Pub Date : 2024-12-05DOI: 10.1016/j.chemgeo.2024.122547
S. Jung, R.L. Romer, J.A. Pfänder
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<ce:inf loc="post">2</ce:inf> alkaline melts with low TiO<ce:inf loc="post">2</ce:inf> and K<ce:inf loc="post">2</ce:inf>O/Na<ce:inf loc="post">2</ce:inf>O < 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 <ce:sup loc="post">87</ce:sup>Sr/<ce:sup loc="post">86</ce:sup>Sr (0.7044–0.7046), εNd (+1.2 to +1.7) and Pb isotope ratios (<ce:sup loc="post">206</ce:sup>Pb/<ce:sup loc="post">204</ce:sup>Pb, 17.86–18.00, <ce:sup loc="post">207</ce:sup>Pb/<ce:sup loc="post">204</ce:sup>Pb: 15.54–15.56, <ce:sup loc="post">208</ce:sup>Pb/<ce:sup loc="post">204</ce:sup>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<ce:glyph name="sbnd"></ce:glyph>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<ce:inf loc="post">2</ce:inf>, 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 <ce:cross-ref ref>Lee et al. (2009)</ce:cross-ref> (<ce:cross-ref ref>Lee et al., 2009</ce:cross-ref>. 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 composi
{"title":"Petrology of alkali gabbro from the Messum Crater, Namibia (Paranà-Etendeka Province) – Melting of metasomatized carbonated lithospheric mantle sources","authors":"S. Jung, R.L. Romer, J.A. Pfänder","doi":"10.1016/j.chemgeo.2024.122547","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122547","url":null,"abstract":"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<ce:inf loc=\"post\">2</ce:inf> alkaline melts with low TiO<ce:inf loc=\"post\">2</ce:inf> and K<ce:inf loc=\"post\">2</ce:inf>O/Na<ce:inf loc=\"post\">2</ce:inf>O < 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 <ce:sup loc=\"post\">87</ce:sup>Sr/<ce:sup loc=\"post\">86</ce:sup>Sr (0.7044–0.7046), εNd (+1.2 to +1.7) and Pb isotope ratios (<ce:sup loc=\"post\">206</ce:sup>Pb/<ce:sup loc=\"post\">204</ce:sup>Pb, 17.86–18.00, <ce:sup loc=\"post\">207</ce:sup>Pb/<ce:sup loc=\"post\">204</ce:sup>Pb: 15.54–15.56, <ce:sup loc=\"post\">208</ce:sup>Pb/<ce:sup loc=\"post\">204</ce:sup>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<ce:glyph name=\"sbnd\"></ce:glyph>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<ce:inf loc=\"post\">2</ce:inf>, 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 <ce:cross-ref ref>Lee et al. (2009)</ce:cross-ref> (<ce:cross-ref ref>Lee et al., 2009</ce:cross-ref>. 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 composi","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"7 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-05","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":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-04DOI: 10.1016/j.chemgeo.2024.122529
A. Entrena, L.F. Auqué, M.J. Gimeno, J.J. Fornós
Calcite-aragonite bimineralic rafts and related waters from hypogenic lakes in the coastal cave of “Cova dets Ases”, Mallorca, have been analyzed for mineralogy and geochemistry (including stable isotopes). The main objective has been the evaluation of the factors that control the chemical and mineralogical composition of the precipitates as they are another natural example of the classical problem of aragonite vs calcite precipitation.
