Stable Isotope Geochemistry Research Articles

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

Genesis of the Florida Canyon Nonsulfide Zn Ores (Northern Peru): New Insights Into the Supergene Mineralizing Events of the Bongará District

Abstract The Florida Canyon Zn deposit in the Bongará Province of northern Peru consists of sulfide and nonsulfide mineralizations within dolomitized strata of the Triassic Chambará Formation, a member of the Triassic-Jurassic Pucará Group. The nonsulfide mineralization, which represents one third of the total resource, formed by supergene alteration of Mississippi Valley-type sulfide bodies. The nonsulfide assemblages occur in stratabound or fault-related, discordant zones that mimic the shapes of the former sulfide zones. Two nonsulfide facies can be distinguished: facies 1 – red zinc ores, which are characterized by smithsonite, or hemimorphite-dominant bodies that formed by direct replacement of primary sulfide assemblages, and facies 2 – white zinc ores, which are characterized by masses of colloform smithsonite formed by replacement of wall rock. The facies are distinct in bulk chemical composition and stable isotope geochemistry. Facies 1 shows high concentrations of Zn, Pb, Fe, Ge, Mn, and As, whereas facies 2 shows only high Zn and Cd concentrations. Enrichments in Ge, which have been reported in other Zn deposits of the Bongará Province, are associated with hemimorphite, Fe hydroxides, and remnant sphalerite in facies 1. The δ13C and δ18O signatures of smithsonite in both facies suggest that meteoric waters infiltrating the precursor sulfide bodies were affected by kinetic fractionation and originated from multiple sources at different altitude.

Metal source and hydrothermal evolution of the Jiaoxi quartz vein-type tungsten deposit (Tibet): Insights from textural and compositional variations of wolframite and scheelite

Abstract We evaluate the controlling factors of hydrothermal wolframite and scheelite precipitation in the quartz vein-type Jiaoxi tungsten deposit situated in the western part of the Lhasa terrane (Tibet, China) using texture, major and trace element mineral geochemistry, and sulfur stable isotope geochemistry. Pyrite and chalcopyrite that are intergrown with Fe-enriched wolframite and siderite, have distinct in situ S isotope compositions (δ34SV-CDT) of −31.38 to +1.77‰, and +2.07 to +2.30‰, respectively. Major and trace element contents and in situ S isotope compositions of pyrite and chalcopyrite indicate that the hydrothermal evolution involved fluid-fluid mixing and greisenization. We report evidence for an early magmatic fluid, which is characterized by the enrichment of W, Mn, Zr, Ti, Sc, and Sn and depletion of Fe. This magmatic fluid was diluted by meteoric water and interacted with biotite monzogranite porphyry to leach Fe, Mg, and Zn into the system to form wolframites with variable Fe/(Fe+Mn) ratios ranging between 0.06–0.84. The late Fe-enriched magmatic fluid released from the muscovite granite mixed with meteoric water that leached minor Fe and S from shale to form late shale-hosted wolframite with a Fe/(Fe+Mn) mass ratio of >0.75 and coeval siderite and sulfides. This study highlights that multiple Fe sources were present in the system, including muscovite granite-released Fe through fluid exsolution, biotite monzogranite porphyry-released Fe during greisenization, and minor Fe released from the shale as a result of meteoric water leaching.

Stable isotope (C, O, S, and Zn) geochemistry of marble-hosted exhalative zinc deposits in the Central Metasedimentary Belt, Grenville Province, Canada: Insights into ore deposition and tectonic setting

