Chemical Geology最新文献

{"title":"Calibration of the relationship between oxygen fugacity and the oxidation state of iron in anorthitic plagioclase","authors":"W.L. Ofierska , P.A. Sossi , C. Liebske , D.F. Sanchez , D. Grolimund , M.W. Schmidt","doi":"10.1016/j.chemgeo.2025.122814","DOIUrl":"10.1016/j.chemgeo.2025.122814","url":null,"abstract":"<div><div>Plagioclase, one of the major minerals comprising igneous rocks in the Earth's crust and those of other rocky planetary bodies, incorporates minor quantities of iron in its structure. While ferric iron is known to preferentially partition into plagioclase over its ferrous counterpart, there have been few efforts to characterise its oxidation state directly. To do so, we collected X-ray Absorption Near-Edge Structure (XANES) spectra at the Fe K-edge for a series of quenched anorthite-melt pairs synthesised in the CaO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>-FeO* (CMAS+Fe) system over a range of oxygen fugacities from two log<sub>10</sub>-units below the iron-wüstite (IW) buffer; ΔIW-2, to air (ΔIW + 12) at 1140 °C and 1 bar, with an additional experiment up to ΔIW + 16.7 at 1200 °C and 1 GPa. We show that the range of area mean-weighted centroid energies and integrated intensities of the pre-edge feature in glass and anorthite overlap, permitting Fe<sup>3+</sup>/∑Fe ratios for both phases to be determined using existing calibrations for glasses. The equilibrium constant for the reaction FeO + 1/4O<sub>2</sub> = FeO<sub>1.5</sub> in plagioclase determined by XANES is found to be log<em>K</em><sup>plg</sup> = 1.60 ± 0.09 with a stoichiometric coefficient for oxygen, <em>n</em> = 0.25 ± 0.01, indistinguishable from the ideal value, independent of plagioclase orientation. This permits the <em>f</em>O<sub>2</sub> under which anorthite crystallised to be determined from its Fe<sup>3+</sup>/Fe<sup>2+</sup> ratio alone, by the equation <span><math><msub><mi>f</mi><msub><mi>o</mi><mn>2</mn></msub></msub></math></span> = (<em>K</em><sup>plg</sup>/[Fe<sup>3+</sup>/Fe<sup>2+</sup>]<sub>plg</sub>)<sup>(−1/<em>n</em>)</sup>. Fits to partition coefficients between coexisting melt and anorthite as a function of <span><math><msub><mi>f</mi><msub><mi>o</mi><mn>2</mn></msub></msub></math></span> yield <span><math><msubsup><mi>D</mi><mrow><mi>plg</mi><mo>/</mo><mi>melt</mi></mrow><msup><mi>Fe</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></msubsup></math></span> (0.055 ± 0.005) and <span><math><msubsup><mi>D</mi><mrow><mi>plg</mi><mo>/</mo><mi>melt</mi></mrow><msup><mi>Fe</mi><mrow><mn>3</mn><mo>+</mo></mrow></msup></msubsup></math></span> (0.43 ± 0.04), which, together with log<em>K</em><sup>melt</sup> (0.78 ± 0.03), lead to a second oxybarometer that can be applied to plagioclase-melt pairs, <span><math><msub><mi>f</mi><msub><mi>o</mi><mn>2</mn></msub></msub></math></span> = ([Fe<sup>3+</sup>/Fe<sup>2+</sup>]<sub>plg</sub>/([<span><math><msubsup><mi>D</mi><mrow><mi>plg</mi><mo>/</mo><mi>melt</mi></mrow><msup><mi>Fe</mi><mrow><mn>3</mn><mo>+</mo></mrow></msup></msubsup></math></span>/<span><math><msubsup><mi>D</mi><mrow><mi>plg</mi><mo>/</mo><mi>melt</mi></mrow><msup><mi>Fe</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></msubsup></math></span>]<em>K</em><sup>melt</sup>))<sup>1/<em>n</em></sup>. For a typical precision of ±0.02 on Fe<sup>3+</sup>/∑Fe in ","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"686 ","pages":"Article 122814"},"PeriodicalIF":3.6,"publicationDate":"2025-04-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143918408","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Inhomogeneous porewater geochemistry in unconsolidated argillaceous sediments: Implications for porewater extraction protocols and characterization of solute mobilization","authors":"Wenkai Qiu ,&nbsp;Teng Ma ,&nbsp;Yao Du ,&nbsp;Bolin Zheng ,&nbsp;Ziqi Peng ,&nbsp;Ruihua Shang ,&nbsp;Cong Xiao ,&nbsp;Kewen Luo ,&nbsp;Philippe Van Cappellen","doi":"10.1016/j.chemgeo.2025.122813","DOIUrl":"10.1016/j.chemgeo.2025.122813","url":null,"abstract":"<div><div>Unconsolidated argillaceous sediments play important roles in water-rock interactions