Chemical Geology最新文献

{"title":"Microscale sulfur isotope imaging analysis with NanoSIMS: A new methodology for arbitrary area selection and its application to Archean sedimentary pyrite","authors":"Kohei Sasaki ,&nbsp;Akizumi Ishida ,&nbsp;Kenichiro Sugitani ,&nbsp;Naoto Takahata","doi":"10.1016/j.chemgeo.2025.122770","DOIUrl":"10.1016/j.chemgeo.2025.122770","url":null,"abstract":"<div><div>We present a method for in situ sulfur (S) isotope analysis in arbitrarily small areas (&lt;1 μm²) within pyrite using ion imaging mode of nanoscale secondary ion mass spectrometry (NanoSIMS). The precision and accuracy of δ³⁴S values obtained using this method were evaluated with reference pyrite with homogeneous S isotope ratios. The internal precision of the δ³⁴S values in any region of interest (ROI) as small as 0.94 μm² was approximately 1.0 ‰ at the 1σ weighted standard deviation (termed wSD) level for a single ROI. The in situ ion imaging method developed here enables the selection of any ROI smaller than 1 μm² within a raster area and is accurate and precise enough to detect δ³⁴S variations in small pyrite (&lt;10 μm) commonly found in ancient sedimentary rocks. This technique was applied to measure δ³⁴S values of sedimentary pyrite from early Archean cherts, yielding results consistent with those obtained through conventional spot analysis. The δ³⁴S values of small pyrite exhibited a fractionation of over 20 ‰ from those of Archean seawater sulfate. One plausible explanation for this fractionation is that these pyrites formed from sulfur that underwent isotopic fractionation via biological metabolism, such as microbial sulfate reduction or/and microbial sulfur disproportionation. Although further data are needed to strengthen interpretations of origins of the analyzed pyrite, our results demonstrate that the imaging mode of NanoSIMS is a powerful tool for high spatial resolution isotope analysis. This approach has the potential to provide insights into sulfur metabolizing activity on the early Earth.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122770"},"PeriodicalIF":3.6,"publicationDate":"2025-04-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143800376","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":"Tracing quartz provenance: A multi-method investigation of luminescence sensitisation mechanisms of quartz from granite source rocks and derived sediments","authors":"Daniela Constantin ,&nbsp;Aditi K. Dave ,&nbsp;Șerban Grecu ,&nbsp;Zuzanna Kabacińska ,&nbsp;Andris Antuzevics ,&nbsp;Anca Barla ,&nbsp;Petru Urdea ,&nbsp;Mihai N. Ducea ,&nbsp;Alida Timar-Gabor","doi":"10.1016/j.chemgeo.2025.122774","DOIUrl":"10.1016/j.chemgeo.2025.122774","url":null,"abstract":"<div><div>Quartz optically stimulated luminescence (OSL) sensitivity as well as some electron spin resonance (ESR) and cathodoluminescence (CL) signals have been empirically proposed as indicators for sediment provenance. Sensitivity is defined as luminescence emitted in response to a given dose per unit mass. While it is largely believed to be acquired by earth surface processes, recent studies bring evidence that sensitisation processes depend on source geology.</div><div>Here we combine OSL and thermoluminescence (TL), ESR and CL analyses to understand the mechanisms of quartz OSL sensitisation. We investigate granites and their derived sediments from catchments draining simple lithologies of known age that display contrasting OSL sensitisation behaviour both in nature and during irradiation and light exposure laboratory experiments. The sample displaying increased OSL sensitisation is characterised by TL emission at intermediate temperatures (150–250 °C), Ti-related signals in CL, and Ti and Ge lithium compensated signals in ESR. The insensitive samples either lack or exhibit very weak such characteristics and contain several times less amount of trace titanium measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).