对马略卡岛“Cova dets Ases”沿海洞穴中方解石-文石双矿物筏体及相关水体进行了矿物学和地球化学（包括稳定同位素）分析。主要目的是评价控制沉淀物的化学和矿物学组成的因素，因为它们是文石与方解石沉淀的经典问题的另一个自然例子。
{"title":"Bimineralic calcite-aragonite rafts in the hypogeous lakes of Cova dets Ases (Mallorca): Controls on precipitation and polymorphism","authors":"A. Entrena, L.F. Auqué, M.J. Gimeno, J.J. Fornós","doi":"10.1016/j.chemgeo.2024.122529","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122529","url":null,"abstract":"Calcite-aragonite bimineralic rafts and related waters from hypogenic lakes in the coastal cave of “Cova dets Ases”, Mallorca, have been analyzed for mineralogy and geochemistry (including stable isotopes). The main objective has been the evaluation of the factors that control the chemical and mineralogical composition of the precipitates as they are another natural example of the classical problem of aragonite vs calcite precipitation.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"51 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142857728","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}
Pub Date : 2024-12-03DOI: 10.1016/j.chemgeo.2024.122526
Rakesh K. Tiwari, Tarun K. Dalai, Rupak Samadder, Waliur Rahaman, Sunil K. Singh
This study is focussed on understanding of molybdenum (Mo) cycling in the mesotidal to macrotidal estuary of the Ganga (Hooghly) River in India. Our investigation encompasses the composition of water and suspended particulate matter (SPM) samples collected in three separate periods (pre-monsoon: PrM, monsoon: M and post-monsoon: PoM) over two consecutive years, surface sediments, exchangeable phases of surface and suspended sediments, one sediment core, urban and industrial effluent waters and ground waters. The observed dissolved Mo concentrations in the estuary are in excess of those expected from conservative mixing of seawater and river water. The “excess” Mo values, observed in the mid-to-high salinity regions (4–27 ‰) for four out of the six study periods, are the highest in the monsoon samples. The bulk and exchangeable phase compositions are supportive of release of Mo from the Fe-Mn oxyhydroxide phases of the SPM and surface sediments in the estuary. Mass balance calculations indicate that Mo release from the SPM and surface sediments can account for up to 40 % of the median value of the “excess” Mo in the estuary. The compositions of the sediment core provide evidence for the mobilization of Mo and its release to porewater due to reductive dissolution of Fe-Mn oxyhydroxides. The tidally induced sediment resuspension likely facilitates the transport of porewater Mo to the overlying water. Mass balance calculations indicate that Mo loss from the sediment column can account for the bulk of the “excess” Mo in the estuary waters. This study further demonstrates that industrial effluents, urban waste waters and groundwaters do not constitute significant sources of dissolved Mo in the estuary.
{"title":"Non-conservative behaviour of molybdenum in the Ganga (Hooghly) River estuary, India: Role of solute-particle interaction and sediment diagenesis","authors":"Rakesh K. Tiwari, Tarun K. Dalai, Rupak Samadder, Waliur Rahaman, Sunil K. Singh","doi":"10.1016/j.chemgeo.2024.122526","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122526","url":null,"abstract":"This study is focussed on understanding of molybdenum (Mo) cycling in the mesotidal to macrotidal estuary of the Ganga (Hooghly) River in India. Our investigation encompasses the composition of water and suspended particulate matter (SPM) samples collected in three separate periods (pre-monsoon: PrM, monsoon: M and post-monsoon: PoM) over two consecutive years, surface sediments, exchangeable phases of surface and suspended sediments, one sediment core, urban and industrial effluent waters and ground waters. The observed dissolved Mo concentrations in the estuary are in excess of those expected from conservative mixing of seawater and river water. The “excess” Mo values, observed in the mid-to-high salinity regions (4–27 ‰) for four out of the six study periods, are the highest in the monsoon samples. The bulk and exchangeable phase compositions are supportive of release of Mo from the Fe-Mn oxyhydroxide phases of the SPM and surface sediments in the estuary. Mass balance calculations indicate that Mo release from the SPM and surface sediments can account for up to 40 % of the median value of the “excess” Mo in the estuary. The compositions of the sediment core provide evidence for the mobilization of Mo and its release to porewater due to reductive dissolution of Fe-Mn oxyhydroxides. The tidally induced sediment resuspension likely facilitates the transport of porewater Mo to the overlying water. Mass balance calculations indicate that Mo loss from the sediment column can account for the bulk of the “excess” Mo in the estuary waters. This study further demonstrates that industrial effluents, urban waste waters and groundwaters do not constitute significant sources of dissolved Mo in the estuary.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"119 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815873","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}
Pub Date : 2024-12-03DOI: 10.1016/j.chemgeo.2024.122521
Chao Wu, Guoxiong Chen, Huayong Chen