and can significantly impact the geochemistry of groundwater and connected surface water bodies. Characterizing porewater geochemistry in these sediments and the geochemical processes regulating solute mobilization is challenging, however. While various porewater extraction methods have been used, the representativeness of the analytical results remains largely untested. Here, we carried out continuous and segmented porewater extraction experiments on clay-rich sediments from the Jianghan Plain in central China. Our data clearly show that the porewaters are mixtures, rather than homogeneous solutions. A first type of porewater inhomogeneity was associated with the abrupt increase in porewater binding strength separating free from bound porewater. The change in binding strength occurred at a loading stress of around 0.20–0.30 MPa, with the threshold value dependent on the particle size distribution of the sediments. The second type of inhomogeneity featured smooth changes in solute concentrations and isotopic compositions, likely reflecting variable contributions of water-sediment reactions. Solutes whose mobilization under increasing loading stress was primarily affected by the first type of inhomogeneity included ammonium and arsenic, those primarily affected by the second type of homogeneity included dissolved organic carbon, alkalinity, and strontium. Overall, the macroscopically observed geochemical variability of porewater discharged during the extractions was related to changes in moisture content and pore stress. Our findings shed light on the multiplicity of processes regulating solute mobilization from unconsolidated argillaceous sediments and emphasize the importance of selecting appropriate porewater extraction procedures for these sediments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122813"},"PeriodicalIF":3.6,"publicationDate":"2025-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143895842","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrogen diffusion in apatite normal to the c-axis: Estimation of hydrogen diffusing species","authors":"Chikashi Yoshimoto ,&nbsp;Shoichi Itoh ,&nbsp;Isao Sakaguchi","doi":"10.1016/j.chemgeo.2025.122812","DOIUrl":"10.1016/j.chemgeo.2025.122812","url":null,"abstract":"<div><div>Hydrogen isotope retention of apatite is important in discussing hydrogen isotopic compositions in natural apatite. In order to estimate hydrogen isotopic variations associated with thermal processes, experiments on the normal diffusion of fluorapatite along the c-axis using ²H₂O vapor as a diffusion source were performed at 550–700 °C. The diffusion coefficients of fluorapatite were determined by secondary ion mass spectrometry (SIMS). Depth profiles of diffusion were obtained by SIMS, and diffusion coefficients were obtained by model fitting. The Arrhenius relationship for fluorapatite normal to the c-axis yielded a pre-exponential factor (<em>D</em>₀) = 9.77 × 10⁻⁶ [m²/s] and activation energy (<em>E</em>_a) = 208 ± 27 [kJ/mol] over the temperature range of the experiment. The diffusion coefficient normal to the c-axis was approximately three times greater than that parallel to the c-axis and the activation energies from both crystal orientations were consistent within the error margin. The difference in diffusion coefficients can be explained by using the nearest oxygen site and considering the diffusion mechanism with OH⁻ as the diffusing species. Based on the similarity of the diffusion mechanism with crystallographic orientation, a closure temperature was calculated using the results of this study. This updated closure temperature is useful for discussing the retention of hydrogen isotopes and for determining the cooling rate from the hydrogen isotopic compositions in natural water-rock interaction under asteroids, planets, and terrestrial geological setting.