</div><div>We demonstrate that the OSL sensitisation by irradiation and light exposure results as an effect of the existence of certain defects and impurities in the quartz crystal in the parent rock, such as titanium and germanium. However, the degree of sensitisation reached in nature is significantly higher than in the laboratory. As such, the existence of this precursor represents the potential for sensitisation by irradiation and illumination. Sensitivity can later be amplified by other environmental factors that remain to be identified.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122774"},"PeriodicalIF":3.6,"publicationDate":"2025-04-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143807161","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":"Subduction and melting of biogenic and ferromanganese sediments as evidenced by sub-Moho granitoids","authors":"Tiago Valim Angelo ,&nbsp;Christopher J. Spencer ,&nbsp;Hong-Yan Li ,&nbsp;Derek Knaack ,&nbsp;Ziyi Zhu ,&nbsp;Marina Seraine ,&nbsp;Nick M.W. Roberts ,&nbsp;Evelyne Leduc ,&nbsp;Sophie Divilek ,&nbsp;Anna Ren ,&nbsp;Brian Joy ,&nbsp;Gui-Mei Lu","doi":"10.1016/j.chemgeo.2025.122759","DOIUrl":"10.1016/j.chemgeo.2025.122759","url":null,"abstract":"<div><div>Mantle-hosted granitoids (MHG) from the supra-subduction Samail ophiolite in Oman and the United Arab Emirates exhibit diverse compositions, highlighting variations in petrogenesis and source contributions. Previous isotopic data indicate these MHG originated through the interaction of sediment-derived with basaltic melts from an underthrust oceanic plate within the mantle wedge. The sedimentary contribution was attributed to the partial melting of pelitic to siliceous (bio-siliceous) material atop the subducted plate based on elevated zircon δ¹⁸O values (∼14–28 ‰). To further evaluate this hypothesis on Samail MHG petrogenesis and source contribution, we present new and compiled radiogenic (Sr-Nd-Hf-Pb) and stable (O-Li-H) isotopes, along with zircon trace element analyses. The variable Sr and Pb isotopic signature support a mixed origin involving altered mafic and sedimentary sources in the formation of the MHG. Negative whole-rock εNd, coupled with elevated δ⁷Li in muscovite suggest the involvement of sedimentary sources and particularly those resembling deep-sea ferromanganese-rich sediments. We propose a new model identifying ferromanganese sediments as a potential source given their widespread distribution across the ocean floor, broad range of δ¹⁸O (up to 29.5 ‰), slightly positive Hf values, seawater-like δ⁷Li signatures (median of ∼27 ‰), and zircon trace element compositions lacking a signature of monazite co-precipitation, which match the signatures required for the genesis of the Samail MHG. Preservation of oceanic lithosphere in the geological record is limited, and MHG in ophiolites are uncommon. Therefore, the Samail MHG are key examples of crustal materials transported to the mantle, with implications for mantle heterogeneity and arc mantle redox budget.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122759"},"PeriodicalIF":3.6,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143814983","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":"Effects of adsorption and oxidation of dissolved organic matter by ferrihydrite and/or birnessite on its binding characteristics to copper","authors":"Zecong Ding ,&nbsp;Yang Ding ,&nbsp;Lanlan Zhu ,&nbsp;Kai Cui ,&nbsp;Jiang Xiao ,&nbsp;Qianting Ye ,&nbsp;Zhenqing Shi","doi":"10.1016/j.chemgeo.2025.122766","DOIUrl":"10.1016/j.chemgeo.2025.122766","url":null,"abstract":"<div><div>Dissolved organic matter (DOM) is a natural ligand for heavy metal binding. Adsorption and oxidation of DOM by iron and/or manganese oxides may change its molecular composition and affect its binding with heavy metals, but the underlying mechanisms are still poorly understood. In this study, the effects of adsorption and oxidation of DOM by ferrihydrite and/or birnessite on its binding characteristics with copper ions (Cu²⁺) were investigated using fluorescence spectroscopy and Fourier transform infrared spectroscopies combined with two-dimensional correlation analysis. Results showed that the adsorption and oxidation of DOM on birnessite significantly weakened