Chemical Geology最新文献

{"title":"Grassland woody encroachment alters subsurface mineral weathering and groundwater composition in a carbonate system","authors":"Christa Anhold , Camden Hatley , Eresay Alcantar-Velasquez , Rachel M. Keen , Kayalvizhi Sadayappan , Karla M. Jarecke , Pamela L. Sullivan , Jesse B. Nippert , Li Li , G.L. Macpherson , Matthew F. Kirk","doi":"10.1016/j.chemgeo.2024.122522","DOIUrl":"10.1016/j.chemgeo.2024.122522","url":null,"abstract":"<div><div>Displacement of grasses by woody plants (woody encroachment) is occurring in grasslands worldwide. Previous studies indicate that encroachment can alter subsurface carbon dioxide (CO<sub>2</sub>) concentrations and mineral weathering, though these impacts are still poorly understood. To address this knowledge gap, we sampled groundwater and stream water every three weeks during the 2022 water year from two watersheds at Konza Prairie Biological Station, a native tallgrass prairie underlain by limestone and mudrock units in Kansas, USA. Amounts of woody encroachment differ between the watersheds primarily because of differences in fire frequency. One watershed is burned annually and contains 6 % and 45 % woody plant coverage in its upland and riparian areas, respectively, whereas the other is burned every four years and contains 28 % and 74 % woody plant coverage, respectively. We expected to find higher CO<sub>2</sub> levels in the more encroached watershed, assuming the deep roots of woody plants increase inputs of CO<sub>2</sub> to bedrock. However, we found the opposite. Our results indicate that groundwater from a single limestone aquifer contained an average of 1.4 mM CO<sub>2</sub> in the less encroached watershed and 1.0 mM CO<sub>2</sub> in the more encroached watershed. Similarly, stream water CO<sub>2</sub> concentrations at the outlet of the less encroached watershed (0.25 mM) were more than twice that of the more encroached watershed (0.12 mM) on average. Despite these differences in CO<sub>2</sub> concentration, amounts of mineral weathering per liter of groundwater differed little between watersheds. We hypothesize that encroachment is causing differences in CO<sub>2</sub> concentrations between watersheds by decreasing the proportion of mineral weathering that occurs under conditions that are open with respect to CO<sub>2</sub> exchange. During open-system weathering, dissolved CO<sub>2</sub> consumed by weathering reactions can be replaced from an adjacent gas phase, allowing CO<sub>2</sub> concentrations to remain elevated as weathering progresses. In contrast, during closed-system weathering, CO<sub>2</sub> is not replaced and decreases in concentration as weathering progresses. If weathering primarily occurs under open-system conditions within the study area soils, which are unsaturated, and closed-system conditions within the underlying bedrock, where pores are more commonly saturated, then woody encroachment has the potential to decrease the proportion of open-system weathering by increasing soil permeability and thus decreasing soil water residence times. This hypothesis is consistent with our findings and implies that a shortening of soil water residence time with woody encroachment lowers the proportion of CO<sub>2</sub> delivered from the soil to the subsurface and creates a more aggressive weathering engine at depth and along deeper flow paths. Encroachment may also be altering soil CO<sub>2</sub> production an","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"673 ","pages":"Article 122522"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815833","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":"Application of Ni, Cu and Fe isotopes as indicators of ore genesis - New insights from the epigenetic-hydrothermal Rajapalot AuCo prospect, Finnish Lapland","authors":"J. Tepsell ,&nbsp;Y. Lahaye ,&nbsp;F. Molnár ,&nbsp;O.T. Rämö ,&nbsp;N. Cook","doi":"10.1016/j.chemgeo.2024.122513","DOIUrl":"10.1016/j.chemgeo.2024.122513","url":null,"abstract":"<div><div>Application