Chemical Geology最新文献

{"title":"Trace elements and iron speciation analysis of Paleoproterozoic phyllite from the Gandarela Syncline: Implications for salinity and redox conditions in the Quadrilátero Ferrífero, Brazil","authors":"Janaina Rodrigues de Paula ,&nbsp;Wei Wei ,&nbsp;Fabricio A. Caxito ,&nbsp;Katherine N. Snihur ,&nbsp;Cody N. Lazowski ,&nbsp;Kurt O. Konhauser ,&nbsp;Erik A. Sperling ,&nbsp;Daniel S. Alessi","doi":"10.1016/j.chemgeo.2025.122839","DOIUrl":"10.1016/j.chemgeo.2025.122839","url":null,"abstract":"<div><div>The Quadrilátero Ferrífero, Brazil, is among the most significant iron ore provinces. This study utilizes geochemical proxies, including elemental paleosalinity ratios (B/Ga, Sr/Ba, S/TOC), redox-sensitive elements (RSE), and iron speciation, to investigate water mass evolution. Samples were analyzed from three drill cores and four outcrops in the Gandarela Syncline, providing new insights into the Paleoproterozoic phyllites associated with the Cauê BIF. Although metamorphism of these rocks might affect the application of the proxy, consistent directional trends in proxy results indicate a refined and/or robust paleoenvironmental reconstruction of the Minas Basin. Paleosalinity data indicate a transition from freshwater to low-brackish conditions during early sedimentation (Caraça Formation), followed by predominantly marine conditions during the deposition of the Cauê, Gandarela, and Cercadinho Formations. This model revises earlier interpretations suggesting an initial dominance of marine facies during fine sediment deposition in the Batatal Formation. Elevated concentrations of trace elements (Cu, Ni, and Zn) in carbonaceous phyllites highlight the role of microbial activity in bioproductivity before BIF deposition. Additionally, the decreasing enrichment factor _(EF) of Mo_EF/U_EF ratio and RSE concentrations toward younger rocks, coupled with variations in Fe_HR (highly reactive iron), suggest that the early basin waters were characterized by anoxic to suboxic conditions, with free O₂ likely present in surface waters. These results point to a stratified water column, featuring an oxic surface layer overlying ferruginous, anoxic water. This study advances understanding of the paleoenvironmental evolution of the Minas Basin and the depositional processes that facilitated the formation of the Quadrilátero Ferrífero's giant iron deposits. It also underscores the value of paleosalinity proxies for reconstructing environmental conditions in Paleoproterozoic systems, while also highlighting the need for more studies of how paleoenvironmental proxies behave during metamorphism.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"690 ","pages":"Article 122839"},"PeriodicalIF":3.6,"publicationDate":"2025-05-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144071869","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":"Stable cerium isotopes as a potential new redox proxy: insight from first-principles calculations","authors":"Fang Liu ,&nbsp;Yining Zhang ,&nbsp;Yun Liu ,&nbsp;Zhaofeng Zhang ,&nbsp;Xuefang Li","doi":"10.1016/j.chemgeo.2025.122856","DOIUrl":"10.1016/j.chemgeo.2025.122856","url":null,"abstract":"<div><div>Redox fluctuation plays a key role in shaping the evolution of the atmosphere, biosphere, lithosphere and other planets in the solar system. Cerium (Ce) isotope fractionation has recently been suggested to be redox sensitive due to the high reduction potential of Ce. However, the mechanism of Ce isotope behavior is not yet clear. Therefore, we use first-principles calculations to investigate mass-dependent equilibrium Ce isotope fractionations among Ce(III) and Ce(IV) species in aqueous systems and adsorption process of surface complexes of Ce onto goethite. Our results show that the equilibrium isotope fractionation factors between Ce(III)-bearing species are relatively insignificant (mostly less than 0.14 ‰ at 25 °C). In contrast, the fractionations between Ce(III) and Ce(IV) species are significantly larger (e.g., 10³lnα_{Ce(IV)-Ce(III)} is −1.04 ‰ at 25°C), highlighting the importance of chemical bonding environment. During the adsorption onto goethite, our results show that the Ce isotope fractionation direction is opposite for Ce (III) and Ce (IV) species, respectively. Although the nuclear volume effect (NVE) induces negligible fractionation on the ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce ratios, it has a significant effect on the commonly used ¹⁴²Ce/¹⁴⁰Ce ratio which is opposite to that of mass dependent fractionation (MDF). The contrasting isotopic behaviors of MDF and NVE on the ¹⁴²Ce/¹⁴⁰Ce ratio result in minimum total isotopic fractionation at equilibrium. Given the lack of NVE on the ¹³⁶Ce/¹⁴⁰Ce ratio, it could be fractionated significantly under surface environments due to redox fluctuation, suggesting the strong potential of the Ce stable isotopes to serve as a valuable redox tracer.