the Cu²⁺ binding ability of DOM especially in the presence ferrihydrite, which was ascribed to the preferentially adsorption/oxidation of phenolic compounds and the adsorption of carboxylic compounds. Specifically, the reactions of DOM with birnessite enhanced the sensitivity of fulvic-like fraction to Cu²⁺ binding, being less favorable in the presence of a high concentration of ferrihydrite. Despite the unchanged binding order of Cu²⁺ with fluorescence components after adsorption and oxidation of DOM on birnessite, the binding of Cu²⁺ with carboxylic group became later compared with carbohydrate C–O, phenolic and aryl groups and aliphatic C–H. However, with the increase in the content of ferrihydrite, the binding of Cu²⁺ with carboxylic groups was more advanced. Our results highlighted the importance of the ratio of birnessite and ferrihydrite in controlling the binding characteristics of Cu²⁺ with DOM fractions, which helps to understand the reactions of heavy metals with natural organic matter under the impacts of complex mineral assemblages.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122766"},"PeriodicalIF":3.6,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143759992","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":"Characterizing ancient seep environments by in-situ sulfur isotope composition of authigenic pyrite","authors":"Qinyi Zhang ,&nbsp;Zhiyong Lin ,&nbsp;Harald Strauss ,&nbsp;Tingting Chen ,&nbsp;Yang Lu ,&nbsp;James L. Goedert ,&nbsp;Jörn Peckmann","doi":"10.1016/j.chemgeo.2025.122767","DOIUrl":"10.1016/j.chemgeo.2025.122767","url":null,"abstract":"<div><div>Authigenic pyrite commonly coexists with authigenic carbonate forming at marine methane seeps. Both pyrite and carbonate minerals are by-products of sulfate-driven anaerobic oxidation of methane (SD-AOM), the dominant biogeochemical process at seeps. The sulfur isotopic composition of pyrite has been extensively used to study the sulfur cycle and seepage activity in modern seep environments. However, to what extent δ³⁴S_py values can aid the interpretation of ancient seep environments is poorly known. To improve on this knowledge gap, secondary ion mass spectroscopy (SIMS) was applied to investigate the sulfur isotope patterns of different types of pyrite (framboidal and euhedral) on a microscale from seep deposits of Beauvoisin (Jurassic, France) and Whiskey Creek (Eocene, USA), which is supplemented by the sulfur isotope measurement of bulk pyrite (chromium reducible sulfur; CRS). Framboids of the Beauvoisin seep deposits show low δ³⁴S_py values (−40.1 ‰ to −23.9 ‰), those of the Whiskey Creek deposit exhibit a wider range of values (−40.7 ‰ to 9.9 ‰). Most of the euhedral pyrite from both sites is typified by higher δ³⁴S_py values (4.4 ‰ to 5.7 ‰ for Beauvoisin; 0.4 ‰ to 3.6 ‰ for Whiskey Creek). The δ³⁴S_CRS values of Beauvoisin deposits (−23.0 ‰ to 1.80 ‰) are also lower than those of Whiskey Creek (−10.8 ‰ to 2.9 ‰). The large variations in δ³⁴S_py values, reflecting different types of pyrite, suggest that pyrite formed episodically over a prolonged period of seepage during early diagenesis. Overall, high rates of sulfate reduction and high replenishment of the pore water pool of sulfate with seawater sulfate at active methane seeps controlled the crystal habit and the sulfur isotope composition of the studied pyrite. Higher δ³⁴S_py values of euhedral pyrite record progressively sulfate-limited conditions where sulfate consumption by SD-AOM exceeded sulfate replenishment – as observed for later-stage pyrite extracted from modern seepage-affected sediment with δ³⁴S_py values higher than 100 ‰. The lack of such extremely high δ³⁴S_py values within the studied seep limestones is probably not only controlled by the degree of replenishment of seawater sulfate but also by the engulfment of pyrite by authigenic seep carbonate, impeding the formation of later stages of pyrite. Overall, the identified sulfur isotope patterns of pyrite preserved in the Beauvoisin and Whiskey Creek seep deposits resemble observations made on pyrite from modern seeps, confirming that the sulfur