Adakite-like rocks, as an important recorder of magmatic evolution in mantle and crust, are closely associated with major porphyry Cu deposits. However, the underlying mechanism connecting these associations remains insufficiently elucidated. This study compiles ca. 7000 whole-rock geochemical data from Phanerozoic adakite-like rock samples and from ore-forming porphyries in porphyry Cu deposits. Machine learning investigations are performed to characterize the geochemical variations of adakite-like rocks from various orogenic or cratonic systems and investigate any geochemical similarities with rocks associated with porphyry ore systems. Principal component analysis (PCA), along with t-distributed Stochastic Neighbor Embedding (t-SNE), highlights elemental distinctions between adakite-like rocks formed in mature and juvenile crust and the similarities of rocks from porphyry CuMo deposits with the former and rocks from porphyry CuAu deposits with the latter. Extreme Gradient Boosting (XGBoost) and Support Vector Machine (SVM) analyses are applied to the original dataset and the PCA data derived from the dataset to discriminate adakite-like rocks from various geodynamic settings from each other and from rocks associated with porphyry ore deposits. Our models show that XGBoost and SVM can identify the tectonic regions of adakite-like rocks and rocks from porphyry copper systems with high efficiency and confidence e.g., accuracy = 0.88–0.94 and area under curve (AUC) = 0.69–0.86. The application of SHapley Additive exPlanations (SHAP) values is employed to elucidate the parameters that discriminate the different environments, providing insight into the underlying petrogenetic processes, including diverse levels of formation depth and extent of mantle interaction. Additionally, when examining ore-forming porphyries and adakite-like rocks, the incorporation of heatmaps into taxonomic classification (HTC) provides significant distinguishing factors with substantial geological implications. These findings suggest potential geological processes (e.g., remelting of Cu-sulfide-bearing cumulates or flotation of Cu-sulfide phases attached to vapor bubbles) that may enrich the source magma of ore-forming porphyries beyond the conventional geological processes responsible for adakite-like rock formation, which could serve as an exploration indicator.
{"title":"Unraveling the link between worldwide adakite-like rocks and porphyry Cu deposits","authors":"Chao Wu, Guoxiong Chen, Huayong Chen","doi":"10.1016/j.chemgeo.2024.122521","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122521","url":null,"abstract":"Adakite-like rocks, as an important recorder of magmatic evolution in mantle and crust, are closely associated with major porphyry Cu deposits. However, the underlying mechanism connecting these associations remains insufficiently elucidated. This study compiles ca. 7000 whole-rock geochemical data from Phanerozoic adakite-like rock samples and from ore-forming porphyries in porphyry Cu deposits. Machine learning investigations are performed to characterize the geochemical variations of adakite-like rocks from various orogenic or cratonic systems and investigate any geochemical similarities with rocks associated with porphyry ore systems. Principal component analysis (PCA), along with t-distributed Stochastic Neighbor Embedding (t-SNE), highlights elemental distinctions between adakite-like rocks formed in mature and juvenile crust and the similarities of rocks from porphyry Cu<ce:glyph name=\"sbnd\"></ce:glyph>Mo deposits with the former and rocks from porphyry Cu<ce:glyph name=\"sbnd\"></ce:glyph>Au deposits with the latter. Extreme Gradient Boosting (XGBoost) and Support Vector Machine (SVM) analyses are applied to the original dataset and the PCA data derived from the dataset to discriminate adakite-like rocks from various geodynamic settings from each other and from rocks associated with porphyry ore deposits. Our models show that XGBoost and SVM can identify the tectonic regions of adakite-like rocks and rocks from porphyry copper systems with high efficiency and confidence e.g., accuracy = 0.88–0.94 and area under curve (AUC) = 0.69–0.86. The application of SHapley Additive exPlanations (SHAP) values is employed to elucidate the parameters that discriminate the different environments, providing insight into the underlying petrogenetic processes, including diverse levels of formation depth and extent of mantle interaction. Additionally, when examining ore-forming porphyries and adakite-like rocks, the incorporation of heatmaps into taxonomic classification (HTC) provides significant distinguishing factors with substantial geological implications. These findings suggest potential geological processes (e.g., remelting of Cu-sulfide-bearing cumulates or flotation of Cu-sulfide phases attached to vapor bubbles) that may enrich the source magma of ore-forming porphyries beyond the conventional geological processes responsible for adakite-like rock formation, which could serve as an exploration indicator.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"22 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815835","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}
Pub Date : 2024-12-03DOI: 10.1016/j.chemgeo.2024.122525
Mara Limonta, Christian France-Lanord, Albert Galy, Andrey Gurenko, Nordine Bouden, Eduardo Garzanti
Quartz is the most abundant mineral in sediments and sedimentary rocks but efforts to reliably identify its provenance have been only partially fruitful so far. Even advanced methods such as cathodo-luminescence, Raman spectroscopy, synchrotron X-ray absorption spectroscopy, and laser ablation spectrometry have led to limited success. This article demonstrates how the δ18O of detrital quartz provides useful additional insight. The oxygen-isotope signature primarily depends on source rocks and their formation conditions, being highly different for different crustal sources and highest for carbonate rocks.
