Pub Date : 2025-01-16DOI: 10.1016/j.chemgeo.2025.122628
N.G. Rudraswami, V.P. Singh, M. Pandey
In a recent publication by Loeb et al. (2024), the collection of spherules from the Pacific Ocean was classified as extraterrestrial, specifically micrometeorites. We critically evaluate their findings and conclusions, pointing out inaccuracies in their data interpretation. A comprehensive re-examination of these spherules is essential to classify them as micrometeorites accurately. We argue that the spherules they studied are primarily volcanic or terrestrial in origin, and we emphasize that a revision of their classification is crucial given the flawed observations. This work highlights the challenges faced by researchers and specialists who are not focused on micrometeorites when attempting to locate specific meteor particles from the ocean's depths, a task often regarded as nearly impossible due to the complexities involved in identifying and retrieving these objects. It is advisable to adopt standardized protocols to reduce the possibilities of unreliable interpretations in micrometeorite research. Additionally, “cosmic spherules” should not be used indiscriminately to describe recovered particles, underscoring the importance of adhering to specific chemical and textural characteristics.
{"title":"Re-evaluation of the spherules proposed origin recovered from the Pacific Ocean site of the CNEOS 2014-01-08 (IM1) bolide","authors":"N.G. Rudraswami, V.P. Singh, M. Pandey","doi":"10.1016/j.chemgeo.2025.122628","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2025.122628","url":null,"abstract":"In a recent publication by Loeb et al. (2024), the collection of spherules from the Pacific Ocean was classified as extraterrestrial, specifically micrometeorites. We critically evaluate their findings and conclusions, pointing out inaccuracies in their data interpretation. A comprehensive re-examination of these spherules is essential to classify them as micrometeorites accurately. We argue that the spherules they studied are primarily volcanic or terrestrial in origin, and we emphasize that a revision of their classification is crucial given the flawed observations. This work highlights the challenges faced by researchers and specialists who are not focused on micrometeorites when attempting to locate specific meteor particles from the ocean's depths, a task often regarded as nearly impossible due to the complexities involved in identifying and retrieving these objects. It is advisable to adopt standardized protocols to reduce the possibilities of unreliable interpretations in micrometeorite research. Additionally, “cosmic spherules” should not be used indiscriminately to describe recovered particles, underscoring the importance of adhering to specific chemical and textural characteristics.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"12 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2025-01-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143021232","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 : 2025-01-14DOI: 10.1016/j.chemgeo.2025.122623
Shixiang Li, Zhilin Wang, Youwei Chen, Daniel D. Gregory, Erke Peng, Deru Xu, Yufei Wang, Shaohao Zou, Huan Li
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<ce:glyph name="sbnd"></ce:glyph>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<ce:glyph name="sbnd"></ce:glyph>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<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">V-CDT</ce:inf> 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<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">V-CDT</ce:inf> 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<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">V-CDT</ce:inf>) and the rims marked by comparatively light sulfur (−7.1 to 7.2 ‰ δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">V-CDT</ce:inf>). 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<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">V-CDT
{"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":"https://doi.org/10.1016/j.chemgeo.2025.122623","url":null,"abstract":"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<ce:glyph name=\"sbnd\"></ce:glyph>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<ce:glyph name=\"sbnd\"></ce:glyph>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<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">V-CDT</ce:inf> 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<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">V-CDT</ce:inf> 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<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">V-CDT</ce:inf>) and the rims marked by comparatively light sulfur (−7.1 to 7.2 ‰ δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">V-CDT</ce:inf>). 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<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">V-CDT","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"74 1","pages":""},"PeriodicalIF":3.9,"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}
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 #Ringstetra, 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.