isotopic composition of pyrite can serve as a potent tool for reconstructing the biogeochemical cycling of sulfur in ancient seep environments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122767"},"PeriodicalIF":3.6,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143768390","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":"Salinity variation in Carboniferous cyclothemic successions","authors":"Thomas J. Algeo ,&nbsp;Wei Wei ,&nbsp;Dustin E. Sweet","doi":"10.1016/j.chemgeo.2025.122760","DOIUrl":"10.1016/j.chemgeo.2025.122760","url":null,"abstract":"<div><div>Late Paleozoic seas on the North American Craton were subject to large fluctuations in elevation and facies in response to Gondwanan glacio-eustasy, but quantification of salinity variation has proven difficult. Here, we reconstruct salinity variation using two proxies (B/Ga and “excess boron” [B_xs]) through ten cyclothems spanning the upper Desmoinesian through Missourian stages (∼308–303 Ma) in a single core from the Late Pennsylvanian Midcontinent Shelf. These proxies record cyclic variation between normal-marine (30–40 psu) and high-brackish conditions (15–30 psu), with occasional excursions to low-brackish conditions (1–15 psu). Most limestone formations accumulated under marine salinity conditions although a few (i.e., the argillaceous Exline and Raytown limestones) were brackish. Shales exhibit a wider range of salinity variation: organic-rich core shales are associated with fully marine conditions and outside shales mostly with reduced-salinity conditions. The lithofacies and salinity history of the study core provide insights into contemporaneous tectono-glacial events of the Middle to Late Pennsylvanian transition: the Pawnee-Altamont cyclothems record a major basinal subsidence event followed by large-scale influx of deltaic siliciclastics of the Nowata Shale linked to latest Desmoinesian uplift in the Ouachita-Marathon orogens. This North American tectonic event, coupled with other concurrent orogens globally, may have triggered a deep glacially driven sea-level fall around the Desmoinesian/Missourian stage boundary. This event heralded the onset of large-amplitude, high-frequency glacio-eustatic fluctuations of the Missourian Stage, signaling a fundamental shift toward increased Gondwanan icesheet instability.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122760"},"PeriodicalIF":3.6,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143783851","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":"OH-isoGDGTs as a paleothermometer for bottom seawater temperature in the eastern China marginal seas","authors":"Yuning Wang ,&nbsp;Huamao Yuan ,&nbsp;Jinming Song ,&nbsp;Junhui Chen ,&nbsp;Fengmin Pan ,&nbsp;Qidong Wang ,&nbsp;Xuegang Li ,&nbsp;Liqin Duan ,&nbsp;Ning Li ,&nbsp;Yueqi Wang","doi":"10.1016/j.chemgeo.2025.122768","DOIUrl":"10.1016/j.chemgeo.2025.122768","url":null,"abstract":"<div><div>Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-isoGDGTs), which exhibit similar behavior to regular isoprenoid GDGTs (isoGDGTs), have shown potential for reconstructing past seawater temperature than isoGDGTs in marginal seas due to their reduced impact from terrestrial inputs. However, there are marginal seas exhibit regional variability, such as water depth and seasonality bias, in terms of how isoGDGTs and OH-isoGDGTs reflect the marine environment. In this study, we investigated OH-isoGDGTs in both suspended particulate matters and surface sediments of the eastern China marginal seas (ECMS), to constrain which water layer temperature the sedimentary OH-isoGDGTs-based indices better represents. Our results showed that OH-isoGDGTs concentrations were generally higher in bottom waters than in surface waters. Additionally, the composition of OH-isoGDGTs (and also isoGDGTs) in surface sediments showed no significant differences (<em>p</em> &gt; 0.05) with that of bottom waters and significant differences (<em>p</em> &lt; 0.05) with that of surface waters, indicating that OH-isoGDGTs in surface sediments were mainly derived from the deposition of bottom