Chemical Geology最新文献

{"title":"Metal enrichment in the Cambrian black shale: Evidence from pyrite overgrowth and NanoSIMS sulfur isotopes","authors":"Shixiang Li , Zhilin Wang , Youwei Chen , Daniel D. Gregory , Erke Peng , Deru Xu , Yufei Wang , Shaohao Zou , Huan Li","doi":"10.1016/j.chemgeo.2025.122623","DOIUrl":"10.1016/j.chemgeo.2025.122623","url":null,"abstract":"<div><div>The Lower Cambrian black shale has attracted much attention due to its role in elucidating the chemical evolution and sulfur cycles of the global ocean, as well as its economic significance as a source of phosphate, barite, and high-grade Ni<img>Mo polymetallic sulfide ores. However, the occurrence of abnormally high metal accumulation within thin ore layers across extensive areas remains contentious. Pyrite, that occurs as framboids or fine- to coarse-grained euhedral-subhedral crystals, is a ubiquitous sulfide found in both host black shale and metalliferous ore horizons. This study delineates a three-stage growth of pyrite within the Chuanyanping and Sancha Ni<img>Mo sulfide layers in western Hunan Province, South China, based on the micro-textures, chemical compositions, and sulfur isotopes. The earliest pyrite (PyI) appears as framboids (3–7 μm) or as small euhedral-subhedral grains (<20 μm). It exhibits a depletion in trace elements and negative δ<sup>34</sup>S<sub>V-CDT</sub> values spanning from −34.2 to −17.4 ‰, indicating the formation in a euxinic water column via microbial sulfate reduction (MSR) during syngenesis. The later pyrite (PyII), which is associated with gersdorffite, appears as thin overgrowth rings encircling PyI or euhedral to subhedral crystals displaying complex oscillatory and lacy zoning patterns. It is characterized by elevated Cu (up to 5.84 wt%), Ni (up to 3.56 wt%), and As (up to 1.70 wt%) concentrations and highly variable sulfur isotopic compositions. The overgrowth rings exhibit δ<sup>34</sup>S<sub>V-CDT</sub> values spanning from −9.6 to 12.4 ‰, while the euhedral to subhedral crystals have inhomogeneous sulfur isotopic signatures, with the cores enriched in heavy sulfur (31.7 to 33.7 ‰ δ<sup>34</sup>S<sub>V-CDT</sub>) and the rims marked by comparatively light sulfur (−7.1 to 7.2 ‰ δ<sup>34</sup>S<sub>V-CDT</sub>). This isotopic variation reflects the joint effect of thermochemical sulfate reduction (TSR) and hydrothermal-derived sulfur. Specifically, the initial influx of high-temperature hydrothermal fluids produced minimal isotopic fractionation between sulfide and seawater sulfate via TSR, facilitating the formation of heavy‑sulfur PyII. Subsequently, hydrothermal sulfur became the dominant source for the deposition of light‑sulfur PyII, with a minor contribution from dissolution of PyI. Given the close association of organic matter with ores, hydrocarbons may significantly facilitate metal transport via organometallic complexing, with Ni primarily derived from hydrothermal fluids and Mo from seawater as indicated by previous Ni and Mo isotopic analyses. The barren pyrite (PyIII), which presents as overgrowth matrix cementing early PyI and PyII, has positive δ<sup>34</sup>S<sub>V-CDT</sub> values (15.5 to 34.6 ‰), suggesting formation via MSR in closed, sulfate-limited environment, most likely following deep burial. Collectively, the chemical and sulfur isotopic evidence, coupled with petrogr","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122623"},"PeriodicalIF":3.6,"publicationDate":"2025-01-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143021234","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":"BrGDGTs sources in eastern China marginal seas and their constraints on seawater temperature reconstruction","authors":"Fengmin Pan ,&nbsp;Huamao Yuan ,&nbsp;Jinming Song ,&nbsp;Xuegang Li ,&nbsp;Liqin Duan ,&nbsp;Yueqi Wang","doi":"10.1016/j.chemgeo.2025.122624","DOIUrl":"10.1016/j.chemgeo.2025.122624","url":null,"abstract":"<div><div>Branched glycerol dialkyl glycerol tetraethers (brGDGTs) serve as promising proxies for paleotemperature in both terrestrial and marine settings. However, their application in reconstructing