of stable transition metal isotopes as indicators of ore genesis is becoming more popular, yet the fractionation mechanisms and isotopic distribution in these unconventional systems remain poorly understood. In this study, we present an analysis of sulphide Ni, Cu and Fe isotopes measured from solution using multicollector ICP-MS. The data were collected from the dominant sulphide phases in the Raja prospect within the epigenetic-hydrothermal Rajapalot Au<img>Co deposit, Finnish Lapland. Our main goal was to gain new information on the systematics and behaviour of the isotopes in high-temperature ore-forming environments, with implications for ore genesis. The Raja prospect is hosted by a Paleoproterozoic volcanic-sedimentary sequence and was formed by multi-stage hydrothermal processes during the Svecofennian orogeny. Pyrite shows significant variation in δ⁵⁶Fe (-2.08 to +3.29 ‰), including the heaviest iron isotopes thus far observed in natural pyrite. The δ⁵⁶Fe values in pyrrhotite vary less (-0.74 to +0.80 ‰) but are unusually heavy compared to those of magmatic pyrrhotite. δ⁵⁶Fe in chalcopyrite ranges from +0.10 to +1.45 ‰, δ⁶⁰Ni in pyrrhotite from -1.03 to +0.18 ‰, and δ⁶⁵Cu in chalcopyrite from -0.30 to +0.23 ‰. The δ⁵⁶Fe values in co-existing sulphide phases suggest both equilibrium and kinetic fractionation effects. The extreme Fe fractionation in pyrite implies that kinetic fractionation played a major role in the precipitation of isotopically light pyrite. Moreover, inheritance of low δ⁵⁶Fe values from a pyrrhotite precursor is likely. The heavy Fe isotopic composition of some of the pyrrhotite and pyrite is probably the result of preferential leaching of light isotopes by late hydrothermal fluids.</div><div>Systematic correlations between the composition of the examined isotope systems, Co and Au concentrations, and textural features link the sulphide isotopes to multi-stage ore formation. Gradual trends in the isotope compositions suggest Rayleigh fractionation. Early Co deposition is attributed to isotopically heavy fluid, probably derived from a sedimentary formation with abundant iron oxides. The isotopically lighter Au mineralising fluids point to a separate fluid source, probably involving evolved granites. The late hydrothermal Au-carrying fluids overprinted the early Co mineralisation forming Au<img>Co enriched zones. Our study highlights the potential of multiple isotope systematics in sulphides as a useful diagnostic tool for tracing mineralisation processes in and source regions of hydrothermal Au and Co.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"673 ","pages":"Article 122513"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142777461","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":"The role of scavenging and early diagenesis in controlling hydrothermal fluxes into the ocean along the Endeavour segment of the Juan de Fuca ridge","authors":"Isabelle Beaupré-Olsen ,&nbsp;Steve Mihaly ,&nbsp;Hannah Robutka ,&nbsp;Jody Spence ,&nbsp;Kathryn M. Gillis ,&nbsp;Laurence A. Coogan","doi":"10.1016/j.chemgeo.2025.122675","DOIUrl":"10.1016/j.chemgeo.2025.122675","url":null,"abstract":"<div><div>Mixing of high-temperature hydrothermal vent fluids with seawater leads to the precipitation of a large fraction of the non-conservative element load and scavenging of elements from seawater onto the particles formed. This substantially modifies hydrothermal fluxes into the ocean relative to the flux across the seafloor at vents. However, the particles formed in the plume are rarely in equilibrium within the uppermost sediments, and scavenged elements may be loosely bound to particle surfaces, meaning that the net flux from the ocean can be readily modified by processes operating during early diagenesis. Using bulk geochemistry, and progressive leaching experiments, we characterize sediments from around the Endeavour segment of the Juan