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"690 ","pages":"Article 122856"},"PeriodicalIF":3.6,"publicationDate":"2025-05-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144083746","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":"Characteristics and evolution of iron speciation during early diagenesis in the sand-mud transition zone of the East China Sea Shelf","authors":"Yumeng Zhu ,&nbsp;Daidu Fan ,&nbsp;Jianfeng Su ,&nbsp;Yijing Wu ,&nbsp;Xiang-Zhou Meng","doi":"10.1016/j.chemgeo.2025.122841","DOIUrl":"10.1016/j.chemgeo.2025.122841","url":null,"abstract":"<div><div>The early diagenesis of iron is a critical component of the biogeochemical cycling within shelf sediments; however, its variability across different sedimentary environments has been less known. This study analyzed two sediment cores (EH2-23 A and EH2-23B) from the sand-mud transition zone of the East China Sea (ECS) shelf, with a focus on sediment grain size, total organic carbon (TOC) content, ²¹⁰Pb and ¹³⁷Cs dating, iron speciation and porewater chemistry. The results reveal that the upper section of EH2-23 A and the lower section of EH2-23B comprise palimpsest sandy sediments, while the middle section of EH2-23 A consists of relict sandy sediments, all of which are characterized by low TOC%, older ages, and limited contents of highly reactive iron (Fe_HR), indicating minimal diagenetic activity. The lower section of EH2-23 A features interbedded sandy and muddy sediments, shaped by fluctuating hydrodynamic conditions and influenced by submarine groundwater discharge. In contrast, the upper section of EH2-23B comprises younger, TOC-rich muddy sediments, where early diagenesis is predominantly driven by sulfate reduction. Sandy sediments consistently exhibit higher degrees of pyritization (DOP) compared to muddy sediments, reflecting prolonged diagenetic processes. Depth-dependent patterns of the ratios of Fe_HR to total iron (Fe_HR/Fe_T) and pyrite to Fe_HR (Fe_py/Fe_HR) are classified into four distinct types. These findings contribute to a deeper understanding of early diagenesis and the C-S-Fe cycle in river-dominated shelf environments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"687 ","pages":"Article 122841"},"PeriodicalIF":3.6,"publicationDate":"2025-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143943720","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":"Mineralogical controls on Li, Sr and oxygen isotope composition of mixed CaMg carbonate phases with implications for sedimentary dolomites","authors":"Colton J. Vessey ,&nbsp;Yinuo Li ,&nbsp;Maija J. Raudsepp ,&nbsp;Jean-Michel Brazier ,&nbsp;Anna L. Harrison ,&nbsp;Sasha Wilson ,&nbsp;Sylvia Riechelmann ,&nbsp;Vasileios Mavromatis","doi":"10.1016/j.chemgeo.2025.122836","DOIUrl":"10.1016/j.chemgeo.2025.122836","url":null,"abstract":"<div><div>The formation of ordered dolomite is unlikely to occur under ambient Earth's surface conditions, yet “disordered dolomite,” has been shown to crystallize at temperatures as low as 40 °C. Such synthetic precipitates have a similar <em>d</em>(104) spacing as dolomite, and have been studied previously to determine their O isotope compositions as a function of temperature with the goal of using oxygen isotopes as a temperature proxy. However, laboratory synthesis yields mineralogical assemblages that transform to more stable phase assemblages over time. Previous studies however have not thoroughly addressed how this transformation proceeds and how it affects O isotope compositions of the precipitates. To better understand the relationship between temperature and δ¹⁸O values of Ca-Mg‑carbonates at temperatures &lt;100 °C, in this study, Ca<img>Mg carbonates were synthesized at 40, 60 and 80 °C and incubated up