{"title":"Single quartz δ18O: A new frontier in detrital provenance analysis (Bengal Fan, IODP Expedition 354)","authors":"Mara Limonta, Christian France-Lanord, Albert Galy, Andrey Gurenko, Nordine Bouden, Eduardo Garzanti","doi":"10.1016/j.chemgeo.2024.122525","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122525","url":null,"abstract":"Quartz is the most abundant mineral in sediments and sedimentary rocks but efforts to reliably identify its provenance have been only partially fruitful so far. Even advanced methods such as cathodo-luminescence, Raman spectroscopy, synchrotron X-ray absorption spectroscopy, and laser ablation spectrometry have led to limited success. This article demonstrates how the δ<ce:sup loc=\"post\">18</ce:sup>O of detrital quartz provides useful additional insight. The oxygen-isotope signature primarily depends on source rocks and their formation conditions, being highly different for different crustal sources and highest for carbonate rocks.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"64 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142858013","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}
Pub Date : 2024-12-03DOI: 10.1016/j.chemgeo.2024.122523
Bo Li, Kun Lin, Xu Liu, Xudong Ma, Xuezhen Li, Zhiliang Wu, Cheng Li, Tao Yu, Tiansheng Wu, Zhongfang Yang
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.
{"title":"Mechanism of cadmium (Cd) enrichment in the soil of karst areas with high geochemical background in Southwest China","authors":"Bo Li, Kun Lin, Xu Liu, Xudong Ma, Xuezhen Li, Zhiliang Wu, Cheng Li, Tao Yu, Tiansheng Wu, Zhongfang Yang","doi":"10.1016/j.chemgeo.2024.122523","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122523","url":null,"abstract":"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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"5 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-03","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}
Pub Date : 2024-12-02DOI: 10.1016/j.chemgeo.2024.122527
James A. Kidder, Christopher E. Beckett-Brown, Alexandre Voinot, Zhaoping Yang, Pierre Pelchat, Duane C. Petts, Matthew Polivchuk, John B. Chapman, Scott Casselman, Matthew I. Leybourne
The fate of metals, such as Cu, in stream waters draining porphyry mineralization is commonly controlled by several natural processes such as sorption, microbial processes, and ligand availability. Isotopes of Cu offer a novel approach to understanding these processes and determining metal sources within complicated mineralogical systems. Drainages at the Casino Cu-Au-Mo porphyry deposit, Yukon, Canada exhibit circumneutral (pH > 5) in Casino Creek and natural acid rock drainage (pH < 3.5) in Proctor Gulch with the precipitation of schwertmannite (Fe3+). Isotopic systems δ65Cu and δ34Ssulfate indicate different metal sources, with signatures of both hypogene and supergene mineralization. Waters from Proctor Gulch contain δ65Cu values (< −0.5 ‰) consistent with supergene Cu sources from the leached or oxide portion of the mineralization. Comparatively, drainage in the upper part of Casino Creek contains a δ65Cu composition (> 0.5 ‰) characteristic of Cu sourced from hypogene sulfide mineral oxidation. Variation in metal sources is similarly supported by aqueous δ34Ssulfate values in the stream waters, which suggest mixing of S derived from a sulfide mineral phase and a much heavier sulfate mineral (e.g., gypsum or anhydrite). Isotopic fractionation of Cu in the dissolved (<0.45 μm) phase presents two predominant controls on Cu dispersion. The natural acid conditions in Proctor Gulch favor the preferential co-precipitation of 63Cu with schwertmannite but could be influenced by intracellular assimilation or adsorption by microbes, which also has been shown to preferentially favor 63Cu. Copper isotopic fractionation results in a gradient of increasing δ65Cu values in waters downstream. In Casino Creek, higher pH conditions favor the precipitation of Fe(OH)3 and the preferential adsorption of 65Cu, resulting in decreasing δ65Cu values downstream. Copper concentrations in stream waters remain elevated (up to 4.1 μg/L) above ambient background (1.9 μg/L) levels up to 11 km downstream of the deposit. Given the abundance of surface water in many parts of northern Canada, hydrogeochemical prospecting using broad scale stream water catchment analysis is clearly a viable greenfield exploration methodology.