de Fuca ridge to investigate scavenging and early diagenetic processes. Changes in bulk sediment composition with distance from the ridge (e.g., decreases in Fe/Ti) support a decreased hydrothermal input off-axis. Bulk-sediment and leachate data are interpreted as indicating substantial scavenging of elements such as P, V, Cr and REEs as previously suggested, with different elements scavenged with different efficiencies in different parts of the plume. Correlations of these elements with Fe in the HCl leachates, suggests their uptake is associated with Fe-oxyhydroxides. Water column scavenging is overprinted by fluxes associated with benthic scavenging for V and REE. In contrast, early diagenesis leads to complete loss of scavenged P back to the ocean. Leaching the samples with NaOH and acetic acid provides evidence for recrystallization of reactive (biogenic and hydrothermal) silica and CaCO₃ during early diagenesis in all locations. The fraction of Fe and Mn leached by 1 M HCl in 1 h versus 24 h, and the amount leached in 24 h, vary systematically; these data can be explained by progressive recrystallization of hydrothermally-derived Fe and Mn minerals during early diagenesis. These early diagenetic reactions release almost all hydrothermally-derived Mn back into the ocean, while Fe remains largely fixed in the sediment. Large-scale loss of Co, Zn, Mo, Ag and Cd also occurs during early diagenesis, with some evidence As, Pb and U are also lost. In deeper, more reducing, sediments there is uptake of As, Mo, Ag, Cd and U, presumably via diffusion through the shallow oxic sediments. Such diagenetic processes mean that the net fluxes of elements into and out of the ocean associated with hydrothermal systems differ from those estimated from studies of either vent fluids or hydrothermal plumes.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"678 ","pages":"Article 122675"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143369703","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":"Petrology of alkali gabbro from the Messum Crater, Namibia (Paranà-Etendeka Province) – Melting of metasomatized carbonated lithospheric mantle sources","authors":"S. Jung ,&nbsp;R.L. Romer ,&nbsp;J.A. Pfänder","doi":"10.1016/j.chemgeo.2024.122547","DOIUrl":"10.1016/j.chemgeo.2024.122547","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Foid gabbros (alkali gabbros) are late-stage intrusive rocks into the alkaline core of the Cretaceous Messum complex, an anorogenic ring complex belonging to the Etendeka Large Igneous Province in NW Namibia. The mineralogy of the alkali gabbros is characterized by primary olivine, clinopyroxene (Ti-augite), nepheline and plagioclase and late-stage or secondary brown Ti-amphibole and biotite plus magnetite. The bulk rock compositions resemble low-SiO&lt;sub&gt;2&lt;/sub&gt; alkaline melts with low TiO&lt;sub&gt;2&lt;/sub&gt; and K&lt;sub&gt;2&lt;/sub&gt;O/Na&lt;sub&gt;2&lt;/sub&gt;O &lt; 1. Some of them have Cr (up to 490 ppm) and Ni (up to 265 ppm) contents that approach values characteristic for primary mantle melts. The alkali gabbros are characterized by variable enrichment in incompatible trace elements (Ba: 750–1020 ppm, Zr: 131–173 ppm; Hf: 3.0–3.4 ppm, Nb: 66–90 ppm, Ta: 3.7–5.2 ppm, La: 32–43 ppm, Th: 4.0–5.5 ppm). Zr/Hf and Nb/Ta ratios are high and negatively correlated. The alkali gabbros display a small spread in initial &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr (0.7044–0.7046), εNd (+1.2 to +1.7) and Pb isotope ratios (&lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;204&lt;/sup&gt;Pb, 17.86–18.00, &lt;sup&gt;207&lt;/sup&gt;Pb/&lt;sup&gt;204&lt;/sup&gt;Pb: 15.54–15.56, &lt;sup&gt;208&lt;/sup&gt;Pb/&lt;sup&gt;204&lt;/sup&gt;Pb: 37.65–37.81). Variations of major and trace element data and Sr, Nd, and Pb isotopic compositions indicate that the more differentiated alkali gabbro samples evolved by fractional crystallization from the most mafic parent involving