Chemical Geology最新文献

{"title":"Origin of the coeval Nb-enriched and arc mafic igneous rocks: Implications for Paleozoic tectonic evolution of the Southwestern Tianshan","authors":"Li-Tao Ma ,&nbsp;Li-Qun Dai ,&nbsp;Ye-Dan Hu ,&nbsp;Zi-Fu Zhao","doi":"10.1016/j.chemgeo.2024.122582","DOIUrl":"10.1016/j.chemgeo.2024.122582","url":null,"abstract":"<div><div>Constraining the origin of the Nb-enriched mafic igneous rocks and associated rock assemblages is critical for understanding the tectonic evolution and geodynamic processes in the subduction zone. In this study, we focus on Early Devonian normal arc gabbros and Nb-enriched gabbros from the Central Tianshan Block (CTB), along with Early Carboniferous Nb-enriched basalts from the southern margin of Yili Block. The Early Devonian arc gabbros display pronounced Nb and Ta troughs, relatively depleted Sr–Nd–Hf isotopes, and low zircon δ¹⁸O value. In contrast, the Nb-enriched gabbros show both arc- and OIB-like trace element features, more enriched Sr–Nd–Hf isotopes, higher zircon δ¹⁸O value, and elevated Nb and Nb/La ratio. These rocks are coeval with N-MORB, <em>E</em>-MORB and OIB in the South Tianshan Belt and Nb-enriched mafic igneous rocks in the northern Tarim, suggesting their formation during back-arc opening of the South Tianshan Ocean, triggered by the southward subduction of Terskey Ocean beneath the Central Tianshan-Northern Tarim Craton. In such a circumstance, the normal arc gabbros represent mantle melts modified by aqueous solutions derived from subducting slab, while the Nb-enriched gabbros were generated by partial melting of mantle metasomatized by subducting oceanic slab-derived melts with rutile breakdown. The Early Carboniferous Nb-enriched basalts also display arc-like trace element signatures and depleted Sr–Nd–Hf isotope features, as well as high Nb and Nb/La. These basalts, coexisting with adakitic granite porphyry and sanukitic high-Mg andesites. These suggest their formation was primarily related to interactions between a mantle wedge and adakitic melts derived from basaltic oceanic crust, with or without sediment melts, in a back-arc setting developed during the northward subduction of South Tianshan Ocean. Thus, this study provides crucial insights into the origin of Nb-enriched mafic rocks and tectonic transition from Ordovician-Early Devonian subduction of the Terskey Ocean to Late Devonian-Late Carboniferous subduction of the South Tianshan Ocean.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122582"},"PeriodicalIF":3.6,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142887290","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 link between ophiolitic chromitites, natural hydrogen and methane: Insights from 3D microtomography","authors":"Lucia Pappalardo ,&nbsp;Gianmarco Buono ,&nbsp;Monia Procesi ,&nbsp;Giuseppe Etiope","doi":"10.1016/j.chemgeo.2024.122575","DOIUrl":"10.1016/j.chemgeo.2024.122575","url":null,"abstract":"<div><div>Ultramafic rocks in ophiolites are known as source rocks of abiotic hydrogen (H₂) and methane (CH₄), due to serpentinization and successive CO₂ hydrogenation. Ophiolites are therefore key targets in natural hydrogen exploration. While serpentinized peridotites are the main sources of H₂, chromitites host both hydrogen and large quantities of methane, as revealed by analyses of direct gas extraction from rocks and micro-Raman analyses. However, the fluid bearing properties of chromitites, as well as their mineralogical phases that are correlated to the gas genesis and evolution, are unclear. We conducted high-resolution X-ray computed micro-tomography (microCT) on chromitite samples from two ophiolites in Greece. The microCT analysis, using the X-ray attenuation coefficient (which is density-dependent), combined with 3D image analysis and pore-scale permeability simulations, revealed the geometry and distribution of pores and microfractures. This approach provided insights into their flow properties and spatial relationships with solid phases that could act as catalysts for CH₄ production (Platinum Group Elements - PGM), H₂ flow (altered PGM), and CO₂ hydrogenation (amorphous carbon). Microfractures appear as potential sites or microreactors for H₂-CO₂ conversion into CH₄, while also retaining residual, unreacted H₂. The microCT technique provides insights into the in-situ textural relationship between microfractures, gas pores and solid phases, unattainable through 2D traditional techniques, thus offering a valuable support for natural hydrogen exploration.