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122812"},"PeriodicalIF":3.6,"publicationDate":"2025-04-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143900276","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The stable Zr isotope composition of the martian mantle","authors":"Ninna K. Jensen ,&nbsp;Zhengbin Deng ,&nbsp;Martin Bizzarro","doi":"10.1016/j.chemgeo.2025.122811","DOIUrl":"10.1016/j.chemgeo.2025.122811","url":null,"abstract":"<div><div>We report high-precision stable Zr isotope data for a wide selection of martian meteorites using the double-spike method. The investigated samples comprise shergottites, nakhlites, a single chassignite, and several clasts from the regolith breccia meteorite Northwest Africa (NWA) 7533. These samples are mostly of mafic compositions, and they exhibit limited, yet resolvable, stable Zr isotope compositions. Specifically, the range of <em>δ</em>⁹⁴Zr_IPGP-Zr compositions recorded by the SNC meteorites extends from 0.021 ± 0.003 ‰ (2SE) in Dhofar 019 to 0.093 ± 0.018 ‰ (2SE) in Shergotty, and the NWA 7533 clasts (except for one monzonitic sample) have compositions overlapping with this range. The total <em>δ</em>⁹⁴Zr_IPGP-Zr range of &lt;0.1 ‰ indicates that the formation of the mafic martian samples was not associated with significant mass dependent Zr isotope fractionation. Hence, we suggest that the mean isotope composition of these samples reflects the best estimate of the martian mantle <em>δ</em>⁹⁴Zr_IPGP-Zr composition of 0.062 ± 0.043 ‰ (2SD, <em>n</em> = 37), which is indistinguishable from the current best estimate of Earth's mantle (<em>δ</em>⁹⁴Zr_IPGP-Zr = 0.40 ± 0.44 ‰, 2SD, <em>n</em> = 72).</div><div>An evolved monzonitic clast from NWA 7533, on the other hand, records an extreme <em>δ</em>⁹⁴Zr_IPGP-Zr value of 0.362 ± 0.008 ‰ (2SE). This isotopically heavy signature likely reflects fractional crystallisation of ilmenite, baddeleyite, and/or zircon. Lastly, we suggest that the limited <em>δ</em>⁹⁴Zr variability observed for the NWA 7533 basaltic igneous clasts, which contain a puzzling abundance of zircon considering their zircon-undersaturated bulk compositions, reflects late-stage zircon saturation in pockets of residual melt.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"688 ","pages":"Article 122811"},"PeriodicalIF":3.6,"publicationDate":"2025-04-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144084341","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Salinity and hydrology reconstructions using B/Ga proxy records in the Pearl River Estuary (China)","authors":"Jingyu Zhang ,&nbsp;Wei Wei ,&nbsp;Yi Zhong ,&nbsp;Xing Wei ,&nbsp;Wout Krijgsman ,&nbsp;Yingci Feng ,&nbsp;Yuanjie Li ,&nbsp;Wei Cao ,&nbsp;Yiquan Ma ,&nbsp;Hu Yang ,&nbsp;Yanan Zhang ,&nbsp;Hai Li ,&nbsp;Mengtian Gao ,&nbsp;Wenyue Xia ,&nbsp;Yuxing Liu ,&nbsp;Qingsong Liu","doi":"10.1016/j.chemgeo.2025.122809","DOIUrl":"10.1016/j.chemgeo.2025.122809","url":null,"abstract":"<div><div>Salinity plays a critical role in the hydrological conditions and environmental changes in marginal marine settings. However, bulk-sediment paleosalinity proxies require further refinement, and their sensitivity in detecting changes in ancient sediments needs to be better constrained. In this study, we evaluate the applicability of a boron (B)/gallium (Ga) salinity proxy to an analysis of sediments of the Pearl River Estuary (PRE). PRE bottom water salinity analyses featured a well-defined gradual salinity gradient of 8.6 psu (practical salinity unit) to 32.2 psu, making PRE an ideal archive to study the response of salinity proxies to salinity change. Our results show a significant positive linear correlation (<em>r</em> = 0.796, <em>P</em> &lt; 0.01) detected between B/Ga in surface sediments (from 2.90 to 4.76) and salinity in bottom water, suggesting the sensitivity of B/Ga proxy for salinity reconstruction in marginal seas and estuarine environments. Additionally, we find significant correlations between salinity, grain size, and chemical weathering proxies, revealing that grain size and chemical weathering can be affected by terrestrial–marine environmental transitions. We propose using the salinity proxy B/Ga to help distinguish whether changes in grain size and chemical weathering are caused by the depositional environment changes, particularly in estuaries with complex environmental variations. Furthermore, B/Ga records from Holocene F01 