to 104 days. Mineralogical composition was quantified using Rietveld refinement of X-ray diffraction patterns, while concomitantly monitoring fluid and solid compositions to assess the utility of the δ¹⁸O_solid, Li, and Sr compositions as paleo-proxies in complex Ca-Mg‑carbonate assemblages. The results suggest a continuous transformation of the mineralogy of the samples throughout the duration of the experimental runs, although Mg/Ca of the bulk solids remained quasi-constant at ∼1, and between 40 and 104 days of reaction the bulk δ¹⁸O_solid values did not exhibit significant variations. These δ¹⁸O_solid values were used to estimate temperature-dependent oxygen isotope fractionation between bulk solid and fluid that can be expressed as:<span><span><span><math><msup><mn>10</mn><mn>3</mn></msup><mi>ln</mi><msub><mi>α</mi><mrow><mi>solid</mi><mo>−</mo><mi>fluid</mi></mrow></msub><mo>=</mo><mn>1.78</mn><mspace></mspace><mfenced><mrow><mo>±</mo><mn>0.13</mn></mrow></mfenced><mfrac><msup><mn>10</mn><mn>6</mn></msup><msup><mi>T</mi><mn>2</mn></msup></mfrac><mo>+</mo><mn>8.47</mn><mfenced><mrow><mo>±</mo><mn>1.20</mn></mrow></mfenced></math></span></span></span></div><div>where T is temperature in Kelvin.</div><div>The use of this relation yields significantly different temperature dependence compared to that reported earlier by <span><span>Schmidt et al. (2005)</span></span> using the same synthesis procedure. The difference can be assigned to mineralogical changes occurring in the precipitates over the course of the 104-day runs that may not have occurred in the earlier study owing to a shorter experimental duration. Here we discuss in detail the role mineralogy has on the chemical and isotopic compositions of Ca<img>Mg carbonates, and the implications for using Ca<img>Mg carbonate minerals as paleoarchives.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"690 ","pages":"Article 122836"},"PeriodicalIF":3.6,"publicationDate":"2025-05-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143942050","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":"Evaluation of sample pretreatment methods for boron concentration analysis","authors":"Wei Wei ,&nbsp;Thomas Algeo ,&nbsp;Lin Chen ,&nbsp;Zhiquan Li ,&nbsp;Cody Lazowski ,&nbsp;Katherine Snihur ,&nbsp;Daniel Alessi ,&nbsp;Kurt Konhauser","doi":"10.1016/j.chemgeo.2025.122838","DOIUrl":"10.1016/j.chemgeo.2025.122838","url":null,"abstract":"<div><div>Elemental boron (B) concentrations have been shown to be useful in reconstruction of paleosalinity conditions. Accurate measurement of the B content of a sample is critical for this purpose, but B is a light and relatively volatile element, substantial amounts of which can be lost through sample preparation protocols that fail to take this property of B into consideration. Here, we evaluate the effects of four commonly used sample preparation protocols on measured B concentrations. The Conventional Acid Method uses concentrated HNO₃ and HF, but the latter acid can cause volatilization of B. A second method, the modified Conventional Acid Method, uses the same acids but with the addition of H₃PO₄ to reduce B volatility. Two further methods eliminate HF entirely: the Ammonium Digestion Method makes use of NH₄F/NH₄HF₂, and the Alkali Fusion Method fuses the sample with Na₂O₂. We tested 9 samples and 4 standards using these four pretreatment methods. Our results show that the Alkali Fusion Method consistently has the highest B yields, with an average recovery rate of ∼98 %, whereas the other methods have average yields ranging from 72 % to 92 %. Owing to the potential for incomplete B recovery using these other methods (especially the Conventional Acid Method, which has the lowest recovery rate), salinity values based on those methods are likely to be underestimated. Given the dominant use of the Conventional Acid Method in older studies reporting B concentrations, paleosalinity estimates based on those studies will require re-evaluation. In the future, we recommend that all studies measuring B concentrations make use of the Alkali Fusion Method in order to achieve near-quantitative recovery, and to provide for maximum robustness and comparability of paleosalinity estimates among studies.