seawater temperature in marginal seas is complicated by uncertainties arising from mixed land and marine sources, isomer effect, and variability in depth and seasonality. To explore these factors, we examined brGDGTs in surface sediments from the eastern China marginal seas (ECMS) and reanalyzed previously published brGDGT data from the ECMS and Chinese soils. Using a three-end-member mixing model based on source indicators of ∑IIIa/IIa and #Rings_tetra, we found that the autochthonous fraction of sedimentary brGDGTs accounted for 59.6 ± 14.4 %, 47.8 ± 21.7 %, and 33.2 ± 17.7 % in the Bohai Sea (BS), Yellow Sea (YS) and East China Sea (ECS), respectively. For temperature reconstruction, the relationship with annual mean surface seawater temperature (SST) was better captured by the methylation index of 6-methyl brGDGTs (MBT’_6Me) than by that of 5-methyl brGDGTs (MBT’_5Me). This is due to the higher abundance of 6-methyl brGDGT producers compared to those producing 5-methyl isomers, resulting in the predominance of 6-methyl brGDGTs and thus a better response of MBT’_6Me to SST changes. Additionally, brGDGTs in the ECMS were shown to reflect the annual mean rather than seasonal SST, as SST remains well above freezing throughout the year and supports continuous brGDGT production. In contrast to deep oceans, production in deep waters or sediments appears to have a negligible effect on sedimentary brGDGTs in the ECMS, as brGDGT-based indexes responded better to SST than to bottom seawater temperature (BWT). Furthermore, the methylation index of 6-methyl brGDGTs containing two rings (defined as MBT’_6Me-c) emerged as a robust potential indicator for annual mean SST in the ECMS, showing improved correlation with SST and reduced residual mean error (RMSE) compared to MBT’_6Me. The components (Ic, IIc’, and IIIc’) used to establish MBT’_6Me-c are mainly derived from in-situ production rather than terrestrial input. Therefore, the index was less influenced by terrestrial inputs and is a reliable tool for SST reconstruction in the China marginal seas.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122624"},"PeriodicalIF":3.6,"publicationDate":"2025-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143021235","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":"Sulfate concentration and redox state control the pyrite formation and sulfur cycle in a T-OAE lake, Sichuan Basin, China","authors":"Xiang Qin ,&nbsp;Jian Cao ,&nbsp;Bing Luo ,&nbsp;Jinchao Liu ,&nbsp;Guang Hu","doi":"10.1016/j.chemgeo.2025.122622","DOIUrl":"10.1016/j.chemgeo.2025.122622","url":null,"abstract":"<div><div>The sedimentary pyrite formation and sulfur cycle have been extensively investigated, especially in marine systems. The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) was marked by significant shifts in Earth’s climate and paleoceanographic conditions, and its global carbon cycle perturbations have been extensively studied. However, constraints on terrestrial sulfur cycling within the Earth system during this period are underexplored. To fill the knowledge gap, this study presents new constraints on the Da'anzhai Member of the Sichuan Basin in South China, a terrestrial record of the T-OAE, to explore the sedimentary pyrite formation and sulfur cycle in ancient lake environments. Results show that pyrite sulfur contents (S_pyr) of most samples are &lt;0.1 wt.%, and the stable sulfur isotopic compositions of pyrite (δ³⁴S_pyr) change from a low negative value of −11.5‰ to a positive value of 20.3‰ before the T-OAE. This variation was interpreted to be influenced by the oxygenated water and a relatively high sedimentation rate; the former confined microbial sulfate reduction (MSR) to the porewater space, while the latter reduced the efficiency of diffusive resupply of porewater sulfate. The T-OAE was associated with a marked increase in S_pyr values (mean = 0.52 wt%), and δ³⁴S_pyr values fluctuated by ∼10‰ (5.4‰–15.6‰), consistent with higher sulfate reduction rates driven by increased organic carbon contents and sulfate concentration ([SO₄²⁻]) during this stage. After the T-OAE, the