mainly olivine, clinopyroxene, and minor plagioclase and Fe&lt;img&gt;Ti oxides, whereas crustal contamination was negligible. In primitive mantle-normalized multi-element diagrams, alkali gabbros show depletions in Rb, Th, U, Pb, Hf, Zr and Ti and enrichments in Sr. Notably, high molar Ca/Al, high Ti/Eu, high Zr/Hf and Nb/Ta, low TiO&lt;sub&gt;2&lt;/sub&gt;, low Zr/Nb at high Nb concentrations and low Zr/Sm in samples with high Lu/Hf and low Hf/Sm point to a carbonatitic component in the source of the alkali gabbros. REE modelling suggests that the parental melts form within the garnet stability field and may be modified either by partial re-equilibration or mixing with melts generated in the spinel stability field. Application of the thermobarometer proposed by &lt;span&gt;&lt;span&gt;Lee et al. (2009)&lt;/span&gt;&lt;/span&gt; (&lt;span&gt;&lt;span&gt;Lee et al., 2009&lt;/span&gt;&lt;/span&gt;. Constraints on the depths and temperatures of basaltic magma generation on Earth, and other terrestrial planets using new thermobarometers for mafic magmas. Earth Planet. Sci. Lett. 279, 20–33.) implies generation of the most primitive alkali gabbros at ∼4.5 GPa and ∼ 1480 °C, the latter value is in agreement with previous temperature estimates using Al-in-olivine thermometry and melt inclusion studies. The Sr-Nd-Pb isotope compositions of the alkali gabbros do not overlap with those of mafic lavas attributed to the Tristan or Gough hot spot or the newly defined Doros component. Trace element and isotope data indicate derivation of the Messum alkali gabbros from a heterogeneous m","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"673 ","pages":"Article 122547"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142858012","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":"Triple oxygen and hydrogen stable isotope composition of water in Murchison carbonaceous chondrite accessed by thermogravimetry-enabled laser spectroscopy","authors":"Erik J. Oerter ,&nbsp;Alexandre Garenne ,&nbsp;Miguel Cisneros ,&nbsp;George D. Cody ,&nbsp;Michael Singleton ,&nbsp;Conel M. O’D. Alexander","doi":"10.1016/j.chemgeo.2024.122557","DOIUrl":"10.1016/j.chemgeo.2024.122557","url":null,"abstract":"<div><div>We have developed the capability to measure ¹⁷O/¹⁶O in H₂O and OH⁻ of hydrous minerals by a technique using thermogravimetric analysis coupled with isotope ratio infrared spectroscopy (TGA-IRIS) on a suite of liquid water standards, terrestrial mineral standards, multi-mineral synthetic mixtures of terrestrial samples, and Murchison carbonaceous chondrite. This ¹⁷O/¹⁶O analytical capability is combined with the previously developed TGA-IRIS capability of ¹⁸O/¹⁶O, which enables us to calculate Δ`<sup>17</sup>O values. Additionally, we have made <sup>2</sup>H/<sup>1</sup>H measurements by TGA-IRIS on the suite of materials. Analyses of 328 standard water samples sealed into silver tubes (0.25 μL, 174 VSMOW and 154 GISP) analyzed over the course of three years yielded δ<sup>18</sup>O values by TGA-IRIS with precision of ±0.30 ‰, δ<sup>17</sup>O of ±0.24 ‰, Δ`¹⁷O of ±0.126 ‰, and δ²H ± 7.5 ‰ (± 1 Std. Dev.). TGA-IRIS analyses of a suite of hydrous minerals (oxy-hydroxide, mica, serpentines, sulfide, phyllosillicates) yields similar precision. TGA-IRIS analyses of the CM2 carbonaceous chondrite Murchison resulted in δ¹⁸O, δ¹⁷O, Δ`<sup>17</sup>O, and δ<sup>2</sup>H values of the H<sub>2</sub>O and/or OH<sup>−</sup> from its hydrous minerals. Δ`¹⁷O values of Murchison H₂O range from 1.034 ‰ at 350 °C, to −0.368 ‰ at 650 °C, to −1.75 ‰ at 850 °C. TGA-IRIS can generate Δ`¹⁷O values of H₂O and/or OH⁻ in hydrous materials that are well within the precision needed for the analysis of meteoritic samples, as well as samples returned from asteroids, such as Ryugu and Bennu.