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122575"},"PeriodicalIF":3.6,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140535","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Co-application of rhenium, vanadium, uranium and molybdenum as paleo-redox proxies: Insight from modern and ancient environments","authors":"Sen Li ,&nbsp;Paul B. Wignall ,&nbsp;Simon W. Poulton","doi":"10.1016/j.chemgeo.2024.122565","DOIUrl":"10.1016/j.chemgeo.2024.122565","url":null,"abstract":"<div><div>Numerous approaches have been developed for determining past redox conditions in marine settings (e.g., Fe speciation, redox sensitive trace metal (RSTM) systematics, pyrite morphologies, I/(Ca + Mg) ratios), enabling a broad range of redox conditions, from fully oxic to euxinic, to be potentially identified. However, many points along this spectrum remain difficult to constrain, including dysoxic and highly versus weakly euxinic conditions. This limits the broader scale inferences that can be drawn from paleoredox studies, including links between oxygen availability and biological evolution, and the potential for isotope systems (e.g., Mo) to record water column signals. Here, we develop a new approach using RSTM ratios (Re/Mo, Re/U, Re/V, Mo/U), in combination with modified RSTM enrichment factors (EF^⁎) and Mo_EF^⁎-U_EF^⁎ cross-plots, that potentially enables a robust, highly resolved reconstruction of ancient water column redox conditions to be achieved. We initially document the differential behaviour of RSTM EF^⁎ values and ratios in modern settings that range from fully oxygenated, through weakly (30–90 μM O₂) and highly (&lt;30 μM O₂) dysoxic, to anoxic non-sulfidic and euxinic conditions. This redox behaviour is further resolved when the drawdown mechanisms for Mo are evaluated by Mo_EF^⁎-U_EF^⁎ cross-plots. We subsequently ground-truth this approach by considering samples from the Carboniferous Bowland Basin, which have previously been studied for redox conditions via independent geochemical and mineralogical techniques, as well as the Jurassic Kimmeridge Clay Formation, where redox conditions have been defined based on paleoecological characteristics. A strong degree of consistency between RSTM behaviour in modern and ancient settings highlights that weakly to highly dysoxic conditions are characterized by increases in Re/Mo, Re/U and Re/V ratios, accompanied by RSTM EF^⁎ values that only become notably enriched under highly dysoxic conditions. Non-sulfidic water column anoxia is indicated by increased U_EF^⁎ values and low Re/Mo ratios, while euxinia is readily identified by high Mo/U, low Re/U and very low Re/Mo ratios, alongside high Mo_EF^⁎ values. In addition, highly euxinic conditions may be distinguished from weak euxinia by particularly high Mo/U ratios and Mo_EF^⁎ values. This combined approach has the potential to provide a hitherto unprecedented level of insight into paleodepositional redox conditions, and consequently the chemical evolution of the biosphere.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122565"},"PeriodicalIF":3.6,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142887304","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":"Source versus crustal processing and the evolution of the mantle wedge in the Trans-Mexican Volcanic Belt: Constraints from Os-O-He isotope systematics in olivine","authors":"J. Ahmadi ,&nbsp;E. Widom ,&nbsp;S.M. Straub ,&nbsp;R. Sanchez ,&nbsp;D.C. Kuentz ,&nbsp;A. Gómez-Tuena ,&nbsp;R. Espinasa-Perena ,&nbsp;I.N. Bindeman ,&nbsp;F.M. Stuart","doi":"10.1016/j.chemgeo.2024.122574","DOIUrl":"10.1016/j.chemgeo.2024.122574","url":null,"abstract":"<div><div>We have integrated Os isotope systematics in olivine phenocrysts with published O and He isotope data from a suite of well-characterized high-Mg olivine-phyric basalts to andesites across the Trans-Mexican Volcanic Belt (TMVB) to address the relative roles of subduction-related crustal input to the mantle source versus shallow fractional crystallization and/or crustal