S_pyr contents were &lt; 0.1 wt% and δ³⁴S_pyr values were ∼10‰. The δ³⁴S_pyr values correlated well with the salinity proxy Sr/Ba ratios, suggesting that sulfate levels on system openness were likely the dominant way controlling δ³⁴S_pyr. Compared with the marine systems, relatively lower sulfate concentrations are the main limiting factor on pyrite formation in freshwater lacustrine systems, and the sedimentation rate and sulfate contents may be the main factors responsible for the higher δ³⁴S_pyr values in lacustrine systems. During the T-OAE, elevated atmospheric <em>p</em>CO₂ and global climate warming triggered a series of chemical changes in lake systems (particularly in [O₂] levels), which in turn significantly disrupted lacustrine sulfur cycling. This represents a notable terrestrial response to the T-OAE. This study highlights the dynamic control of sedimentary environmental factors on lacustrine sulfur cycling and the broader impact of the T-OAE on lacustrine environments by focusing on the first documented case of a large lake system.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122622"},"PeriodicalIF":3.6,"publicationDate":"2025-01-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142988038","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":"Kimberlite segregation from an uppermost asthenospheric thermal boundary and the longevity of cold craton roots","authors":"Hong-Kun Dai ,&nbsp;Jian-Ping Zheng ,&nbsp;Qing Xiong ,&nbsp;William L. Griffin ,&nbsp;Philip E. Janney ,&nbsp;Suzanne Y. O'Reilly","doi":"10.1016/j.chemgeo.2025.122621","DOIUrl":"10.1016/j.chemgeo.2025.122621","url":null,"abstract":"<div><div>Long-lived (&gt;2.5 Ga) cratons usually preserve ancient cold and refractory mantle roots, but how the deep roots survive from recycling back to the convective mantle remains open to debate. Here, the mechanism for preservation of Archean mantle roots is explored using the major-, trace-element and Sr<img>Nd isotopic systematics of kimberlites, the asthenosphere-derived magmas under cratons. A case study on ∼480 Ma kimberlites of the North China Craton suggests that their segregation domains have pressures (∼5 GPa) shallower than the lower boundaries of typical craton roots and potential temperatures (<em>T</em>_p) between those of the ambient asthenosphere (<em>T</em>_p = ∼1400 °C) and the cold lithospheric roots of cratons (∼1200 °C). The dataset of primary kimberlites worldwide records similar temperature variation in their segregation domains, which likely represent the lowermost (asthenospheric) part of a thick thermal boundary layer between conductive lithosphere and convective asthenosphere. Our calculation on mantle viscosity suggests that the asthenospheric part of the thermal boundary layer would show marked viscosity increase due to thermal offset from normal mantle adiabat. The resultant resistant uppermost asthenosphere can serve as a protective sheath that can protect the cratonic roots from being eroded and removed. Our proposed model emphasizes the longevity of cratons provided simply by the thermal contrast between the cold craton roots and the asthenosphere.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122621"},"PeriodicalIF":3.6,"publicationDate":"2025-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143021245","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":"Apatite texture, trace elements and SrNd isotope geochemistry of the Koga carbonatite-alkaline complex, NW Pakistan: Implications for petrogenesis and mantle source","authors":"Asad Khan ,&nbsp;Zaheen Ullah ,&nbsp;Huan Li ,&nbsp;Shah Faisal ,&nbsp;Yasin Rahim","doi":"10.1016/j.chemgeo.2025.122611","DOIUrl":"10.1016/j.chemgeo.2025.122611","url":null,"abstract":"&lt;div&gt;&lt;div&gt;The Koga Carbonatite-Alkaline Complex (KCAC) in northwestern Pakistan is emplaced within the Early Paleozoic Greater Himalayan Sequence of the region, marking a significant magmatic event. The complex primarily comprises quartz syenite, nepheline syenite, and carbonatite lithologies. Apatite grains extracted from these lithologies were systematically analyzed using