Mesoproterozoic marbles of the southwestern Grenville Province contain numerous stratiform zinc deposits, which include the world-class Balmat-Edwards Zn-Pb district in the Adirondack Lowlands and the Franklin-Sterling Zn-Fe-Mn district in the New Jersey Highlands. Ten small deposits in the Central Metasedimentary Belt (CMB) of Ontario and Quebec were selected for stable isotope study (C and O in carbonates and S and Zn in sulfides) to help constrain depositional conditions, ore genesis, and tectonic setting. These deposits and the Balmat-Edwards and Franklin-Sterling districts are interpreted to have formed in contemporaneous back-arc basins associated with rifting of the Andean-style margin of Laurentia. Stable isotope systematics and the geology of these deposits indicate two distinct sedimentary exhalative (Sedex) settings: marble-hosted deposits associated with volcanic rocks, and marble-hosted deposits that are not associated with volcanic rocks and sometimes occur with evaporites. Volcanic-associated Sedex deposits (the Long Lake, Slave Lake, Wilkinson, Lafontaine, Bouchette, and Leitch deposits, and the Franklin-Sterling district) have COS isotope systematics that are consistent with water/rock interaction between marine carbonate protoliths and fluids having an igneous component at ca. 150 ± 25 °C. In contrast, Sedex deposits not associated with volcanic rocks (the Cadieux, Spry, Thirty Island Lake, and Salerno Lake deposits, and the Balmat-Edwards district) have C and O isotope ratios that are consistent with formation at ambient or low (<80 °C) temperatures. Sulfur isotopes of sulfides in some evaporite-associated deposits are similar to those of other Proterozoic Sedex deposits (δ34S = 5–15 ‰ VCDT), with the Cadieux and Spry having lower values (δ34S ≈ 0‰) which might reflect differences in depositional settings. Zinc isotope ratios of CMB Zn deposits are relatively constrained (δ66Zn = 0.17 ± 0.04 ), and are similar to other seafloor deposits. An exception to this is the Thirty Island Lake deposit in the Frontenac terrane, which has δ66Zn = 0.46 ± 0.05 ‰, which may indicate an exotic origin relative to other CMB deposits. The recognition of the volcanic association for some Grenville Sedex deposits and not for others (and the relationship with evaporites for this group) has implications for Zn exploration programs in this region.

Stable isotope geochemistry and microfossil assemblages of carbonate rocks in the ophiolite mélange zone of the Indo–Myanmar orogenic Belt, NE India: Implications on age and depositional environment

A comprehensive data of whole‐rock and stable isotopic geochemistry along with microfossil assemblages of the carbonate rocks of the ophiolite mélange zone of the Indo–Myanmar orogenic Belt (IMOB), north‐east India are discussed, to determine the influence of terrigenous contamination during the formation of these carbonate rocks and also to understand their depositional environment and ages. These carbonate rocks contain a diverse fauna with the dominance of foraminiferal assemblages of planktonic foraminifera (Globotruncana sp. and Heterohelix sp., etc.) which indicates they were formed during the Santonian to Maastrichtian age. Based on chemical compositions, these carbonate rocks have been identified as limestone (CaO/MgO &gt; 50.1) to slightly dolomitic limestone (9.1 &lt; CaO/MgO &lt; 50.1). Total rare earth element (REE) contents in these carbonates are variable (22.39–146.05 ppm). The Post‐Archean Australian Shale (PAAS)‐normalized REE + Y patterns of these carbonates exhibit seawater‐like REE patterns with LREE depletion and relative HREE enrichment with negative Ce anomalies (Ce/Ce* = 0.32–0.79) and positive Y (4.42–27.81 ppm) and Eu anomalies (Eu/Eu* = 1.11–1.86), suggesting that they were deposited under an oxygenated environment with contamination by hydrothermal activity. They are also depleted in δ13C 0/00 (PDB) (1.02–1.570/00) and δ18O 0/00 (PDB) (−6.37 to −9.00%) values which characterize marine precipitates. Eu anomalies and spread in negative δ18O 0/00 (PDB) values to a lesser extent of δ13C 0/00 (PDB) values of these carbonates suggest their formation was altered by diagenesis in the shallow marine environment. Our new whole‐rock and stable isotope geochemical characteristics, in conjunction with microfacies, suggest that the investigated carbonate rocks might have been formed in low‐energy environments, and deposited in neritic to bathyal palaeoenvironments during the Santonian to Maastrichtian interval.