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"687 ","pages":"Article 122838"},"PeriodicalIF":3.6,"publicationDate":"2025-05-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143948997","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":"Formation and biogeochemical potential of ferromanganese oxide deposited on dead sponges","authors":"Wenzhu Li ,&nbsp;Hengchao Xu ,&nbsp;Dominic Papineau ,&nbsp;Kaiwen Ta ,&nbsp;Shun Chen ,&nbsp;Shamik Dasgupta ,&nbsp;Shuangquan Liu ,&nbsp;Bohao Yin ,&nbsp;Xiaotong Peng","doi":"10.1016/j.chemgeo.2025.122840","DOIUrl":"10.1016/j.chemgeo.2025.122840","url":null,"abstract":"<div><div>Ferromanganese oxides that coat dead marine biological remains such as porous sponges, fish bones, and cold-water corals are widespread in deep-sea environments. However, their mineralization mechanisms and the intrinsic connections between ferromanganese oxides and biological remains are poorly understood. In this study, we examine ferromanganese oxide coatings (FMC) on five dead sponge specimens collected from seamounts in the Central Basin of the South China Sea (SCS) and in the West Mariana Ridge (WMR). Sponge FMC are friable and loosely attached precipitates grown onto the reticulate skeletal structure and they have a narrow range of thicknesses. High-resolution microscopic observations show that sponge FMC are composed of porous, spheroidal, and relatively compact sheath-like Mn oxides with variable oxidation state, which possibly imply a different formation pathway from general ferromanganese deposits (FMD). New observations further reveal the position of sponge FMC spatially overlapping with sponge collagen fibers, as well as reveal widespread presence of microbial cells, Nitrogen-enriched OM with abundant amide groups, and nanoscopic apatite in sponge FMC. The observations provide a new insight into the precipitation of sponge FMC that is related to the decomposition of skeleton-associated proteins of the sponge. The degradation of organic matter contributes to localized enrichment of metal cations and micro-scale oxic-suboxic transitions. Meanwhile, small-molecule carbon compounds derived from protein decomposition are favorable to the activity of heterotrophic microorganisms, which modulates localized physicochemical conditions. These redox variations in microenvironments are proposed to trigger the oxidation and reduction of metal cations and their precipitation into FMC around sponge remains. While the composition of sponge FMC resembles that of hydrogenic crusts, their rare element and yttrium (REY) patterns display notable discrepancies with hydrogenic processes, including weak negative or significantly positive Ce anomaly. The discrepancies reflect the unique geochemical characteristics of sponge FMC, possibly attributed to differences in the activity of oxic-suboxic transitions and in the redox behavior of Mn and Ce metals. Biological remains in the deep-sea have a widespread occurrence and most of them similarly serve also as FMC substrates, which may point to a broader implication for marine polymetallic cycling than previously considered.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"687 ","pages":"Article 122840"},"PeriodicalIF":3.6,"publicationDate":"2025-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143943719","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":"(UTh)/He Geochronology of Fe and FeAl Lateritic Duricrusts and Bauxites in Central-Northern Amazonia: Implications for Paleoclimate and Landscape Evolution","authors":"Márcio Fernando dos Santos Albuquerque ,&nbsp;Adriana Maria Coimbra Horbe ,&nbsp;Cécile Gautheron ,&nbsp;João Gabriel Cavalcante Vieira ,&nbsp;Pedro Val ,&nbsp;Rosella Pinna-Jamme ,&nbsp;Carlos Alejandro Salazar","doi":"10.1016/j.chemgeo.2025.122835","DOIUrl":"10.1016/j.chemgeo.2025.122835","url":null,"abstract":"<div><div>Over the past 100 million years, intense lateritization has resulted in the widespread development of Fe, Fe<img>Al, and bauxite lateritic duricrusts across Amazonia. In Central-Northern Amazonia, Fe-rich lateritic duricrusts formed around 44 Ma, with peaks at 11–10 Ma and 5–4 Ma, while bauxite formation occurred between 25 and 15 Ma. Expanding this interpretation to the Amazonian region, two main bauxitization episodes have been identified: (i) between 30 and 15 Ma and (ii) post-15 Ma, most pronounced at ∼3–4 Ma. These lateritic duricrusts suggest that lateritization persisted from the Eocene to the Pliocene. This indicates a predominantly humid, well-drained climate conducive to goethite (bauxite) formation, overprinted by drier ferruginization pulses associated with global climatic fluctuations.