{"title":"BrGDGTs sources in eastern China marginal seas and their constraints on seawater temperature reconstruction","authors":"Fengmin Pan, Huamao Yuan, Jinming Song, Xuegang Li, Liqin Duan, Yueqi Wang","doi":"10.1016/j.chemgeo.2025.122624","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2025.122624","url":null,"abstract":"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<ce:inf loc=\"post\">tetra</ce:inf>, 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’<ce:inf loc=\"post\">6Me</ce:inf>) than by that of 5-methyl brGDGTs (MBT’<ce:inf loc=\"post\">5Me</ce:inf>). 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’<ce:inf loc=\"post\">6Me</ce:inf> 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’<ce:inf loc=\"post\">6Me-c</ce:inf>) 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’<ce:inf loc=\"post\">6Me</ce:inf>. The components (Ic, IIc’, and IIIc’) used to establish MBT’<ce:inf loc=\"post\">6Me-c</ce:inf> 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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"15 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2025-01-10DOI: 10.1016/j.chemgeo.2025.122622
Xiang Qin, Jian Cao, Bing Luo, Jinchao Liu, Guang Hu
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<ce:inf loc="post">pyr</ce:inf>) of most samples are <0.1 wt.%, and the stable sulfur isotopic compositions of pyrite (δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf>) 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<ce:inf loc="post">pyr</ce:inf> values (mean = 0.52 wt%), and δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf> values fluctuated by ∼10‰ (5.4‰–15.6‰), consistent with higher sulfate reduction rates driven by increased organic carbon contents and sulfate concentration ([SO<ce:inf loc="post">4</ce:inf><ce:sup loc="post">2−</ce:sup>]) during this stage. After the T-OAE, the S<ce:inf loc="post">pyr</ce:inf> contents were < 0.1 wt% and δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf> values were ∼10‰. The δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf> values correlated well with the salinity proxy Sr/Ba ratios, suggesting that sulfate levels on system openness were likely the dominant way controlling δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf>. 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 δ<ce:sup loc="post">34</ce:sup>S<ce:inf loc="post">pyr</ce:inf> values in lacustrine systems. During the T-OAE, elevated atmospheric <ce:italic>p</ce:italic>CO<ce:inf loc="post">2</ce:inf> and global climate warming triggered a series of chemical changes in lake systems (particularly in [O<ce:inf loc="post">2</ce:inf>] 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
{"title":"Sulfate concentration and redox state control the pyrite formation and sulfur cycle in a T-OAE lake, Sichuan Basin, China","authors":"Xiang Qin, Jian Cao, Bing Luo, Jinchao Liu, Guang Hu","doi":"10.1016/j.chemgeo.2025.122622","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2025.122622","url":null,"abstract":"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<ce:inf loc=\"post\">pyr</ce:inf>) of most samples are <0.1 wt.%, and the stable sulfur isotopic compositions of pyrite (δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf>) 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<ce:inf loc=\"post\">pyr</ce:inf> values (mean = 0.52 wt%), and δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf> values fluctuated by ∼10‰ (5.4‰–15.6‰), consistent with higher sulfate reduction rates driven by increased organic carbon contents and sulfate concentration ([SO<ce:inf loc=\"post\">4</ce:inf><ce:sup loc=\"post\">2−</ce:sup>]) during this stage. After the T-OAE, the S<ce:inf loc=\"post\">pyr</ce:inf> contents were < 0.1 wt% and δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf> values were ∼10‰. The δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf> values correlated well with the salinity proxy Sr/Ba ratios, suggesting that sulfate levels on system openness were likely the dominant way controlling δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf>. 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 δ<ce:sup loc=\"post\">34</ce:sup>S<ce:inf loc=\"post\">pyr</ce:inf> values in lacustrine systems. During the T-OAE, elevated atmospheric <ce:italic>p</ce:italic>CO<ce:inf loc=\"post\">2</ce:inf> and global climate warming triggered a series of chemical changes in lake systems (particularly in [O<ce:inf loc=\"post\">2</ce:inf>] 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 ","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"55 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2025-01-09DOI: 10.1016/j.chemgeo.2025.122621
Hong-Kun Dai, Jian-Ping Zheng, Qing Xiong, William L. Griffin, Philip E. Janney, Suzanne Y. O'Reilly
Long-lived (>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 SrNd 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 (Tp) between those of the ambient asthenosphere (Tp = ∼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.