Fluid Inclusions and Stable Isotope Geochemistry of Gold Mineralization Associated with Fine-Grained Granite: A Case Study of the Xiawolong Gold Deposit, Jiaodong Peninsula, China

The Xiawolong gold deposit, located in the Muping–Rushan gold metallogenic belt (eastern Jiaodong Peninsula), is a newly discovered deposit that developed in the late Early Cretaceous as fine-grained granite. Gold mineralization, which mainly occurs in the middle of fresh fine-grained granite dikes, consists of stockwork-style and disseminated ores. They are characterized by middle-high-temperature mineral assemblages, such as molybdenite and magnetite, associated with gold-bearing pyrite. Four types of primary fluid inclusions, contained within the quartz grains from the gold-bearing disseminated and stockwork-style fine-grained granitic ores, were identified based on microthermometry and Raman spectroscopy. The types identified were type 1 aqueous inclusions with middle-high temperature (201 to 480 °C) and middle-low salinity of 0.18 to 17.00 wt.% NaCl equiv.; type 2 H2O–CO2 inclusions, which show middle-high temperatures (218 to 385 °C), middle-low salinities (1.23 to 13.26 wt.% equiv. NaCl), and variable XCO2 (0.031 to 0.044); type 3 daughter mineral-bearing inclusions with high temperature (416 to 446 °C) and relatively constant and high salinity (28.59 to 32.87 wt.% NaCl equiv.); and type 4 CO2 fluid inclusions, which possess a bulk density of 0.405 to 0.758 g/cm3 and a constant XCO2 (0.952 to 0.990) (according to the decreasing abundance of fluid inclusions). The δ18Owater range is between 3.4 and 5.9‰, and the range of the δD is from −97.1 to −77.4‰, which indicates that the ore-forming process is of a magmatic water origin. The δ34S values possess a narrow range between 4.5 and 9.3‰, indicating the source of the Mesozoic Kunyushan granitoids. The Pb isotopic compositions of pyrite show that the Mesozoic Kunyushan granitoids are the main lead source for pyrites. Types 1, 2, and 3 fluid inclusions coexist in the same view field of the quartz grain, which are suggested to occur as the result of fluid immiscibility because of the boiling of a single homogeneous NaCl-CaCl2-KCl-CO2-H2O system. The fluid immiscibility, rather the fluid mixing and wall-rock sulfidation, is the mechanism of gold precipitation in the Xiawolong deposit. Compared with both the “Linglong-type” and “Jiaojia-type” gold deposits in the Jiaodong Peninsula in terms of geological–petrographic evidence and all of the available geochemical data, it can be concluded the Xiawolong gold deposit is of magmatic hydrothermal origin, having a genetic relation to the fine-grained granite.

Integrating sedimentologic data and stable isotope geochemistry to comprehend the sequence stratigraphic framework of the Middle–Upper Jurassic Sargelu and Najmah formations in NW Kermanshah (Howramanat, Dowdan and Kezi sections)

Integrating sedimentologic data and stable isotope geochemistry to comprehend the sequence stratigraphic framework of the Middle–Upper Jurassic Sargelu and Najmah formations in NW Kermanshah (Howramanat, Dowdan and Kezi sections)

Geology, U-Pb geochronology and stable isotope geochemistry of the Heihaibei gold deposit in the southern part of the Eastern Kunlun Orogenic Belt, China: A granitic intrusion-related gold deposit?