waters. Furthermore, OH-isoGDGTs-based indices in surface sediments correlated better with bottom seawater temperature (BST) than with surface seawater temperature (SST), reinforcing the notion that sedimentary OH-isoGDGTs mainly derive from bottom waters. Moreover, OH-isoGDGTs-based indices showed stronger correlations with summer and autumn temperatures compared to other seasons, and exhibited improved accuracy in BST reconstruction than isoGDGTs-based TEX₈₆ indices, suggesting that sedimentary OH-isoGDGTs-based indices are potential BST proxies for warm seasons in the ECMS. Our findings enhance the understanding of the potential of OH-isoGDGTs as paleothermometer proxies in marginal seas and offer insights into their application for more precise paleoceanographic reconstructions.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122768"},"PeriodicalIF":3.6,"publicationDate":"2025-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143820233","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":"Reactivity of chlorite and gneiss from ONKALO (Western Finland): Laboratory experiments and reactive transport modeling","authors":"Josep M. Soler ,&nbsp;Jordi Cama ,&nbsp;Tiina Lamminmäki ,&nbsp;Orlando Silva ,&nbsp;José A. Jiménez ,&nbsp;Atefeh Vafaie ,&nbsp;Antti Joutsen ,&nbsp;Lasse Koskinen ,&nbsp;Antti Poteri","doi":"10.1016/j.chemgeo.2025.122762","DOIUrl":"10.1016/j.chemgeo.2025.122762","url":null,"abstract":"<div><div>The Finnish geological repository for spent nuclear fuel is being constructed at the ONKALO® underground facility in Olkiluoto Island. The bedrock is mainly migmatitic gneiss, including brittle fault zones where groundwater flows. An important process is the dissolution of chlorite, which releases Fe(II) that can lead to FeS precipitates and regulate dissolved HS⁻ concentrations. Elevated HS⁻ concentrations could induce corrosion of the copper canisters containing the spent fuel.</div><div>Two powder chlorite samples from faults in the gneiss and provided by Posiva Oy (Finnish nuclear waste management company) were characterized. They contained both muscovite and chlorite. Laboratory batch and flow-through experiments were performed to study the solubilities and dissolution reaction rates of the chlorite (and muscovite) samples. The results were consistent with previous results reported in the literature.</div><div>Two core-infiltration experiments were also performed, using a gneiss sample containing a fracture (also provided by Posiva Oy). The results were interpreted by means of one- and two-dimensional reactive transport models. The 1D model included flow, solute transport and reaction only along the fracture. It needed unrealistically large mineral surface areas to reproduce the experimental results. In addition to the fracture, the 2D model included transport by diffusion in the rock matrix, with mineral reactions in both fracture and matrix. With this approach the large surface areas in the fracture were no longer needed, highlighting the reactivity of the rock matrix despite the small porosities (about 1 %) and diffusivities. The calculated dissolution of chlorite could indeed release Fe(II) under repository conditions.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122762"},"PeriodicalIF":3.6,"publicationDate":"2025-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143768391","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 evolution beneath a back-arc basin: Highly siderophile elements and Os isotope of Dongco ophiolitic peridotites, Tibet","authors":"Xiao-Ni Li ,&nbsp;Wei-Qi Zhang ,&nbsp;Tong Liu ,&nbsp;Chang Zhang ,&nbsp;Yin-Zheng Lin ,&nbsp;Zhen Zhang ,&nbsp;Jia-Ning Zhu ,&nbsp;Hong-Yu Li ,&nbsp;Chuan-Zhou Liu","doi":"10.1016/j.chemgeo.2025.122757","DOIUrl":"10.1016/j.chemgeo.2025.122757","url":null,"abstract":"<div><div>Coexistence of mantle peridotites with distinct evolutionary histories is commonly observed in global ophiolites, especially in supra-subduction zone (SSZ) settings. The Dongco ophiolite in central Tibet has been interpreted to form within a Jurassic slow-spreading back-arc basin. This study presents comprehensive