assimilation. Osmium concentrations in the olivines across all the samples range from 7.3 to 2200 pg/g and, with the exception of one anomalous sample with ¹⁸⁷Os/¹⁸⁸Os_(ol) = 0.532, ¹⁸⁷Os/¹⁸⁸Os_(ol) ranges from 0.125 to 0.259. Olivines from the rear-arc samples are relatively unradiogenic in Os (¹⁸⁷Os/¹⁸⁸Os = 0.122 to 0.136) compared to the arc front olivines (¹⁸⁷Os/¹⁸⁸Os ≥ 0.130), which are more radiogenic than primitive upper mantle and largely overlap with mantle xenoliths from arc settings. The arc front olivines exhibit distinctly heavier δ¹⁸O than those of the rear-arc, but a significant role for crustal assimilation in the evolution of most TMVB magmas can be precluded due to the lack of correlation between ¹⁸⁷Os/¹⁸⁸Os_(ol) or δ¹⁸O_(ol) with indices of fractionation (e.g., Fo#, Ni_(ol), and Mg#_(WR)), as well as the mantle-like He isotope signatures of the olivines. This suggests that the radiogenic Os and heavy δ¹⁸O are inherited from the mantle source region. A mixing model between mantle and sediment-rich slab-derived components, as proposed previously for other areas of the TMVB, can explain the ⁸⁷Sr/⁸⁶Sr_(wr) - ²⁰⁶Pb/²⁰⁴Pb_(wr) - δ¹⁸O_(ol) systematics. However, the radiogenic ¹⁸⁷Os/¹⁸⁸Os_(ol) requires an unexpectedly high degree of fluid mobility for Os in this model. Instead, the Os data suggest that serial subduction fluxing and melting of the mantle wedge result in an accumulation of radiogenic Os in the mantle wedge through progressive slab flux, consistent with models from earlier studies based on olivine chemistry and the positive correlation of δ¹⁸O_(ol) with mantle depletion proxies. The decoupling of ¹⁸⁷Os/¹⁸⁸Os_(ol) and δ¹⁸O_(ol) may be influenced by the presence of primary and secondary sulfides in the mantle wedge, which control the Os budget.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122574"},"PeriodicalIF":3.6,"publicationDate":"2024-12-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142887291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Immobilization of Sb(V) by secondary Fe (oxyhydr)oxides during Fe(II) oxygenation: Insights into Sb(V) incorporation and Fe(II) mineralization mechanisms","authors":"Yiqing Wang ,&nbsp;Mengchang He ,&nbsp;Chunye Lin ,&nbsp;Wei Ouyang ,&nbsp;Xitao Liu","doi":"10.1016/j.chemgeo.2024.122567","DOIUrl":"10.1016/j.chemgeo.2024.122567","url":null,"abstract":"<div><div>Abiotic Fe(II) oxygenation to form secondary Fe (oxyhydr)oxides commonly occurs in natural environments and critically affects the mobility and fate of metalloids such as antimony (Sb). However, the Sb(V) immobilization process and mechanism during Fe(II) oxygenation are not well understood, and the interactions between Sb(V) and formed Fe (oxyhydr)oxides need further study. This study comprehensively investigated Sb(V) immobilization and secondary Fe (oxyhydr)oxides formation during Fe(II) oxygenation for 10 h in the presence of Sb(V). The results indicated that Sb(V) was immobilized by secondary Fe (oxyhydr)oxides mainly via coprecipitation rather than adsorption. Extended X-ray absorption fine structure (EXAFS) analysis further verified that Sb(V) was structurally incorporated into the formed lepidocrocite mainly via edge-sharing linkage and into goethite via edge-sharing and double corner-sharing linkages between SbO₆ and FeO₆ octahedra, thus resulting in the formation of various secondary Fe (oxyhydr)oxides. Additionally, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD) characterization demonstrated that Sb(V) incorporation inhibited lepidocrocite formation and favored goethite formation at pH 6 and 7 with initial Sb(V)/Fe(II) molar ratios above 0.01 and 0.04, respectively, and it also hindered magnetite formation at pH 8. Transmission electron microscopy (TEM) suggested that Sb(V) incorporation affected the morphologies of formed Fe (oxyhydr)oxides. Overall, our findings provide valuable insights into Sb(V) immobilization and Fe (oxyhydr)oxides formation during Fe(II) oxygenation, and are conducive to clarifying the geochemical behavior of Sb(V) coupled with Fe(II) at dynamic redox interfaces in Sb(V)-contaminated environments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122567"},"PeriodicalIF":3.6,"publicationDate":"2024-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142857734","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":"Intrinsic