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"673 ","pages":"Article 122557"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142857885","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":"Mechanism of cadmium (Cd) enrichment in the soil of karst areas with high geochemical background in Southwest China","authors":"Bo Li ,&nbsp;Kun Lin ,&nbsp;Xu Liu ,&nbsp;Xudong Ma ,&nbsp;Xuezhen Li ,&nbsp;Zhiliang Wu ,&nbsp;Cheng Li ,&nbsp;Tao Yu ,&nbsp;Tiansheng Wu ,&nbsp;Zhongfang Yang","doi":"10.1016/j.chemgeo.2024.122523","DOIUrl":"10.1016/j.chemgeo.2024.122523","url":null,"abstract":"<div><div>Karst areas are formed from the dissolution of carbonate rocks and are present worldwide. The soil found in such areas is enriched in heavy metals, such as cadmium (Cd), lead (Pb) and Arsenic (As), and has geological high-background characteristics. However, this enrichment is anomalous depending on the type of bedrock, and the mechanism of enrichment has not yet been comprehensively elucidated. To explore the mechanisms of Cd enrichment in these soils, we investigated the distribution patterns and migration characteristics of Cd during the weathering and pedogenesis of carbonate rocks, and the causes of Cd enrichment and associated ecological risks were determined. The conclusions were as follows: (1) In the process of natural evolution, the boundary between the dissolution of carbonate rocks and the weathering of insoluble substances is unclear, resulting in the overlap of and interaction between the processes of dissolution-accumulation-weathering and pedogenesis. (2) In addition to the enrichment effects caused by the dissolution of carbonate rocks and the inheritance, the readsorption of insoluble substances and the retention of secondary carrier minerals are the main driving forces of Cd enrichment. (3) During the late stage of weathering of insoluble materials, soil acidification significantly promotes the Cd activation process. When the proportion of active Cd significantly increases, the ecological risk potential increases, especially in geological high-background areas rich in Cd.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"673 ","pages":"Article 122523"},"PeriodicalIF":3.6,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815874","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":"A model explaining the anomalous enrichment of Pt in phosphatized crusts: The influence of phosphates and organic acids","authors":"Zhengkun Li ,&nbsp;Xiaoming Sun ,&nbsp;Dengfeng Li ,&nbsp;Yongjia Liang","doi":"10.1016/j.chemgeo.2025.122673","DOIUrl":"10.1016/j.chemgeo.2025.122673","url":null,"abstract":"<div><div>Numerous studies have reported a remarkable enrichment of platinum (Pt) in phosphatized ferromanganese crusts, with concentrations several times higher than in non-phosphatized crusts. Understanding the mechanism behind this extraordinary Pt enrichment requires investigating the influence of anoxic water layers, which are rich in various ions, on the accumulation and preservation of Pt in ferromanganese minerals during phosphorization events. This study combines adsorption/desorption experiments, Fourier transform infrared (FTIR) spectrum analysis, transmission electron microscope (TEM) observations, and X-ray absorption fine structure (XAFS) analysis using feroxyhyte (δ-FeOOH) and vernadite (δ-MnO2) as representative ferromanganese minerals in Fe<img>Mn crusts to examine the fixation and reactivation of Pt during phosphorization events. Our research findings reveal that changes in the ionic composition of water layers during phosphorization events have a significant impact on the adsorption and preservation of Pt by ferromanganese minerals. Fe²⁺, Mn²⁺, and phosphate ions were found to strongly inhibit the adsorption of Pt by feroxyhyte. On the other hand, citric acid enhances Pt adsorption by creating additional adsorption sites at the edges of vernadite through dissolution. Regarding Pt preservation, phosphate ions can lead to a substantial release of Pt adsorbed in feroxyhyte, while only a small amount of Pt in vernadite can be released by phosphate. FTIR analysis indicates that phosphate can reactivate Pt in feroxyhyte by competing for surface hydroxyl groups. XAFS analysis further reveals that phosphate ions weaken the bonding between Pt and vernadite, although not to an extent that causes significant Pt release.