</div><div>These lateritic duricrusts have acted as effective erosion-resistant surfaces, forming three distinct geomorphic levels. Surface 1 (S1, &gt;240 m a.s.l.) and Surface 2 (S2, 100–240 m a.s.l.), characterized by extensive bauxite and Fe<img>Al lateritic duricrusts, developed at ∼20 Ma and between ∼17 and ∼ 11–5 Ma, respectively. Surface 3 (S3, &lt;100 m a.s.l.) formed under intense ferruginous lateritization at ∼4 Ma. A progressive decrease in goethite ages and Al content from S1 to S3 reflects increasing landscape dissection. Furthermore, (U<img>Th)/He data reveal a significant fluvial incision rate of 28 m/Ma, suggesting pronounced landscape rejuvenation, particularly between 10 and 3 Ma, which overprints older weathering phases preserved in southern Amazonia. Neotectonic activity facilitated the preservation of complete lateritic profiles on hilltops and plateaus, whereas truncated profiles with stone lines characterize valley slopes.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"689 ","pages":"Article 122835"},"PeriodicalIF":3.6,"publicationDate":"2025-05-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143948465","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":"Constraints on Fe-Ca metasomatism in mineralized mantle rocks: Insights from in-situ geochemistry and thermodynamic modeling","authors":"R. Coltat ,&nbsp;C.G.C. Patten ,&nbsp;J.A. Padrón-Navarta ,&nbsp;F. Hochscheid ,&nbsp;M. Ulrich ,&nbsp;Y. Branquet ,&nbsp;P. Boulvais ,&nbsp;C.J. Garrido","doi":"10.1016/j.chemgeo.2025.122826","DOIUrl":"10.1016/j.chemgeo.2025.122826","url":null,"abstract":"<div><div>Ultramafic-hosted seafloor massive sulfide deposits have been reported in present-day oceanic settings for nearly thirty years. However, the development of comprehensive genetic models that account for deep-seated hydrothermal processes is largely hindered by the limited availability of seafloor observations and their reliance on large-scale geophysical studies. The Platta nappe (Swiss Alps) preserves a Jurassic hydrothermal system (the Marmorera-Cotschen Hydrothermal System; MCHS), where Cu-Fe-Co-Zn-Ni mineralization is associated with Fe-Ca silicates (ilvaite, hydrogarnet, and diopside). Petrographic analyses and thermodynamic modeling indicate that Fe-Ca metasomatism occurred between 300 and 360 °C and at low <em>f</em>O₂ (from FMQ −6 to +1), likely coeval with early-stage serpentinization. The composition of Fe-Ca silicates (Co, Ni, and REE contents, measured by in-situ LA-ICP-MS) indicates fluid-rock interaction from an ultramafic-dominated system to an open-system, involving fluids derived from both mafic and ultramafic rocks. Mineralogical and geochemical signatures of Fe-Ca silicates in the MCHS do not support genetic relationships with common rodingitization. Our results highlight that Fe-Ca metasomatism may be a widespread deep-seated alteration along mafic–ultramafic rock contacts or in mantle rocks modified through melt-rock interaction accompanying mantle exhumation.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"687 ","pages":"Article 122826"},"PeriodicalIF":3.6,"publicationDate":"2025-05-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143949054","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":"Trace element and sulfur isotope study of sulfide chimneys from the mafic-hosted Daxi Vent Field: Implications on the genesis of seafloor massive sulfide deposit at a segment end","authors":"Yejian Wang ,&nbsp;Ming Yang ,&nbsp;Xiqiu Han ,&nbsp;Sven Petersen ,&nbsp;Yiyang Cai ,&nbsp;Xinkai Hu ,&nbsp;Anxia Chen ,&nbsp;Zhongyan Qiu ,&nbsp;Xing Yu ,&nbsp;Hang Hu ,&nbsp;Matthias Frische ,&nbsp;Honglin Li","doi":"10.1016/j.chemgeo.2025.122825","DOIUrl":"10.1016/j.chemgeo.2025.122825","url":null,"abstract":"<div><div>The mafic-hosted Daxi Vent Field (DVF) on the Carlsberg Ridge hosts an actively forming massive sulfide deposit on the neovolcanic ridge at a non-transform offset (NTO). Here, we present mineralogy, geochemistry and in situ sulfur isotope analysis of the sulfide chimneys sample from the DVF, which can be classified into Fe-rich and Zn-rich types based on the