The Heihaibei gold deposit is a newly discovered gold deposit in the southern part of the Eastern Kunlun Orogenic Belt. Its most distinctive features are that the gold mineralization is hosted in monzogranite, and that the presence of pre-ore (possibly syn-ore) monzogranite and post-ore gabbro allows to constrain the mineralization’s formation age. Zircons from the monzogranites yield U-Pb ages of 454 ± 3 Ma, while zircons separated from the gabbro dikes cutting the monzogranites and gold mineralized body yield U-Pb ages of 439 ± 3 Ma, which is interpreted to be the minimum age of the Au mineralizing event. Combined with the regional geological background, we proposed that the Heihaibei Au mineralization occurred during the subduction stage of the Early Paleozoic Proto-Tethys ocean.The ore assemblage is dominated by pyrite, arsenopyrite and native gold. The hydrothermal alteration that has led to the peculiar enrichment of Au is not systematically distributed and displays no clear concentric zoning pattern. The main mineralization formed during three stages: the K-feldspar-quartz-pyrite (Py1)-arsenopyrite-sericite-epidote stage (I), the quartz-pyrite (Py2)-native gold-chlorite stage (Ⅱ), and the quartz-carbonate stage (III). The main gold mineralization occurred during stage Ⅱ. Fluid inclusion homogenization temperature and salinities decrease from stage I (Th., 268–412 °C; W., 6.87–16.63 wt% NaCl equiv.) to stage Ⅱ (Th., 183–288 °C; W., 3.69–14.84 wt% NaCl equiv.). The δ18O and δD values (δ18OH2O = 4.9 to 9.7‰; δDV-SMOW = -84.1‰ to −81.1‰) of quartz samples from stage I and stage Ⅱ are comparable to a magmatic-hydrothermal ore-forming fluid that possibly underwent fluid-rock interaction with the Nachitai Group metamorphic rocks during the early ore-forming stage. The relatively uniform δ34S values (δ34SV-CDT = 7.7 to 8.5‰) are slightly elevated compared to magmatic δ34S values, but could be derived from a magma if a significant crustal melt component is present. Moreover, the δ34S values are within the S isotopic composition range of a granitic reservoir, suggesting that they are probably inherited from the Heihaibei monzogranites. The Pb and Hf isotope compositions imply a close genetic association between the gold mineralization and granitic magmatism, which are both the products of the mixing of crustal and mantle sources. The trace element compositions of pyrite provide additional evidence that the gold mineralization in the Heihaibei deposit was related to the magmatism. Compared with the typical characteristics of orogenic gold and intrusion-related gold systems (IRGS) deposits, the Heihaibei gold deposit may instead be classified as a granitic intrusion-related gold deposit.

Detection of diagenetic alteration in bones and teeth for migration and dietary studies \u2014 a combined FTIR and C-N\u2013O-Sr isotope study on tenth century CE cemeteries in northern and northeastern Hungary

Stable isotope-based dietary information on the tenth century CE Hungarian population serves as a valuable comparative tool to other dietary stable isotope studies in the region. This study presents a multiproxy approach involving skeletal samples with the least diagenetic alteration and the best-preserved primary signals. Dental and bone samples collected from three cemeteries in northeastern Hungary were investigated using a combination of Fourier-transform infrared spectroscopy (FTIR), stable isotope geochemistry, and strontium isotope analysis. FTIR measurements were used to detect the addition of secondary carbonate to the skeletal apatite carbonate component and to differentiate diagenetically altered samples. Our results suggest that all but the bone apatite carbonate stable isotope values and 87Sr/86Sr ratios are trustworthy and may be used to interpret the diet and migration of the researched individuals. We also determined that the inhabitants of the three tenth century CE Hungarian settlements relied primarily on C3 plants, but that C4 plants, most likely broomcorn millet (P. miliaceum), comprised a moderate portion of their plant-based diet and that animal products were generally consumed in moderate-high amounts. Coupled analyses of carbonate oxygen and strontium isotope compositions revealed that most of the bone apatite carbonate was heavily altered, whereas enamel apatite carbonate samples largely preserved their primary compositions. The bones showing the least amount of diagenetic alteration may partially preserve their primary apatite carbonate compositions, as well as show a combination of primary and secondary signals, which should be taken into consideration when bone apatite carbonate data are interpreted.