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"678 ","pages":"Article 122673"},"PeriodicalIF":3.6,"publicationDate":"2025-02-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143369702","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":"Clay Li isotopes reveal contrasting responses of chemical weathering intensity to hydroclimate changes in South Asia","authors":"Jingrui Li ,&nbsp;Xuefa Shi ,&nbsp;Shengfa Liu ,&nbsp;Hui Zhang ,&nbsp;Zhaowei Jing ,&nbsp;Xiaoming Miao ,&nbsp;Kaikai Wu ,&nbsp;Rui Jiang ,&nbsp;Somkiat Khokiattiwong ,&nbsp;Narumol Kornkanitnan","doi":"10.1016/j.chemgeo.2025.122652","DOIUrl":"10.1016/j.chemgeo.2025.122652","url":null,"abstract":"<div><div>Weathering processes in orogenic belts play a pivotal role in Earth's climate regulation. However, the link between silicate weathering and hydroclimatic fluctuations remains elusive. Here, we investigate the lithium‑neodymium isotopic compositions (δ⁷Li, εNd) and clay mineralogy of sediments in the northeastern Bay of Bengal to determine if the δ⁷Li can be used as a reliable tracer of the chemical weathering intensity and to assess how weathering responses to hydroclimate changes. δ⁷Li and εNd values range from −2.4 ‰ to 0.4 ‰ and − 12.5 to −8.9, respectively, with clay mineralogy dominated by illite (57 % on average). A decline in δ⁷Li since 13.5 ka, averaging −1.3 ‰, compared to −0.7 ‰ between 25 ka and 13.5 ka, indicates intensified weathering aligned with monsoon variations. Prominent millennial-scale δ⁷Li fluctuations (2.8 ‰ in maximum) during the last deglaciation, showing lower values during the cold events (Younger Drys and Heinrich stadial 1), imply a different mechanism of weathering response to hydroclimate changes. Shift of high mountain- versus floodplain-dominated weathering regimes and hydrology induced changes in water-rock interaction time or water residence time, are suggested to be responsible for δ⁷Li changes at glacial-interglacial scale and millennial scale, respectively. Our study provides key evidence for weathering response to short-term climate change, and highlights the carbon sink role of silicate weathering in floodplains at short-term scales and the potential of Li isotopes in reconstructing past hydroclimate changes.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"678 ","pages":"Article 122652"},"PeriodicalIF":3.6,"publicationDate":"2025-02-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143207238","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":"Using oxygen (δ18O, δ17O) and hydrogen (δ2H) isotopes in the Northern Limb of the Bushveld Igneous Complex to assess the extent of footwall contamination","authors":"Sharlotte C. Mkhonto ,&nbsp;Matthew I. Leybourne ,&nbsp;Derek Knaack ,&nbsp;Pedro J. Jugo ,&nbsp;Daniel Layton-Matthews ,&nbsp;Danie F. Grobler","doi":"10.1016/j.chemgeo.2025.122665","DOIUrl":"10.1016/j.chemgeo.2025.122665","url":null,"abstract":"<div><div>The Platreef in the Northern Limb is in the same stratigraphic position as the Critical Zone (i.e. Merensky Reef) in the Eastern and Western limbs of the Bushveld Igneous Complex (BIC). However, in some areas, the Platreef is in direct contact with Archean granitic gneisses and Transvaal Supergroup metasedimentary rocks. Although there is significant evidence of assimilation of Transvaal Supergroup metasedimentary rocks in the southern parts of the Platreef, there is no consensus on the extent of contamination and if contamination is proximal or distal to the Pt, Pd, Au mineralized zone/Merensky Reef equivalent. For this study, we report δ’¹⁸O, Δ’¹⁷O, and δ²H values of samples from the downdip portion of the Flatreef (part of the Platreef) on Turfspruit. The results show that δ’¹⁸O