cathode-luminescence (CL) imaging, trace elements and Sr&lt;img&gt;Nd isotopic compositions, and LA-ICP-MS U&lt;img&gt;Pb geochronology. The homogeneous cathode luminescence of apatite grains from quartz syenite (Q-Ap1), nepheline syenite (N-Ap1) and carbonatite (C-Ap1) and their high concentrations of REE (2400 ppm – 22,130 ppm; 3832 ppm – 14,532 ppm; and 2494–9050, respectively), particularly light REE (with La&lt;sub&gt;N&lt;/sub&gt;/Yb&lt;sub&gt;N&lt;/sub&gt; = 12 to 78; 22 to 113; and 30–166; respectively), indicate their magmatic origin. Texturally and chemically zoned apatite from nepheline syenite (N-Ap2) and carbonatite (C-Ap2) are also enriched in light REE (La&lt;sub&gt;N&lt;/sub&gt;/Yb&lt;sub&gt;N&lt;/sub&gt; = 38 to 97 and 24 to 108; respectively) relative to heavy REE, which is consistent with a magmatic origin. The Q-Ap1 apatite are showing distinct negative Eu anomalies (0.18–0.75) indicative of crystallization from an evolved melt that experienced significant plagioclase fractionation. The N-Ap2 apatite exhibit a rim-ward increase in REE + Y contents, which can be attributed to fractional crystallization of pyroxene, alkali feldspar and biotite. While in C-Ap2 the rim-ward increase in REE + Y is coupled with increasing Na and decreasing Mn, suggesting early crystallization of calcite. Moreover, patchy texture and depleted REE + Y contents of N-Ap3 apatite from coarse-grained nepheline syenite compared to N-Ap1, reflect their metasomatic nature by interacting with fluids. However, N-Ap3 show similar initial Sr and Nd composition with magmatic apatite from nepheline syenite (i.e., N-Ap1) suggesting that the fluids involved in metasomatic alteration were likely originated from the cooling host magma. The LA-ICP-MS U&lt;img&gt;Pb apatite geochronology data from KCAC align closely with previously published zircon U&lt;img&gt;Pb ages, indicating rapid cooling following the emplacement of the complex. The U&lt;img&gt;Pb dating of apatite yields an emplacement age of 287 ± 16 Ma, 279 ± 9 Ma and 274 ± 9 Ma for the quartz syenite, nepheline syenite and carbonatite, respectively; implying their emplacement during the Permian rifting and breakup of the northern margin of Gondwana. The Sr and Nd isotopic compositions of apatite from the KCAC imply a sub-lithospheric mantle source, with minimal crustal contamination or interaction with metasomatized sub-continental lithospheric mantle. The study demonstrates that apatite textural features combined with in-situ trace elements and Sr&lt;img&gt;Nd isotopic compositions can be an effective tool to unravel complex magmatic and hydrothermal processes and nature of the magma source of carbonatite- alkaline compl","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122611"},"PeriodicalIF":3.6,"publicationDate":"2025-01-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142939817","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":"Insights into magma dynamics from daily OP-FTIR gas compositions throughout the 2021 Tajogaite eruption, La Palma, Canary Islands","authors":"María Asensio-Ramos ,&nbsp;Ana Pardo Cofrades ,&nbsp;Mike Burton ,&nbsp;Alessandro La Spina ,&nbsp;Patrick Allard ,&nbsp;José Barrancos ,&nbsp;Catherine Hayer ,&nbsp;Ben Esse ,&nbsp;Luca D’Auria ,&nbsp;Pedro A. Hernández ,&nbsp;Eleazar Padrón ,&nbsp;Gladys V. Melián ,&nbsp;Nemesio M. Pérez","doi":"10.1016/j.chemgeo.2024.122605","DOIUrl":"10.1016/j.chemgeo.2024.122605","url":null,"abstract":"<div><div>From September 19 to December 12, 2021, a mixed explosive-effusive eruption impacted La Palma Island, in the Canary archipelago, leading to the growth of Tajogaite volcanic cone. Daily Open-Path Fourier Transform Infrared (OP-FTIR) measurements from October 3 until the end of the eruption allowed us to capture the molar proportions of H₂O, CO₂, SO₂, HCl and CO (plus COS occasionally) in magmatic gases emitted from summit and flank vents of the new cone. Our results reveal high CO₂/SO₂ ratios ranging from 11 to 53 in explosive gas emissions throughout most of the eruption, with a time-averaged value of ∼30. In contrast, effusive degassing at