core, located near the PRE, align with diatom-based salinity reconstructions and precipitation records from southern China, supporting its reliability as a salinity proxy and its potential to capture monsoonal precipitation variability through freshwater input changes. These findings highlight the sensitivity and validity of the B/Ga ratio as an indicator of salinity and hydrology in marginal marine environments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122809"},"PeriodicalIF":3.6,"publicationDate":"2025-04-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143890919","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cryogenic cave carbonates: New insights from alpine ice caves","authors":"Christoph Spötl,&nbsp;Gabriella Koltai","doi":"10.1016/j.chemgeo.2025.122808","DOIUrl":"10.1016/j.chemgeo.2025.122808","url":null,"abstract":"<div><div>Cryogenic cave carbonates (CCC) are speleothems that form under freezing conditions and provide crucial insights into past cold environments and cryogenic processes. Traditionally, CCC have been classified into two categories: fine-grained CCC (hereafter CCC-f) and coarse-grained CCC (hereafter CCC-c), primarily based on particle size and stable isotopic composition. However, recent observations from alpine ice caves in Austria suggest the existence of an intermediate class, termed CCC-i, characterized by small grain sizes (&lt;1 mm) and stable isotope values that overlap with CCC-c. This study documents the occurrence, morphology, and stable isotopic composition of CCC-i found in seven ice caves, providing insights into their formation mechanism. CCC-i are associated with small pools in ice and form in a cave climate that is transitional between the fully heterothermic regime close to the entrance and the homothermic regime of the inner cave passages characterized by high thermal stability. The stable isotopic signatures of CCC-i indicate a Rayleigh-type fractionation process similar to CCC-c but at a smaller spatial and temporal scale. This study highlights the paleoclimatic significance of CCC-i, as they may improve reconstructions of past cave glaciations. However, the ongoing retreat of cave ice due to climate warming threatens the preservation of CCC deposits, emphasizing the urgency of further research.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122808"},"PeriodicalIF":3.6,"publicationDate":"2025-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143873022","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geochemical pattern of chemoautotrophic mussels from cold seeps and hydrothermal vents: Implications for fluid and trophic type","authors":"Zice Jia ,&nbsp;Zhaoshan Zhong ,&nbsp;Xudong Wang ,&nbsp;Hongfei Lai ,&nbsp;Hongyu Zhang ,&nbsp;Na Cui ,&nbsp;Zenggui Kuang ,&nbsp;Minxiao Wang ,&nbsp;Chaolun Li","doi":"10.1016/j.chemgeo.2025.122799","DOIUrl":"10.1016/j.chemgeo.2025.122799","url":null,"abstract":"<div><div>Hydrothermal vents and cold seeps occur in tectonically active areas and along continental margins. They harbor diverse chemoautotrophic organisms, providing valuable insights into fluid types and trophic dynamics. Interestingly, certain mussel species can inhabit both ecosystems, deriving energy from the aerobic oxidation of methane (AeOM) or sulfur oxidation. However, the geochemical fingerprinting of methanotrophic mussels in hydrothermal vents areas remains largely unexplored, especially compared with cold seeps. In this study, stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S) and rare earth elements (REEs) in <em>Gigantidas platifrons</em> collected from hydrothermal vents at Iheya North Knoll in the Okinawa Trough and the cold seep at Site F in the South China Sea were examined. The δ¹³C values demonstrate carbon fixation via AeOM in both environments. The average δ¹³C_tissue values of seep <em>G. platifrons</em> (−75.4‰, <em>n</em> = 28) is similar to δ¹³C_methane. However, the average δ¹³C_tissue of hydrothermal <em>G. platifrons</em> (−25.8‰, <em>n</em> = 13) exhibits enhanced fractionation compared with δ¹³C_methane, potentially because of the abundance of methanotrophs. The δ³⁴S and δ¹⁵N values of