values range from ∼4 to 7 ‰ in intervals with little to no evidence of assimilation and can reach up to 17 ‰ in intervals with evidence of pervasive assimilation. The δ’¹⁸O values in areas with no evidence of assimilation are similar to previously reported values in the Eastern and Western limbs of the BIC. The Δ’¹⁷O values range from −0.200 to 0.050 ‰ throughout the stratigraphy. Most of the positive Δ’¹⁷O values are associated with intervals showing evidence of assimilation. The δ²H values (which range from −92 to −30 ‰) show no correlation between intervals with pervasive assimilation and those with little to no contamination. Samples from the Turfspruit area of the Northern Limb have δ’¹⁸O values of -5.53 to 17.14 ‰ which shows the extent of footwall assimilation of the Transvaal metasedimentary rocks. The triple oxygen isotope values show that the Northern Limb of the BIC records two separate episodes of crustal contamination: (1) a proximal contamination by local footwall lithologies; and (2) a deeper, more pervasive (with respect to the entire BIC) contamination, prior to injection of the BIC magmas into the middle/upper crust.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"678 ","pages":"Article 122665"},"PeriodicalIF":3.6,"publicationDate":"2025-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143377007","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":"Fluorapatite fingerprints on magmatic-hydrothermal fluid evolution of the Lianyunshan Nb-Ta-Sn deposit, NE Hunan (South China)","authors":"Nuerkanati Madayipu ,&nbsp;Huan Li ,&nbsp;Thomas J. Algeo ,&nbsp;Safiyanu Muhammad Elatikpo ,&nbsp;Han Zheng ,&nbsp;Biao Liu ,&nbsp;Da-Peng Zhu ,&nbsp;Nuo Li ,&nbsp;Victor Ikechukwu Vincent","doi":"10.1016/j.chemgeo.2025.122669","DOIUrl":"10.1016/j.chemgeo.2025.122669","url":null,"abstract":"<div><div>The textures and compositions of apatite grains have been reported for many types of economic and non-economic deposits, but apatites in hydrothermal Nb<img>Ta (rare-metal) deposits have received scant attention to date. Apatite is common in magmatic–pegmatites and metasedimentary cover rocks of the Lianyunshan Complex (LYSC) of northeastern Hunan Province in South China, potentially providing insights into Nb<img>Ta metallogenic systems. Here, we present integrated textural, major- and trace-elemental, and in situ U-Pb-Sr isotopic data for hydrothermal apatite grains from the metasedimentary (schist)–magmatic (muscovite monzogranite)–pegmatite systems of the LYSC, to investigate its crystallization and mineralization processes in detail. Four types of fluorapatite (S-ap1 and S-ap2 in schist, G-ap in muscovite monzogranite, and P-ap in pegmatite) are recognized. The first three types crystallized in a magmatic-hydrothermal (transitional-stage) setting at ∼132–129 Ma, whereas the P-ap crystallized during a late-stage hydrothermal Nb<img>Ta mineralization event at ∼123 Ma. All four types show similar and low Cl content (&lt; 0.01 wt%) and high F content (2.70–4.49 wt%), suggesting their precipitation from a melt derived through partial melting of crustal material rich in F-bearing minerals. Relatively homogeneous (⁸⁷Sr/⁸⁶Sr)_i ratios are shown by the S-ap1 and S-ap2 grains (0.75067 ± 0.00010), contrasting with higher and variable values in the G-ap (0.75242 to 0.75943, mean 0.75610) and P-ap grains (0.75350–0.75776, mean 0.75533). The Sr isotope signatures of the LYSC appear to be inherited from Lengjiaxi Group metasedimentary strata. Our study highlights application of integrated petrographic-geochemical analyses of fluorapatites to trace the nature of ore-forming fluids in hydrothermal Nb<img>Ta (rare-metal) deposits of metasedimentary–magmatic–pegmatite systems.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"678 ","pages":"Article 122669"},"PeriodicalIF":3.6,"publicationDate":"2025-02-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143378317","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}