activity of organic acids controlling photochemical behavior and transformation of schwertmannite in acid mine drainage","authors":"Shishu Zhu ,&nbsp;Xiaokang Hou ,&nbsp;Huanxin Ma ,&nbsp;Hengyi Fu ,&nbsp;Jeng-Lung Chen ,&nbsp;Tsung-Yi Chen ,&nbsp;Zhi Dang ,&nbsp;Chunhua Feng","doi":"10.1016/j.chemgeo.2024.122569","DOIUrl":"10.1016/j.chemgeo.2024.122569","url":null,"abstract":"<div><div>Occurrence and transformation of schwertmannite (Sch) widely influence the speciation and distributions of iron and sulfur as well as pollutants in acid mine drainage (AMD)-impacted ecosystems. Despite extensive research on the biogeochemical fate of Sch in terrestrial systems, the mechanisms underlying its phase transformation mediated by redox processes in acidic waters remain inadequately understood. This study investigates how the intrinsic activity of naturally abundant organic acids in AMD-impacted waters affects the photochemical behavior and transformation of Sch under oxic and anoxic conditions through comprehensive characterizations. Solid product characterization results showed that the addition of oxalic acid (OA) and tartaric acid (TA), rather than formic acid (FA), significantly accelerated photochemical transformation of Sch into goethite (Gt) and magnetite (Mt), increasing by 29%–−47% and 35% under anoxic condition, respectively. Comparison analyses suggested the photoactivated interfacial electron transfer could be accelerated by the organic acids with the stronger complexing and electron-donating abilities, further enhancing photoreductive dissolution of structural Fe(III) to initiate Fe(II)-catalyzed transformation of Sch. Such transformation pathway of Sch was inhibited due to oxygenation of Fe(II) and only 21% Gt newly formed in the Sch/TA system under oxic conditions. It is further found that carbon-centered radicals (CCR^•), derived from organic acids containing electron-withdrawing groups with lower dissociation enthalpy, efficiently protected Fe(II) from oxygenation by competing with oxidants, thus enhancing Sch transformation. The study provides new insights into the expanded transformation pathways of Sch, advancing the understanding of iron cycling and reactive species production in the euphotic zone of acidic waters.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122569"},"PeriodicalIF":3.6,"publicationDate":"2024-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142912323","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":"Carbonatite evolution at St Honoré (Canada), the apatite record","authors":"O.V. Vasyukova ,&nbsp;A.E. Williams-Jones ,&nbsp;D.C. Petts ,&nbsp;B.A. Kjarsgaard","doi":"10.1016/j.chemgeo.2024.122568","DOIUrl":"10.1016/j.chemgeo.2024.122568","url":null,"abstract":"<div><div>The purpose of our study was to test the hypothesis that apatite provides a detailed record of the evolution of carbonatitic magmas. To this end, we investigated the chemistry and textures of apatite in the various rock units (banded carbonatite, biotitised syenite, biotitite, magnetite-biotite rock and massive carbonatite) of the St Honoré carbonatite using a combination of micro-analytical and imaging techniques. Subtle changes in the uptake of a variety of trace elements during growth led to corresponding changes in the cathodoluminescence response that are recorded as distinct zones in the apatite. In the banded carbonatite, the first apatite to crystallise was fluid/mineral inclusion-bearing and was followed by apatite displaying oscillatory zoning and, in turn, by apatite that replaced the earlier apatite through dissolution-reprecipitation. In contrast, the earliest apatite in the biotitised syenite displays oscillatory zoning and was variably replaced by later apatite. The subsequent crystallisation stages duplicate those of apatite in the banded carbonatite. Apatite crystallisation in the magnetite-biotite rock duplicated the stages recorded by apatite in the banded carbonatite. Finally, the apatite of the massive carbonatite contains representatives of all the apatite types mentioned above.