C, Sr, Nd isotope chemostratigraphy and zircon provenance of the Witvlei Group (Namibia): Neoproterozoic glaciations and seawater evolution

The Neoproterozoic witnessed several low latitude glaciations (i.e., the Sturtian, Marinoan and Gaskiers glaciations) and the geodynamic reorganization of cratonic blocks after the breakup of Rodinia. Trace element and stable isotope geochemistry from approximately coeval carbonate deposits on continental marginal sea and foreland basins (Yangtze Platform, China; Otavi and Nama Groups, Namibia) have shown fluctuating oxygen increase on the marine shelves. To gain an increasingly global picture of the bio-geochemical conditions in late Neoproterozoic seawater, we here present new redox-sensitive trace element, stable (C, O) and radiogenic (Nd, Sr) isotope records of carbonates from the Cryogenian Blaubeker and Court, and the Ediacaran Buschmannsklippe Formations (Witvlei Group, Namibia). Shale-normalised REE + Y patterns of post-Sturtian and post-Marinoan carbonates parallel modern seawater showing positive La, Gd and Y anomalies. Negative Ce anomalies argue for their preservation in increasingly more oxidising sea/porewater conditions in the Witvlei Basin from the Ediacaran on. While Cryogenian carbonates underwent radiogenic basin-fluid type overprinting, Ediacaran carbonates upsection record pristine Sr isotopic compositions that match the global Neoproterozoic seawater curve. Partly coupled negative correlations between Sr and Nd suggest long-term shifts in continental weathering and short-term changes in ocean circulation patterns. The δ13Ccarb values range from −7.2 to +3.5 ‰ and record a negative isotope excursion in the upper part of the Witvlei stratigraphy. This excursion might be equivalent to the ‘Shuram’ carbon isotope excursion (CIE). However, the causes, global extent and correlation of the Shuram CIE are still debated. Typical for carbonates of the putative Shuram excursion are less pronounced Ce anomalies, lower Y/Ho ratios, and lower bio-essential and redox-sensitive trace metal concentrations, arguing for periodic redoxcline oscillations in a redox-stratified late Neoproterozoic shelf environment. The overall long-term decrease in redox-sensitive element enrichments throughout the Witvlei Group argues for a progressively increasing, presumably biologically-driven metal cycling towards values typical for Phanerozoic carbonates. The combination of changes in local weathering flux and ambient redox conditions in the late Neoproterozoic ocean may have caused dynamic (bio)geochemical metal cycling, predating (and possibly promoting) the metazoan radiation documented in the overlying Nama Group.

Provenancing Nineteenth Century Saltpetre From British India Using Nitrogen, Oxygen, and Strontium Isotope Ratios

Highly purified specimen of saltpetre, kept in two glass jars are stored in the collections of the Natural History Museum, London, United Kingdom. Historic catalogs of the specimen collection record their acquisition date as 17th March, 1881. The specimen represent heritage of the saltpetre industry during the times of the colonial British India. Chemical analysis shows that they are highly refined, containing only traces of impurities. Dual isotopic composition (δ15N and δ18O) of nitrate implies their intrinsic origin as British India. In addition, relative to highly radiogenic signature of Sr widespread throughout India, less radiogenic strontium (87Sr/86Sr = 0.707859–0.714168) in the specimens suggests that final stages of the refining took place in western India where relevant waters containing pristine Sr were used. Stable isotope geochemistry in combination with history and socio-economy provide new insights into this research area which is not yet fully understood.

Geochemical and petrographic approach for the origin of the limestone blocks of thewalls of the Great Temple of Hattusha, Çorum (N-Turkey)

The socle stones of the Great Temple in the Lower City of the Hittite capital city, Hattusha (Çorum Province) have been studied for the first time by petrographic and geochemical analysis (Sr isotope, stable isotope, and trace element geochemistry). Study objectives were to determine the carbonate facies of the stone samples used in the Great Temple and investigate their possible sources. Petrographic analysis of 10 limestone samples presents five clusters of different microfacies. Stable isotope (δ13C and δ18O) analysis displayed the presence of 5 groups in parallel with petrographic analysis. Trace element analysis of the same groups displayed considerable difference among the stone samples and 87Sr/86Sr isotope ratios change between 0.70697 and 0.706867. Based on the results, it can be stated that the building using stones are from probably five different sources. This is of great importance to better understand the construction process and the acquisition of the building materials.