{"title":"Using δ65Cu and δ34S to determine the fate of copper in stream waters draining porphyry mineralization: Implications for exploration targeting","authors":"James A. Kidder, Christopher E. Beckett-Brown, Alexandre Voinot, Zhaoping Yang, Pierre Pelchat, Duane C. Petts, Matthew Polivchuk, John B. Chapman, Scott Casselman, Matthew I. Leybourne","doi":"10.1016/j.chemgeo.2024.122527","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122527","url":null,"abstract":"The fate of metals, such as Cu, in stream waters draining porphyry mineralization is commonly controlled by several natural processes such as sorption, microbial processes, and ligand availability. Isotopes of Cu offer a novel approach to understanding these processes and determining metal sources within complicated mineralogical systems. Drainages at the Casino Cu-Au-Mo porphyry deposit, Yukon, Canada exhibit circumneutral (pH > 5) in Casino Creek and natural acid rock drainage (pH < 3.5) in Proctor Gulch with the precipitation of schwertmannite (Fe<ce:sup loc=\"post\">3+</ce:sup>). Isotopic systems δ<ce:sup loc=\"post\">65</ce:sup>Cu and δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">sulfate</ce:inf> indicate different metal sources, with signatures of both hypogene and supergene mineralization. Waters from Proctor Gulch contain δ<ce:sup loc=\"post\">65</ce:sup>Cu values (< −0.5 ‰) consistent with supergene Cu sources from the leached or oxide portion of the mineralization. Comparatively, drainage in the upper part of Casino Creek contains a δ<ce:sup loc=\"post\">65</ce:sup>Cu composition (> 0.5 ‰) characteristic of Cu sourced from hypogene sulfide mineral oxidation. Variation in metal sources is similarly supported by aqueous δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">sulfate</ce:inf> values in the stream waters, which suggest mixing of S derived from a sulfide mineral phase and a much heavier sulfate mineral (e.g., gypsum or anhydrite). Isotopic fractionation of Cu in the dissolved (<0.45 μm) phase presents two predominant controls on Cu dispersion. The natural acid conditions in Proctor Gulch favor the preferential co-precipitation of <ce:sup loc=\"post\">63</ce:sup>Cu with schwertmannite but could be influenced by intracellular assimilation or adsorption by microbes, which also has been shown to preferentially favor <ce:sup loc=\"post\">63</ce:sup>Cu. Copper isotopic fractionation results in a gradient of increasing δ<ce:sup loc=\"post\">65</ce:sup>Cu values in waters downstream. In Casino Creek, higher pH conditions favor the precipitation of Fe(OH)<ce:inf loc=\"post\">3</ce:inf> and the preferential adsorption of <ce:sup loc=\"post\">65</ce:sup>Cu, resulting in decreasing δ<ce:sup loc=\"post\">65</ce:sup>Cu values downstream. Copper concentrations in stream waters remain elevated (up to 4.1 μg/L) above ambient background (1.9 μg/L) levels up to 11 km downstream of the deposit. Given the abundance of surface water in many parts of northern Canada, hydrogeochemical prospecting using broad scale stream water catchment analysis is clearly a viable greenfield exploration methodology.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"3 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815876","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}
