth. However, the transportation of excess Zn was more complicated due to the intricate processes involved in Zn dynamics. These findings contribute to our understanding of the sources and transport mechanisms of bioactive trace metals in the marine environment. Full article

Towards a better understanding of factors controlling carbon (C) exchange between inland waters and atmosphere, we addressed the inorganic carbon cycle in semiarid lakes of Central Eurasia, subjected to the strong impact of on-going climate change. As such, we assessed the hydrochemical variability and quantified its control on the formation of authigenic carbonate minerals, occurring within the upper layer of sediments in 43 semiarid lakes located in the southwest of Western Siberia (Central Eurasia). Based on measurements of pH, total dissolved solids (TDS), cationic and anionic composition, dissolved organic and inorganic C, as well as textural and mineralogical characterization of bottom sediments using X-ray diffraction and scanning electron microscopy, we demonstrate that lake water pH and TDS are primarily controlled by both the lithological and climatic context of the lake watershed. We have not revealed any direct relationships between lake morphology and water chemistry. The most common authigenic carbonates scavenging atmospheric CO₂ in the form of insoluble minerals in lake sediments were calcite, aragonite, Mg-calcite, dolomite and hydromagnesite. The calcite was the most common component, aragonite mainly appears in lakes with sediments enriched in gastropod shells or artemia cysts, while hydromagnesite was most common in lakes with high Mg/Ca molar ratios, as well as at high DIC concentrations. The relationships between mineral formation and water chemistry established in this study can be generalized to a wide suite of arid and semiarid lakes in order to characterize the current status of the inorganic C cycle and predict its possible modification under on-going climate warming such as a rise water temperature and a change in hydrological connectivity, primary productivity and nutrient regime. Full article