Geochemical and petrographic approach for the origin of the limestone blocks of thewalls of the Great Temple of Hattusha, Çorum (N-Turkey)

The socle stones of the Great Temple in the Lower City of the Hittite capital city, Hattusha (Çorum Province) have been studied for the first time by petrographic and geochemical analysis (Sr isotope, stable isotope, and trace element geochemistry). Study objectives were to determine the carbonate facies of the stone samples used in the Great Temple and investigate their possible sources. Petrographic analysis of 10 limestone samples presents five clusters of different microfacies. Stable isotope (δ13C and δ18O) analysis displayed the presence of 5 groups in parallel with petrographic analysis. Trace element analysis of the same groups displayed considerable difference among the stone samples and 87Sr/86Sr isotope ratios change between 0.70697 and 0.706867. Based on the results, it can be stated that the building using stones are from probably five different sources. This is of great importance to better understand the construction process and the acquisition of the building materials.

Hydrocarbon seepage in the mid-Cretaceous greenhouse world: A new perspective from southern Tibet

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained δ13Ccarb value of −30.5‰ is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including 13C depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates – the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system – agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous.

Trace Elements and Stable Isotopic Geochemistry of Two Sedimentary Sections in the Lower Cambrian Strata from the Tarim Basin, Northwest China: Implications for Silicification of Cherts and Biological Evolution

Trace Elements and Stable Isotopic Geochemistry of Two Sedimentary Sections in the Lower Cambrian Strata from the Tarim Basin, Northwest China: Implications for Silicification of Cherts and Biological Evolution

Mineralogy, Fluid Inclusions, and Oxygen Isotope Geochemistry Signature of Wolframite to Scheelite and Fe,Mn Chlorite Veins from the W, (Cu,Mo) Ore Deposit of Borralha, Portugal

Scheelitization of Mn-bearing wolframite, scheelite, quartz, and Fe,Mn-chlorite veins was identified in the W, (Cu,Mo) ore deposits of Borralha, by optical microscopy, electron-microprobe analysis, and stable isotope geochemistry. Fluid inclusions derived scheelite crystallization temperature was compared with the oxygen isotope temperature estimated. Scheelite was formed mainly during stage I from a low salinity aqueous-carbonic fluid dominated by CO2, where the homogenization temperature (Th) decreased from 380 °C to 200 °C (average of 284 °C). As temperature decreased further, the aqueous-carbonic fluid became dominated by CH4 (Stage II; (average Th = 262 °C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average Th = 218 °C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The δ18OFluid values calculated for quartz-water and wolframite-water fractionation fall within the calculated magmatic water range. The ∆quartz-scheelite fractionation occurred at about 350–400 °C. The ∆chlorite-water fractionation factor calculated is about +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. Estimated crystallizing temperatures based on semi-empirical chlorite geothermometers range from 373 °C to 458 °C and 435 °C to 519 °C. A narrower temperature range of 375 °C to 410 °C was estimated for Fe,Mn-chlorite crystallization.

Stable Isotope Geochemistry of the Organic Elements within Shales and Crude Oils: A Comprehensive Review.

Over time, stable isotopes have proven to be a useful tool in petroleum geochemistry. However, there is currently insufficient literature on stable isotope geochemistry of the organic elements within shales and crude oils in many petroleum systems around the world. As a result, this paper critically reviews the early and recent trends in stable isotope geochemistry of organic elements in shales and crude oils. The bulk and compound-specific stable isotopes of H, C, and S, as well as their uses as source facies, depositional environments, thermal maturity, geological age, and oil–oil and oil–source rock correlation studies, are all taken into account. The applications of the stable isotopes of H and C in gas exploration are also discussed. Then, the experimental and instrumental approaches to the stable isotopes of H, C, and S, are discussed.