There is still much controversy over whether apatite and graphite can be used as fingerprint mineral assemblages to preserve biosignatures in ancient sedimentary rocks that experienced medium- to high-grade metamorphism. Herein, we present a comprehensive analysis of graphite and associated phases from meta-phosphorites of the Huangmailing Formation in the South Dabie Orogen to assess possible preservation of biosignatures. Stratigraphic correlations and previous geochronological data suggest that the protoliths of Huangmailing Formation was deposited in the Ediacaran, and was metamorphosed during the Triassic Dabie Orogeny as documented by our apatite U<ce:glyph name="sbnd"></ce:glyph>Pb dating. Microcrystalline graphite occurs both as inclusions within metamorphic minerals like apatite, and as matrix-hosted disseminations at grain boundaries of other phases. Bulk organic carbon isotopic analyses yield δ<ce:sup loc="post">13</ce:sup>C values between −26.0 ‰ and − 15.2 ‰ and total organic carbon contents from 0.33 to 3.27 wt%. Raman spectra of the two types of graphites yield an average peak metamorphic temperature of 452 °C and 473 °C, consistent with the upper greenschist facies to lower amphibolite facies metamorphic conditions in the area. However, the matrix graphites show a wider range of peak temperature variations, likely containing graphitic carbon with lower crystallinity that was affected by retrograde metamorphic fluids. Notably, two prominent Raman bands at around 325 cm<ce:sup loc="post">−1</ce:sup> and 395 cm<ce:sup loc="post">−1</ce:sup> are observed for the graphite hosted in apatite. This, along with the presence of a peak at 2473 eV in the sulfur <ce:italic>K</ce:italic>-edge XANES spectra of some graphite in apatite, and the co-occurrence of <ce:sup loc="post">12</ce:sup>C and <ce:sup loc="post">14</ce:sup>N and local association of <ce:sup loc="post">32</ce:sup>S with <ce:sup loc="post">12</ce:sup>C in graphite inclusions determined by TOF-SIMS, suggest the possible presence of C-S-Fe and N-bearing compounds and functional groups. TEM and EDS analysis reveal that graphite inclusions in apatite are closely associated with amorphous silica, and mainly occur as well crystallized, polygonal grains around 100 to 200 nm across and has an expanded (002) lattice spacing between 3.43 Å and 3.64 Å. This lattice expansion is attributable to the presence of heteroatoms such as S, Fe and Si, and N, in the graphene interlayers. In contrast, matrix graphites exhibit a wider range of structural and compositional variations than their counterpart as inclusion in apatite, which probably arise from interactions with metamorphic fluids or incorporation of fluid-deposited graphitic carbon. Our study documents the ubiquitous preservation of biological carbon as graphite inclusions in apatite from Huangmailing meta-phosphorites, likely as a result of metamorphic recrystallization of organic-bearing Ediacaran phosphorites. This suggests that the org
磷灰石和石墨能否作为指纹矿物组合保存中、高等级变质作用的古沉积岩的生物特征，目前还存在很大的争议。本文对南大别造山带黄麦岭组变质磷岩的石墨及其伴生物相进行了综合分析，以评估其生物特征保存的可能性。地层对比和前人的年代学资料表明，黄麦岭组原岩沉积于埃迪卡拉系，并在三叠纪大别造山运动期间变质。微晶石墨既可以作为磷灰石等变质矿物的包裹体存在，也可以作为基质在其他相的晶界处传播。整体有机碳同位素分析的δ13C值在−26.0‰~−15.2‰之间，总有机碳含量在0.33 ~ 3.27 wt%之间。两类石墨的拉曼光谱显示其平均变质峰温度分别为452℃和473℃，与该区上部绿片岩相至下部角闪岩相的变质条件一致。然而，基体石墨显示出更大范围的峰值温度变化，可能含有受逆行变质流体影响的结晶度较低的石墨碳。值得注意的是，在磷灰石中，石墨在325 cm - 1和395 cm - 1附近观察到两个突出的拉曼带。再加上磷灰石中部分石墨的硫k边XANES光谱在2473 eV处出现一个峰，以及TOF-SIMS测定的石墨包裹体中12C和14N的共存以及32S和12C的局部结合，表明可能存在含C-S-Fe和含n的化合物和官能团。透射电镜和能谱分析表明，磷灰石中的石墨包裹体与无定形二氧化硅密切相关，主要以直径为100 ~ 200 nm的多角形晶粒形态出现，晶格间距在3.43 Å ~ 3.64 Å之间。这种晶格膨胀是由于石墨烯中间层中存在S、Fe、Si和N等杂原子。相比之下，基质石墨在结构和成分上的变化范围更广，这可能是由于与变质流体的相互作用或流体沉积石墨碳的掺入。我们的研究表明，黄麦岭变质磷灰岩磷灰石中普遍存在石墨包裹体形式的生物碳，这可能是埃迪卡拉系含有机质磷灰岩变质再结晶的结果。这表明沉积磷灰石在共沉淀或早期成岩作用中被捕获的有机质可能在变质重结晶过程中被修改较少，从而有利于生物特征的保存。