major sulfide minerals. Our results show that the Fe-rich chimneys, primarily consisting of pyrrhotite and isocubanite, exhibit enrichment in Co, Se, and Ni, with heavy δ³⁴S in pyrite (up to 17.38 ‰). The highly saline fluids resulting from phase separation processes lead to the significant Co enrichment. The precipitation of pyrite in a relatively closed environment intensified by the Rayleigh fractionation, leading to significant heavy S isotopic composition. On the other hand, the Zn-rich chimneys are predominantly sphalerite, and enriched in Sn, Ag, As, Ga, Ge, and Cd. The fluid-seawater mixing processes contribute to the elevated Sn levels and the wide range of S isotopic compositions (δ³⁴S = 1.47–8.34 ‰) in sphalerite. The unusual Co and Sn enrichment as well as unique S isotopic composition in the DVF are linked to the thin ocean crust at NTOs, where the thin ocean crust may favor interaction of fluids circulating in the subsurface with ultramafic rocks, despite the absence of ultramafic lithologies on the seafloor. This study demonstrates the exploration potential for polymetallic resources in the NTO along slow-spreading mid-ocean ridges.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"686 ","pages":"Article 122825"},"PeriodicalIF":3.6,"publicationDate":"2025-05-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143931887","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":"Toward a better understanding of the controlling factors on the Mg isotope compositions of shallow-water carbonates: Insights from core samples in the South China Sea","authors":"Pan Zhang ,&nbsp;Kang-Jun Huang ,&nbsp;Dong Feng ,&nbsp;Wen Yan","doi":"10.1016/j.chemgeo.2025.122827","DOIUrl":"10.1016/j.chemgeo.2025.122827","url":null,"abstract":"<div><div>The Mg isotope composition (δ²⁶Mg) of shallow-water carbonates provides valuable insights into the marine Mg cycle, past climate, and diagenesis. However, the effects of the mixing zone and meteoric diagenesis on the δ²⁶Mg of carbonates remain poorly understood, and the variations of δ²⁶Mg during diagenesis differ globally under diverse environmental settings. In this study, we investigate the controlling factors affecting δ²⁶Mg of shallow-water carbonates during diagenesis across different mineralogies (aragonite, calcite, and dolomite) and diagenetic realms (meteoric, marine, and mixing zone diagenesis, as well as dolomitization) using bulk carbonate samples from Well NK-1, Meiji Atoll, South China Sea, by integrating mineralogical, geochemical analyses with numerical modeling. Our results reveal that δ²⁶Mg of carbonate is primarily controlled by both mineralogy and diagenetic realms, with varying degrees of diagenetic alteration. Dolomites with a fluid-buffered origin and well-preserved high-Mg calcite exhibit δ²⁶Mg offsets of ∼ −2 ‰ and ∼ −2.4 ‰ from coeval seawater, respectively, making them reliable archives for seawater δ²⁶Mg reconstruction. In contrast, the δ²⁶Mg of aragonite-dominated samples is easily influenced by mixed calcite due to the low Mg content in aragonite. Moreover, calcite displays a large δ²⁶Mg variation from −5.1 ‰ to −3.1 ‰, with the constrained Mg isotope fractionation during meteoric, mixing zone, and marine diagenesis overlapping between −4.5 ‰ and − 4 ‰. This suggests that aragonite and low-Mg calcite are not ideal archives of seawater δ²⁶Mg, whereas the least altered limestone sample may represent a lower limit for coeval seawater δ²⁶Mg with an offset of ∼ −2.4 ‰. By comparing our geochemical data with other modern shallow-water carbonates, we propose that variations of limestone δ²⁶Mg from global sites are primarily controlled by both diagenetic realms and the degree of alteration. This finding underscores the importance of using more severely altered samples to assess the fidelity of carbonates in recording seawater chemistry. Our study enhances the understanding of the behavior of Mg isotopes in carbonates during diagenesis, evaluates the reliability of carbonate minerals as seawater δ²⁶Mg archives, and clarifies key controls on carbonate δ²⁶Mg across global settings.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"687 ","pages":"Article 122827"},"PeriodicalIF":3.6,"publicationDate":"2025-05-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143948998","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}