mussels indicate two different sulfur and nitrogen sources. In both environments, <em>G. platifrons</em> consistently exhibits positive lanthanum (La) anomalies because it serves as an enzyme co-element in AeOM. Seep mussels have more positive La/La^⁎ ratios compared with hydrothermal mussels, indicating an increased number of methanotrophs on their gills. Positive europium (Eu) anomalies exist in hydrothermal mussels and are influenced by ascending hydrothermal fluids. Four REE pattern fingerprints are summarized, highlighting the potential of stable isotopes and REEs as powerful analytical proxies for studying the complex interactions between fluid sources, microbial symbiosis, and trophic structures near hydrothermal vents and cold seeps.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122799"},"PeriodicalIF":3.6,"publicationDate":"2025-04-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143868597","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Study of the sources and dispersion of sediments on the Sunda Shelf based on the investigation of rare earth element concentrations and Nd isotope compositions","authors":"Kaikai Wu ,&nbsp;Shengfa Liu ,&nbsp;Xuefa Shi ,&nbsp;Che Abd. Rahim Mohamed ,&nbsp;Hui Zhang ,&nbsp;Rosella Pinna-Jamme ,&nbsp;Arnaud Dapoigny ,&nbsp;Christophe Colin","doi":"10.1016/j.chemgeo.2025.122807","DOIUrl":"10.1016/j.chemgeo.2025.122807","url":null,"abstract":"<div><div>The Sunda Shelf receives substantial sediment inputs from the Tibetan Plateau and Southeast Asia's mountainous islands, impacting southern South China Sea sedimentation during Quaternary sea-level changes. Due to multiple river basins contributing to the shelf, the origin and distribution of its surface sediments remain poorly defined. Here, we analyzed major and trace element concentrations and Nd isotope compositions (¹⁴³Nd/¹⁴⁴Nd expressed in epsilon units, εNd) of sediments from rivers around the Sunda Shelf and its surface sediments, focusing on &lt;63 μm and &lt; 2 μm fractions to evaluate grain size effects on rare earth element (REE) patterns and εNd compositions and to identify sediment sources. Significant differences in UCC-normalized REE patterns between &lt;2 μm and &lt; 63 μm river sediment fractions indicate a notable grain size effect. However, εNd variations between these fractions are typically less than 1 εNd unit, suggesting minimal grain size impact on Nd isotope compositions of the detrital fractions. Surface sediments on the western Sunda Shelf derive from the Malay Peninsula, Mekong River, and northern Thai rivers based on the δEu_UCC-(Gd/Yb)_UCC diagram. εNd spatial distribution of the detrital fraction indicates nearshore sediments of the Sunda Shelf are influenced by neighboring rivers (from −15.8 to −6.6), while the central region is affected by the Mekong and northern Thai rivers (from −15.8 to −10.3). Notably, unradiogenic εNd in central shelf sediments indicates Thai river sediments were transported there via paleo-channels during low sea levels before the Gulf of Thailand fully flooded. A large eddy in the central Sunda Shelf would have conserved these relic sediments and redistributed modern sediments by sorting and transporting sediments from nearby rivers.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122807"},"PeriodicalIF":3.6,"publicationDate":"2025-04-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143869173","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Thallium isotope data suggest minimal recycled material within the proto-Iceland mantle plume source","authors":"J. Kaare-Rasmussen ,&nbsp;F. Horton ,&nbsp;C.M. Ostrander ,&nbsp;S.G. Nielsen","doi":"10.1016/j.chemgeo.2025.122798","DOIUrl":"10.1016/j.chemgeo.2025.122798","url":null,"abstract":"<div><div>Mantle plume isotopic compositions are useful for understanding the processes that produce heterogeneity within the mantle. Thallium (Tl) isotopic compositions are uniquely well suited for tracing pelagic sediments and low-temperature altered oceanic crust in the mantle. We measured the Tl isotopic composition of Paleocene lavas on Baffin Island, the earliest lavas associated with the