{"title":"Preservation of biosignatures in Neoproterozoic phosphorites metamorphosed at temperatures >450 °C","authors":"Kenan Cao, Zhenbing She, Dominic Papineau, Jingbo Nan, Chao Chen, Reza Deevsalar, Ayetullah Tunc, Qunfeng Xiao, Kang-Jun Huang, Yuanming Pan","doi":"10.1016/j.chemgeo.2024.122519","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122519","url":null,"abstract":"There is still much controversy over whether apatite and graphite can be used as fingerprint mineral assemblages to preserve biosignatures in ancient sedimentary rocks that experienced medium- to high-grade metamorphism. Herein, we present a comprehensive analysis of graphite and associated phases from meta-phosphorites of the Huangmailing Formation in the South Dabie Orogen to assess possible preservation of biosignatures. Stratigraphic correlations and previous geochronological data suggest that the protoliths of Huangmailing Formation was deposited in the Ediacaran, and was metamorphosed during the Triassic Dabie Orogeny as documented by our apatite U<ce:glyph name=\"sbnd\"></ce:glyph>Pb dating. Microcrystalline graphite occurs both as inclusions within metamorphic minerals like apatite, and as matrix-hosted disseminations at grain boundaries of other phases. Bulk organic carbon isotopic analyses yield δ<ce:sup loc=\"post\">13</ce:sup>C values between −26.0 ‰ and − 15.2 ‰ and total organic carbon contents from 0.33 to 3.27 wt%. Raman spectra of the two types of graphites yield an average peak metamorphic temperature of 452 °C and 473 °C, consistent with the upper greenschist facies to lower amphibolite facies metamorphic conditions in the area. However, the matrix graphites show a wider range of peak temperature variations, likely containing graphitic carbon with lower crystallinity that was affected by retrograde metamorphic fluids. Notably, two prominent Raman bands at around 325 cm<ce:sup loc=\"post\">−1</ce:sup> and 395 cm<ce:sup loc=\"post\">−1</ce:sup> are observed for the graphite hosted in apatite. This, along with the presence of a peak at 2473 eV in the sulfur <ce:italic>K</ce:italic>-edge XANES spectra of some graphite in apatite, and the co-occurrence of <ce:sup loc=\"post\">12</ce:sup>C and <ce:sup loc=\"post\">14</ce:sup>N and local association of <ce:sup loc=\"post\">32</ce:sup>S with <ce:sup loc=\"post\">12</ce:sup>C in graphite inclusions determined by TOF-SIMS, suggest the possible presence of C-S-Fe and N-bearing compounds and functional groups. TEM and EDS analysis reveal that graphite inclusions in apatite are closely associated with amorphous silica, and mainly occur as well crystallized, polygonal grains around 100 to 200 nm across and has an expanded (002) lattice spacing between 3.43 Å and 3.64 Å. This lattice expansion is attributable to the presence of heteroatoms such as S, Fe and Si, and N, in the graphene interlayers. In contrast, matrix graphites exhibit a wider range of structural and compositional variations than their counterpart as inclusion in apatite, which probably arise from interactions with metamorphic fluids or incorporation of fluid-deposited graphitic carbon. Our study documents the ubiquitous preservation of biological carbon as graphite inclusions in apatite from Huangmailing meta-phosphorites, likely as a result of metamorphic recrystallization of organic-bearing Ediacaran phosphorites. This suggests that the org","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"41 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142815783","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}
Pub Date : 2024-11-30DOI: 10.1016/j.chemgeo.2024.122520
Yang Xu, Chuan-Zhou Liu, Chang Zhang, Tong Liu
Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young Re-depletion model (TRD) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (∼6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The Re-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest PdN/IrN and ReN/IrN ratios have the least radiogenic 187Os/188Os of 0.11497–0.11537, giving the old TRD ages of 1.89–1.98 Ga. These ages are considerably older than previously reported TRD ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest TRD age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic 187Os/188Os ratio of 0.158 that is similar to the 187Os/188Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high 187Os/188Os of the YTO pyroxenites.