{"title":"Kimberlite segregation from an uppermost asthenospheric thermal boundary and the longevity of cold craton roots","authors":"Hong-Kun Dai, Jian-Ping Zheng, Qing Xiong, William L. Griffin, Philip E. Janney, Suzanne Y. O'Reilly","doi":"10.1016/j.chemgeo.2025.122621","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2025.122621","url":null,"abstract":"Long-lived (>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<ce:glyph name=\"sbnd\"></ce:glyph>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 (<ce:italic>T</ce:italic><ce:inf loc=\"post\">p</ce:inf>) between those of the ambient asthenosphere (<ce:italic>T</ce:italic><ce:inf loc=\"post\">p</ce:inf> = ∼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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"301 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2025-01-04DOI: 10.1016/j.chemgeo.2025.122611
Asad Khan, Zaheen Ullah, Huan Li, Shah Faisal, Yasin Rahim
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<ce:glyph name="sbnd"></ce:glyph>Nd isotopic compositions, and LA-ICP-MS U<ce:glyph name="sbnd"></ce:glyph>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<ce:inf loc="post">N</ce:inf>/Yb<ce:inf loc="post">N</ce:inf> = 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<ce:inf loc="post">N</ce:inf>/Yb<ce:inf loc="post">N</ce:inf> = 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<ce:glyph name="sbnd"></ce:glyph>Pb apatite geochronology data from KCAC align closely with previously published zircon U<ce:glyph name="sbnd"></ce:glyph>Pb ages, indicating rapid cooling following the emplacement of the complex. The U<ce:glyph name="sbnd"></ce:glyph>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
{"title":"Apatite texture, trace elements and Sr[sbnd]Nd isotope geochemistry of the Koga carbonatite-alkaline complex, NW Pakistan: Implications for petrogenesis and mantle source","authors":"Asad Khan, Zaheen Ullah, Huan Li, Shah Faisal, Yasin Rahim","doi":"10.1016/j.chemgeo.2025.122611","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2025.122611","url":null,"abstract":"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<ce:glyph name=\"sbnd\"></ce:glyph>Nd isotopic compositions, and LA-ICP-MS U<ce:glyph name=\"sbnd\"></ce:glyph>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<ce:inf loc=\"post\">N</ce:inf>/Yb<ce:inf loc=\"post\">N</ce:inf> = 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<ce:inf loc=\"post\">N</ce:inf>/Yb<ce:inf loc=\"post\">N</ce:inf> = 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<ce:glyph name=\"sbnd\"></ce:glyph>Pb apatite geochronology data from KCAC align closely with previously published zircon U<ce:glyph name=\"sbnd\"></ce:glyph>Pb ages, indicating rapid cooling following the emplacement of the complex. The U<ce:glyph name=\"sbnd\"></ce:glyph>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","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"75 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2025-01-03DOI: 10.1016/j.chemgeo.2024.122605
María Asensio-Ramos, Ana Pardo Cofrades, Mike Burton, Alessandro La Spina, Patrick Allard, José Barrancos, Catherine Hayer, Ben Esse, Luca D’Auria, Pedro A. Hernández, Eleazar Padrón, Gladys V. Melián, Nemesio M. Pérez
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.