Unlocking the Single-Crystal Record of Heavy Stable Isotopes

Stable isotopes provide deep insights into processes across a wide range of scales, from micron- to cosmic-size systems. Here, we review how continued advances in mass-spectrometry have enabled the analysis of ever-smaller samples and brought the field of heavy stable isotope geochemistry to its next frontier: the single-crystal scale. Accessing this record can be as enlightening as it is challenging. Drawing on novel systematics at different stages of development (from well-established to nascent), we discuss how the isotopes of heavy elements, such as magnesium, iron, zirconium, or uranium, can be used at the single-crystal and subcrystal scales to reconstruct magma thermal histories, crystal growth timescales, or, possibly, magma redox conditions.

Hydrodynamics, Hydrochemistry, and Stable Isotope Geochemistry to Assess Temporal Behavior of Seawater Intrusion in the La Yarada Aquifer in the Vicinity of Atacama Desert, Tacna, Peru

The La Yarada aquifer is the primary water resource for municipal, irrigation, and industrial uses in the semi-arid Tacna, Peru. Presently, over-pumping has caused severe groundwater management problems, including the abandonment of saline water wells. This study presents multivariate analysis and chemical–isotopic trends in water to investigate seawater intrusion and hydrogeological processes affecting water quality. The chemical and isotopic analysis of water samples, collected in two campaigns in the dry (August 2020) and wet (November 2020) seasons, together with the 1988 data, were evaluated with a mixing model, cluster, and factor analysis. The hydrochemical and isotopic mixing model suggested the formation of a wedge with 20% seawater intrusion. The heterogeneity of piezometric map isolines corroborates the wedge formation associated with the groundwater movement. The spatial distributions of factors, FA1 and FA2, suggest two processes of seawater front movement: dispersion (diffusion) of chemical elements and different types of water mixing, respectively. At the edge of the La Yarada aquifer, the water head was relatively low, permitting seawater and freshwater mixing. On the other hand, along the sea-land boundary, the water head of the La Yarada aquifer was relatively high, avoiding seawater and freshwater mixing; however, the chemical species were migrating from the seawater to the groundwater due to the diffusion processes. The cluster 4 samples are in the region corresponding to the isotopic mixing process represented by the FA2, while cluster 4 describes the chemical diffusion process represented by the FA2. Thus, the integrated approach is helpful to assess the seawater intrusion mechanisms in coastal aquifers in a semi-arid region.

Contribution of deep-sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from radiocarbon and stable isotope geochemistry

Sulfate-driven anaerobic oxidation of methane (AOM) limits the release of methane from marine sediments and promotes the formation of carbonates close to the seafloor in seepage areas along continental margins. It has been established that hydrocarbon seeps are a source of methane, dissolved inorganic carbon, and dissolved organic carbon to marine environments. However, questions remain about the contribution of deep-sourced carbon from hydrocarbon seeps to the sedimentary organic carbon pool. In this study, we analyzed carbon quantity, radiocarbon content (as percent modern carbon, pMC), stable carbon isotopic compositions (as δ13C) of organic matter enclosed within seep carbonates from the Gulf of Mexico and the South China Sea to assess if sediment organic matter may be used as a proxy for methane seepage intensity. The δ13C values of organic matter (δ13Corg) exhibited a large range from −81.4‰ to −23.9‰. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% pMC, suggesting organic matter of the carbonates is a mixture of marine particulate organic matter (δ13C = −22‰ VPDB and 90% modern carbon) and biomass resulting from methane oxidation (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity and duration are the most important factors controlling the contribution of methane-derived carbon to the sedimentary column. This study reinforces the potential for using δ13C values of organic carbon to discern methane-rich environments in ancient sedimentary environments where authigenic carbonate is not present and to constrain the record of AOM through Earth history.