lower flank vents systematically displayed lower CO₂/SO₂ ratios (from 3 to 11) and enhanced proportions of H₂O, S and Cl. Combined with solubility data and the magma eruption rates, this chemical contrast suggests gas fractionation in a very shallow conduit branching beneath the Tajogaite cone, were most of the pre-exsolved CO₂-rich gas phase but a minor fraction of the magma explosively escaped through the main eruptive conduit, while CO₂-depleted gas and most of the magma were effusively discharged through lateral branches. The CO/COS/CO₂ ratios for explosive degassing are consistent with petrological evidence of oxidized magmatic conditions (FMQ +1.7 ± 0.3), which enhanced sulfur solubility and late-stage SO₂ outgassing. The high oxidation state, as well as low HCl/HF ratios, retrieved from solar occultation across the volcanic plume, are both typical of Ocean Island Basalt (OIB) magmatism. The apparent increase of CO₂/SO₂ and SO₂/HCl ratios at summit vents during the first half of the eruption is consistent with the influx of progressively more mafic, deeper-derived, basanitic magma, though we cannot entirely rule out artefacts due to more challenging measurement of the pure explosive gas in that period. Our study presents the very first detailed data set for gas geochemistry during a subaerial eruption in the Canary archipelago and highlights the remarkable potential of OP-FTIR spectroscopy for real-time monitoring and study of volcanic eruptions.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122605"},"PeriodicalIF":3.6,"publicationDate":"2025-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142939818","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":"Felsic magmatism and hydrothermal activity of the Central Igneous Complex of Boa Vista (Cape Verde islands): Insights from zircon, titanite and apatite geochronology","authors":"D. Orejana ,&nbsp;J.A. Moreno ,&nbsp;C. Pérez-Soba ,&nbsp;A. Beranoaguirre ,&nbsp;C. de Ignacio ,&nbsp;M. García-Rodríguez ,&nbsp;C. Villaseca","doi":"10.1016/j.chemgeo.2024.122602","DOIUrl":"10.1016/j.chemgeo.2024.122602","url":null,"abstract":"<div><div>Studies focused on the U-Pb geochronology and oxygen isotope composition of magmatic zircon formed in intraplate oceanic islands are very scarce. The Boa Vista Island (Cape Verde archipelago) displays one of the few felsic igneous complexes (alkaline volcanic and plutonic rocks) in an oceanic setting and provides the opportunity to date and characterise magmatic-hydrothermal events based on the isotopic records of zircon. Due to the scarcity of this mineral in these igneous rocks, we have separated detrital zircon from sediments of dry riverbeds of the Central Igneous Complex of the island of Boa Vista, which is dominantly felsic in composition. Zircon has been analysed to determine the U–Pb–O isotopic composition and trace element contents. All zircon grains display complex CL textures indicative of a strong transformation due to interaction with a hydrothermal agent. This reactive event led to a strong disturbance of the U-Pb system, enrichment of trace elements (Ti, Nb, Ta, Th, U, Pb, Y and REE) and decrease of δ¹⁸O values (down to −6 ‰). These features suggest that the magmatic U–Th-rich zircon crystallised from highly evolved melts and experienced partial metamictization prior to its transformation. High-T hydrothermal fluids were likely mixed with groundwater (imprinting such low oxygen isotope composition) and reacted with and transformed zircon crystals from the main felsic igneous formations. The geochronological results obtained by SHRIMP on zircon have been complemented with U-Pb ages obtained by laser ablation SF–ICP–MS on titanite and apatite of syenitic intrusions. Overall, the geochronological results fall within a restricted range of ages from 13.3 to 15 Ma. These results, together with literature data, imply that most of the felsic volcanic and plutonic rocks from Boa Vista were formed in the range ∼ 12–15 Ma (episodes 1 and 2 of the second stage of magmatism). The hydrothermal process observed in the studied detrital zircon likely occurred during the last felsic magmatic events (∼10–12 Ma).