</div><div>A model is presented in which inclusion-rich apatite records aqueous‑carbonic fluid exsolution from the magma, oscillatory zoned apatite records periods of quiescent growth and replacement apatite records dissolution-reprecipitation induced by the differential stresses that accompanied fluid overpressures. Apatite in the massive carbonatite was incorporated from the other units. Based on the above model, we propose that banded carbonatites at St Honoré and other similar complexes formed during an early stage of carbonatitic magma emplacement, when thermal gradients between the magma (hot) and the host rocks (cooler) were steep and the calcite liquidus was reached before significant biotitisation. With the emplacement of additional batches of magma, the thermal gradient was gradually flattened and biotitisation was extensive, producing massive biotitite. This hypothesis explains the occurrence, spatial distribution and genesis of the biotitite (glimmerite) and banded carbonatite observed in many carbonatite complexes.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122568"},"PeriodicalIF":3.6,"publicationDate":"2024-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142887293","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Timing of carbon uptake during seafloor alteration: Insight from in situ U-Pb dating at DSDP sites 417A and 417D","authors":"Zhichao Liu ,&nbsp;Shuo Chen ,&nbsp;Yanhong Chen ,&nbsp;Lifeng Zhong ,&nbsp;Renbiao Tao ,&nbsp;Yaoling Niu ,&nbsp;Zhaojie Yu ,&nbsp;Laurence A. Coogan","doi":"10.1016/j.chemgeo.2024.122571","DOIUrl":"10.1016/j.chemgeo.2024.122571","url":null,"abstract":"<div><div>Formation of carbonate minerals during alteration of seafloor lavas serves as a significant global CO₂ sink. Understanding the timing of carbonate formation is critical for evaluating the role of low-temperature seafloor alteration as a negative feedback on the global carbon cycle. However, whether carbonate mineral formation largely occurs soon after crustal accretion, or continues throughout the entire lifespan of the ocean crust, remains debated. In this study, we use <em>in situ</em> U-Pb dating techniques to investigate the formation ages of carbonate veins and vesicles in ∼120 Ma ocean crust at Deep Sea Drilling Project (DSDP) Sites 417A and 417D, located in the western Atlantic. Our results show that carbonate mineral U concentrations differ between these cores, which we interpret as reflecting a strong dependence of U uptake into calcite on the redox conditions of the aquifer during carbonate growth. The more oxidizing alteration conditions at Site 417A led to growth of carbonate minerals with much lower U concentrations than those formed at Site 417D, which was altered under more reducing conditions. Importantly, through a thorough evaluation of both published and our new carbonate U-Pb age data, we confirm that more than 90 % of carbonate mineral formation during seafloor alteration occurs within &lt;20 m.y. after crustal accretion. Simple models based on this refined timescale of basalt alteration and carbonate mineral formation show that variation in bottom water temperature within the first 10 m.y. after crustal accretion can affect the final carbon content of the upper oceanic crust. Our study provides valuable insights for carbon cycle models and highlights the importance of seafloor alteration in regulating Earth's climate.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122571"},"PeriodicalIF":3.6,"publicationDate":"2024-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142887311","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":"Mercury evidence for volcanism driving environmental changes during the protracted Late Ordovician mass extinction and early Silurian recovery","authors":"Yanfang Li ,&nbsp;Hui Tian ,&nbsp;Tongwei Zhang ,&nbsp;Baojian Shen ,&nbsp;Deyong Shao","doi":"10.1016/j.chemgeo.2024.122566","DOIUrl":"10.1016/j.chemgeo.2024.122566","url":null,"abstract":"<div><div>Volcanism has been proposed as the trigger for the environmental perturbations and associated mass extinction during the Ordovician–Silurian (O<img>S) transition. However, the timing, duration, and intensity of volcanic eruptions during this critical period and their relationships to environmental perturbations and biotic changes remain unresolved. In this study, we use mercury (Hg) concentrations and isotopes from marine sediments in South China