{"title":"Re-Os isotopic evidence for ancient melt depletion in refertilized Neo-Tethyan suboceanic mantle domain","authors":"Yang Xu, Chuan-Zhou Liu, Chang Zhang, Tong Liu","doi":"10.1016/j.chemgeo.2024.122520","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122520","url":null,"abstract":"Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young <ce:italic>Re</ce:italic>-depletion model (T<ce:inf loc=\"post\">RD</ce:inf>) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (∼6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The <ce:italic>Re</ce:italic>-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest Pd<ce:inf loc=\"post\">N</ce:inf>/Ir<ce:inf loc=\"post\">N</ce:inf> and Re<ce:inf loc=\"post\">N</ce:inf>/Ir<ce:inf loc=\"post\">N</ce:inf> ratios have the least radiogenic <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os of 0.11497–0.11537, giving the old T<ce:inf loc=\"post\">RD</ce:inf> ages of 1.89–1.98 Ga. These ages are considerably older than previously reported T<ce:inf loc=\"post\">RD</ce:inf> ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest T<ce:inf loc=\"post\">RD</ce:inf> age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os ratio of 0.158 that is similar to the <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high <ce:sup loc=\"post\">187</ce:sup>Os/<ce:sup loc=\"post\">188</ce:sup>Os of the YTO pyroxenites.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"19 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142777462","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}
Pub Date : 2024-11-30DOI: 10.1016/j.chemgeo.2024.122522
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
Displacement of grasses by woody plants (woody encroachment) is occurring in grasslands worldwide. Previous studies indicate that encroachment can alter subsurface carbon dioxide (CO<ce:inf loc="post">2</ce:inf>) 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<ce:inf loc="post">2</ce:inf> levels in the more encroached watershed, assuming the deep roots of woody plants increase inputs of CO<ce:inf loc="post">2</ce:inf> 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<ce:inf loc="post">2</ce:inf> in the less encroached watershed and 1.0 mM CO<ce:inf loc="post">2</ce:inf> in the more encroached watershed. Similarly, stream water CO<ce:inf loc="post">2</ce:inf> 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<ce:inf loc="post">2</ce:inf> concentration, amounts of mineral weathering per liter of groundwater differed little between watersheds. We hypothesize that encroachment is causing differences in CO<ce:inf loc="post">2</ce:inf> concentrations between watersheds by decreasing the proportion of mineral weathering that occurs under conditions that are open with respect to CO<ce:inf loc="post">2</ce:inf> exchange. During open-system weathering, dissolved CO<ce:inf loc="post">2</ce:inf> consumed by weathering reactions can be replaced from an adjacent gas phase, allowing CO<ce:inf loc="post">2</ce:inf> concentrations to remain elevated as weathering progresses. In contrast, during closed-system weathering, CO<ce:inf loc="post">2</ce:inf> 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<ce:inf loc="p
木本植物取代草类（木本侵蚀）在世界范围内的草地上都有发生。先前的研究表明，侵蚀可以改变地下二氧化碳（CO2）浓度和矿物风化，尽管这些影响仍然知之甚少。为了解决这一知识差距，我们在2022水年期间每三周从康扎草原生物站的两个流域取样地下水和溪流，康扎草原生物站是美国堪萨斯州石灰岩和泥岩单元下的原生高草草原。不同流域的木材侵蚀量不同，主要是因为火灾频率不同。一个流域每年燃烧一次，在其高地和河岸地区分别有6%和45%的木本植物覆盖率，而另一个流域每四年燃烧一次，木本植物覆盖率分别为28%和74%。假设木本植物的深根增加了基岩的二氧化碳输入，我们预计在更受侵蚀的流域会发现更高的二氧化碳水平。然而，我们发现情况恰恰相反。结果表明，受侵蚀程度较轻的单一石灰岩含水层地下水平均含1.4 mM CO2，受侵蚀程度较重的单一石灰岩含水层地下水平均含1.0 mM CO2。同样，受侵程度较轻的流域（0.25 mM）出口的水流CO2浓度平均是受侵程度较重的流域（0.12 mM）的两倍多。尽管二氧化碳浓度存在这些差异，但不同流域每升地下水的矿物风化量差异不大。我们假设，侵蚀通过减少矿物风化的比例导致了流域间二氧化碳浓度的差异，而矿物风化发生在开放的二氧化碳交换条件下。在开放体系风化过程中，被风化反应消耗的溶解二氧化碳可以被邻近的气相所取代，从而使二氧化碳浓度在风化过程中保持升高。而在封闭系统风化过程中，CO2不被取代，随着风化过程的进行，CO2浓度逐渐降低。如果研究区土壤的风化主要发生在非饱和的开放体系条件下，而基岩的封闭体系条件下孔隙通常是饱和的，那么树木侵蚀可能会通过增加土壤渗透性来降低开放体系的风化比例，从而减少土壤水分停留时间。这一假设与我们的发现是一致的，这意味着树木侵蚀导致土壤水分停留时间的缩短，降低了从土壤到地下的二氧化碳比例，并在深度和更深的流动路径上创造了一个更具侵略性的风化引擎。入侵也可能改变土壤二氧化碳的产生和/或排放，尽管这些可能性需要进一步调查。
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