{"title":"Insights into magma dynamics from daily OP-FTIR gas compositions throughout the 2021 Tajogaite eruption, La Palma, Canary Islands","authors":"María Asensio-Ramos, Ana Pardo Cofrades, Mike Burton, Alessandro La Spina, Patrick Allard, José Barrancos, Catherine Hayer, Ben Esse, Luca D’Auria, Pedro A. Hernández, Eleazar Padrón, Gladys V. Melián, Nemesio M. Pérez","doi":"10.1016/j.chemgeo.2024.122605","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122605","url":null,"abstract":"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 H<ce:inf loc=\"post\">2</ce:inf>O, CO<ce:inf loc=\"post\">2</ce:inf>, SO<ce:inf loc=\"post\">2</ce:inf>, HCl and CO (plus COS occasionally) in magmatic gases emitted from summit and flank vents of the new cone. Our results reveal high CO<ce:inf loc=\"post\">2</ce:inf>/SO<ce:inf loc=\"post\">2</ce:inf> 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 CO<ce:inf loc=\"post\">2</ce:inf>/SO<ce:inf loc=\"post\">2</ce:inf> ratios (from 3 to 11) and enhanced proportions of H<ce:inf loc=\"post\">2</ce:inf>O, 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 CO<ce:inf loc=\"post\">2</ce:inf>-rich gas phase but a minor fraction of the magma explosively escaped through the main eruptive conduit, while CO<ce:inf loc=\"post\">2</ce:inf>-depleted gas and most of the magma were effusively discharged through lateral branches. The CO/COS/CO<ce:inf loc=\"post\">2</ce:inf> 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 SO<ce:inf loc=\"post\">2</ce:inf> 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 CO<ce:inf loc=\"post\">2</ce:inf>/SO<ce:inf loc=\"post\">2</ce:inf> and SO<ce:inf loc=\"post\">2</ce:inf>/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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"28 1","pages":""},"PeriodicalIF":3.9,"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":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-01-02DOI: 10.1016/j.chemgeo.2024.122602
D. Orejana, J.A. Moreno, C. Pérez-Soba, A. Beranoaguirre, C. de Ignacio, M. García-Rodríguez, C. Villaseca
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).
{"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, J.A. Moreno, C. Pérez-Soba, A. Beranoaguirre, C. de Ignacio, M. García-Rodríguez, C. Villaseca","doi":"10.1016/j.chemgeo.2024.122602","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122602","url":null,"abstract":"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 δ<ce:sup loc=\"post\">18</ce:sup>O 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).","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"1 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2025-01-02DOI: 10.1016/j.chemgeo.2024.122610
Feng Huang, Jie Li, Jifeng Xu, Yunchuan Zeng
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/86Sri = 0.710260–0.710671, εNd(t) = −10.63 to −8.97, and εHf(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 δ98/95Mo 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.
{"title":"Sediment melts impart extremely light Mo isotopes to arc magmas of central Tibet","authors":"Feng Huang, Jie Li, Jifeng Xu, Yunchuan Zeng","doi":"10.1016/j.chemgeo.2024.122610","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122610","url":null,"abstract":"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 (<ce:sup loc=\"post\">87</ce:sup>Sr/<ce:sup loc=\"post\">86</ce:sup>Sr<ce:inf loc=\"post\">i</ce:inf> = 0.710260–0.710671, ε<ce:inf loc=\"post\">Nd</ce:inf>(t) = −10.63 to −8.97, and ε<ce:inf loc=\"post\">Hf</ce:inf>(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<ce:sup loc=\"post\">98/95</ce:sup>Mo 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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"75 1","pages":""},"PeriodicalIF":3.9,"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}
Pub Date : 2024-12-31DOI: 10.1016/j.chemgeo.2024.122607
Sander Molendijk, Olivier Namur, David A. Neave
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.
{"title":"Mantle sources underlying Nyiragongo volcano","authors":"Sander Molendijk, Olivier Namur, David A. Neave","doi":"10.1016/j.chemgeo.2024.122607","DOIUrl":"https://doi.org/10.1016/j.chemgeo.2024.122607","url":null,"abstract":"The Nyiragongo volcano, part of the Virunga Volcanic Province (VVP) in the Democratic Republic of Congo is a uniquely silica-undersaturated (32–49 wt% SiO<ce:inf loc=\"post\">2</ce:inf>) and alkali-rich (7–17 wt% Na<ce:inf loc=\"post\">2</ce:inf>O + K<ce:inf loc=\"post\">2</ce:inf>O) 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.","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"20 1","pages":""},"PeriodicalIF":3.9,"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}
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