to reconstruct the evolution of volcanism from the Late Ordovician to early Silurian. Our results show that strong Hg enrichment coupled with generally near-zero to slightly positive Δ¹⁹⁹Hg values occurred before, during, and after the classically defined Late Ordovician Mass Extinction (LOME), suggesting a significant influx of volcanogenic Hg. The Hg enrichment intervals coincided with global warming, oceanic anoxia, and negative excursions in carbon and sulfur isotopes, suggesting that volcanism drove the environmental perturbations during the O<img>S transition. The coincidence of Hg enrichment with extinction horizons supports the hypothesis that volcanism may have contributed to LOME. Our study also suggests that volcanism persisted for approximately 3 million years after mass extinction and may have delayed the recovery of marine ecosystems during early Silurian.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122566"},"PeriodicalIF":3.6,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142858002","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":"Hydrothermal origin of platinum-group minerals during serpentinization of the podiform chromitites from the Kızıldağ ophiolite in southern Türkiye","authors":"Chen Chen ,&nbsp;Christina Yan Wang ,&nbsp;Saihong Yang ,&nbsp;İbrahim Uysal","doi":"10.1016/j.chemgeo.2024.122563","DOIUrl":"10.1016/j.chemgeo.2024.122563","url":null,"abstract":"<div><div>Platinum-group minerals (PGMs) in podiform chromitites usually occur in the interior and/or edge of chromite. However, the origin of PGMs in podiform chromitites has long been a matter of debate. Here we examined sub-micro to nanoscale textural features, morphologies, and compositions of PGMs from the disseminated, banded, massive and nodular chromitites in the Kızıldağ ophiolite in southern Türkiye, and found both primary and secondary PGMs. The aim of this study is to reveal the transformation processes from primary to secondary PGMs, thereby taking a thorough examination of the origin of these PGMs. Primary PGMs include laurite and Os-Ir alloy, which are prevalent in all samples. They are typically enclosed within or located at the edge of chromite, and formed either prior to or contemporaneously with the crystallization of chromite at temperature of 1100–1200 °C and logƒS₂ values of -2 to -1. In contrast, PGE-bearing pentlandite are commonly present at the edge of chromite, corresponding to an increase of <em>f</em>S₂ with the progressive crystallization of chromite. These primary PGMs and PGE-bearing pentlandite in the intergranular space of chromite are susceptible to alter and transform into secondary PGMs and base metal mineral assemblages, which include Os-Ru nanophases (Os-Ru nanoparticle and OsRu₃ nanoalloy) + awaruite (FeNi₃) + trevorite (Fe₂NiO₄) in nodular chromitite, Os-rich laurite + Os-Ir(Ru) alloy/oxide + pentlandite + millerite (NiS) in banded and massive chromitite, and Ru(Ir) oxide + heazlewoodite (Ni₃S₂) in disseminated chromitite. The development of these diverse assemblages can be attributed to the degrees of serpentinization of chromitites. The nodular chromitite underwent weak serpentinization and had low water/rock ratios (&lt;∼1), <em>f</em>S₂ and <em>f</em>O₂, leading to the conversion of IPGE (Os, Ir and Ru)-bearing pentlandite into Os-Ru nanoparticle- and OsRu₃ nanoalloy-bearing awaruite. The massive and disseminated chromitites had high water/rock ratios and high <em>f</em>S₂ and <em>f</em>O₂ relative to those of the nodular chromitite during serpentinization, and consequently the corresponding pentlandite was transformed into heazlewoodite and/or millerite associated with S loss. Meanwhile, Ir, Os and possibly Ru were released from laurite to form Os-Ir(Ru) alloy/oxide at the edge of laurite. Our observation highlights that primary PGMs and pentlandite in the chromitites of the Kızıldağ ophiolite have been modified under different physical-chemical conditions during serpentinization, resulting in the formation of complex secondary PGMs and base metal mineral assemblages.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"674 ","pages":"Article 122563"},"PeriodicalIF":3.6,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142857929","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}