</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122602"},"PeriodicalIF":3.6,"publicationDate":"2025-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142939822","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":"Sediment melts impart extremely light Mo isotopes to arc magmas of central Tibet","authors":"Feng Huang ,&nbsp;Jie Li ,&nbsp;Jifeng Xu ,&nbsp;Yunchuan Zeng","doi":"10.1016/j.chemgeo.2024.122610","DOIUrl":"10.1016/j.chemgeo.2024.122610","url":null,"abstract":"<div><div>Subduction zones are pivotal in understanding the interaction between Earth's surface and deep materials. The markedly diverse Mo isotopic compositions observed in arc magmas provide a valuable opportunity to explore the material cycling processes within subduction zones. Arc magma sources altered by slab fluids exhibit heavy Mo isotopes, while the origin of arc lavas with light Mo isotopes remains contentious, which is hypothesized to originate from two potential sources: (1) the dehydrated oceanic crust and (2) subducting sediments. Although the former has been extensively recognized, the latter still poses an enigma. Here, we present the Mo-Sr-Nd-Hf isotopic and elemental data of Jiang Tso andesites in the central Tibetan Plateau to elucidate the chemical compositions of sediment melts. These andesites show elevated Mg# values, along with trace element characteristics reminiscent of typical sediment melts. Their Sr-Nd-Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.710260–0.710671, ε_Nd(t) = −10.63 to −8.97, and ε_Hf(t) = −9.30 to −7.95) closely resemble those of contemporaneous sediments in the central Tibetan Plateau. They exhibit higher Ce/Mo ratios (396–587) and lower δ^98/95Mo values (−1.62 ‰ to −0.69 ‰) compared to the depleted mantle and most arc lavas, suggesting a more plausible explanation lies in the involvement of dehydrated subducting sediments rather than dehydrated oceanic crust in the source. Our findings, integrated with existing research, suggest that the Mo isotopes of arc magmas, in conjunction with trace elemental ratios, can preliminarily constrain the different subduction components (fluid or melt) in their sources. In addition, arc rocks with extremely light Mo isotopes may not be exclusively derived from subducted oceanic crust. Instead, they could originate from dehydrated sediment residues that enter the sub-arc mantle. Sediments with light Mo isotopes that are subducted into and preserved within the continental lithospheric mantle are likely to form localized reservoir characterized by light Mo isotopic signatures. This reservoir could play a crucial role in reconciling the discrepancies in Mo isotopic compositions between the continental crust and the depleted mantle.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"676 ","pages":"Article 122610"},"PeriodicalIF":3.6,"publicationDate":"2025-01-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142939938","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":"Mantle sources underlying Nyiragongo volcano","authors":"Sander Molendijk ,&nbsp;Olivier Namur ,&nbsp;David A. Neave","doi":"10.1016/j.chemgeo.2024.122607","DOIUrl":"10.1016/j.chemgeo.2024.122607","url":null,"abstract":"<div><div>The Nyiragongo volcano, part of the Virunga Volcanic Province (VVP) in the Democratic Republic of Congo is a uniquely silica-undersaturated (32–49 wt% SiO₂) and alkali-rich (7–17 wt% Na₂O + K₂O) volcanic system. Though alkaline volcanism is by no means rare in the western branch of the East African Rift System (EARS), compositions erupted by Nyiragongo are nevertheless notably alkaline, especially given that the other volcanoes in the VVP predominantly erupt silica-saturated compositions. In order to address the origin of the geochemical character of Nyiragongo it is imperative to understand the mantle source feeding its plumbing system. We approach this issue through the combined analysis of primitive whole-rock samples and early-crystallized olivine hosted by these samples. Major and trace element compositional data from olivine indicates a Ca-rich, but otherwise unremarkable geochemical signature indicative of melt derivation from a peridotite restite. However, whole-rock compositions require melting of a K-rich phlogopite bearing source in the presence of apatite, garnet, and clinopyroxene to produce Si-undersaturated, Ca-, and K- rich melts. Geochemical modelling using recently acquired partition coefficients suggests that such melting must occur primarily at high pressures (∼ 3 GPa), outside of the stability field of amphibole, in line with Nyiragongo residing on thick lithosphere bordering the Tanzanian craton.