Iceland mantle plume, to constrain the geochemical evolution of the mantle plume. After correcting for assimilation and degassing, these lavas have ²⁰⁵Tl/²⁰³Tl ratios indistinguishable from the inferred ratio of the upper mantle and from modern Iceland lavas. Therefore, the amount of subducted pelagic sediments or low temperature altered oceanic crust potentially within the head of the Iceland mantle plume is vanishingly small. These components cannot account for the well-documented evolution in other isotopic systems over time in the Iceland mantle plume. Thus, we evaluate other processes that could account for this evolution, including other subducted components and core-mantle interactions.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122798"},"PeriodicalIF":3.6,"publicationDate":"2025-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143855309","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Paleoenvironmental changes and nitrogen cycling of the lacustrine systems following the Toarcian Oceanic Anoxic Event in the Sichuan Basin (China): Insights from nitrogen isotopes","authors":"Yang Bai ,&nbsp;Jianfa Chen ,&nbsp;Jin Wu ,&nbsp;Wenzhe Gang ,&nbsp;Václav Suchý ,&nbsp;Cong Chen ,&nbsp;Xuemin Xu ,&nbsp;Rao Fu ,&nbsp;Jiaqi Zhang ,&nbsp;Shuaiqi Tang","doi":"10.1016/j.chemgeo.2025.122797","DOIUrl":"10.1016/j.chemgeo.2025.122797","url":null,"abstract":"<div><div>The Toarcian Oceanic Anoxic Event (T-OAE) represents a significant disturbance of the Earth's atmosphere-ocean system in the Early Jurassic, and its response in lacustrine systems has been widely studied in recent years. However, the impact of T-OAE on the subsequent environmental changes and nitrogen cycling patterns in lacustrine systems are poorly understood. This study investigated the paleoenvironmental changes following the T-OAE based on elemental geochemistry analyses of organic-rich shales from the Early Jurassic Da'anzhai Member and the Middle Jurassic Lianggaoshan Formation in the Sichuan Basin. The nitrogen cycling during this period was reconstructed drawing on nitrogen isotopes (δ¹⁵N) and their correlations with water column conditions. The results indicated that during the deposition of Da'anzhai shale (i.e., T-OAE period), the lacustrine system was dominated by a temperate-humid climate, with frequent oxic-suboxic variations in the water column. The sedimentary environment of the Lianggaoshan shale was similar to that of the Da'anzhai shale, even though there was a brief arid-cold climate during the sedimentary period at the bottom of Lianggaoshan Formation. The nitrogen isotopes of kerogen (δ¹⁵N_ker) were primarily controlled by the redox conditions of water column, and the silicate-bound inorganic nitrogen isotopes (δ¹⁵N_sil) were influenced by combined factors including climate, input of terrestrial clay minerals, and the redox conditions of the bottom water column. These variations in δ¹⁵N provided foundations for reconstructing nitrogen cycling under diverse lacustrine environmental conditions. Under the suboxic water column, the occurrence of denitrification and anaerobic ammonium oxidation (anammox) in the chemocline resulted in enrichment of ¹⁵N in kerogen. The positive excursion in δ¹⁵N_sil was a cumulative effect attributed to the enhanced influx of exogenous illite under temperate-humid climatic conditions, as well as the combined influence of anammox on NH₄⁺ concentrations. In water column with elevated oxygen concentrations, biogeochemical processes, predominantly driven by nitrification, resulted in lighter δ¹⁵N_ker values, and δ¹⁵N_sil, influenced by the low concentration of NH₄⁺ in the bottom water, also exhibited a lighter isotopic signature. The similar geochemical signatures in the Da'anzhai and Lianggaoshan shales implied that the lacustrine system might be susceptible to variations in localized paleoenvironment, and was not completely consistent with the marine system.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122797"},"PeriodicalIF":3.6,"publicationDate":"2025-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143848291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}