geochemical data of two types of peridotites from the Dongco ophiolite to investigate processes that occurred in the asthenosphere beneath a back-arc basin. Type-1 peridotites contain 5–9 vol% clinopyroxene and have spinel Cr# values &lt;0.21, which are more fertile in compositions compared to Type-2 peridotites. Type-1 peridotites are characterized by relatively flat whole-rock REE patterns, but variably LREE-depleted patterns for pyroxenes. Their HSE patterns are slightly fractionated. Type-1 peridotites are estimated to have experienced 2–8 % degrees of melting under water-poor conditions, similar to mid-ocean ridge (MOR) peridotites. In contrast, Type-2 peridotites contain less than 2 vol% clinopyroxene and have spinel Cr# &gt;0.55. They display U-shaped whole-rock REE patterns and LREE-enriched patterns for pyroxenes. Their HSE patterns are highly fractionated, with positive platinum anomalies and supra-chondritic Pt/Pd and Ru/Ir ratios. Type-2 peridotites are estimated to have undergone 20–24 % degrees of melting under hydrous conditions, resembling typical SSZ peridotites. Both types of Dongco peridotites have similar ranges of ¹⁸⁷Os/¹⁸⁸Os ratios, yielding Re depletion ages (T_RD) of 0.2–1.0 Ga. We interpret the Dongco peridotites represent residues of distinct mantle sources generated at different stages during the evolution of a back-arc basin. Type-2 peridotites represent residues of mantle wedge after hydrous melting and thus represent SSZ- peridotites; Type-1 peridotites are relics of the asthenosphere after low degrees of melting that were negligibly affected by slab-released fluids or hydrous melts, and belong to MOR- peridotites. The tectonic juxtaposition of SSZ- and MOR-type peridotites in the Dongco ophiolite supports the hypothesis of partial replacement of sub-arc mantle by the upwelling asthenosphere during back-arc spreading.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"683 ","pages":"Article 122757"},"PeriodicalIF":3.6,"publicationDate":"2025-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143768389","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":"Reassessing the photochemical contribution to Archean Banded Iron Formations","authors":"Pilar C. Vergeli ,&nbsp;Stephen J. Romaniello ,&nbsp;Hilairy E. Hartnett ,&nbsp;Ariel D. Anbar","doi":"10.1016/j.chemgeo.2025.122763","DOIUrl":"10.1016/j.chemgeo.2025.122763","url":null,"abstract":"<div><div>Banded iron formations (BIFs) are massive iron (Fe) oxide deposits that provide information about the Precambrian environment. A biological origin for BIFs is commonly invoked. Some studies have suggested on theoretical and experimental grounds that an abiotic origin of BIFs is possible, via UV photo-oxidation of dissolved Fe(II) species, but this conclusion has been disputed. Here, we provide an experimental assessment of the viability of the photochemical hypothesis under the most realistic Archean environmental conditions simulated to date. These conditions include realistic seawater salinity and carbonate buffering, and a light source that simulates the solar UV spectrum. We report the wavelength dependence of photo-oxidation rates and calculate the quantum yield for Fe³⁺ production from FeOH⁺ (<span><math><mi>Φ</mi></math></span> = 0.03) and from Fe²⁺(aq) (<span><math><mi>Φ</mi></math></span> = 0.1). Using these quantum yields in a photogeochemical model, we calculate the rate of Fe³⁺ production by photo-oxidation as 96 mg⁻¹ cm⁻² yr⁻¹. Previous studies that do not consider wavelength-dependent effects overestimate the rate of Fe deposition by almost 50 %. Nevertheless, our modeled rate is sufficient to account for the mass of Fe deposited in BIFs at all but the most extreme periods of iron deposition, in the Neoarchean, when other mechanisms such as oxidation by photosynthetically produced O₂ or photoferrotrophy must be invoked.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"685 ","pages":"Article 122763"},"PeriodicalIF":3.6,"publicationDate":"2025-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143902274","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}