</div><div>Combining the peridotite restite-signature from olivine compositions with the phlogopite-pyroxenite melt-source inferred from whole-rock data results in a model of metasomatic vein-melting, wherein veins emplaced in the subcontinental lithospheric mantle during the Pan-African orogeny are preferentially molten through a breakdown reaction of phlogopite at temperatures of 1150 to 1350 °C. These melts subsequently equilibrate with the surrounding peridotite mantle, resulting in a peridotite-restite signature observed in olivine.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"675 ","pages":"Article 122607"},"PeriodicalIF":3.6,"publicationDate":"2024-12-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142940010","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":"Investigating the crystallization age and initial lead composition of Phanerozoic magmatic allanites by LA-MC-ICP-MS","authors":"Etienne Skrzypek ,&nbsp;Daniela Gallhofer ,&nbsp;Christoph Hauzenberger ,&nbsp;Isabella Haas ,&nbsp;Anh Thi Quynh Nong ,&nbsp;Harald Fritz ,&nbsp;Milan Kohút ,&nbsp;Viljem Podgoršek ,&nbsp;Zmago Žorž","doi":"10.1016/j.chemgeo.2024.122603","DOIUrl":"10.1016/j.chemgeo.2024.122603","url":null,"abstract":"<div><div>Allanite is a REE-bearing, epidote-group mineral that incorporates Th, U, but also initial Pb. This results in U<img>Pb isotopic analyses defining a mixing trend between an initial and radiogenic Pb component. So far, only the latter has been given attention because it can reveal the age of various geological events, whereas the significance of the former has been neglected. We explore both the age <em>and</em> initial Pb isotopic composition of nine magmatic samples (volcanic, plutonic, pegmatitic) with emplacement ages that cover nearly the entire Phanerozoic (∼ 540–19 Ma). We first characterize the major-element composition of allanite by EPMA, then measure its (Th–)U–Pb isotopic composition in situ by LA-MC-ICP-MS using zircon as reference material. U<img>Pb data are regressed to determine both the crystallization age and initial ²⁰⁷Pb/²⁰⁶Pb₀ composition. These results are compared to the U<img>Pb age of co-genetic zircon and the ²⁰⁷Pb^/206Pb ratio of coexisting feldspar from the same samples or localities, respectively. Their good agreement (mainly less than ∼4 % and 1 % offset, respectively) supports the validity of our analytical protocol. The accuracy of age and ²⁰⁷Pb/²⁰⁶Pb₀ results is governed by the proportion of initial Pb, <em>f(</em>²⁰⁶Pb₀), especially its minimum (for age)/maximum (for ²⁰⁷Pb/²⁰⁶Pb₀) value, while precision is chiefly controlled by the centroid of <em>f(</em>²⁰⁶Pb₀) distribution. Calculations using published allanite/melt partition coefficients show that a ²³⁰Th disequilibrium correction based on the whole-rock Th/U is not only appropriate for magmatic samples, but also necessary to improve accuracy. In four samples the ²⁰⁷Pb/²⁰⁶Pb₀ ratios obtained from allanite are lower than values predicted by terrestrial Pb evolution models at the time of emplacement, showing that anchoring a regression line to a fixed Pb composition is not always valid. Although it does not affect age determination, it overlooks the potential of allanite U<img>Pb data for constraining the radiogenic character of crustal magma sources.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"675 ","pages":"Article 122603"},"PeriodicalIF":3.6,"publicationDate":"2024-12-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142940014","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}